2024 |
Bhowmick, S., Chaudhary, A., Jishad, M., Seemanth, M., Utkarsh, Agarwal, N., et al. (2024). How Useful Are Mispointing Phase Saral/Altika Geophysical Products For Ocean Applications? Advances In Space Research, 737(1), 414–425.
Abstract: Saral/Altika, The First Microwave Altimeter Operating At Ka -Band Frequency, Recently Completed Nine Years Of Operations In Orbit. During These Years, It Has Catered To Many Applications Related To Operational Oceanography, Climate Sciences, Hydrology And Cryosphere. More Specifically, In Oceanography, Saral Has Contributed Immensely To Operational Wave And Circulation Modelling, Eddy Detection/Tracking, Ocean Current Generation And Many More. However, Since Feb 2019, Saral Has Moved From The Drifting Phase (Dp) To The Mispointing Phase (Mp) Due To The Malfunctioning Of The Star Sensor Of The Spacecraft. In This Study, We Analyse The Instrument'S Performance And Its Waveforms During Its Ongoing Mp. We Find Out That During The Mp, Significant Wave Height (Swh) Measurements Are Anomalously High Between 18 And 24 M, And Wind Speed Measurements Are Between 16 And 19 M/S. In Sea Surface Height Anomaly (Ssha), There Is A Steady Rise In Negative Values During The Mp. In The Return Waveform, -15% Degradation In Brown -Type Waveforms In The Open Ocean Region Is Noticed. These Changes Significantly Impact The Saral Applications. Two Important Applications Of Wave Forecast And Eddy Detection Are Discussed Here As Examples. Following This, We Also Recommend Using Provided Quality Flags So That The Data Can Be Further Explored For Various Ocean Applications. (C) 2023 Cospar. Published By Elsevier B.V. All Rights Reserved.
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Sadhvi, K., Suresh, I., Lengaigne, M., Izumo, T., Penduff, T., Molines, J., et al. (2024). Intrinsic Versus Wind-Forced Great Whirl Non-Seasonal Variability. Journal Of Geophysical Research-Oceans, 1291(2).
Abstract: The Great Whirl (Gw) Is A Quasi-Permanent Anticyclonic Eddy That Appears Every Summer Monsoon Offshore Of The Somalia Upwelling. The Annual Cycle Of The Gw Is Well Described, But Deviations From Its Mean Seasonal Cycle (Hereafter Non-Seasonal Variability) Have Been Less Explored. Satellite Observations Reveal That The Leading Mode Of Summer Non-Seasonal Sea-Level Variability In This Region Is Associated With Similar To 100-Km Northward Or Southward Gw Shifts From Its Climatological Position. Northward Shifts Are Associated With A Stronger Gw, And Two Cold, Productive Coastal Upwelling Wedges At 5 Degrees N And 10 Degrees N. Southward Shifts Are Associated With A Weaker Gw, No Wedge At 5 Degrees N And A Single Stronger-Than-Usual Cold And Productive Wedge At 10 Degrees N. An Eddy-Permitting (25-Km Resolution) 50-Member Ensemble Ocean Simulation Reproduces This Gw Variability Well. It Indicates That The Non-Seasonal Gw Variability Has A Short Similar To 20 Days Timescale Intrinsic Component, Associated With The Gw Interaction With Mesoscale Eddies, And A Lower-Frequency, Similar To 100 Days Externally Forced Component. Intrinsic Variability Dominates At Both Subseasonal (Two Thirds Of The Variance) And Interannual Timescales (57% Of The Variance). The Externally Forced Signal Results From Shifts In The Probability Distribution Of The Subseasonal Gw Position (E.G., More Likely Northward Than Southward Shifted Instantaneous Gw Positions Over A Season). The Mechanism For This External Forcing Is Not Entirely Clear, But It Appears To Be Related To The Rossby Wave Response To Offshore Wind Stress Curl Forcing, Which Evolves Into A North-South Dipole That Projects Onto The Gw Variability Pattern. The Great Whirl (Gw) Is A Similar To 500-Km Diameter Oceanic Eddy That Forms Each Summer Off The Somalia Coast. The Gw Influences The Temperature And Biological Productivity Of The Somalia Coastal Upwelling. The Gw Seasonal Cycle, Including Its Northward Movement At The End Of The Monsoon, Is Known. Here, We Study Deviations From This Mean Seasonal Cycle. The Gw Exhibits Fast (Typically 20 Days Long), Similar To 100-Km Northward Or Southward Displacements. These Displacements Result From Interactions With Smaller Neighboring Oceanic Eddies, And Are Hence The Consequence Of An Intrinsic Ocean Dynamics, Rather Than Atmospheric Forcing. The Gw Is More Pronounced When Displaced Northward, And The Somalia Coastal Upwelling Has Two Clear “Wedges” Of Cold, Plankton-Rich Water At 5 Degrees And 10 Degrees N. When Displaced Southward, The Gw Is Less Pronounced, And There Is A Single, Stronger-Than-Usual Wedge Of Cold Water At 10 Degrees N. We Also Show That Atmospheric Forcing Can Induce Seasonal Gw Displacements, By Making, For Example, Northward Events More Likely Than The Southward. Thus, The Seasonal Mean Gw Position Has Both A Predictable (Due To Atmospheric Forcing) And Slightly Larger Unpredictable (Due To Interactions With Eddies) Component. We Propose That The Atmospheric Forcing Influences The Fast Oscillations Of The Gw Through Slow Oceanic Adjustment To Wind Variations Further East. The Great Whirl (Gw) Non-Seasonal Variability Is Dominated By Similar To 100-Km Northward Or Southward Shifts Relative To Its Climatological Position Those Shifts Induce Sea Level, Surface Temperature And Chlorophyll Signals Those Gw Displacements Have A Fast (Similar To 20 Days) Intrinsic Variability Component And A Slower, Seasonal Forced Component
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Uchida, T., Jamet, Q., Dewar, W., Deremble, B., Poje, A., & Sun, L. (2024). Imprint Of Chaos On The Ocean Energy Cycle From An Eddying North Atlantic Ensemble. Journal Of Physical Oceanography, 545(3), 679–696.
Abstract: We Examine The Ocean Energy Cycle Where The Eddies Are Defined About The Ensemble Mean Of A Partially Air-Sea Coupled, Eddy -Rich Ensemble Simulation Of The North Atlantic. The Decomposition About The Ensemble Mean Leads To A Parameter -Free Definition Of Eddies, Which Is Interpreted As The Expression Of Oceanic Chaos. Using The Ensemble Framework, We Define The Reservoirs Of Mean And Eddy Kinetic Energy (Mke And Eke, Respectively) And Mean Total Dynamic Enthalpy (Mtde). We Opt For The Usage Of Dynamic Enthalpy (De) As A Proxy For Potential Energy Due To Its Dynamically Consistent Relation To Hydrostatic Pressure In Boussinesq Fluids And Nonreliance On Any Reference Stratification. The Curious Result That Emerges Is That The Potential Energy Reservoir Cannot Be Decomposed Into Its Mean And Eddy Components, And The Eddy Flux Of De Can Be Absorbed Into The Eke Budget As Pressure Work. We Find From The Energy Cycle That While Baroclinic Instability, Associated With A Positive Vertical Eddy Buoyancy Flux, Tends To Peak Around February, Eke Takes Its Maximum Around September In The Wind -Driven Gyre. Interestingly, The Energy Input From Mke To Eke, A Process Sometimes Associated With Barotropic Processes, Becomes Larger Than The Vertical Eddy Buoyancy Flux During The Summer And Autumn. Our Results Question The Common Notion That The Inverse Energy Cascade Of Wintertime Eke Energized By Baroclinic Instability Within The Mixed Layer Is Solely Responsible For The Summer -To -Autumn Peak In Eke And Suggest That Both The Eddy Transport Of De And Transfer Of Energy From Mke To Eke Contribute To The Seasonal Eke Maxima.
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Wang, Q., Shu, Q., Bozec, A., Chassignet, E., Fogli, P., Fox-Kemper, B., et al. (2024). Impact Of Increased Resolution On Arctic Ocean Simulations In Ocean Model Intercomparison Project Phase 2 (Omip-2). Geoscientific Model Development, 171(1), 347–379.
Abstract: This Study Evaluates The Impact Of Increasing Resolution On Arctic Ocean Simulations Using Five Pairs Of Matched Low- And High-Resolution Models Within The Omip-2 (Ocean Model Intercomparison Project Phase 2) Framework. The Primary Objective Is To Assess Whether A Higher Resolution Can Mitigate Typical Biases In Low-Resolution Models And Improve The Representation Of Key Climate-Relevant Variables. We Reveal That Increasing The Horizontal Resolution Contributes To A Reduction In Biases In Mean Temperature And Salinity And Improves The Simulation Of The Atlantic Water Layer And Its Decadal Warming Events. A Higher Resolution Also Leads To Better Agreement With Observed Surface Mixed-Layer Depth, Cold Halocline Base Depth And Arctic Gateway Transports In The Fram And Davis Straits. However, The Simulation Of The Mean State And Temporal Changes In Arctic Freshwater Content Does Not Show Improvement With Increased Resolution. Not All Models Achieve Improvements For All Analyzed Ocean Variables When Spatial Resolution Is Increased So It Is Crucial To Recognize That Model Numerics And Parameterizations Also Play An Important Role In Faithful Simulations. Overall, A Higher Resolution Shows Promise In Improving The Simulation Of Key Arctic Ocean Features And Processes, But Efforts In Model Development Are Required To Achieve More Accurate Representations Across All Climate-Relevant Variables.
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2023 |
Belmajdoub, H., El Aouni, A., & Minaoui, K. (2023). Convolutional Neural Networks For Coastal Upwelling Monitoring Along The Atlantic Coast Of Morocco. Remote Sensing Letters, , 77577–78577.
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Belmajdoub, H., Minaoui, K., El Aouni, A., Hilmi, K., Saadane, R., & Chehri, A. (2023). A New Upwelling Index For The Moroccan Atlantic Coast For The Period Between 1982-2021. Remote Sensing, .
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Bendinger, A., Cravatte, S., Gourdeau, L., Brodeau, L., Albert, A., Tchilibou, M., et al. (2023). Regional Modeling Of Internal-Tide Dynamics Around New Caledonia – Part 1: Coherent Internal-Tide Characteristics And Sea Surface Height Signature. Ocean Science, 191(4), 1315–1338.
Abstract: The Southwestern Tropical Pacific Exhibits A Complex Bathymetry And Represents A Hot Spot Of Internal-Tide Generation. Based On A Tailored High-Resolution Regional Model, We Investigate For The First Time The Internal-Tide Field Around The New Caledonia Islands Through Energy Budgets That Quantify The Coherent Internal-Tide Generation, Propagation, And Dissipation. A Total Of 15.27 Gw Is Converted From The Barotropic To The Baroclinic M2 Tide With The Main Conversion Sites Associated With The Most Prominent Bathymetric Structures Such As Continental Slopes And Narrow Passages In The North (2.17 Gw) And Ridges And Seamounts South Of New Caledonia (3.92 Gw). The Bulk Of Baroclinic Energy Is Generated In Shallow Waters Around 500 M Depth And On Critical To Supercritical Slopes, Highlighting The Limitations Of Linear Semi-Analytical Models In Those Areas. Despite The Strongly Dominant Mode-1 Generation, More Than 50 % Of The Locally Generated Energy Either Dissipates In The Near Field Close To The Generation Sites Or Loses Coherence. The Remaining Baroclinic Energy Propagates Within Well-Defined Tidal Beams With Baroclinic Energy Fluxes Of Up To 30 Kw M – 1 Toward The Open Ocean. The New Caledonia Site Represents A Challenge For Swot (Surface Water And Ocean Topography) Observability Of Balanced Motion In The Presence Of Internal Tides With Sea Surface Height (Ssh) Signatures > 6 Cm At Similar Wavelengths. We Show For Our Study Region That A Correction Of Ssh For The Coherent Internal Tide Potentially Increases The Observability Of Balanced Motion From Wavelengths > 160 Km To Well Below 100 Km.
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Burgard, C., Jourdain, N., Mathiot, P., Smith, R., Schaefer, R., Caillet, J., et al. (2023). Emulating Present And Future Simulations Of Melt Rates At The Base Of Antarctic Ice Shelves With Neural Networks. Journal Of Advances In Modeling Earth Systems, 151(121).
Abstract: Melt Rates At The Base Of Antarctic Ice Shelves Are Needed To Drive Projections Of The Antarctic Ice Sheet Mass Loss. Current Basal Melt Parameterizations Struggle To Link Open Ocean Properties To Ice-Shelf Basal Melt Rates For The Range Of Current Sub-Shelf Cavity Geometries Around Antarctica. We Present A Proof Of Concept Exploring The Potential Of Simple Deep Learning Techniques To Parameterize Basal Melt. We Train A Simple Feedforward Neural Network, Or Multilayer Perceptron, Acting On Each Grid Cell Separately, To Emulate The Behavior Of Circum-Antarctic Cavity-Resolving Ocean Simulations. We Find That This Kind Of Emulator Produces Reasonable Basal Melt Rates For Our Training Ensemble, At Least As Close As Or Closer To The Reference Than Traditional Parameterizations. On An Independent Ensemble Of Simulations That Was Produced With The Same Ocean Model But With Different Model Parameters, Cavity Geometries And Forcing, The Neural Network Yields Similar Results To Traditional Parameterizations On Present Conditions. In Much Warmer Conditions, Both Traditional Parameterizations And Neural Network Struggle, But The Neural Network Tends To Produce Basal Melt Rates Closer To The Reference Than A Majority Of Traditional Parameterizations. While This Shows That Such A Neural Network Is At Least As Suitable For Century-Scale Antarctic Ice-Sheet Projections As Traditional Parameterizations, It Also Highlights That Tuning Any Parameterization On Present-Like Conditions Can Introduce Biases And Should Be Used With Care. Nevertheless, This Proof Of Concept Is Promising And Provides A Basis For Further Development Of A Deep Learning Basal Melt Parameterization. A Warmer Ocean Around Antarctica Leads To Higher Melting Of The Floating Ice Shelves, Which Influence The Ice Loss From The Antarctic Ice Sheet And Therefore Sea-Level Rise. In Computer Simulations Of The Ocean, These Ice Shelves Are Often Not Represented. For Simulations Of The Ice Sheet, So-Called Parameterizations Are Used To Link The Oceanic Properties In Front Of The Shelf And The Melt At Their Base. We Show That This Link Can Be Emulated With A Simple Neural Network, Which Performs At Least As Well As Traditional Physical Parameterizations Both For Present And Much Warmer Conditions. This Study Also Proposes Several Potential Ways Of Further Improving The Use Of Deep Learning To Parameterize Basal Melt. We Show That Simple Neural Networks Produce Reasonable Basal Melt Rates By Emulating Circum-Antarctic Cavity-Resolving Ocean Simulationspredicted Melt Rates For Present And Warmer Conditions Are Similar Or Closer To The Reference Simulation Than Traditional Parameterizationswe Show That Neural Networks Are Suited To Be Used As Basal Melt Parameterizations For Century-Scale Ice-Sheet Projections
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Burgard, C., Jourdain, N., Mathiot, P., Smith, R., Schäfer, R., Caillet, J., et al. (2023). Emulating Present And Future Simulations Of Melt Rates At The Base Of Antarctic Ice Shelves With Neural Networks. Journal Of Advances In Modeling Earth Systems, 151(121).
Abstract: Melt Rates At The Base Of Antarctic Ice Shelves Are Needed To Drive Projections Of The Antarctic Ice Sheet Mass Loss. Current Basal Melt Parameterizations Struggle To Link Open Ocean Properties To Ice-Shelf Basal Melt Rates For The Range Of Current Sub-Shelf Cavity Geometries Around Antarctica. We Present A Proof Of Concept Exploring The Potential Of Simple Deep Learning Techniques To Parameterize Basal Melt. We Train A Simple Feedforward Neural Network, Or Multilayer Perceptron, Acting On Each Grid Cell Separately, To Emulate The Behavior Of Circum-Antarctic Cavity-Resolving Ocean Simulations. We Find That This Kind Of Emulator Produces Reasonable Basal Melt Rates For Our Training Ensemble, At Least As Close As Or Closer To The Reference Than Traditional Parameterizations. On An Independent Ensemble Of Simulations That Was Produced With The Same Ocean Model But With Different Model Parameters, Cavity Geometries And Forcing, The Neural Network Yields Similar Results To Traditional Parameterizations On Present Conditions. In Much Warmer Conditions, Both Traditional Parameterizations And Neural Network Struggle, But The Neural Network Tends To Produce Basal Melt Rates Closer To The Reference Than A Majority Of Traditional Parameterizations. While This Shows That Such A Neural Network Is At Least As Suitable For Century-Scale Antarctic Ice-Sheet Projections As Traditional Parameterizations, It Also Highlights That Tuning Any Parameterization On Present-Like Conditions Can Introduce Biases And Should Be Used With Care. Nevertheless, This Proof Of Concept Is Promising And Provides A Basis For Further Development Of A Deep Learning Basal Melt Parameterization. A Warmer Ocean Around Antarctica Leads To Higher Melting Of The Floating Ice Shelves, Which Influence The Ice Loss From The Antarctic Ice Sheet And Therefore Sea-Level Rise. In Computer Simulations Of The Ocean, These Ice Shelves Are Often Not Represented. For Simulations Of The Ice Sheet, So-Called Parameterizations Are Used To Link The Oceanic Properties In Front Of The Shelf And The Melt At Their Base. We Show That This Link Can Be Emulated With A Simple Neural Network, Which Performs At Least As Well As Traditional Physical Parameterizations Both For Present And Much Warmer Conditions. This Study Also Proposes Several Potential Ways Of Further Improving The Use Of Deep Learning To Parameterize Basal Melt. We Show That Simple Neural Networks Produce Reasonable Basal Melt Rates By Emulating Circum-Antarctic Cavity-Resolving Ocean Simulationspredicted Melt Rates For Present And Warmer Conditions Are Similar Or Closer To The Reference Simulation Than Traditional Parameterizationswe Show That Neural Networks Are Suited To Be Used As Basal Melt Parameterizations For Century-Scale Ice-Sheet Projections
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Deremble, B., Uchida, T., Dewar, W., & Samelson, R. (2023). Eddy-Mean Flow Interaction With A Multiple Scale Quasi Geostrophic Model. Journal Of Advances In Modeling Earth Systems, 151(101).
Abstract: Parameterization Of Mesoscale Eddies In Coarse Resolution Ocean Models Is Necessary To Include The Effect Of Eddies On The Large-Scale Oceanic Circulation. We Propose To Use A Multiple-Scale Quasi-Geostrophic (Msqg) Model To Capture The Eddy Dynamics That Develop In Response To A Prescribed Large-Scale Flow. The Msqg Model Consists In Extending The Traditional Quasi Geostrophic (Qg) Dynamics To Include The Effects Of A Variable Coriolis Parameter And Variable Background Stratification. Solutions To This Msqg Equation Are Computed Numerically And Compared To A Full Primitive Equation Model. The Large-Scale Flow Field Permits Baroclinically Unstable Qg Waves To Grow. These Instabilities Saturate Due To Non-Linearities And A Filtering Method Is Applied To Remove Large-Scale Structures That Develop Due To The Upscale Cascade. The Resulting Eddy Field Represents A Dynamically Consistent Response To The Prescribed Background Flow, And Can Be Used To Rectify The Large-Scale Dynamics. Comparisons Between Gent-Mcwilliams Eddy Parameterization And The Present Solutions Show Large Regions Of Agreement, While Also Indicating Areas Where The Eddies Feed Back Onto The Large Scale In A Manner That The Gent-Mcwilliams Parameterization Cannot Capture. Also Of Interest Is The Time Variability Of The Eddy Feedback Which Can Be Used To Build Stochastic Eddy Parameterizations.
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Deremble, B., Uchida, T., Dewar, W., & Samelson, R. (2023). Eddy-Mean Flow Interaction With A Multiple Scale Quasi Geostrophic Model. Journal Of Advances In Modeling Earth Systems, 151(101).
Abstract: Parameterization Of Mesoscale Eddies In Coarse Resolution Ocean Models Is Necessary To Include The Effect Of Eddies On The Large-Scale Oceanic Circulation. We Propose To Use A Multiple-Scale Quasi-Geostrophic (Msqg) Model To Capture The Eddy Dynamics That Develop In Response To A Prescribed Large-Scale Flow. The Msqg Model Consists In Extending The Traditional Quasi Geostrophic (Qg) Dynamics To Include The Effects Of A Variable Coriolis Parameter And Variable Background Stratification. Solutions To This Msqg Equation Are Computed Numerically And Compared To A Full Primitive Equation Model. The Large-Scale Flow Field Permits Baroclinically Unstable Qg Waves To Grow. These Instabilities Saturate Due To Non-Linearities And A Filtering Method Is Applied To Remove Large-Scale Structures That Develop Due To The Upscale Cascade. The Resulting Eddy Field Represents A Dynamically Consistent Response To The Prescribed Background Flow, And Can Be Used To Rectify The Large-Scale Dynamics. Comparisons Between Gent-Mcwilliams Eddy Parameterization And The Present Solutions Show Large Regions Of Agreement, While Also Indicating Areas Where The Eddies Feed Back Onto The Large Scale In A Manner That The Gent-Mcwilliams Parameterization Cannot Capture. Also Of Interest Is The Time Variability Of The Eddy Feedback Which Can Be Used To Build Stochastic Eddy Parameterizations.
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Diaz, E., Varando, G., Johnson, J., & Camps-Valls, G. (2023). Learning Latent Functions For Causal Discovery. Machine Learning-Science And Technology, .
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Hutchinson, K., Deshayes, J., Ethe, C., Rousset, C., De Lavergne, C., Vancoppenolle, M., et al. (2023). Improving Antarctic Bottom Water Precursors In Nemo For Climate Applications. Geoscientific Model Development, , 362933–365033.
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Jenkins, J., Paiement, A., Ourmieres, Y., Le Sommer, J., Verron, J., Ubelmann, C., et al. (2023). A Dnn Framework For Learning Lagrangian Drift With Uncertainty. Applied Intelligence, .
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Li, L., Deremble, B., Lahaye, N., & Memin, E. (2023). Stochastic Data-Driven Parameterization Of Unresolved Eddy Effects In A Baroclinic Quasi-Geostrophic Model. Journal Of Advances In Modeling Earth Systems, 151(2).
Abstract: In This Work, A Stochastic Representation Based On A Physical Transport Principle Is Proposed To Account For Mesoscale Eddy Effects On The Large-Scale Oceanic Circulation. This Stochastic Framework Arises From A Decomposition Of The Lagrangian Velocity Into A Smooth-In-Time Component And A Highly Oscillating Noise Term. One Important Characteristic Of This Random Model, Without Any External Forcing And Damping, Is That It Conserves The Total Energy Of The Resolved Flow For Any Realization. The Proposed Stochastic Formulation Is Successfully Implemented In A Well Established Multi-Layered Quasi-Geostrophic Dynamical Core. The Empirical Spatial Correlation Of The Unresolved Noise Is Calibrated From The Eddy-Resolving Simulation Data. In Particular, A Stationary Correction Drift Can Be Introduced In The Noise Through Girsanov Transformation. This Non-Intuitive Term Appears To Be Important In Reproducing On A Coarse Mesh The Eastward Jet Of The Wind-Driven Double-Gyre Circulation. In Addition, A Projection Method Has Been Proposed To Constrain The Noise To Act Along The Iso-Surfaces Of The Vertical Stratification. The Resulting Noise Enables Us To Improve The Intrinsic Low-Frequency Variability Of The Large-Scale Current.
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Lott, F., Beljaars, A., Pauget, L., & Deremble, B. (2023). Neutral And Stratified Turbulent Boundary-Layer Flow Over Low Mountains. Quarterly Journal Of The Royal Meteorological Society, .
Abstract: A Theory For Flow Over Gentle Hills Using A Mixing-Length Turbulence Closure Is Developed To Describe The Transition From Turbulent Orographic Form Drag To Gravity Wave Drag. It Confirms That The First Is Associated With Downstream Sheltering, And The Second With Upstream Blocking And Strong Downslope Winds. It Shows That The Altitude At Which The Incident Flow Needs To Be Taken To Calculate The Drag Is The Inner Layer Scale At Which Dissipation Equilibrates Disturbance Advection. It Also Shows That The Parameter That Controls The Transition, Here A Richardson Number, Compares The Mountain Length With The Altitude Of The Turning Points Above Which The Upward-Propagating Gravity Waves Become Evanescent. Our Solutions Are Also Used To Show That The Downslope Winds Penetrate Well Into The Inner Layer And That A Good Fraction Of The Drag Is Deposited In The Inner Layer: All Of It In The Neutral Case, A Large Fraction In The Intermediate Cases When There Are Trapped Lee Waves, And Even In Stable Situations Without Trapping Part Of The Gravity Wave Drag Is Eroded In The Inner Layer. Some Discussion On How To Combine Neutral And Stratified Effects In The Parametrization Of Subgrid Scale Orography In Large-Scale Models Is Given. A Theory For The Interaction Between A Boundary Layer And A Low Mountain Is Derived. The Incident Wind Considered (U0$$ {U}_0 $$, Left Panel) Presents A Logarithmic Profile Near The Surface. The Theory Describes The Transition From Neutral To Stratified Flows, And The Systems Of Mountain Waves (Upward Propagating And Trapped, See Right Panel) That Develop During The Transition. The Theory Also Reproduces The Transition From Downstream Sheltering To Downslope Winds (Zoom) As Stratification Increases. The Mountain Drag And Reynolds Stress Profiles Are Also Discussed.Image
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Mathiot, P., & Jourdain, N. (2023). Southern Ocean Warming And Antarctic Ice Shelf Melting In Conditions Plausible By Late 23Rd Century In A High-End Scenario. Ocean Science, 191(6), 1595–1615.
Abstract: How Much Antarctic Ice Shelf Basal Melt Rates Can Increase In Response To Global Warming Remains An Open Question. Here We Describe The Response Of The Southern Ocean And Ice Shelf Cavities To An Abrupt Change To High-End Atmospheric Conditions Plausible By The Late 23Rd Century Under The Ssp5-8.5 Scenario. To Achieve This Objective, We First Present And Evaluate A New 0.25 Circle Global Configuration Of The Nemo Nucleus For European Modelling Of The Ocean Ocean And Sea Ice Model. Our Present-Day Simulations Demonstrate Good Agreement With Observational Data For Key Variables Such As Temperature, Salinity, And Ice Shelf Melt Rates, Despite The Remaining Difficulties To Simulate The Interannual Variability In The Amundsen Sea. The Ocean Response To The High-End Atmospheric Perturbation Includes A Strengthening And Extension Of The Ross And Weddell Gyres And A Quasi-Disappearance Of Sea Ice, With A Subsequent Decrease In Production Of High Salinity Shelf Water And Increased Intrusion Of Warmer Water Onto The Continental Shelves Favoured By Changes In Baroclinic Currents At The Shelf Break. We Propose To Classify The Perturbed Continental Shelf As A “Warm-Fresh Shelf”. This Induces A Substantial Increase In Ice Shelf Basal Melt Rates, Particularly In The Coldest Seas, With A Total Basal Mass Loss Rising From 1180 To 15 700 Gt Yr – 1 And An Antarctica Averaged Melt Rate Increasing From 0.8 To 10.6 M Yr – 1 . In The Perturbed Simulation, Most Ice Shelves Around Antarctica Experience Conditions That Are Currently Found In The Amundsen Sea, While The Amundsen Sea Warms By 2 Circle C. These Idealised Projections Can Be Used As A Base To Calibrate Basal Melt Parameterisations Used In Long-Term Ice Sheet Projections.
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Metref, S., Cosme, E., Le Lay, M., & Gailhard, J. (2023). Snow Data Assimilation For Seasonal Streamflow Supply Prediction In Mountainous Basins. Hydrology And Earth System Sciences, , 228322–229922.
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Polton, J., Harle, J., Holt, J., Katavouta, A., Partridge, D., Jardine, J., et al. (2023). Reproducible And Relocatable Regional Ocean Modelling: Fundamentals And Practices. Geoscientific Model Development, 161(5), 1481–1510.
Abstract: In Response To An Increasing Demand For Bespoke Or Tailored Regional Ocean Modelling Configurations, We Outline Fundamental Principles And Practices That Can Expedite The Process To Generate New Configurations. The Paper Develops The Principle Of Reproducibility And Advocates Adherence By Presenting Benefits To The Community And User. The Elements Of This Principle Are Reproducible Workflows And Standardised Assessment, With Additional Effort Over Existing Working Practices Being Balanced Against The Added Value Generated. The Paper Then Decomposes The Complex Build Process, For A New Regional Ocean Configuration, Into Stages And Presents Guidance, Advice And Insight For Each Component. This Advice Is Compiled From Across The Nemo (Nucleus For European Modelling Of The Ocean) User Community And Sets Out Principles And Practises That Encompass Regional Ocean Modelling With Any Model. With Detailed And Region-Specific Worked Examples In Sects. 3 And 4, The Linked Companion Repositories And Dois All Target Nemov4. The Aim Of This Review And Perspective Paper Is To Broaden The User Community Skill Base And To Accelerate Development Of New Configurations In Order To Increase The Time Available For Exploiting The Configurations.
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Sedakov, R., Osadchiev, A., Barnier, B., Molines, J., & Colombo, P. (2023). Large Chocked Lagoon As A Barrier For River-Sea Flux Of Dissolved Pollutants: Case Study Of The Azov Sea And The Black Sea. Marine Pollution Bulletin, 1871.
Abstract: The Don River Is Among The Largest Rivers In The Eastern Europe And Is Heavily Polluted. This River Inflows Into Small And Semi-Isolated Sea Of Azov, Which Is Connected With The Black Sea By A Narrow Strait. Generally, The Sea Of Azov Is A Large Choked Lagoon, Which Serves As A Barrier For River-Borne Constituents. Using Numerical Modeling, We Reveal That Presence Of The Choked Lagoon Significantly Slows Down The Estuary-Seawater Flux Of Dissolved Pollutants And Slackens Its Discharge-Induced Seasonal Variability. In Particular, The Sea Of Azov Delays The 5 % And 95 % Of The Total Flux Of Riverine Pollution To The Black Sea By 9 And 36 Months, Respectively. The Obtained Results Are Important For Assessment The Influence Of Background And Emergency Pollution Accidents At The Don River On Water Quality In The Study Region. Moreover, These Results Could Be Applied To Many Other Chocked Lagoons In The World Ocean.
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Thiria, S., Sorror, C., Archambault, T., Charantonis, A., Bereziat, D., Mejia, C., et al. (2023). Downscaling Of Ocean Fields By Fusion Of Heterogeneous Observations Using Deep Learning Algorithms. Ocean Modelling, 1821.
Abstract: We Present A Deep Learning Method To Downscale Low-Resolution Geophysical Fields By Merging Them With High-Resolution Data. The Downscaling Was Performed Using An Ensemble Of Convolutional Neural Networks (Cnns), Whose Prediction Values Are The Average Values Of The Outputs Of 20 Cnns. Academic Experiments Were Conducted On Simulated Ocean Data In The Gulf Stream Region, Given By The Outputs Of The Natl60 Model. The Cnns Forced With Low-Resolution (120 X 120 Km) Sea Surface High (Ssh) Data And Mesoscale Resolution (12 X 12 Km) Sea Surface Temperature (Sst) Data Allowed Us To Obtain Mesoscale Resolution Sea Surface Currents With Good Accuracy. Sensitivity Experiments Have Shown That Taking Sst Into Account Significantly Increases The Accuracy Of The High-Resolution Velocity Retrieval, Even When Noise Is Added To The Ssh Data. The Velocity Information Embedded In The Transport Equation Modeling The Sst Advection Is Taken Into Account By The Cnn, Which Greatly Increases The Resolution Of Ocean Currents Provided By Ssh. In The Present Work, We Only Consider Spatial Downscaling By Assuming That Ssh And Sst Are Daily Observations. The Method We Developed Is Generic And Can Be Used To Improve The Resolution Of A Wide Variety Of Large-Scale Fields By Merging Them With High-Resolution Fields.
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Treguier, A., Montegut, C., Bozec, A., Chassignet, E., Fox-Kemper, B., Hogg, A., et al. (2023). The Mixed-Layer Depth In The Ocean Model Intercomparison Project (Omip): Impact Of Resolving Mesoscale Eddies. Geoscientific Model Development, 161(131), 3849–3872.
Abstract: The Ocean Mixed Layer Is The Interface Between The Ocean Interior And The Atmosphere Or Sea Ice And Plays A Key Role In Climate Variability. It Is Thus Critical That Numerical Models Used In Climate Studies Are Capable Of A Good Representation Of The Mixed Layer, Especially Its Depth. Here We Evaluate The Mixed-Layer Depth (Mld) In Six Pairs Of Non Eddying (1 Degrees Grid Spacing) And Eddy-Rich (Up To 1/16 Degrees) Models From The Ocean Model Intercomparison Project (Omip), Forced By A Common Atmospheric State. For Model Evaluation, We Use An Updated Mld Dataset Computed From Observations Using The Omip Protocol (A Constant Density Threshold). In Winter, Low-Resolution Models Exhibit Large Biases In The Deep-Water Formation Regions. These Biases Are Reduced In Eddy-Rich Models But Not Uniformly Across Models And Regions. The Improvement Is Most Noticeable In The Mode Water Formation Regions Of The Northern Hemisphere. Results In The Southern Ocean Are More Contrasted, With Biases Of Either Sign Remaining At High Resolution. In Eddy-Rich Models, Mesoscale Eddies Control The Spatial Variability In Mld In Winter. Contrary To A Hypothesis That The Deepening Of The Mixed Layer In Anticyclones Would Make The Mld Larger Globally, Eddy-Rich Models Tend To Have A Shallower Mixed Layer At Most Latitudes Than Coarser Models Do. In Addition, Our Study Highlights The Sensitivity Of The Mld Computation To The Choice Of A Reference Level And The Spatio-Temporal Sampling, Which Motivates New Recommendations For Mld Computation In Future Model Intercomparison Projects.
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Uchida, T., Jamet, Q., Poje, A., Wienders, N., Dewar, W., & Deremble, B. (2023). Wavelet-Based Wavenumber Spectral Estimate Of Eddy Kinetic Energy: Idealized Quasi-Geostrophic Flow. Journal Of Advances In Modeling Earth Systems, 151(3).
Abstract: A Wavelet-Based Method Is Re-Introduced In An Oceanographic And Spectral Context To Estimate Wavenumber Spectrum And Spectral Flux Of Kinetic Energy And Enstrophy. We Apply This To A Numerical Simulation Of Idealized, Doubly Periodic Quasi-Geostrophic Flows, That Is, The Flow Is Constrained By The Coriolis Force And Vertical Stratification. The Double Periodicity Allows For A Straightforward Fourier Analysis As The Baseline Method. Our Wavelet Spectra Agree Well With The Canonical Fourier Approach But With The Additional Strengths Of Negating The Necessity For The Data To Be Periodic And Being Able To Extract Local Anisotropies In The Flow. Caution Is Warranted, However, When Computing Higher-Order Quantities, Such As Spectral Flux.
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Zhao, M., Ponte, R., & Penduff, T. (2023). Global-Scale Random Bottom Pressure Fluctuations From Oceanic Intrinsic Variability. Science Advances, 9(292).
Abstract: Intrinsic Processes Such As Mesoscale Turbulence Have Recently Been Proved As Important As Atmospheric Variability In Causing Variations In Ocean Bottom Pressure (P(B)). Intrinsic Processes Are Also Known To Generate Random Variability On Scales Larger Than The Mesoscale Through Inverse Energy Cascades Or Large-Scale Baroclinic Instability. Here, Model Analyses Reveal A Truly Global-Scale, Intrinsic P(B) Mode Of Variability At Monthly Time Scales That Relies On A Different Mechanism. The Intrinsic Mode Has Largest Amplitudes Around Drake Passage And Opposite Polarity Between The Southern Ocean And Atlantic/Arctic Oceans. Its Signature Is Consistent With Localized Eddy-Driven P(B) Anomalies Of Opposite Sign Near Drake Passage That Then Adjust Freely In The Rest Of The Ocean Via Barotropic Wave Processes. This Intrinsic Mode Seems Consistent With Observed P(B) Variability.
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2022 |
Amraoui, S., Auroux, D., Blum, J., & Cosme, E. (2022). Back-And-Forth Nudging For The Quasi-Geostrophic Ocean Dynamics With Altimetry: Theoretical Convergence Study And Numerical Experiments With The Future Swot Observations. Discrete And Continuous Dynamical Systems-Series S, .
Abstract: In data assimilation for geophysical problems, the increasing amount of satellite data to analyze makes it more and more challenging to guarantee near real time forecasting. Thus, low time and memory consuming data assimilation methods become very attractive. The back-and-forth nudging (BFN) method is a non-classical data assimilation method that can be seen as a deterministic and smoothing version of the Kalman filter. From a practical point of view, the BFN method is very valuable for its simplicity of implementation (no optimization, no differentiation,...) and its rapidity of convergence. Under observability conditions, we prove the mathematical convergence of BFN at deep layers for a multi-layer quasi-geostrophic (MQG) ocean circulation model using an infinite dimensional variant of LaSalle's invariance principle. We also extend the BFN to the problem of joint state-parameter identification. The numerical experiments, performed on 120km large swath sea surface height (SSH) simulated data of the Surface Water Ocean Topography (SWOT) satellite, show the high robustness of the algorithm to uncertainties and the few iterations needed to reach convergence, whereas some problems remain due to non-reversibility properties in time. We also give a strategy to improve geophysical model accuracy, considering the large number of uncertain parameters inherent to models and their impacts on state estimation performance. We propose here a joint state-parameter estimation, tested on the baroclinic wavenumber as an unobserved parameter.
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Bonino, G., Iovino, D., Brodeau, L., & Masina, S. (2022). The Bulk Parameterizations Of Turbulent Air-Sea Fluxes In Nemo4: The Origin Of Sea Surface Temperature Differences In A Global Model Study. Geoscientific Model Development, 151(171), 6873–6889.
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Burgard, C., Jourdain, N., Reese, R., Jenkins, A., & Mathiot, P. (2022). An Assessment Of Basal Melt Parameterisations For Antarctic Ice Shelves. Cryosphere, 161(121), 4931–4975.
Abstract: Ocean-Induced Ice-Shelf Melt Is One Of The Largest Uncertainty Factors In The Antarctic Contribution To Future Sea-Level Rise. Several Parameterisations Exist, Linking Oceanic Properties In Front Of The Ice Shelf To Melt At The Base Of The Ice Shelf, To Force Ice-Sheet Models. Here, We Assess The Potential Of A Range Of These Existing Basal Melt Parameterisations To Emulate Basal Melt Rates Simulated By A Cavity-Resolving Ocean Model On The Circum-Antarctic Scale. To Do So, We Perform Two Cross-Validations, Over Time And Over Ice Shelves Respectively, And Re-Tune The Parameterisations In A Perfect-Model Approach, To Compare The Melt Rates Produced By The Newly Tuned Parameterisations To The Melt Rates Simulated By The Ocean Model. We Find That The Quadratic Dependence Of Melt To Thermal Forcing Without Dependency On The Individual Ice-Shelf Slope And The Plume Parameterisation Yield The Best Compromise, In Terms Of Integrated Shelf Melt And Spatial Patterns. The Box Parameterisation, Which Separates The Sub-Shelf Circulation Into Boxes, The Picop Parameterisation, Which Combines The Box And Plume Parameterisation, And Quadratic Parameterisations With Dependency On The Ice Slope Yield Basal Melt Rates Further From The Model Reference. The Linear Parameterisation Cannot Be Recommended As The Resulting Integrated Ice-Shelf Melt Is Comparably Furthest From The Reference. When Using Offshore Hydrographic Input Fields In Comparison To Properties On The Continental Shelf, All Parameterisations Perform Worse; However, The Box And The Slope-Dependent Quadratic Parameterisations Yield The Comparably Best Results. In Addition To The New Tuning, We Provide Uncertainty Estimates For The Tuned Parameters.
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Dewar, W., Parfitt, R., & Wienders, N. (2022). Routine Reversal Of The Amoc In An Ocean Model Ensemble. Geophysical Research Letters, 494(242).
Abstract: We Describe A Form Of Atlantic Meridional Overturning Circulation (Amoc) Variability That We Believe Has Not Previously Appeared In Observations Or Models. It Is Found In An Ensemble Of Eddy-Resolving North Atlantic Simulations That The Amoc Frequently Reverses In Sign At Similar To 35 Degrees N With Gyre-Wide Anomalies In Size And That Reach Throughout The Water Column. The Duration Of Each Reversal Is Roughly 1 Month. The Reversals Are Part Of The Annual Amoc Cycle Occurring In Boreal Winter, Although Not All Years Feature An Actual Reversal In Sign. The Occurrence Of The Reversals Appears In Our Ensemble Mean, Suggesting It Is A Forced Feature Of The Circulation. A Partial Explanation Is Found In An Ekman Response To Wind Stress Anomalies. Model Ensemble Simulations Run With Different Combinations Of Climatological And Realistic Forcings Argue That It Is The Atmospheric Forcing Specifically That Results In The Reversals, Despite The Signals Extending Into The Deep Ocean.
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Frezat, H., Le Sommer, J., Fablet, R., Balarac, G., & Lguensat, R. (2022). A Posteriori Learning For Quasi-Geostrophic Turbulence Parametrization. Journal Of Advances In Modeling Earth Systems, 141(111).
Abstract: The Use Of Machine Learning To Build Subgrid Parametrizations For Climate Models Is Receiving Growing Attention. State-Of-The-Art Strategies Address The Problem As A Supervised Learning Task And Optimize Algorithms That Predict Subgrid Fluxes Based On Information From Coarse Resolution Models. In Practice, Training Data Are Generated From Higher Resolution Numerical Simulations Transformed In Order To Mimic Coarse Resolution Simulations. By Essence, These Strategies Optimize Subgrid Parametrizations To Meet So-Called A Priori Criteria. But The Actual Purpose Of A Subgrid Parametrization Is To Obtain Good Performance In Terms Of A Posteriori Metrics Which Imply Computing Entire Model Trajectories. In This Paper, We Focus On The Representation Of Energy Backscatter In Two-Dimensional Quasi-Geostrophic Turbulence And Compare Parametrizations Obtained With Different Learning Strategies At Fixed Computational Complexity. We Show That Strategies Based On A Priori Criteria Yield Parametrizations That Tend To Be Unstable In Direct Simulations And Describe How Subgrid Parametrizations Can Alternatively Be Trained End-To-End In Order To Meet A Posteriori Criteria. We Illustrate That End-To-End Learning Strategies Yield Parametrizations That Outperform Known Empirical And Data-Driven Schemes In Terms Of Performance, Stability, And Ability To Apply To Different Flow Configurations. These Results Support The Relevance Of Differentiable Programming Paradigms For Climate Models In The Future.
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Herbaut, C., Houssais, M. N., Blaizot, A. C., & Molines, J. M. (2022). A Role For The Ocean In The Winter Sea Ice Distribution North Of Svalbard. Journal Of Geophysical Research-Oceans, 1271(6).
Abstract: The ocean is suggested to play a major role in the ongoing winter decay of the sea ice cover in the western Eurasian Basin. Using a high-resolution sea ice-ocean model, we investigate the processes influencing the ice-ocean interactions in winter in the waters north of Svalbard, with a particular focus on those contributing to sea ice melt events of large amplitude. These short term events, lasting 5-10 days, are associated with locally large melt rates mostly found along the pathway of the Atlantic Water. The sum of all these events over the simulation period is found to contribute 40% of the total winter melt. Episodes of strong surface winds, occasionally associated with enhanced velocity shear at the mixed layer base, can trigger enhanced entrainment of Atlantic Water through the relatively shallow upper thermocline in the Atlantic Water boundary current, leading to substantial ocean heat transfer to the sea ice. In some cases, strengthening of the boundary current also contributes to fueling the heat transfer to the ice. Another type of large melt event, not linked to increased ocean vertical heat flux but due to ice being advected over warm surface waters, is also identified, sometimes associated with episodes of ice close-up. Sea ice budget calculations show that, overall, large melt events contribute significantly to the eastward retreat of the winter marginal ice zone on the upper slope east of Svalbard while episodes of northward advection of ice largely dominate the ice edge retreat over the shelf north of Svalbard.
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Hogg, A. M., Penduff, T., Close, S. E., Dewar, W. K., Constantinou, N. C., & Martinez-Moreno, J. (2022). Circumpolar Variations In The Chaotic Nature Of Southern Ocean Eddy Dynamics. Journal Of Geophysical Research-Oceans, 1271(5).
Abstract: Circulation in the Southern Ocean is unique. The strong wind stress forcing and buoyancy fluxes, in concert with the lack of continental boundaries, conspire to drive the Antarctic Circumpolar Current replete with an intense eddy field. The effect of Southern Ocean eddies on the ocean circulation is significant-they modulate the momentum balance of the zonal flow, and the meridional transport of tracers and mass. The strength of the eddy field is controlled by a combination of forcing (primarily thought to be wind stress) and intrinsic, chaotic, variability associated with the turbulent flow field itself. Here, we present results from an eddy-permitting ensemble of ocean model simulations to investigate the relative contribution of forced and intrinsic processes in governing the variability of Southern Ocean eddy kinetic energy. We find that variations of the eddy field are mostly random, even on longer (interannual) timescales. Where correlations between the wind stress forcing and the eddy field exist, these interactions are dominated by two distinct timescales-a fast baroclinic instability response; and a multi-year process owing to feedback between bathymetry and the mean flow. These results suggest that understanding Southern Ocean eddy dynamics and its larger-scale impacts requires an ensemble approach to eliminate intrinsic variability, and therefore may not yield robust conclusions from observations alone.
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Hogg, A. M., Penduff, T., Close, S. E., Dewar, W. K., Constantinou, N. C., & Martinez-Moreno, J. (2022). Circumpolar Variations In The Chaotic Nature Of Southern Ocean Eddy Dynamics. Journal Of Geophysical Research-Oceans, 1271(5).
Abstract: Circulation in the Southern Ocean is unique. The strong wind stress forcing and buoyancy fluxes, in concert with the lack of continental boundaries, conspire to drive the Antarctic Circumpolar Current replete with an intense eddy field. The effect of Southern Ocean eddies on the ocean circulation is significant-they modulate the momentum balance of the zonal flow, and the meridional transport of tracers and mass. The strength of the eddy field is controlled by a combination of forcing (primarily thought to be wind stress) and intrinsic, chaotic, variability associated with the turbulent flow field itself. Here, we present results from an eddy-permitting ensemble of ocean model simulations to investigate the relative contribution of forced and intrinsic processes in governing the variability of Southern Ocean eddy kinetic energy. We find that variations of the eddy field are mostly random, even on longer (interannual) timescales. Where correlations between the wind stress forcing and the eddy field exist, these interactions are dominated by two distinct timescales-a fast baroclinic instability response; and a multi-year process owing to feedback between bathymetry and the mean flow. These results suggest that understanding Southern Ocean eddy dynamics and its larger-scale impacts requires an ensemble approach to eliminate intrinsic variability, and therefore may not yield robust conclusions from observations alone.
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Jamet, Q., Leroux, S., Dewar, W., Penduff, T., Le Sommer, J., Molines, J., et al. (2022). Non-Local Eddy-Mean Kinetic Energy Transfers In Submesoscale-Permitting Ensemble Simulations. Journal Of Advances In Modeling Earth Systems, 141(101).
Abstract: Understanding Processes Associated With Eddy-Mean Flow Interactions Helps Our Interpretation Of Ocean Energetics, And Guides The Development Of Parameterizations. Here, We Focus On The Non-Local Nature Of Kinetic Energy (Ke) Transfers Between Mean And Turbulent Reservoirs. Transfers Are Interpreted As Non-Local When The Energy Extracted From The Mean Flow Does Not Locally Sustain An Growth Of Energy In The Turbulent Flow, Or Vice Versa. The Novelty Of Our Approach Is To Use Ensemble Statistics To Define The Mean And The Turbulent Flow. Based On Ke Budget Considerations, We First Rationalize The Eddy-Mean Separation In The Ensemble Framework, And Discuss The Interpretation Of A Mean Flow < U > Driven By The Prescribed (Surface And Boundary) Forcing And A Turbulent Flow U' Driven By Non-Linear Dynamics Sensitive To Initial Conditions. We Then Analyze 120-Day Long, 20-Member Ensemble Simulations Of The Western Mediterranean Basin Run At 1/60 Degrees Resolution. Our Main Contribution Is To Recognize The Prominent Contribution Of The Cross Energy Term < U(H)> . U(H)' To Explain Non-Local Energy Transfers, Which Provides A Strong Constraint On The Horizontal Organization Of Eddy-Mean Flow Ke Transfers Since The Cross Energy Term Vanishes Identically For Perturbations (U(H)') Orthogonal To The Mean Flow (< U(H)>). We Also Highlight The Prominent Contribution Of Vertical Turbulent Fluxes For Energy Transfers Within The Surface Mixed Layer. Analyzing The Scale Dependence Of Non-Local Energy Transfers Supports The Local Approximation Usually Made In The Development Of Meso-Scale, Energy-Aware Parameterizations For Non-Eddying Models, But Points Out To The Necessity Of Accounting For Non-Local Dynamics In The Meso-To-Submeso Scale Range. Plain Language Summary The Ocean Constantly Exchanges Energy Between Its Mean And Its Turbulent Reservoirs. However, We Are Still Lacking A Clear Understanding Of Eddy-Mean Flow Interactions, Which Limits Our Ability To Represent Them In Numerical Ocean Simulations That Require Turbulent Closures. In Particular, It Has Been Recently Shown That Instabilities Of Midlatitude Jets Do Not Necessarily Sustain The Growth Of Turbulent Eddies Locally. Instead, The Energy Released By The Jet Can Be Transported Over Significant Distances To Either Sustain Turbulence Or To Reinforce The Jet. Here, We Analyze Model Outputs Of Submesoscale-Permitting (Horizontal Resolution Of 1-2 Km) Ensemble Simulations Of The Western Mediterranean Basin With The View Of Better Understanding This Non-Local Dynamics. Starting From 20 Initial Conditions Perturbed By Small, Independent Perturbations, We Analyze The Development Of The Ensemble Spread During 120-Days Long Simulations Exposed To Identical Forcing. We Investigate The Spatiotemporal Structure Of Eddy-Mean Flow Interactions Through Their Kinetic Energy Expression. Our Main Contribution Is To Highlight Turbulent Fluxes Of The Cross Energy Term As A Driving Mechanism To Explain Non-Local Dynamics, A Process That Need To Be Accounted For In The Development Of Submesoscale Parametrizations.
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Jourdain, N., Mathiot, P., Burgard, C., Caillet, J., & Kittel, C. (2022). Ice Shelf Basal Melt Rates In The Amundsen Sea At The End Of The 21St Century. Geophysical Research Letters, 494(222).
Abstract: Antarctic Ice Sheet Projections Show The Highest Sensitivity To Increased Basal Melting In The Amundsen Sea. However, Little Is Known About The Processes That Control Future Increase In Melt Rates. We Build An Ensemble Of Three Ocean-Sea-Ice-Ice-Shelf Simulations For Both The Recent Decades And The Late 21St Century, Constrained By Regional Atmosphere Simulations And The Multi-Model Mean Climate Change Of The Fifth Climate Model Intercomparison Project Under The Rcp8.5 Scenario. The Ice-Shelf Melt Rates Are Typically Multiplied By 1.4-2.2 From Present Day To Future, For A Total Basal Mass Loss Increased By 346 Gt Yr(-1) On Average. This Is Equally Explained By Advection Of Warmer Water From Remote Locations And Regional Changes In Ekman Downwelling And In The Ice-Shelf Melt-Induced Circulation, While Increased Iceberg Melt Plays No Significant Role. Our Simulations Suggest That High-End Melt Projections Previously Used To Constrain Recent Sea Level Projections May Have Been Significantly Overestimated.
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Labreuche, P., Staquet, C., & Le Sommer, J. (2022). Resonant Growth Of Inertial Oscillations From Lee Waves In The Deep Ocean. Geophysical And Astrophysical Fluid Dynamics, 1161(5-6), 351–373.
Abstract: The Interactions Between Inertial Oscillations (Io) And Lee Waves (Lw) Close To The Bottom Of The Ocean And The Role Of Io In Energy Dissipation Are Addressed For A Range Of Physical Parameters Typical Of Southern Ocean Conditions. Idealized Numerical Simulations In A Vertical Plane And Resonant Interaction Theory Are Combined For This Purpose. The Lee Waves Are Emitted By A Uniform Geostrophic Flow Over A Sinusoidal Topography For A Constant Buoyancy Frequency At Mid-Latitude. We Show That Io Can Grow By Triadic Resonant Interactions With The Lw. Two Triads Are Dominant, Which Involve Waves With Frequency Omega(Lw), F And Omega(Lw)-F, Where Omega(Lw) Is The Intrinsic Frequency Of The Lw And F The Coriolis Frequency (Assumed Positive). These Triads Differ By The Sign And Value Of The Io Vertical Wavenumber. Results From The Numerical Simulations Show That The Triad Associated With The Upward Phase Propagation Of The Io Is Selected, Consistent With Oceanic Observations, That A Good Agreement Is Obtained With The Io Growth Rate Predicted Theoretically And That The Io Develop In A Bottom Layer Of Height Less Than 1000 M. A Quasi-Steady Flow Regime Is Eventually Reached, With The Io Amplitude Being Of The Same Order As The Geostrophic Flow Speed. During This Regime, Depending Upon The Flow Parameters, The Io Kinetic Energy Is Equal To 30-70% Of The Lw Energy Flux During One Inertial Period. This Large Range Of Values Is Not Reflected In The Turbulent Kinetic Energy (Tke) Dissipation Rate, Which Is Comprised Between 10 And 30% Of The Lw Energy Flux, Whatever The Io Amplitude, Even If Vanishingly Small. Therefore, For The Set Of Parameters We Consider, The Tke Dissipation Rate Cannot Be Inferred From The Io Amplitude. Yet, The Nonlinear Interactions Between The Lee Waves And The Io Are Critical In Setting The Energy Spectrum, And Similarly For The Internal Tide And The Io At Low Latitudes According To The Literature. This Implies That Io Should Be Taken Into Account In The Parameterisation Of Mixing In The Ocean.
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Leroux, S., Brankart, J., Albert, A., Brodeau, L., Molines, J., Jamet, Q., et al. (2022). Ensemble Quantification Of Short-Term Predictability Of The Ocean Dynamics At A Kilometric-Scale Resolution: A Western Mediterranean Test Case. Ocean Science, 181(6), 1619–1644.
Abstract: We Investigate The Predictability Properties Of The Ocean Dynamics Using An Ensemble Of Short-Term Numerical Regional Ocean Simulations Forced By Prescribed Atmospheric Conditions. In That Purpose, We Developed A Kilometric-Scale, Regional Model For The Western Mediterranean Sea (Med-West60, At 1/60 Degrees Horizontal Resolution). A Probabilistic Approach Is Then Followed, Where A Stochastic Parameterization Of Model Uncertainties Is Introduced In This Setup To Initialize Ensemble Predictability Experiments. A Set Of Three Ensemble Experiments (20 Members And 2 Months) Are Performed, One With The Deterministic Model Initiated With Perturbed Initial Conditions And Two With The Stochastic Model, For Two Different Amplitudes Of Stochastic Model Perturbations. In All Three Experiments, The Spread Of The Ensemble Is Shown To Emerge From The Smallest Scales (Kilometric Scale) And Progressively Upscales To The Largest Structures. After 2 Months, The Ensemble Variance Saturates Over Most Of The Spectrum, And The Small Scales (< 100 Km) Have Become Fully Decorrelated Across The Ensemble Members. These Ensemble Simulations Can Provide A Statistical Description Of The Dependence Between Initial Accuracy And Forecast Accuracy For Time Lags Between 1 And 20 D. The Predictability Properties Are Assessed Using A Cross-Validation Algorithm (I.E., Using Alternatively Each Ensemble Member As The Reference Truth And The Remaining 19 Members As The Ensemble Forecast) Together With A Given Statistical Score To Characterize The Initial And Forecast Accuracy. From The Joint Distribution Of Initial And Final Scores, It Is Then Possible To Quantify The Probability Distribution Of The Forecast Score Given The Initial Score Or Reciprocally To Derive Conditions On The Initial Accuracy To Obtain A Target Forecast Accuracy. The Misfit Between Ensemble Members Is Quantified In Terms Of Overall Accuracy (Crps Score), Geographical Position Of The Ocean Structures (Location Score) And Spatial Spectral Decorrelation Of The Sea Surface Height 2-D Fields (Decorrelation Score). With This Approach, We Estimate For Example That, In The Region And Period Of Interest, The Initial Location Accuracy Required (Necessary Condition) With A Perfect Model (No Model Uncertainty) To Obtain A Location Accuracy Of The Forecast Of 10 Km With A 95 % Confidence Is About 8 Km For A 1 D Forecast, 4 Km Fora 5 D Forecast And 1.5 Km Fora 10 D Forecast, And This Requirement Cannot Be Met With A 15 D Or Longer Forecast.
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Light, C. X., Arbic, B. K., Martin, P. E., Brodeau, L., Farrar, J. T., Griffies, S. M., et al. (2022). Effects of grid spacing on high-frequency precipitation variance in coupled high-resolution global ocean-atmosphere models. Climate Dynamics, .
Abstract: High-frequency precipitation variance is calculated in 12 different free-running (non-data-assimilative) coupled high resolution atmosphere-ocean model simulations, an assimilative coupled atmosphere-ocean weather forecast model, and an assimilative reanalysis. The results are compared with results from satellite estimates of precipitation and rain gauge observations. An analysis of irregular sub-daily fluctuations, which was applied by Covey et al. (Geophys Res Lett 45:12514-12522, 2018. http://doi.org/10.1029/2018GL078926) to satellite products and low-resolution climate models, is applied here to rain gauges and higher-resolution models. In contrast to lower-resolution climate simulations, which Covey et al. (2018) found to be lacking with respect to variance in irregular sub-daily fluctuations, the highest-resolution simulations examined here display an irregular sub-daily fluctuation variance that lies closer to that found in satellite products. Most of the simulations used here cannot be analyzed via the Covey et al. (2018) technique, because they do not output precipitation at sub-daily intervals. Thus the remainder of the paper focuses on frequency power spectral density of precipitation and on cumulative distribution functions over time scales (2-100 days) that are still relatively “high-frequency” in the context of climate modeling. Refined atmospheric or oceanic model grid spacing is generally found to increase high-frequency precipitation variance in simulations, approaching the values derived from observations. Mesoscale-eddy-rich ocean simulations significantly increase precipitation variance only when the atmosphere grid spacing is sufficiently fine (<0.5 degrees). Despite the improvements noted above, all of the simulations examined here suffer from the “drizzle effect”, in which precipitation is not temporally intermittent to the extent found in observations.
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Llovel, W., Kolodziejczyk, N., Close, S., Penduff, T., Molines, J. M., & Terray, L. (2022). Imprint of intrinsic ocean variability on decadal trends of regional sea level and ocean heat content using synthetic profiles. Environmental Research Letters, 171(4).
Abstract: The global ocean is warming and has absorbed 90% of the Earth Energy Imbalance over 2010-2018 leading to global mean sea level rise. Both ocean heat content (OHC) and sea level trends show large regional deviations from their global means. Both quantities have been estimated from in-situ observations for years. However, in-situ profile coverage is spatially uneven, leading to uncertainties when assessing both OHC and sea level trends, especially at regional scale. Recently, a new possible driver of regional sea level and OHC trends has been highlighted using eddy-permitting ensemble ocean simulations over multiple decades: non-linear ocean processes produce chaotic fluctuations, which yield random contributions to regional decadal OHC and sea level trends. In-situ measurements capture a combination of the atmospherically-forced response and this intrinsic ocean variability. It is therefore important to understand the imprint of the chaotic ocean variability recorded by the in-situ measurement sampling in order to assess its impact and associated uncertainty on regional budgets. A possible approach to investigate this problem is to use a set of synthetic in-situ-like profiles extracted from an ensemble of forced ocean simulations started from different states and integrated with the same atmospheric forcing. Comparisons between the original ensemble outputs and the remapped, subsampled, in-situ-like profiles elucidate the contribution of chaotic ocean variability to OHC and regional sea level trends. Our results show that intrinsic variability may be large in eddy-active regions in the gridded model outputs, and remains substantial when using the in-situ sampling-based estimates. Using the latter, the same result is also found on large scales, for which atmospheric forcing has been identified as the main driver. Our results suggest accounting for this intrinsic ocean variability when assessing regional OHC and sea level trend budgets on decadal time scales.
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Range, M., Arbic, B., Johnson, B., Moore, T., Titov, V., Adcroft, A., et al. (2022). The Chicxulub Impact Produced A Powerful Global Tsunami. Agu Advances, 3(5).
Abstract: The Chicxulub Crater Is The Site Of An Asteroid Impact Linked With The Cretaceous-Paleogene (K-Pg) Mass Extinction At Similar To 66 Ma. This Asteroid Struck In Shallow Water And Caused A Large Tsunami. Here We Present The First Global Simulation Of The Chicxulub Impact Tsunami From Initial Contact Of The Projectile To Global Propagation. We Use A Hydrocode To Model The Displacement Of Water, Sediment, And Crust Over The First 10 Min, And A Shallow-Water Ocean Model From That Point Onwards. The Impact Tsunami Was Up To 30,000 Times More Energetic Than The 26 December 2004 Indian Ocean Tsunami, One Of The Largest Tsunamis In The Modern Record. Flow Velocities Exceeded 20 Cm/S Along Shorelines Worldwide, As Well As In Open-Ocean Regions In The North Atlantic, Equatorial South Atlantic, Southern Pacific And The Central American Seaway, And Therefore Likely Scoured The Seafloor And Disturbed Sediments Over 10,000 Km From The Impact Origin. The Distribution Of Erosion And Hiatuses In The Uppermost Cretaceous Marine Sediments Are Consistent With Model Results.
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Sharma, R., Chaudhary, A., Seemanth, M., Bhowmick, S. A., Agarwal, N., Verron, J., et al. (2022). SARAL/AltiKa data analysis for oceanographic research: Impact of drifting and post star sensor anomaly phases. Advances In Space Research, 696(6), 2349–2361.
Abstract: Y AltiKa, first ever high frequency Ka-band altimeter on board SARAL (Satellite with ARgos and ALtiKa) has gone through different phases of operations, viz. Exact Repeat Mission, (ERM, March 2013 – July 2016), Drifting phase, (DP, July 2016 – January 2018) and then to Mispointing phase, (MP, February 2018 – till date). A detailed assessment of Sea level anomaly (SLA), Significant Wave Height (SWH) and Ocean Surface Wind Speed (WS) has been carried out during these different phases with a focus on the North Indian Ocean. Crossover analysis using the Jason series of satellites available during various phases of SARAL suggest high quality of SARAL/AltiKa data during the ERM and DP with root mean square differences of the order of 0.080 m, 0.25 m and 1 m/s for SLA, SWH and WS respectively. These differences are more during MP, being 0.095 m, 0.45 m and 1.72 m/s for SLA, SWH and WS respectively. Wavenumber Power spectrum computed from the along-track AltiKa SLA reveals that slopes in the mesoscale band (70-250 km) in different phases of operations are not very different. Errors in gridded SARAL/AltiKa SLA with respect to standard AVISO product remains unchanged during DP, but degrade by nearly 9.3% in the MP as compared to ERM. To assess the effect of assimilating along track SWH and SLA from different phases, two set of wave and circulation model simulations, with and without SARAL AltiKa data assimilation, were performed. Assimilation of SWH improved the wave height simulation by similar to 12.8% during the DP and similar to 8% during ERM and MP. As regards to circulation modeling, no significant difference of assimilating SLA from different phases was observed in the mesoscale range. These results indicate the usefulness of SLA from SARAL AltiKa during DP and MP for studying the mesoscale dynamics. (C) 2021 COSPAR. Published by Elsevier B.V. All rights reserved.
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Siahaan, A., Smith, R., Holland, P., Jenkins, A., Gregory, J., Lee, V., et al. (2022). The Antarctic Contribution To 21St-Century Sea-Level Rise Predicted By The Uk Earth System Model With An Interactive Ice Sheet. Cryosphere, 161(101), 4053–4086.
Abstract: The Antarctic Ice Sheet Will Play A Crucial Role In The Evolution Of Global Mean Sea Level As The Climate Warms. An Interactively Coupled Climate And Ice Sheet Model Is Needed To Understand The Impacts Of Ice-Climate Feed-Backs During This Evolution. Here We Use A Two-Way Coupling Between The Uk Earth System Model And The Bisicles (Berkeley Ice Sheet Initiative For Climate At Extreme Scales) Dynamic Ice Sheet Model To Investigate Antarctic Ice-Climate Interactions Under Two Climate Change Scenarios. We Perform Ensembles Of Ssp1-1.9 And Ssp5-8.5 (Shared Socioeconomic Pathway) Scenario Simulations To 2100, Which We Believe Are The First Such Simulations With A Climate Model That Include Two-Way Coupling Of Atmosphere And Ocean Models To Dynamic Models Of The Greenland And Antarctic Ice Sheets. We Focus Our Analysis On The Latter. In Ssp1-1.9 Simulations, Ice Shelf Basal Melting And Grounded Ice Mass Loss From The Antarctic Ice Sheet Are Generally Lower Than Present Rates During The Entire Simulation Period. In Contrast, The Responses To Ssp5-8.5 Forcing Are Strong. By The End Of The 21St Century, These Simulations Feature Order-Of-Magnitude Increases In Basal Melting Of The Ross And Filchner-Ronne Ice Shelves, Caused By Intrusions Of Masses Of Warm Ocean Water. Due To The Slow Response Of Ice Sheet Drawdown, This Strong Melting Does Not Cause A Substantial Increase In Ice Discharge During The Simulations. The Surface Mass Balance In Ssp5-8.5 Simulations Shows A Pattern Of Strong Decrease On Ice Shelves, Caused By Increased Melting, And Strong Increase On Grounded Ice, Caused By Increased Snowfall. Despite Strong Surface And Basal Melting Of The Ice Shelves, Increased Snowfall Dominates The Mass Budget Of The Grounded Ice, Leading To An Ensemble Mean Antarctic Contribution To Global Mean Sea Level Of A Fall Of 22 Mm By 2100 In The Ssp5-8.5 Scenario. We Hypothesise That This Signal Would Revert To Sea-Level Rise On Longer Timescales, Caused By The Ice Sheet Dynamic Response To Ice Shelf Thinning These Results Demonstrate The Need For Fully Coupled Ice-Climate Models In Reducing The Substantial Uncertainty In Sea-Level Rise From The Antarctic Ice Sheet.
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Tilinina, N., Ivonin, D., Gavrikov, A., Sharmar, V., Gulev, S., Suslov, A., et al. (2022). Wind Waves In The North Atlantic From Ship Navigational Radar: Seavision Development And Its Validation With The Spotter Wave Buoy And Wavewatch Iii. Earth System Science Data, 141(8), 3615–3633.
Abstract: Wind Waves Play An Important Role In The Climate System, Modulating The Energy Exchange Between The Ocean And The Atmosphere And Effecting Ocean Mixing. However, Existing Ship-Based Observational Networks Of Wind Waves Are Still Sparse, Limiting Therefore The Possibilities Of Validating Satellite Missions And Model Simulations. In This Paper We Present Data Collected On Three Research Cruises In The North Atlantic And Arctic In 2020 And 2021 And The Seavision System For Measuring Wind Wave Characteristics Over The Open Ocean With A Standard Marine Navigation X-Band Radar. Simultaneously With The Seavision Wind Wave Characteristic Measurements, We Also Collected Data From The Spotter Wave Buoy At The Same Locations, And We Ran The Wavewatch Iii Model In A Very High-Resolution Configuration Over The Observational Domain Seavision Measurements Were Validated Against Co-Located Spotter Wave Buoy Data And Intercompared With The Output Of Wavewatch Iii Simulations. Observations Of The Wind Waves With The Navigation X-Band Radar Were Found To Be In Good Agreement With Buoy Data And Model Simulations With The Best Match For The Wave Propagation Directions. Supporting Datasets Consist Of Significant Wave Heights, Wave Directions, Wave Periods And Wave Energy Frequency Spectra Derived From Both Seavision And The Spotter Buoy. All Supporting Data Are Available Through The Pangaea Repository – Https://Doi.Org/10.1594/Pangaea.939620 (Gavrikov Et Al., 2021). The Dataset Can Be Further Used For Validation Of Satellite Missions And Regional Wave Model Experiments. Our Study Shows The Potential Of Ship Navigation X-Band Radars (When Assembled With Seavision Or Similar Systems) For The Development Of A New Near-Global Observational Network Providing A Much Larger Number Of Wind Wave Observations Compared To E.G. Voluntary Observing Ship (Vos) Data And Research Vessel Campaigns.
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Uchida, T., Deremble, B., & Popinet, S. (2022). Deterministic Model Of The Eddy Dynamics For A Midlatitude Ocean Model. Journal Of Physical Oceanography, 525(6), 1133–1154.
Abstract: Mesoscale Eddies, Although Being On Scales Of O(20-100) Km, Have A Disproportionate Role In Shaping The Mean Stratification, Which Varies On The Scale Of O(1000) Km. With The Increase In Computational Power, We Are Now Able To Partially Resolve The Eddies In Basin-Scale And Global Ocean Simulations, A Model Resolution Often Referred To As Mesoscale Permitting. It Is Well Known, However, That Due To Gridscale Numerical Viscosity, Mesoscale-Permitting Simulations Have Less Energetic Eddies And Consequently Weaker Eddy Feedback Onto The Mean Flow. In This Study, We Run A Quasigeostrophic Model At Mesoscale-Resolving Resolution In A Double Gyre Configuration And Formulate A Deterministic Closure For The Eddy Rectification Term Of Potential Vorticity (Pv), Namely, The Eddy Pv Flux Divergence. Our Closure Successfully Reproduces The Spatial Patterns And Magnitude Of Eddy Kinetic And Potential Energy Diagnosed From The Mesoscale-Resolving Model. One Novel Point About Our Approach Is That We Account For Nonlocal Eddy Feedbacks Onto The Mean Flow By Solving The “Subgrid” Eddy Pv Equation Prognostically In Addition To The Mean Pv.
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Uchida, T., Jamet, Q., Dewar, W. K., Le Sommer, J., Penduff, T., & Balwada, D. (2022). Diagnosing The Thickness-Weighted Averaged Eddy-Mean Flow Interaction From an Eddying North Atlantic Ensemble: The Eliassen-Palm Flux. Journal Of Advances In Modeling Earth Systems, 141(5).
Abstract: The thickness-weighted average (TWA) framework, which treats the residual-mean flow as the prognostic variable, provides a clear theoretical formulation of the eddy feedback onto the residual-mean flow. The averaging operator involved in the TWA framework, although in theory being an ensemble mean, in practice has often been approximated by a temporal mean. Here, we analyze an ensemble of North Atlantic simulations at mesoscale-permitting resolution (1/12 degrees). We therefore recognize means and eddies in terms of ensemble means and fluctuations about those means. The ensemble dimension being orthogonal to the temporal and spatial dimensions negates the necessity for an arbitrary temporal or spatial scale in defining the eddies. Eddy-mean flow feedbacks are encapsulated in the Eliassen-Palm (E-P) flux tensor and its convergence indicates that eddy momentum fluxes dominate in the separated Gulf Stream. The eddies can be interpreted to contribute to the zonal meandering of the Gulf Stream and a northward migration of it in the meridional direction. Downstream of the separated Gulf Stream in the North Atlantic Current region, the interfacial form stress convergence becomes leading order in the E-P flux convergence.
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Uchida, T., Jamet, Q., Dewar, W. K., Le Sommer, J., Penduff, T., & Balwada, D. (2022). Diagnosing The Thickness-Weighted Averaged Eddy-Mean Flow Interaction From an Eddying North Atlantic Ensemble: The Eliassen-Palm Flux. Journal Of Advances In Modeling Earth Systems, 141(5).
Abstract: The thickness-weighted average (TWA) framework, which treats the residual-mean flow as the prognostic variable, provides a clear theoretical formulation of the eddy feedback onto the residual-mean flow. The averaging operator involved in the TWA framework, although in theory being an ensemble mean, in practice has often been approximated by a temporal mean. Here, we analyze an ensemble of North Atlantic simulations at mesoscale-permitting resolution (1/12 degrees). We therefore recognize means and eddies in terms of ensemble means and fluctuations about those means. The ensemble dimension being orthogonal to the temporal and spatial dimensions negates the necessity for an arbitrary temporal or spatial scale in defining the eddies. Eddy-mean flow feedbacks are encapsulated in the Eliassen-Palm (E-P) flux tensor and its convergence indicates that eddy momentum fluxes dominate in the separated Gulf Stream. The eddies can be interpreted to contribute to the zonal meandering of the Gulf Stream and a northward migration of it in the meridional direction. Downstream of the separated Gulf Stream in the North Atlantic Current region, the interfacial form stress convergence becomes leading order in the E-P flux convergence.
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Uchida, T., Jamet, Q., Poje, A., & Dewar, W. K. (2022). An Ensemble-Based Eddy and Spectral Analysis, With Application to the Gulf Stream. Journal Of Advances In Modeling Earth Systems, 141(4).
Abstract: The “eddying” ocean, recognized for several decades, has been the focus of much observational and theoretical research. We here describe a generalization for the analysis of eddy energy, based on the use of ensembles, that addresses two key related issues: the definition of an “eddy” and the general computation of energy spectra. An ensemble identifies eddies as the unpredictable component of the flow, and permits the scale decomposition of their energy in inhomogeneous and non-stationary settings. We present two distinct, but equally valid, spectral estimates: one is similar to classical Fourier spectra, the other reminiscent of classical empirical orthogonal function analysis. Both satisfy Parseval's equality and thus can be interpreted as length-scale dependent energy decompositions. The issue of “tapering” or “windowing” of the data, used in traditional approaches, is also discussed. We apply the analyses to a mesoscale “resolving” (1/12 degrees) ensemble of the separated North Atlantic Gulf Stream. Our results reveal highly anisotropic spectra in the Gulf Stream and zones of both agreement and disagreement with theoretically expected spectral shapes. In general, we find spectral slopes that fall off faster than the steepest slope expected from quasi-geostrophic theory.
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Uchida, T., Le Sommer, J., Stern, C., Abernathey, R., Holdgraf, C., Albert, A., et al. (2022). Cloud-Based Framework For Inter-Comparing Submesoscale-Permitting Realistic Ocean Models. Geoscientific Model Development, 151(141), 5829–5856.
Abstract: With The Increase In Computational Power, Ocean Models With Kilometer-Scale Resolution Have Emerged Over The Last Decade. These Models Have Been Used For Quantifying The Energetic Exchanges Between Spatial Scales, Informing The Design Of Eddy Parametrizations, And Preparing Observing Networks. The Increase In Resolution, However, Has Drastically Increased The Size Of Model Outputs, Making It Difficult To Transfer And Analyze The Data. It Remains, Nonetheless, Of Primary Importance To Assess More Systematically The Realism Of These Models. Here, We Showcase A Cloud-Based Analysis Framework Proposed By The Pangeo Project That Aims To Tackle Such Distribution And Analysis Challenges. We Analyze The Output Of Eight Submesoscale-Permitting Simulations, All On The Cloud, For A Crossover Region Of The Upcoming Surface Water And Ocean Topography (Swot) Altimeter Mission Near The Gulf Stream Separation. The Cloud-Based Analysis Framework (I) Minimizes The Cost Of Duplicating And Storing Ghost Copies Of Data And (Ii) Allows For Seamless Sharing Of Analysis Results Amongst Collaborators. We Describe The Framework And Provide Example Analyses (E.G., Sea-Surface Height Variability, Submesoscale Vertical Buoyancy Fluxes, And Comparison To Predictions From The Mixed-Layer Instability Parametrization). Basin- To Global-Scale, Submesoscale-Permitting Models Are Still At Their Early Stage Of Development; Their Cost And Carbon Footprints Are Also Rather Large. It Would, Therefore, Benefit The Community To Document The Different Model Configurations For Future Best Practices. We Also Argue That An Emphasis On Data Analysis Strategies Would Be Crucial For Improving The Models Themselves.
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2021 |
Abdalla, S., Kolahchi, A., Ablain, M., Adusumilli, S., Bhowmick, S., Alou-Font, E., et al. (2021). Altimetry for the future: Building on 25 years of progress. Advances In Space Research, 68(2), 319–363.
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Ajayi, A., Le Sommer, J., Chassignet, E., Molines, J., Xu, X., Albert, A., et al. (2021). Diagnosing Cross-Scale Kinetic Energy Exchanges From Two Submesoscale Permitting Ocean Models. Journal Of Advances In Modeling Earth Systems, 13(6).
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Carret, A., Llovel, W., Penduff, T., & Molines, J. (2021). Atmospherically Forced and Chaotic Interannual Variability of Regional Sea Level and Its Components Over 1993-2015. Journal Of Geophysical Research-Oceans, 126(4).
Abstract: Satellite altimetry data have revealed a global mean sea level rise of 3.1 mm/yr since 1993 with large regional sea level variability. These remote data highlight complex structures especially in strongly eddying regions. A recent study showed that over 38% of the global ocean area, the chaotic variability may hinder the attribution to the atmospheric forcing of regional sea level trends from 1993 to 2015. This study aims to complement this work by focusing on the atmospherically forced and chaotic interannual variability of regional sea level and its components. At interannual time scales, variability can hamper the detection of regional sea level trends. A global 1/4 degrees ocean/sea-ice 50-member ensemble simulation is analyzed to disentangle the imprints of the atmospheric forcing and of the chaotic ocean variability on the interannual variability of regional sea level and of its steric and manometric components over 1993-2015. The atmospherically forced and chaotic interannual variabilities of sea level mainly have a steric origin, except in coastal areas. The chaotic part of the interannual variability of sea level and its components is stronger in the Pacific and Atlantic Oceans than in the Indian Ocean. The chaotic part of the interannual variance of sea level and of its steric component exceeds 20% over 48% of the global ocean area; this fractional area reduces to 26% for the manometric component. These results confirm the substantial imprint of the chaotic interannual variability on sea level components, questioning in several regions the attribution of their observed evolution to atmospheric causes. Plain Language Summary Since the early 1990s, satellite altimetry has become the main observing system for continuously measuring the sea level variations with a near global coverage. It has revealed a global mean sea level rise of 3.1 mm/yr since 1993 with large regional sea level variability that differs from the mean estimate. These measurements highlight complex structures especially for the western boundary currents (Gulf Stream or Kuroshio) or the Antarctic Circumpolar Current. Recent studies based on numerical modeling showed that the ocean spontaneously generates a chaotic intrinsic variability that substantially impacts the sea level interannual-to-decadal variability and its long-term trends. It is important to note that sea level observations simultaneously record these chaotic variations in the ocean but also the response to the atmospheric forcings. Here, we use a 50-member ensemble ocean simulation to disentangle the atmospherically forced and chaotic parts of the interannual variability of sea level and of its steric and manometric components. We found that, in several regions, the chaotic interannual variability has a large imprint on sea level components. While these results do not question the anthropic origin of global mean sea level rise, they give new insights into the oceanic vs. nonoceanic origin of regional interannual variability.
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Cluzet, B., Lafaysse, M., Cosme, E., Albergel, C., Meunier, L., & Dumont, M. (2021). CrocO_v1.0: a particle filter to assimilate snowpack observations in a spatialised framework. Geoscientific Model Development, 14(3), 1595–1614.
Abstract: Monitoring the evolution of snowpack properties in mountainous areas is crucial for avalanche hazard forecasting and water resources management. In situ and remotely sensed observations provide precious information on the state of the snowpack but usually offer limited spatiotemporal coverage of bulk or surface variables only. In particular, visible-near-infrared (Vis-NIR) reflectance observations can provide information about the snowpack surface properties but are limited by terrain shading and clouds. Snowpack modelling enables the estimation of any physical variable virtually anywhere, but it is affected by large errors and uncertainties. Data assimilation offers a way to combine both sources of information and to propagate information from observed areas to non-observed areas. Here, we present CrocO (Crocus-Observations), an ensemble data assimilation system able to ingest any snowpack observation (applied as a first step to the height of snow (HS) and Vis-NIR reflectances) in a spatialised geometry. CrocO uses an ensemble of snowpack simulations to represent modelling uncertainties and a particle filter (PF) to reduce them. The PF is prone to collapse when assimilating too many observations. Two variants of the PF were specifically implemented to ensure that observational information is propagated in space while tackling this issue. The global algorithm ingests all available observations with an iterative inflation of observation errors, while the klocal algorithm is a localised approach performing a selection of the observations to assimilate based on background correlation patterns. Feasibility testing experiments are carried out in an identical twin experiment setup, with synthetic observations of HS and Vis-NIR reflectances available in only one-sixth of the simulation domain. Results show that compared against runs without assimilation, analyses exhibit an average improvement of the snow water equivalent continuous rank probability score (CRPS) of 60 % when assimilating HS with a 40-member ensemble and an average 20 % CRPS improvement when assimilating reflectance with a 160-member ensemble. Significant improvements are also obtained outside the observation domain. These promising results open a possibility for the assimilation of real observations of reflectance or of any snowpack observations in a spatialised context.
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Cravatte, S., Serazin, G., Penduff, T., & Menkes, C. (2021). Imprint of chaotic ocean variability on transports in the southwestern Pacific at interannual timescales. Ocean Science, 17(2), 487–507.
Abstract: The southwestern Pacific Ocean sits at a bifurcation where southern subtropical waters are redistributed equatorward and poleward by different ocean currents. The processes governing the interannual variability of these currents are not completely understood. This issue is investigated using a probabilistic modeling strategy that allows disentangling the atmospherically forced deterministic ocean variability and the chaotic intrinsic ocean variability. A large ensemble of 50 simulations performed with the same ocean general circulation model (OGCM) driven by the same realistic atmospheric forcing and only differing by a small initial perturbation is analyzed over 1980-2015. Our results show that, in the southwestern Pacific, the interannual variability of the transports is strongly dominated by chaotic ocean variability south of 20 degrees S. In the tropics, while the interannual variability of transports and eddy kinetic energy modulation are largely deterministic and explained by the El Nino-Southern Oscillation (ENSO), ocean nonlinear processes still explain 10% to 20% of their interannual variance at large scale. Regions of strong chaotic variance generally coincide with regions of high mesoscale activity, suggesting that a spontaneous inverse cascade is at work from the mesoscale toward lower frequencies and larger scales. The spatiotemporal features of the low-frequency oceanic chaotic variability are complex but spatially coherent within certain regions. In the Subtropical Countercurrent area, they appear as interannually varying, zonally elongated alternating current structures, while in the EAC (East Australian Current) region, they are eddy-shaped. Given this strong imprint of large-scale chaotic oceanic fluctuations, our results question the attribution of interannual variability to the atmospheric forcing in the region from pointwise observations and one-member simulations.
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Guillou, F., Metref, S., Cosme, E., Ubelmann, C., Ballarotta, M., Sommer, J., et al. (2021). Mapping Altimetry in the Forthcoming SWOT Era by Back-and-Forth Nudging a One-Layer Quasigeostrophic Model. Journal Of Atmospheric And Oceanic Technology, 38(4), 697–710.
Abstract: During the past 25 years, altimetric observations of the ocean surface from space have been mapped to provide two dimensional sea surface height (SSH) fields that are crucial for scientific research and operational applications. The SSH fields can be reconstructed from conventional altimetric data using temporal and spatial interpolation. For instance, the standard Developing Use of Altimetry for Climate Studies (DUACS) products are created with an optimal interpolation method that is effective for both low temporal and low spatial resolution. However, the upcoming next-generation SWOT mission will provide very high spatial resolution but with low temporal resolution. The present paper makes the case that this temporal-spatial discrepancy induces the need for new advanced mapping techniques involving information on the ocean dynamics. An algorithm is introduced, dubbed the BFN-QG, that uses a simple data assimilation method, the back-and-forth nudging (BNF), to interpolate altimetric data while respecting quasigeostrophic (QG) dynamics. The BFN-QG is tested in an observing system simulation experiments and compared to the DUACS products. The experiments consider as reference the high-resolution numerical model simulation NATL60 from which are produced realistic data: four conventional altimetric nadirs and SWOT data. In a combined nadirs and SWOT scenario, the BFN-QG substantially improves the mapping by reducing the root-mean-square errors and increasing the spectral effective resolution by 40 km. Also, the BFN-QG method can be adapted to combine large-scale corrections from nadir data and small-scale corrections from SWOT data so as to reduce the impact of SWOT correlated noises and still provide accurate SSH maps.
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Jamet, Q., Deremble, B., Wienders, N., Uchida, T., & Dewar, W. (2021). On Wind-Driven Energetics of Subtropical Gyres. Journal Of Advances In Modeling Earth Systems, 13(4).
Abstract: The flow of energy in the wind-driven circulation is examined in a combined theoretical and numerical study. Based on a multiple-scale analysis, we find the mesoscale field in the ocean interior is strongly affected by, but does not feed back onto, the ventilated thermocline. In the western boundary region, the associated currents first appear as coastal jets, conserving mean energy, and later as separated jet extensions where the mesoscale is energized by the mean field. It is in the separated jet zone where the primary loss of general circulation energy to the mesoscale occurs. These ideas are tested by an analysis of a regional 1/12 degrees primitive equation numerical model of the North Atlantic. The predictions of the theory are generally supported by the numerical results. The one exception is that topographic irregularities in the coastal jet spawn eddies, although these eddies contribute modestly to the energy budget. We therefore conclude the primary sink of wind input into the mean circulation is in the separated jet, and not the interior. The analysis also shows wind energy input to be much smaller than the interior energy fluxes; thus, the general circulation largely recirculates energy.
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Khatri, H., Griffies, S., Uchida, T., Wang, H., & Menemenlis, D. (2021). Role of Mixed-Layer Instabilities in the Seasonal Evolution of Eddy Kinetic Energy Spectra in a Global Submesoscale Permitting Simulation. Geophysical Research Letters, 48(18).
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Kleinherenbrink, M., Korosov, A., Newman, T., Theodosiou, A., Komarov, A., Li, Y., et al. (2021). Estimating instantaneous sea-ice dynamics from space using the bi-static radar measurements of Earth Explorer 10 candidate Harmony. Cryosphere, 15(7), 3101–3118.
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Le Guillou, F., Lahaye, N., Ubelmann, C., Metref, S., Cosme, E., Ponte, A., et al. (2021). Joint Estimation of Balanced Motions and Internal Tides From Future Wide-Swath Altimetry. Journal Of Advances In Modeling Earth Systems, 13(12).
Abstract: Wide-swath altimetry, for example, the Surface Water and Ocean Topography mission is expected to provide Sea Surface Height (SSH) measurements resolving scales of a few tens of kilometers. Over a large fraction of the globe, the SSH signal at these scales is essentially a superposition of a component due to balanced motions (BMs) and another component due to internal tides (ITs). Several oceanographic applications require the separation of these components and their mapping on regular grids. For that purpose, the paper introduces an alternating minimization algorithm that iteratively implements two data assimilation techniques, each specific to the mapping of one component: a quasi-geostrophic model with Back-and-Forth Nudging for BMs, and a linear shallow-water model with 4-Dimensional Variational assimilation for ITs. The algorithm is tested with Observation System Simulation Experiments where the truth is provided by a primitive-equation ocean model in an idealized configuration simulating a turbulent jet and mode-one ITs. The algorithm reconstructs almost 80% of the variance of BMs and ITs, the remaining 20% being mostly due to dynamics that cannot be described by the simple models used. Importantly, in addition to the reconstruction of stationary ITs, the amplitude and phase of nonstationary ITs are reconstructed. Sensitivity experiments show that the quality of reconstruction significantly depends upon the timing of observations. Although idealized, this study represents a step forward towards the disentanglement of BMs and ITs signals from real wide-swath altimetry data.
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Pietri, A., Capet, X., D'Ovidio, F., Levy, M., Le Sommer, J., Molines, J., et al. (2021). Skills and Limitations of the Adiabatic Omega Equation: How Effective Is It to Retrieve Oceanic Vertical Circulation at Mesoscale and Submesoscale? Journal Of Physical Oceanography, 51(3), 931–954.
Abstract: The quasigeostrophic and the generalized omega equations are the most widely used methods to reconstruct vertical velocity w from in situ data. As observational networks with much higher spatial and temporal resolutions are being designed, the question arises of identifying the approximations and scales at which an accurate estimation of w through the omega equation can be achieved and what critical scales and observables are needed. In this paper we test different adiabatic omega reconstructions of w over several regions representative of main oceanic regimes of the global ocean in a fully eddy-resolving numerical simulation with a 1/60 degrees horizontal resolution. We find that the best reconstructions are observed in conditions characterized by energetic turbulence and/or weak stratification where near-surface frontal processes are felt deep into the ocean interior. The quasigeostrophic omega equation gives satisfactory results for scales larger than similar to 10 km horizontally while the improvements using a generalized formulation are substantial only in conditions where frontal turbulent processes are important (providing improvements with satisfactory reconstruction skill down to similar to 5 km in scale). The main sources of uncertainties that could be identified are related to processes responsible for ocean thermal wind imbalance (TWI), which is particularly difficult to account for (especially in observation-based studies) and to the deep flow that is generally improperly accounted for in omega reconstructions through the bottom boundary condition. Nevertheless, the reconstruction of mesoscale vertical velocities may be sufficient to estimate vertical fluxes of oceanic properties in many cases of practical interest.
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Revuelto, J., Cluzet, B., Duran, N., Fructus, M., Lafaysse, M., Cosme, E., et al. (2021). Assimilation of surface reflectance in snow simulations: Impact on bulk snow variables. Journal Of Hydrology, 603.
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Uchida, T., Deremble, B., & Penduff, T. (2021). The Seasonal Variability of the Ocean Energy Cycle from a Quasi-Geostrophic Double Gyre Ensemble. Fluids, 6(6).
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Verron, J., Bonnefond, P., Andersen, O., Ardhuin, F., Berge-Nguyen, M., Bhowmick, S., et al. (2021). The SARAL/AltiKa mission: A step forward to the future of altimetry. Advances In Space Research, 68(2), 808–828.
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Zhao, M., Ponte, R., Penduff, T., Close, S., Llovel, W., & Molines, J. (2021). Imprints of Ocean Chaotic Intrinsic Variability on Bottom Pressure and Implications for Data and Model Analyses. Geophysical Research Letters, 48(24).
Abstract: Variations in ocean bottom pressure are important for understanding ocean circulation and climate. While most studies have focused on atmospherically driven variability, here we use eddy-permitting large ensemble simulation output from the OceaniC Chaos-ImPacts, strUcture, predicTability (OCCIPUT) project to isolate chaotic intrinsic variability generated by nonlinear oceanic processes. Analyzing separately the mean seasonal cycle and remainder variability in intra-annual (60-365 days) and subseasonal (2-60 days) bands, we find intrinsic variations larger than atmospherically driven ones over eddy-active regions across all timescales, particularly in the intra-annual range, where intrinsic variations dominate in almost 25% of the oceans. At scales larger than mesoscale, intrinsic variability is still considerable, supporting the process of energy inverse cascade toward lower frequency and larger scales. Results highlight the importance of intrinsic variability over a range of spatiotemporal scales and provide new insights on the interpretation of GRACE-like observations and their de-aliasing procedures.
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2020 |
Ajayi, A., Le Sommer, J., Chassignet, E., Molines, J., Xu, X., Albert, A., et al. (2020). Spatial and Temporal Variability of the North Atlantic Eddy Field From Two Kilometric-Resolution Ocean Models. Journal Of Geophysical Research-Oceans, 125(5).
Abstract: Ocean circulation is dominated by turbulent geostrophic eddy fields with typical scales ranging from 10 to 300 km. At mesoscales (>50 km), the size of eddy structures varies regionally following the Rossby radius of deformation. The variability of the scale of smaller eddies is not well known due to the limitations in existing numerical simulations and satellite capability. Nevertheless, it is well established that oceanic flows (<50 km) generally exhibit strong seasonality. In this study, we present a basin-scale analysis of coherent structures down to 10km in the North Atlantic Ocean using two submesoscale-permitting ocean models, a NEMO-based North Atlantic simulation with a horizontal resolution of 1/60 (NATL60) and an HYCOM-based Atlantic simulation with a horizontal resolution of 1/50 (HYCOM50). We investigate the spatial and temporal variability of the scale of eddy structures with a particular focus on eddies with scales of 10 to 100km, and examine the impact of the seasonality of submesoscale energy on the seasonality and distribution of coherent structures in the North Atlantic. Our results show an overall good agreement between the two models in terms of surface wave number spectra and seasonal variability. The key findings of the paper are that (i) the mean size of ocean eddies show strong seasonality; (ii) this seasonality is associated with an increased population of submesoscale eddies (10-50km) in winter; and (iii) the net release of available potential energy associated with mixed layer instability is responsible for the emergence of the increased population of submesoscale eddies in wintertime. Plain Language Summary The ocean is dominated by circular currents of water in swirling motion called oceanic eddies. This class of motion is by far the largest reservoir of oceanic kinetic energy. Much is known about this oceanic eddies at scale >50 km while we are yet to fully comprehend their distribution in terms of size and dynamics at scales <50 km. This is due to the lack of sufficient observational data sets at these scales in the ocean. In this study, we use two kilometric-resolving models of the North Atlantic ocean to investigate the spatial and temporal variability of oceanic eddies down to 10-km scale. Our results show that the distribution of oceanic eddies at spatial scale <100 km undergo strong seasonality and that this seasonality is as a result of an increased population of smaller eddies (10-50 km) often called submesoscales eddies in wintertime. We found that submesoscale turbulence (a class of oceanic turbulence at fine scale) is responsible for the increase in smaller-scale eddy distribution in winter. Key Points The scale of North Atlantic eddies with scale <100 km is studied using two kilometric-resolution ocean models The mean size of these eddies varies across the basin and shows a strong seasonality This seasonality is driven by mixed layer instability and is associated with an increased population of submesoscale eddies in winter
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Barnier, B., Domina, A., Gulev, S., Molines, J., Maitre, T., Penduff, T., et al. (2020). Modelling the impact of flow-driven turbine power plants on great wind-driven ocean currents and the assessment of their energy potential. Nature Energy, 5(3), 240–249.
Abstract: The persistence in the strength and direction of western boundary great ocean currents suggests that flow-driven turbines implemented in these currents have great potential for energy exploitation. However, technological developments in the design and installation of power-generating plants in the ocean are tied to our capacity to accurately identify the most favourable sites and provide practical assessments of the potentially recoverable energy. Here we use a global eddy-resolving ocean model to demonstrate that large ocean power plants may exert feedback on oceanic circulation that results in highly unpredictable changes in ocean currents. Regionally, these changes can drastically modify the path of the current. In extreme cases this corresponds to a decrease in the available power by more than 80% from initial expectations. Ocean currents offer a potential source of power, but identification of the best sites requires a detailed understanding of their variability. Barnier et al. undertake global eddy-resolving ocean modelling to gain insight into the feedback from ocean power plants on currents and the changes they can induce.
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Bricaud, C., Le Sommer, J., Madec, G., Calone, C., Deshayes, J., Ethe, C., et al. (2020). Multi-grid algorithm for passive tracer transport in the NEMO ocean circulation model: a case study with the NEMO OGCM (version 3.6). Geoscientific Model Development, 13(11), 5465–5483.
Abstract: Ocean biogeochemical models are key tools for both scientific and operational applications. Nevertheless the cost of these models is often expensive because of the large number of biogeochemical tracers. This has motivated the development of multi-grid approaches where ocean dynamics and tracer transport are computed on grids of different spatial resolution. However, existing multi-grid approaches to tracer transport in ocean modelling do not allow the computation of ocean dynamics and tracer transport simultaneously. This paper describes a new multi-grid approach developed for accelerating the computation of passive tracer transport in the Nucleus for European Modelling of the Ocean (NEMO) ocean circulation model. In practice, passive tracer transport is computed at runtime on a grid with coarser spatial resolution than the hydrodynamics, which reduces the CPU cost of computing the evolution of tracers. We describe the multi-grid algorithm, its practical implementation in the NEMO ocean model, and discuss its performance on the basis of a series of sensitivity experiments with global ocean model configurations. Our experiments confirm that the spatial resolution of hydrodynamical fields can be coarsened by a factor of 3 in both horizontal directions without significantly affecting the resolved passive tracer fields. Overall, the proposed algorithm yields a reduction by a factor of 7 of the overhead associated with running a full biogeochemical model like PISCES (with 24 passive tracers). Propositions for further reducing this cost without affecting the resolved solution are discussed.
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Buckingham, C., Lucas, N., Belcher, S., Rippeth, T., Grant, A., Le Sommer, J., et al. (2020). The Contribution of Surface and Submesoscale Processes to Turbulence in the Open Ocean Surface Boundary Layer. Journal Of Advances In Modeling Earth Systems, .
Abstract: The ocean surface boundary layer is a critical interface across which momentum, heat, and trace gases are exchanged between the oceans and atmosphere. Surface processes (winds, waves, and buoyancy forcing) are known to contribute significantly to fluxes within this layer. Recently, studies have suggested that submesoscale processes, which occur at small scales (0.1-10 km, hours to days) and therefore are not yet represented in most ocean models, may play critical roles in these turbulent exchanges. While observational support for such phenomena has been demonstrated in the vicinity of strong current systems and littoral regions, relatively few observations exist in the open-ocean environment to warrant representation in Earth system models. We use novel observations and simulations to quantify the contributions of surface and submesoscale processes to turbulent kinetic energy (TKE) dissipation in the open-ocean surface boundary layer. Our observations are derived from moorings in the North Atlantic, December 2012 to April 2013, and are complemented by atmospheric reanalysis. We develop a conceptual framework for dissipation rates due to surface and submesoscale processes. Using this framework and comparing with observed dissipation rates, we find that surface processes dominate TKE dissipation. A parameterization for symmetric instability is consistent with this result. We next employ simulations from an ocean front-resolving model to reestablish that dissipation due to surface processes exceeds that of submesoscale processes by 1-2 orders of magnitude. Together, these results suggest submesoscale processes do not dramatically modify vertical TKE budgets, though such dynamics may be climatically important owing to their ability to remove energy from the ocean.
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Close, S., Penduff, T., Speich, S., & Molines, J. (2020). A means of estimating the intrinsic and atmospherically-forced contributions to sea surface height variability applied to altimetric observations. Progress In Oceanography, 184.
Abstract: Drawing on a 50-member ocean ensemble hindcast, the magnitude and characteristic temporal and spatial scales of intrinsic and forced sea surface height (SSH) variability are evaluated over a 37-year period. The intrinsic and forced contributions derived from the ensemble are found to have similar temporal spectra, but different characteristic spatial scales. These results suggest that, with an appropriate choice of cutoff scales, simple spatial filtering can be used to estimate the forced and intrinsic contributions given either a single model run, or an observational data set. The method is tested using a single member drawn from the ensemble, before being applied to the observed altimetric record. Two sample applications with relevance to large-scale climate are used to illustrate the method's potential utility. Firstly, the long-term trends calculated from the total and recreated forced components using the altimetric record are compared and local differences highlighted. Second, the recreated forced SSH is shown to covary with the North Atlantic Oscillation at seasonal time scales in regions where no such influence can be found using the original SSH signal. Some limitations and uses for which the method may prove unsuitable are also briefly considered.
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Cluzet, B., Revuelto, J., Lafaysse, M., Tuzet, F., Cosme, E., Picard, G., et al. (2020). Towards the assimilation of satellite reflectance into semi-distributed ensemble snowpack simulations. Cold Regions Science And Technology, 170.
Abstract: Uncertainties of snowpack models and of their meteorological forcings limit their use by avalanche hazard forecasters, or for glaciological and hydrological studies. The spatialized simulations currently available for avalanche hazard forecasting are only assimilating sparse meteorological observations. As suggested by recent studies, their forecasting skills could be significantly improved by assimilating satellite data such as snow reflectances from satellites in the visible and the near-infrared spectra. Indeed, these data can help constrain the microstructural properties of surface snow and light absorbing impurities content, which in turn affect the surface energy and mass budgets. This paper investigates the prerequisites of satellite data assimilation into a detailed snowpack model. An ensemble version of Meteo-France operational snowpack forecasting system (named S2M) was built for this study. This operational system runs on topographic classes instead of grid points, so-called 'semi-distributed' approach. Each class corresponds to one of the 23 mountain massifs of the French Alps (about 1000 km(2) each), an altitudinal range (by step of 300 m) and aspect (by step of 45 degrees). We assess the feasability of satellite data assimilation in such a semi-distributed geometry. Ensemble simulations are compared with satellite observations from MODIS and Sentinel-2, and with in-situ reflectance observations. The study focuses on the 2013-2014 and 2016-2017 winters in the Grandes-Rousses massif. Substantial Pearson R-2 correlations (0.75-0.90) of MODIS observations with simulations are found over the domain. This suggests that assimilating it could have an impact on the spatialized snowpack forecasting system. However, observations contain significant biases (0.1-0.2 in reflectance) which prevent their direct assimilation. MODIS spectral band ratios seem to be much less biased. This may open the way to an operational assimilation of MODIS reflectances into the Meteo-France snowpack modelling system.
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Colombo, P., Barnier, B., Penduff, T., Chanut, J., Deshayes, J., Molines, J., et al. (2020). Representation of the Denmark Strait overflow in a z-coordinate eddying configuration of the NEMO (v3.6) ocean model: resolution and parameter impacts. Geoscientific Model Development, 13(7), 3347–3371.
Abstract: We investigate in this paper the sensitivity of the representation of the Denmark Strait overflow produced by a regional z-coordinate configuration of NEMO (version 3.6) to the horizontal and vertical grid resolutions and to various numerical and physical parameters. Three different horizontal resolutions, 1/12, 1/36, and 1/60 degrees, are respectively used with 46, 75, 150, and 300 vertical levels. In the given numerical set-up, the increase in the vertical resolution did not bring improvement at eddy-permitting resolution (1/12 degrees). We find a greater dilution of the overflow as the number of vertical level increases, and the worst solution is the one with 300 vertical levels. It is found that when the local slope of the grid is weaker than the slope of the topography the result is a more diluted vein. Such a grid enhances the dilution of the plume in the ambient fluid and produces its thickening. Although the greater number of levels allows for a better resolution of the ageostrophic Ekman flow in the bottom layer, the final result also depends on how the local grid slope matches the topographic slope. We also find that for a fixed number of levels, the representation of the overflow is improved when horizontal resolution is increased to 1/36 and 1/60 degrees, with the most drastic improvements being obtained with 150 levels. With such a number of vertical levels, the enhanced vertical mixing associated with the step-like representation of the topography remains limited to a thin bottom layer representing a minor portion of the overflow. Two major additional players contribute to the sinking of the overflow: the breaking of the overflow into boluses of dense water which contribute to spreading the overflow waters along the Greenland shelf and within the Irminger Basin, and the resolved vertical shear that results from the resolution of the bottom Ekman boundary layer dynamics. This improves the accuracy of the calculation of the entrainment by the turbulent kinetic energy mixing scheme (as it depends on the local shear) and improves the properties of the overflow waters such that they more favourably compare with observations. At 300 vertical levels the dilution is again increased for all horizontal resolutions. The impact on the overflow representation of many other numerical parameters was tested (momentum advection scheme, lateral friction, bottom boundary layer parameterization, closure parameterization, etc.), but none had a significant impact on the overflow representation.
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Gavrikov, A., Gulev, S., Markina, M., Tilinina, N., Verezemskaya, P., Barnier, B., et al. (2020). RAS-NAAD: 40-yr High-Resolution North Atlantic Atmospheric Hindcast for Multipurpose Applications (New Dataset for the Regional Mesoscale Studies in the Atmosphere and the Ocean). Journal Of Applied Meteorology And Climatology, 59(5), 793–817.
Abstract: We present in this paper the results of the Russian Academy of Sciences North Atlantic Atmospheric Downscaling (RAS-NAAD) project, which provides a 40-yr 3D hindcast of the North Atlantic (10 degrees-80 degrees N) atmosphere at 14-km spatial resolution with 50 levels in the vertical direction (up to 50 hPa), performed with a regional setting of theWRF-ARW3.8.1model for the period 1979-2018 and forced by ERA-Interim as a lateral boundary condition. The dataset provides a variety of surface and free-atmosphere parameters at sigma model levels and meets many demands of meteorologists, climate scientists, and oceanographers working in both research and operational domains. Three-dimensional model output at 3-hourly time resolution is freely available to the users. Our evaluation demonstrates a realistic representation of most characteristics in both datasets and also identifies biases mostly in the ice-covered regions. High-resolution and nonhydrostatic model settings in NAAD resolve mesoscale dynamics first of all in the subpolar latitudes. NAAD also provides a new view of the North Atlantic extratropical cyclone activity with a much larger number of cyclones as compared with most reanalyses. It also effectively captures highly localized mechanisms of atmospheric moisture transports. Applications of NAAD to ocean circulation and wave modeling are demonstrated.
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Gehlen, M., Berthet, S., Seferian, R., Ethe, C., & Penduff, T. (2020). Quantification of Chaotic Intrinsic Variability of Sea-Air CO2 Fluxes at Interannual Timescales. Geophysical Research Letters, 47(22).
Abstract: Chaotic intrinsic variability (CIV) emerges spontaneously from nonlinear ocean dynamics even without any atmospheric variability. Eddy-permitting numerical simulations suggest that CIV is a significant contributor to the interannual to decadal variability of physical properties. Here we show from an ensemble of global ocean eddy-permitting simulations that large-scale interannual CIV propagates from physical properties to sea-air CO2 fluxes in areas of high mesoscale eddy activity (e.g., Southern Ocean and western boundary currents). In these regions and at scales larger than 500 km (similar to 5 degrees), CIV contributes significantly to the interannual variability of sea-air CO2 fluxes. Between 35 degrees S and 45 degrees S (midlatitude Southern Ocean), CIV amounts to 23.76 TgC yr(-1) or one half of the atmospherically forced variability. Locally, its contribution to the total interannual variance of sea-air CO2 fluxes exceeds 76%. Outside eddy-active regions its contribution to total interannual variability is below 16%. Plain Language Summary Sea-air CO2 fluxes undergo substantial regional and interannual fluctuations. These fluctuations are mostly forced by changes in large-scale atmospheric patterns, but ocean internal dynamics could also contribute to them. This study quantifies these two sources of variability and their contributions to fluctuations of sea-air CO2 fluxes over large oceanic regions. It relies on the analyses of three ocean numerical simulations driven by the same atmospheric forcing but starting from small differences in initial conditions, and including a simplified representation of marine ecosystems. Simulations are run at a horizontal resolution allowing to model part of the effect of ocean mesoscale activity on physical and chemical tracers. We demonstrate that nonlinear oceanic processes drive fluctuations of sea-air CO2 fluxes at interannual timescales that are inherently random. The magnitude of these fluctuations is substantial over areas of high kinetic energy and locally exceeds 76% of the total interannual variance of sea-air CO2 fluxes.
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Gomez-Navarro, L., Cosme, E., Le Sommer, J., Papadakis, N., & Pascual, A. (2020). Development of an Image De-Noising Method in Preparation for the Surface Water and Ocean Topography Satellite Mission. Remote Sensing, 12(4).
Abstract: In the near future, the Surface Water Ocean Topography (SWOT) mission will provide images of altimetric data at kilometric resolution. This unprecedented 2-dimensional data structure will allow the estimation of geostrophy-related quantities that are essential for studying the ocean surface dynamics and for data assimilation uses. To estimate these quantities, i.e., to compute spatial derivatives of the Sea Surface Height (SSH) measurements, the uncorrelated, small-scale noise and errors expected to affect the SWOT data must be smoothed out while minimizing the loss of relevant, physical SSH information. This paper introduces a new technique for de-noising the future SWOT SSH images. The de-noising model is formulated as a regularized least-square problem with a Tikhonov regularization based on the first-, second-, and third-order derivatives of SSH. The method is implemented and compared to other, convolution-based filtering methods with boxcar and Gaussian kernels. This is performed using a large set of pseudo-SWOT data generated in the western Mediterranean Sea from a 1/60 simulation and the SWOT simulator. Based on root mean square error and spectral diagnostics, our de-noising method shows a better performance than the convolution-based methods. We find the optimal parametrization to be when only the second-order SSH derivative is penalized. This de-noising reduces the spatial scale resolved by SWOT by a factor of 2, and at 10 km wavelengths, the noise level is reduced by factors of for summer and winter, respectively. This is encouraging for the processing of the future SWOT data.
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Hirschi, J., Barnier, B., Boning, C., Biastoch, A., Blaker, A., Coward, A., et al. (2020). The Atlantic Meridional Overturning Circulation in High-Resolution Models. Journal Of Geophysical Research-Oceans, 125(4).
Abstract: The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26 degrees N-a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high-resolution models. Furthermore, we will describe how high-resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.
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Jamet, Q., Ajayi, A., Le Sommer, J., Penduff, T., Hogg, A., & Dewar, W. (2020). On Energy Cascades in General Flows: A Lagrangian Application. Journal Of Advances In Modeling Earth Systems, 12(12).
Abstract: An important characteristic of geophysically turbulent flows is the transfer of energy between scales. Balanced flows pass energy from smaller to larger scales as part of the well-known upscale cascade, while submesoscale and smaller scale flows can transfer energy eventually to smaller, dissipative scales. Much effort has been put into quantifying these transfers, but a complicating factor in realistic settings is that the underlying flows are often strongly spatially heterogeneous and anisotropic. Furthermore, the flows may be embedded in irregularly shaped domains that can be multiply connected. As a result, straightforward approaches like computing Fourier spatial spectra of nonlinear terms suffer from a number of conceptual issues. In this paper, we develop a method to compute cross-scale energy transfers in general settings, allowing for arbitrary flow structure, anisotropy, and inhomogeneity. We employ Green's function approach to the kinetic energy equation to relate kinetic energy at a point to its Lagrangian history. A spatial filtering of the resulting equation naturally decomposes kinetic energy into length-scale-dependent contributions and describes how the transfer of energy between those scalestakes place. The method is applied to a doubly periodic simulation of vortex merger, resulting in the demonstration of the expected upscale energy cascade. Somewhat novel results are that the energy transfers are dominated by pressure work, rather than kinetic energy exchange, and dissipation is a noticeable influence on the larger scale energy budgets. We also describe, but do not employ here, a technique for developing filters to use in complex domains.
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Jamet, Q., Dewar, W., Wienders, N., Deremble, B., Close, S., & Penduff, T. (2020). Locally and Remotely Forced Subtropical AMOC Variability: A Matter of Time Scales. Journal Of Climate, 33(12), 5155–5172.
Abstract: Mechanisms driving the North Atlantic meridional overturning circulation (AMOC) variability at low frequency are of central interest for accurate climate predictions. Although the subpolar gyre region has been identified as a preferred place for generating climate time-scale signals, their southward propagation remains under consideration, complicating the interpretation of the observed time series provided by the Rapid Climate Change-Meridional Overturning Circulation and Heatflux Array-Western Boundary Time Series (RAPID-MOCHA-WBTS) program. In this study, we aim at disentangling the respective contribution of the local atmospheric forcing from signals of remote origin for the subtropical low-frequency AMOC variability. We analyze for this a set of four ensembles of a regional (20 degrees S-55 degrees N), eddy-resolving (1/12 degrees) North Atlantic oceanic configuration, where surface forcing and open boundary conditions are alternatively permuted from fully varying (realistic) to yearly repeating signals. Their analysis reveals the predominance of local, atmospherically forced signal at interannual time scales (2-10 years), whereas signals imposed by the boundaries are responsible for the decadal (10-30 years) part of the spectrum. Due to this marked time-scale separation, we show that, although the intergyre region exhibits peculiarities, most of the subtropical AMOC variability can be understood as a linear superposition of these two signals. Finally, we find that the decadal-scale, boundary-forced AMOC variability has both northern and southern origins, although the former dominates over the latter, including at the site of the RAPID array (26.5 degrees N).
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Largeron, C., Dumont, M., Morin, S., Boone, A., Lafaysse, M., Metref, S., et al. (2020). Toward Snow Cover Estimation in Mountainous Areas Using Modern Data Assimilation Methods: A Review. Frontiers In Earth Science, 8.
Abstract: The snow cover is a key component of land surface hydrology, especially in mountain areas where it governs the amount and timing of water availability in downstream areas. It is involved in relevant climate feedbacks and natural hazards such as avalanches and floods. Monitoring and forecasting snow cover characteristics is challenging. While snow cover extent is relatively easy to retrieve from satellite data, remote sensing retrievals of the snow water equivalent (SWE) is often inaccurate, particularly in complex mountainous terrain. Model-based snow cover estimates, driven by meteorological data, often bear significant uncertainties due to both input data and model errors. Data assimilation can combine both approaches to improve SWE estimates. In this paper, we review current state-of-the-art data assimilation methodologies used to optimally combine measurements with snow cover models in order to reduce uncertainties. The suitability of a given data assimilation method varies with the numerical complexity of snow models as well as the availability and the type of observations. This review describes the issues and challenges associated with data assimilation applied to the mountain snow cover, providing recommendations for existing and upcoming monitoring and prediction systems of snow hydrology in mountainous regions.
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Metref, S., Cosme, E., Le Guillou, F., Le Sommer, J., Brankart, J., & Verron, J. (2020). Wide-Swath Altimetric Satellite Data Assimilation With Correlated-Error Reduction. Frontiers In Marine Science, 6.
Abstract: For decades now, satellite altimetric observations have been successfully integrated in numerical oceanographic models using data assimilation (DA). So far, sea surface height (SSH) data were provided by one-dimensional nadir altimeters. The next generation Surface Water and Ocean Topography (SWOT) satellite altimeter will provide two-dimensional wide-swath altimetric information with an unprecedented high resolution. This new type of SSH data is expected to strongly improve altimetric assimilation. However, the SWOT data is also expected to be affected by spatially correlated errors and, hence, can not be assimilated as easily as nadir altimeters. The present paper proposes to embed a state-of-the-art correlated-error reduction (CER) method for the SWOT data into an ensemble-based DA scheme. The DA with the new correlated-error reduced-data (CER-data) is implemented and tested in a simple SSH reconstruction problem using artificial SWOT data and a quasi-geostrophic model. The results show that, in an energetic large scale region, the DA with CER-data – in comparison to the classical DA- reduces the root-mean-square-error (RMSE) of the reconstruction in SSH by approximately 10%, in relative vorticity by 5% and in surface currents by 5-10%, and also slightly improves the noise-to-signal ratio and spectral coherence of the SSH signal at mesoscale (100-200 km) but with a small degradation on the large scales (>300 km). In a less energetic region, the DA with CER-data cuts down the RMSE in SSH by more than 50% on average therefore allowing a significantly more accurate reconstruction of SSH at mesoscale in terms of noise-to-signal ratio, spectral coherence, and power spectral density.
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Santana-Falcon, Y., Brasseur, P., Brankart, J., & Garnier, F. (2020). Assimilation of chlorophyll data into a stochastic ensemble simulation for the North Atlantic Ocean. Ocean Science, 16(5), 1297–1315.
Abstract: Satellite-derived surface chlorophyll data are assimilated daily into a three-dimensional 24-member ensemble configuration of an online-coupled NEMO (Nucleus for European Modeling of the Ocean)-PISCES (Pelagic Interaction Scheme of Carbon and Ecosystem Stu dies) model for the North Atlantic Ocean. A 1-year multivariate assimilation experiment is performed to evaluate the impacts on analyses and forecast ensembles. Our results demonstrate that the integration of data improves surface analysis and forecast chlorophyll representation in a major part of the model domain, where the assimilated simulation outperforms the probabilistic skills of a non-assimilated analogous simulation. However, improvements are dependent on the reliability of the prior free ensemble. A regional diagnosis shows that surface chlorophyll is overestimated in the northern limit of the subtropical North Atlantic, where the prior ensemble spread does not cover the observation's variability. There, the system cannot deal with corrections that alter the equilibrium between the observed and unobserved state variables producing instabilities that propagate into the forecast. To alleviate these inconsistencies, a 1-month sensitivity experiment in which the assimilation process is only applied to model fluctuations is performed. Results suggest the use of this methodology may decrease the effect of corrections on the correlations between state vectors. Overall, the experiments presented here evidence the need of refining the description of model's uncertainties according to the biogeochemical characteristics of each oceanic region.
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Zavialov, I., Osadchiev, A., Sedakov, R., Barnier, B., Molines, J., & Belokopytov, V. (2020). Water exchange between the Sea of Azov and the Black Sea through the Kerch Strait. Ocean Science, 16(1), 15–30.
Abstract: The Sea of Azov is a small, shallow, and freshened sea that receives a large freshwater discharge. Under certain external forcing conditions low-salinity waters from the Sea of Azov flow into the north-eastern part of the Black Sea through the narrow Kerch Strait and form a surface-advected buoyant plume. Water flow in the Kerch Strait also regularly occurs in the opposite direction, which results in the spreading of a bottom-advected plume of saline and dense waters from the Black Sea into the Sea of Azov. In this study we focus on the physical mechanisms that govern water exchange through the Kerch Strait and analyse the dependence of its direction and intensity on external forcing conditions. Analysis of satellite imagery, wind data, and numerical modelling shows that water exchange in the Kerch Strait is governed by a wind-induced barotropic pressure gradient. Water flow through the shallow and narrow Kerch Strait is a one-way process for the majority of the time. Outflow from the Sea of Azov to the Black Sea is induced by moderate and strong north-easterly winds, while flow into the Sea of Azov from the Black Sea occurs during wind relaxation periods. The direction and intensity of water exchange have wind-governed synoptic and seasonal variability, and they do not depend on the rate of river discharge to the Sea of Azov on an intra-annual timescale. The analysed data reveal dependencies between wind forcing conditions and spatial characteristics of the buoyant plume formed by the outflow from the Sea of Azov.
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Zhen, Y., Tandeo, P., Leroux, S., Metref, S., Penduff, T., & Le Sommer, J. (2020). An Adaptive Optimal Interpolation Based on Analog Forecasting: Application to SSH in the Gulf of Mexico. Journal Of Atmospheric And Oceanic Technology, 37(9), 1697–1711.
Abstract: Because of the irregular sampling pattern of raw altimeter data, many oceanographic applications rely on information from sea surface height (SSH) products gridded on regular grids where gaps have been filled with interpolation. Today, the operational SSH products are created using the simple, but robust, optimal interpolation (OI) method. If well tuned, the OI becomes computationally cheap and provides accurate results at low resolution. However, OI is not adapted to produce high-resolution and high-frequency maps of SSH. To improve the interpolation of SSH satellite observations, a data-driven approach (i.e., constructing a dynamical forecast model from the data) was recently proposed: analog data assimilation (AnDA). AnDA adaptively chooses analog situations from a catalog of SSH scenes-originating from numerical simulations or a large database of observations-which allow the temporal propagation of physical features at different scales, while each observation is assimilated. In this article, we review the AnDA and OI algorithms and compare their skills in numerical experiments. The experiments are observing system simulation experiments (OSSE) on the Lorenz-63 system and on an SSH reconstruction problem in the Gulf of Mexico. The results show that AnDA, with no necessary tuning, produces comparable reconstructions as does OI with tuned parameters. Moreover, AnDA manages to reconstruct the signals at higher frequencies than OI. Finally, an important additional feature for any interpolation method is to be able to assess the quality of its reconstruction. This study shows that the standard deviation estimated by AnDA is flow dependent, hence more informative on the reconstruction quality, than the one estimated by OI.
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2019 |
Buckingham, C., Lucas, N., Belcher, S., Rippeth, T., Grant, A., Le Sommer, J., et al. (2019). The Contribution of Surface and Submesoscale Processes to Turbulence in the Open Ocean Surface Boundary Layer. Journal Of Advances In Modeling Earth Systems, 11(12), 4066–4094.
Abstract: The ocean surface boundary layer is a critical interface across which momentum, heat, and trace gases are exchanged between the oceans and atmosphere. Surface processes (winds, waves, and buoyancy forcing) are known to contribute significantly to fluxes within this layer. Recently, studies have suggested that submesoscale processes, which occur at small scales (0.1-10 km, hours to days) and therefore are not yet represented in most ocean models, may play critical roles in these turbulent exchanges. While observational support for such phenomena has been demonstrated in the vicinity of strong current systems and littoral regions, relatively few observations exist in the open-ocean environment to warrant representation in Earth system models. We use novel observations and simulations to quantify the contributions of surface and submesoscale processes to turbulent kinetic energy (TKE) dissipation in the open-ocean surface boundary layer. Our observations are derived from moorings in the North Atlantic, December 2012 to April 2013, and are complemented by atmospheric reanalysis. We develop a conceptual framework for dissipation rates due to surface and submesoscale processes. Using this framework and comparing with observed dissipation rates, we find that surface processes dominate TKE dissipation. A parameterization for symmetric instability is consistent with this result. We next employ simulations from an ocean front-resolving model to reestablish that dissipation due to surface processes exceeds that of submesoscale processes by 1-2 orders of magnitude. Together, these results suggest submesoscale processes do not dramatically modify vertical TKE budgets, though such dynamics may be climatically important owing to their ability to remove energy from the ocean.
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Fennel, K., Gehlen, M., Brasseur, P., Brown, C., Ciavatta, S., Cossarini, G., et al. (2019). Advancing Marine Biogeochemical and Ecosystem Reanalyses and Forecasts as Tools for Monitoring and Managing Ecosystem Health. Frontiers In Marine Science, 6.
Abstract: Ocean ecosystems are subject to a multitude of stressors, including changes in ocean physics and biogeochemistry, and direct anthropogenic influences. Implementation of protective and adaptive measures for ocean ecosystems requires a combination of ocean observations with analysis and prediction tools. These can guide assessments of the current state of ocean ecosystems, elucidate ongoing trends and shifts, and anticipate impacts of climate change and management policies. Analysis and prediction tools are defined here as ocean circulation models that are coupled to biogeochemical or ecological models. The range of potential applications for these systems is broad, ranging from reanalyses for the assessment of past and current states, and short-term and seasonal forecasts, to scenario simulations including climate change projections. The objectives of this article are to illustrate current capabilities with regard to the three types of applications, and to discuss the challenges and opportunities. Representative examples of global and regional systems are described with particular emphasis on those in operational or pre-operational use. With regard to the benefits and challenges, similar considerations apply to biogeochemical and ecological prediction systems as do to physical systems. However, at present there are at least two major differences: (1) biogeochemical observation streams are much sparser than physical streams presenting a significant hinderance, and (2) biogeochemical and ecological models are largely unconstrained because of insufficient observations. Expansion of biogeochemical and ecological observation systems will allow for significant advances in the development and application of analysis and prediction tools for ocean biogeochemistry and ecosystems, with multiple societal benefits.
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Fox-Kemper, B., Adcroft, A., Boning, C., Chassignet, E., Curchitser, E., Danabasoglu, G., et al. (2019). Challenges and Prospects in Ocean Circulation Models. Frontiers In Marine Science, 6.
Abstract: We revisit the challenges and prospects for ocean circulation models following Griffies et al. (2010). Over the past decade, ocean circulation models evolved through improved understanding, numerics, spatial discretization, grid configurations, parameterizations, data assimilation, environmental monitoring, and process-level observations and modeling. Important large scale applications over the last decade are simulations of the Southern Ocean, the Meridional Overturning Circulation and its variability, and regional sea level change. Submesoscale variability is now routinely resolved in process models and permitted in a few global models, and submesoscale effects are parameterized in most global models. The scales where nonhydrostatic effects become important are beginning to be resolved in regional and process models. Coupling to sea ice, ice shelves, and high-resolution atmospheric models has stimulated new ideas and driven improvements in numerics. Observations have provided insight into turbulence and mixing around the globe and its consequences are assessed through perturbed physics models. Relatedly, parameterizations of the mixing and overturning processes in boundary layers and the ocean interior have improved. New diagnostics being used for evaluating models alongside present and novel observations are briefly referenced. The overall goal is summarizing new developments in ocean modeling, including: how new and existing observations can be used, what modeling challenges remain, and how simulations can be used to support observations.
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Fujii, Y., Remy, E., Zuo, H., Oke, P., Halliwell, G., Gasparin, F., et al. (2019). Observing System Evaluation Based on Ocean Data Assimilation and Prediction Systems: On-Going Challenges and a Future Vision for Designing and Supporting Ocean Observational Networks. Frontiers In Marine Science, 6.
Abstract: This paper summarizes recent efforts on Observing System Evaluation (OS-Eval) by the Ocean Data Assimilation and Prediction (ODAP) communities such as GODAE OceanView and CLIVAR-GSOP. It provides some examples of existing OS-Eval methodologies, and attempts to discuss the potential and limitation of the existing approaches. Observing System Experiment (OSE) studies illustrate the impacts of the severe decrease in the number of TAO buoys during 2012-2014 and TRITON buoys since 2013 on ODAP system performance. Multi-system evaluation of the impacts of assimilating satellite sea surface salinity data based on OSEs has been performed to demonstrate the need to continue and enhance satellite salinity missions. Impacts of underwater gliders have been assessed using Observing System Simulation Experiments (OSSEs) to provide guidance on the effective coordination of the western North Atlantic observing system elements. OSSEs are also being performed under H2020 AtlantOS project with the goal to enhance and optimize the Atlantic in-situ networks. Potential of future satellite missions of wide-swath altimetry and surface ocean currents monitoring is explored through OSSEs and evaluation of Degrees of Freedomfor Signal (DFS). Forecast Sensitivity Observation Impacts (FSOI) are routinely evaluated for monitoring the ocean observation impacts in the US Navy's ODAP system. Perspectives on the extension of OS-Eval to coastal regions, the deep ocean, polar regions, coupled data assimilation, and biogeochemical applications are also presented. Based on the examples above, we identify the limitations of OS-Eval, indicating that the most significant limitation is reduction of robustness and reliability of the results due to their system-dependency. The difficulty of performing evaluation in near real time is also critical. A strategy to mitigate the limitation and to strengthen the impact of evaluations is discussed. In particular, we emphasize the importance of collaboration within the ODAP community for multi-system evaluation and of communication with ocean observational communities on the design of OS-Eval, required resources, and effective distribution of the results. Finally, we recommend further developing OS-Eval activities at international level with the support of the international ODAP (e.g., OceanPredict and CLIVAR-GSOP) and observational communities.
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Germineaud, C., Brankart, J., & Brasseur, P. (2019). An Ensemble-Based Probabilistic Score Approach to Compare Observation Scenarios: An Application to Biogeochemical-Argo Deployments. Journal Of Atmospheric And Oceanic Technology, 36(12), 2307–2326.
Abstract: A cross-validation algorithm is developed to perform probabilistic observing system simulation experiments (OSSEs). The use of a probability distribution of “true” states is considered rather than a single “truth” using a cross-validation algorithm in which each member of an ensemble simulation is alternatively used as the “truth” and to simulate synthetic observation data that reflect the observing system to be evaluated. The other available members are used to produce an updated ensemble by assimilating the specific data, while a probabilistic evaluation of the observation impacts is obtained using a comprehensive set of verification skill scores. To showcase this new type of OSSE studies with tractable numerical costs, a simple biogeochemical application under the Horizon 2020 AtlantOS project is presented for a single assimilation time step, in order to investigate the value of adding biogeochemical (BGC)-Argo floats to the existing satellite ocean color observations. Further experiments must be performed in time as well for a rigorous and effective evaluation of the BGC-Argo network design, though some evidence from this preliminary work suggests that assimilating chlorophyll data from a BGC-Argo array of 1000 floats can provide additional error reduction at the surface, where the use of spatial ocean color data is limited (due to cloudy conditions), as well at depths ranging from 50 to 150 m.
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Hirschi, J., Frajka-Williams, E., Blaker, A., Sinha, B., Coward, A., Hyder, P., et al. (2019). Loop Current Variability as Trigger of Coherent Gulf Stream Transport Anomalies. Journal Of Physical Oceanography, 49(8), 2115–2132.
Abstract: Satellite observations and output from a high-resolution ocean model are used to investigate how the Loop Current in the Gulf of Mexico affects the Gulf Stream transport through the Florida Straits. We find that the expansion (contraction) of the Loop Current leads to lower (higher) transports through the Straits of Florida. The associated surface velocity anomalies are coherent from the southwestern tip of Florida to Cape Hatteras. A simple continuity-based argument can be used to explain the link between the Loop Current and the downstream Gulf Stream transport: as the Loop Current lengthens (shortens) its path in the Gulf of Mexico, the flow out of the Gulf decreases (increases). Anomalies in the surface velocity field are first seen to the southwest of Florida and within 4 weeks propagate through the Florida Straits up to Cape Hatteras and into the Gulf Stream Extension. In both the observations and the model this propagation can be seen as pulses in the surface velocities. We estimate that the Loop Current variability can be linked to a variability of several Sverdrups (1Sv = 10(6) m(3) s(-1)) through the Florida Straits. The exact timing of the Loop Current variability is largely unpredictable beyond a few weeks and its variability is therefore likely a major contributor to the chaotic/intrinsic variability of the Gulf Stream. However, the time lag between the Loop Current and the flow downstream of the Gulf of Mexico means that if a lengthening/shortening of the Loop Current is observed this introduces some predictability in the downstream flow for a few weeks.
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Jourdain, N., Molines, J., Le Sommer, J., Mathiot, P., Chanut, J., De Lavergne, C., et al. (2019). Simulating or prescribing the influence of tides on the Amundsen Sea ice shelves. Ocean Modelling, 133, 44–55.
Abstract: The representation of tides in regional ocean simulations of the Amundsen Sea enhances ice-shelf melting, with weakest effects for Pine Island and Thwaites (< +10%) and strongest effects for Dotson, Cosgrove and Abbot (> +30%). Tides increase vertical mixing throughout the water column along the continental shelf break. Diurnal tides induce topographically trapped vorticity waves along the continental shelf break, likely underpinning the tidal rectification (residual circulation) simulated in the Dotson-Getz Trough. However, the primary effect by which tides affect ice-shelf melting is the increase of ice/ocean exchanges, rather than the modification of water masses on the continental shelf. Tide-induced velocities strengthen turbulent heat fluxes at the ice/ocean interface, thereby increasing melt rates. Approximately a third of this effect is counterbalanced by the resulting release of cold melt water that reduces melt downstream along the meltwater flow. The relatively weak tide-induced melting underneath Pine Island and Thwaites could be partly related to their particularly thick water column, which limits the presence of quarter wavelength tidal resonance. No sensitivity to the position of Pine Island and Thwaites with respect to the M-2 critical latitude is found. We refine and evaluate existing methodologies to prescribe the effect of tides on ice-shelf melt rates in ocean models that do not explicitely include tidal forcing. The best results are obtained by prescribing spatially-dependent tidal top-boundary-layer velocities in the melt equations. These velocities can be approximated as a linear function of existing barotropic tidal solutions. A correction factor needs to be applied to account for the additional melt-induced circulation associated with tides and to reproduce the relative importance of dynamical and thermodynamical processes.
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Lguensat, R., Viet, P., Sun, M., Chen, G., Tian, F., Chapron, B., et al. (2019). Data-Driven Interpolation of Sea Level Anomalies Using Analog Data Assimilation. Remote Sensing, 11(7).
Abstract: From the recent developments of data-driven methods as a means to better exploit large-scale observation, simulation and reanalysis datasets for solving inverse problems, this study addresses the improvement of the reconstruction of higher-resolution Sea Level Anomaly (SLA) fields using analog strategies. This reconstruction is stated as an analog data assimilation issue, where the analog models rely on patch-based and Empirical Orthogonal Functions (EOF)-based representations to circumvent the curse of dimensionality. We implement an Observation System Simulation Experiment (OSSE) in the South China Sea. The reported results show the relevance of the proposed framework with a significant gain in terms of Root Mean Square Error (RMSE) for scales below 100 km. We further discuss the usefulness of the proposed analog model as a means to exploit high-resolution model simulations for the processing and analysis of current and future satellite-derived altimetric data with regard to conventional interpolation schemes, especially optimal interpolation.
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Lopez-Radcenco, M., Pascual, A., Gomez-Navarro, L., Aissa-El-Bey, A., Chapron, B., & Fablet, R. (2019). Analog Data Assimilation of Along-Track Nadir and Wide-Swath SWOT Altimetry Observations in the Western Mediterranean Sea. Ieee Journal Of Selected Topics In Applied Earth Observations And Remote Sensing, 12(7), 2530–2540.
Abstract: The growing availability of ocean data brought forth by recent advancements in remote sensing, in situ measurements, and numerical models supports the development of data-driven strategies as a powerful, computationally efficient alternative to model-based approaches for the interpolation of high-resolution, gap-free, regularly gridded sea surface geophysical fields from partial satellite-derived observations. In this paper, we investigate such data-driven strategies for the spatio-temporal interpolation of sea level anomaly (SLA) fields in the Western Mediterranean Sea from satellite-derived altimetry data. We introduce and evaluate the analog data assimilation (AnDA) framework, which exploits patch-based analog forecasting operators within a classic Kalman-based data assimilation scheme. With a view toward the upcoming wide-swath surface water and ocean topography (SWOT) mission, two different types of altimetry data are assimilated: along-track nadir data and wide-swath SWOT altimetry data. Using an observing system simulation experiment, we demonstrate the relevance of AnDA as an improved interpolation method, particularly for mesoscale features in the 20- to 100-km horizontal scale range. Results report an SLA reconstruction RMSE (correlation) improvement of 42% (14%) with respect to optimal interpolation, and show a clear gain when the joint assimilation of SWOT and along-track nadir observations are considered.
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Metref, S., Cosme, E., Le Sommer, J., Poel, N., Brankart, J., Verron, J., et al. (2019). Reduction of Spatially Structured Errors in Wide-Swath Altimetric Satellite Data Using Data Assimilation. Remote Sensing, 11(11).
Abstract: The Surface Water and Ocean Topography (SWOT) mission is a next generation satellite mission expected to provide a 2 km-resolution observation of the sea surface height (SSH) on a two-dimensional swath. Processing SWOT data will be challenging because of the large amount of data, the mismatch between a high spatial resolution and a low temporal resolution, and the observation errors. The present paper focuses on the reduction of the spatially structured errors of SWOT SSH data. It investigates a new error reduction method and assesses its performance in an observing system simulation experiment. The proposed error-reduction method first projects the SWOT SSH onto a subspace spanned by the SWOT spatially structured errors. This projection is removed from the SWOT SSH to obtain a detrended SSH. The detrended SSH is then processed within an ensemble data assimilation analysis to retrieve a full SSH field. In the latter step, the detrending is applied to both the SWOT data and an ensemble of model-simulated SSH fields. Numerical experiments are performed with synthetic SWOT observations and an ensemble from a North Atlantic, 1/60 degrees simulation of the ocean circulation (NATL60). The data assimilation analysis is carried out with an ensemble Kalman filter. The results are assessed with root mean square errors, power spectrum density, and spatial coherence. They show that a significant part of the large scale SWOT errors is reduced. The filter analysis also reduces the small scale errors and allows for an accurate recovery of the energy of the signal down to 25 km scales. In addition, using the SWOT nadir data to adjust the SSH detrending further reduces the errors.
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Morrow, R., Fu, L., Ardhuin, F., Benkiran, M., Chapron, B., Cosme, E., et al. (2019). Global Observations of Fine-Scale Ocean Surface Topography With the Surface Water and Ocean Topography (SWOT) Mission. Frontiers In Marine Science, 6.
Abstract: The future international Surface Water and Ocean Topography (SWOT) Mission, planned for launch in 2021, will make high-resolution 2D observations of sea-surface height using SAR radar interferometric techniques. SWOT will map the global and coastal oceans up to 77.6 degrees latitude every 21 days over a swath of 120 km (20 km nadir gap). Today's 2D mapped altimeter data can resolve ocean scales of 150 km wavelength whereas the SWOT measurement will extend our 2D observations down to 15-30 km, depending on sea state. SWOT will offer new opportunities to observe the oceanic dynamic processes at scales that are important in the generation and dissipation of kinetic energy in the ocean, and that facilitate the exchange of energy between the ocean interior and the upper layer. The active vertical exchanges linked to these scales have impacts on the local and global budgets of heat and carbon, and on nutrients for biogeochemical cycles. This review paper highlights the issues being addressed by the SWOT science community to understand SWOT's very precise sea surface height (SSH)/surface pressure observations, and it explores how SWOT data will be combined with other satellite and in situ data and models to better understand the upper ocean 4D circulation (x, y, z, t) over the next decade. SWOT will provide unprecedented 2D ocean SSH observations down to 15-30 km in wavelength, which encompasses the scales of “balanced” geostrophic eddy motions, high-frequency internal tides and internal waves. This presents both a challenge in reconstructing the 4D upper ocean circulation, or in the assimilation of SSH in models, but also an opportunity to have global observations of the 2D structure of these phenomena, and to learn more about their interactions. At these small scales, ocean dynamics evolve rapidly, and combining SWOT 2D SSH data with other satellite or in situ data with different space-time coverage is also a challenge. SWOT's new technology will be a forerunner for the future altimetric observing system, and so advancing on these issues today will pave the way for our future.
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Penduff, T., Llovel, W., Close, S., Garcia-Gomez, I., & Leroux, S. (2019). Trends of Coastal Sea Level Between 1993 and 2015: Imprints of Atmospheric Forcing and Oceanic Chaos. Surveys In Geophysics, 40(6), 1543–1562.
Abstract: The observation and simulation of the variability of coastal sea level are impacted by various uncertainties, such as measurement errors and sampling biases, unresolved processes, and model and forcing biases. Ocean model simulations suggest that another uncertainty should be taken into account for the attribution of sea-level changes. Global ocean simulations indeed show that resolving mesoscale turbulence (even partly) promotes the emergence of low-frequency (LF) chaotic intrinsic variability (CIV) which causes substantial random fluctuations of sea level up to multiple decades in eddy-active regions of the world ocean. This random LFCIV is superimposed on the atmospherically forced (or simply “forced”) fluctuations, which are directly controlled by the atmospheric variability. We show from a large ensemble of global oceanic hindcasts that this multi-decadal LFCIV leaves a substantial imprint on the long-term trends (1993-2015) of coastal sea level: over 17-20% of the global ocean coastal area, in particular along the coastlines of the northwestern Pacific and Indian Oceans, and around the Gulf of Mexico, random sea-level trends may blur their atmospherically forced counterpart, such that simulated (and potentially observed) coastal sea-level trends cannot be unambiguously attributed to atmospheric or anthropic causes. The steric and manometric sea-level change contributions of these uncertainties are discussed, suggesting that they mostly come from the manometric sea-level trends near the coasts.
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Pham, V., Grenier, M., Cravatte, S., Michael, S., Jacquet, S., Belhadj, M., et al. (2019). Dissolved rare earth elements distribution in the Solomon Sea. Chemical Geology, 524, 11–36.
Abstract: Trace Elements and Isotopes (TEIs) were measured as part of the GEOTRACES PANDORA cruise (July-August 2012, R/V L'Atalante), among them Rare Earth Elements (REEs) as pertinent tracers of land-ocean inputs and water mass transformations. This work discusses results of 19 dissolved REE (dREE) profiles measured using a trispike method in the Coral Sea and inside and at the exits of the Solomon Sea, a semi-enclosed sea with complex topography and straits. Overall, dREEs -except the insoluble Ce- show nutrient like profiles, i.e. depleted at the surface and enriched at depth. Illustrative Nd concentrations range from similar to 5 pmol/kg at the surface to > 25 pmol/kg at 5000 m depth. However, local dREE enrichments are observed, mostly in the Straits (Indispensable, Solomon and Vitiaz Straits) and along the island coasts. A box model allows calculating and discussing the fate of the dREEs in the different water layers flowing through the Solomon Sea. Finally, subtle variations revealed by La, Ce, Eu anomalies and the normalized light versus heavy REE ratio (expressed as Nd-n/Yb-n) allows the identification of specific mechanisms affecting the distribution of the different dREEs. The positive Eu anomaly observed in the surface layers reflects the basaltic origin of external inputs, consistent with the intensive weathering and/or volcanic activity affecting the surrounding islands. These data also confirm that the distributions of heavy dREEs (like Yb) are better correlated to the dSi concentrations than that of the other REEs. This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOT-RACES and beyond” – edited by Tim M. Conway, Tristan Homer, Yves Plancherel, and Aridane G. Gonzalez.
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Ponte, R., Carson, M., Cirano, M., Domingues, C., Jevrejeva, S., Marcos, M., et al. (2019). Towards Comprehensive Observing and Modeling Systems for Monitoring and Predicting Regional to Coastal Sea Level. Frontiers In Marine Science, 6.
Abstract: A major challenge for managing impacts and implementing effective mitigation measures and adaptation strategies for coastal zones affected by future sea level (SL) rise is our limited capacity to predict SL change at the coast on relevant spatial and temporal scales. Predicting coastal SL requires the ability to monitor and simulate a multitude of physical processes affecting SL, from local effects of wind waves and river runoff to remote influences of the large-scale ocean circulation on the coast. Here we assess our current understanding of the causes of coastal SL variability on monthly to multi-decadal timescales, including geodetic, oceanographic and atmospheric aspects of the problem, and review available observing systems informing on coastal SL. We also review the ability of existing models and data assimilation systems to estimate coastal SL variations and of atmosphere-ocean global coupled models and related regional downscaling efforts to project future SL changes. We discuss (1) observational gaps and uncertainties, and priorities for the development of an optimal and integrated coastal SL observing system, (2) strategies for advancing model capabilities in forecasting short-term processes and projecting long-term changes affecting coastal SL, and (3) possible future developments of sea level services enabling better connection of scientists and user communities and facilitating assessment and decision making for adaptation to future coastal SL change.
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Prants, S., Reznik, G., & Verron, J. (2019). The international conference “Vortices and coherent structures: from ocean to microfluids”, Vladivostok, Russia, 28-31 August 2017. Ocean Dynamics, 69(4), 509–512.
Abstract: The international conference Vortices and coherent structures: from ocean to microfluids was held at the Pacific Oceanological Institute of the Russian Academy of Sciences in Vladivostok (Russia) from August 28 to August 31, 2017. The event gathered experimentalists and theoreticians, oceanographers and physicists, with a common interest in observing and modelling fluid flows in different media, from the ocean to laboratory flows. It was a fruitful idea to bring together researchers from a variety of backgrounds to benefit from a vigorous discussion of concepts across different disciplines in oceanography and hydrodynamics.
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Prieur, C., Viry, L., Blayo, E., & Brankart, J. (2019). A global sensitivity analysis approach for marine biogeochemical modeling. Ocean Modelling, 139.
Abstract: This paper introduces the Sobol' indices approach for global sensitivity analysis (SA), in the context of marine biogeochemistry. Such an approach is particularly well suited for ocean biogeochemical models, which make use of numerous parameters within large sets of differential equations with complex dependencies. This SA allows for a detailed study of the relative influence of a large number of input parameters on output quantities of interest to be chosen. It is able to distinguish between direct effects of these parameters and effects due to interaction between two or more parameters. Although demanding in terms of computation, such a tool is now becoming affordable, thanks to the development of distributed computing environments. An applicative example is presented with the MODECOGeL biogeochemical model, and illustrates the advantages of this approach over standard local SA.
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Rivière, P., Jaud, T., Siegelman, L., Klein, P., Cott?, C., Le Sommer, J., et al. (2019). Sub-mesoscale fronts modify elephant seals foraging behavior. Limnology And Oceanography Letters, 4(6), 193–204.
Abstract: Sub-mesoscale fronts-with scales from 1 to 50 km are ubiquitous in satellite images of the world oceans. They are known to generate strong vertical velocities with significant impacts on biogeochemical fluxes and pelagic ecosystems. Here, we use a unique data set, combining high-resolution behavioral and physical measurements, to determine the effects of sub-mesoscale structures on the foraging behavior of 12 instrumented female southern elephant seals. These marine mammals make long voyages (several months over more than 2000 km), diving and feeding continuously in the Antarctic Circumpolar Current. Our results show that elephant seals change their foraging behavior when crossing sub-mesoscale fronts: They forage more and at shallower depths inside sub-mesoscale fronts compared to nonfrontal areas, and they also reduce their horizontal velocity likely to concentrate on their vertical diving activity. The results highlight the importance of sub-mesoscale fronts in enhancing prey accessibility for upper trophic levels, and suggest that trophic interactions are stimulated in these structures.
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Sanchez-Roman, A., Gomez-Navarro, L., Fablet, R., Oro, D., Mason, E., Arcos, J., et al. (2019). Rafting behaviour of seabirds as a proxy to describe surface ocean currents in the Balearic Sea. Scientific Reports, 9.
Abstract: Spatio-temporal variability of surface geostrophic mesoscale currents in the Balearic Sea (western Mediterranean) is characterized from satellite altimetry in combination with in-situ velocity measurements collected, among others, by drifting buoys, gliders and high-frequency radar. Here, we explore the use of tracking data from living organisms in the Balearic Sea as an alternative way to acquire in-situ velocity measurements. Specifically, we use GPS-tracks of resting Scopoli's shearwaters Calonectris diomedea, that act as passive drifters, and compare them with satellite-derived velocity patterns. Results suggest that animal-borne GPS data can be used to identify rafting behaviour outside of the breeding colonies and, furthermore, as a proxy to describe local sea surface currents. Four rafting patterns were identified according to the prevailing driving forces responsible for the observed trajectories. We find that 76% of the bird trajectories are associated with the combined effects of slippage and Ekman drift and/or surface drag; 59% are directly driven by the sea surface currents. Shearwaters are therefore likely to be passively transported by these driving forces while resting. The tracks are generally consistent with the mesoscale features observed in satellite data and identified with eddy-tracking software.
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Timko, P., Arbic, B., Hyderd, P., Richman, J., Zamudio, L., O'Dea, E., et al. (2019). Assessment of shelf sea tides and tidal mixing fronts in a global ocean model. Ocean Modelling, 136, 66–84.
Abstract: Tidal mixing fronts, which represent boundaries between stratified and tidally mixed waters, are locations of enhanced biological activity. They occur in summer shelf seas when, in the presence of strong tidal currents, mixing due to bottom friction balances buoyancy production due to seasonal heat flux. In this paper we examine the occurrence and fidelity of tidal mixing fronts in shelf seas generated within a global 3-dimensional simulation of the HYbrid Coordinate Ocean Model (HYCOM) that is simultaneously forced by atmospheric fields and the astronomical tidal potential. We perform a first order assessment of shelf sea tides in global HYCOM through comparison of sea surface temperature, sea surface tidal elevations, and tidal currents with observations. HYCOM was tuned to minimize errors in M-2 sea surface heights in deep water. Over the global coastal and shelf seas (depths < 200 m) the area-weighted root mean square error of the M-2 sea surface amplitude in HYCOM represents 35% of the 50 cm root mean squared M-2 sea surface amplitude when compared to satellite constrained models TPXO8 and FES2014. HYCOM and the altimeter constrained tidal models TPXO8 and FES2014 exhibit similar skill in reproducing barotropic tidal currents estimated from in-situ current meter observations. Through comparison of a global HYCOM simulation with tidal forcing to a global HYCOM simulation with no tides, and also to previous regional studies of tidal mixing fronts in shelf seas, we demonstrate that HYCOM with embedded tides exhibits quite high skill in reproducing known tidal mixing fronts in shelf seas. Our results indicate that the amount of variability in the location of the tidal mixing fronts in HYCOM, estimated using the Simpson-Hunter parameter, is consistent with previous studies when the differences in the net downward heat flux, on a global scale, are taken into account. We also provide evidence of tidal mixing fronts on the North West Australian Shelf for which we have been unable to find references in the existing scientific literature.
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Tissier, A., Brankart, J., Testut, C., Ruggiero, G., Cosme, E., & Brasseur, P. (2019). A multiscale ocean data assimilation approach combining spatial and spectral localisation. Ocean Science, 15(2), 443–457.
Abstract: Ocean data assimilation systems encompass a wide range of scales that are difficult to control simultaneously using partial observation networks. All scales are not observable by all observation systems, which is not easily taken into account in current ocean operational systems. The main reason for this difficulty is that the error covariance matrices are usually assumed to be local (e.g. using a localisation algorithm in ensemble data assimilation systems), so that the large-scale patterns are removed from the error statistics. To better exploit the observational information available for all scales in the assimilation systems of the Copernicus Marine Environment Monitoring Service, we investigate a new method to introduce scale separation in the assimilation scheme. The method is based on a spectral transformation of the assimilation problem and consists in carrying out the analysis with spectral localisation for the large scales and spatial localisation for the residual scales. The target is to improve the observational update of the large-scale components of the signal by an explicit observational constraint applied directly on the large scales and to restrict the use of spatial localisation to the small-scale components of the signal. To evaluate our method, twin experiments are carried out with synthetic altimetry observations (simulating the Jason tracks), assimilated in a 1/4 degrees model configuration of the North Atlantic and the Nordic Seas. Results show that the transformation to the spectral domain and the spectral localisation provides consistent ensemble estimates of the state of the system (in the spectral domain or after backward transformation to the spatial domain). Combined with spatial localisation for the residual scales, the new scheme is able to provide a reliable ensemble update for all scales, with improved accuracy for the large scale; and the performance of the system can be checked explicitly and separately for all scales in the assimilation system.
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Zanna, L., Brankart, J., Huber, M., Leroux, S., Penduff, T., & Williams, P. (2019). Uncertainty and scale interactions in ocean ensembles: From seasonal forecasts to multidecadal climate predictions. Quarterly Journal Of The Royal Meteorological Society, 145, 160–175.
Abstract: The ocean plays an important role in the climate system on time-scales of weeks to centuries. Despite improvements in ocean models, dynamical processes involving multiscale interactions remain poorly represented, leading to errors in forecasts. We present recent advances in understanding, quantifying, and representing physical and numerical sources of uncertainty in novel regional and global ocean ensembles at different horizontal resolutions. At coarse resolution, uncertainty in 21st century projections of the upper overturning cell in the Atlantic is mostly a result of buoyancy fluxes, while the uncertainty in projections of the bottom cell is driven equally by both wind and buoyancy flux uncertainty. In addition, freshwater and heat fluxes are the largest contributors to Atlantic Ocean heat content regional projections and their uncertainties, mostly as a result of uncertain ocean circulation projections. At both coarse and eddy-permitting resolutions, unresolved stochastic temperature and salinity fluctuations can lead to significant changes in large-scale density across the Gulf Stream front, therefore leading to major changes in large-scale transport. These perturbations can have an impact on the ensemble spread on monthly time-scales and subsequently interact nonlinearly with the dynamics of the flow, generating chaotic variability on multiannual time-scales. In the Gulf Stream region, the ratio of chaotic variability to atmospheric-forced variability in meridional heat transport is larger than 50% on time-scales shorter than 2 years, while between 40 and 48 degrees S the ratio exceeds 50% on on time-scales up to 28 years. Based on these simulations, we show that air-sea interaction and ocean subgrid eddies remain an important source of error for simulating and predicting ocean circulation, sea level, and heat uptake on a range of spatial and temporal scales. We discuss how further refinement of these ensembles can help us assess the relative importance of oceanic versus atmospheric uncertainty in weather and climate.
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2018 |
Amores, A., Jorda, G., Arsouze, T., & Le Sommer, J. (2018). Up to What Extent Can We Characterize Ocean Eddies Using Present-Day Gridded Altimetric Products? Journal Of Geophysical Research-Oceans, 123(10), 7220–7236.
Abstract: The most common methodology used to detect and characterize mesoscale eddies in the global ocean is to analyze altimetry-based sea-level gridded products with an automatic eddy detection and tracking algorithm. However, a careful look at the location of altimetry tracks shows that their separation is often larger than the Rossby radius of deformation. This implies that gridded products based on the information obtained along track would potentially be unable to characterize the mesoscale variability and, in particular, the eddy field. In this study, we analyze up to what extent sea-level gridded products are able to characterize mesoscale eddies with a special focus on the North Atlantic Ocean and the Mediterranean Sea. In order to perform this task, we have generated synthetic sea level anomaly maps using along-track data extracted from realistic high-resolution ocean model simulations and applying an optimal interpolation procedure. Then, we have used an eddy detection and tracking algorithm to the gridded synthetic product and to the original model outputs and compared the characteristics of the resulting eddy fields. Our results suggest that gridded products largely underestimate the density of eddies, capturing only between 6% and 16% of the total number of eddies. The main reason is that the spatial resolution of the gridded products is not enough to capture the small-scale eddies that are the most abundant. Also, the unresolved structures are aliased into larger structures in the gridded products, so those products show an unrealistic number of large eddies with overestimated amplitudes. Mesoscale eddies are ocean vortexes that are found all across the global ocean. These structures can move water inside their interiors and stir the surrounding waters, resulting in a net transport of water properties, such as heat and salt. The most common way to study these eddies is by using satellite-based observations of the signature that most of eddies have on the sea surface. We show, by using a new generation of numerical models and mimicking the measuring process that satellites do, that the vast majority of the eddy field is missed because the available observations do not have enough resolution to resolve the smaller vortexes. Moreover, the mapping procedure used tends to merge several smaller eddies into a larger one, making the true detection of eddies with size enough to be correctly captured by satellite measurements not reliable.
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Bonnefond, P., Verron, J., Aublanc, J., Babu, K. N., Berge-Nguyen, M., Cancet, M., et al. (2018). The Benefits of the Ka-Band as Evidenced from the SARAL/AltiKa Altimetric Mission: Quality Assessment and Unique Characteristics of AltiKa Data. Remote Sensing, 10(1).
Abstract: The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. The mission ended its nominal phase after 3 years in orbit and began a new phase (drifting orbit) in July 2016. The objective of this paper is to provide a state of the art of the achievements of the SARAL/AltiKa mission in terms of quality assessment and unique characteristics of AltiKa data. It shows that the AltiKa data have similar accuracy at the centimeter level in term of absolute water level whatever the method (from local to global) and the type of water surfaces (ocean and lakes). It shows also that beyond the fact that AltiKa data quality meets the expectations and initial mission requirements, the unique characteristics of the altimeter and the Ka-band offer unique contributions in fields that were previously not fully foreseen.
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Cretaux, J., Berge-Nguyen, M., Calmant, S., Jamangulova, N., Satylkanov, R., Lyard, F., et al. (2018). Absolute Calibration or Validation of the Altimeters on the Sentinel-3A and the Jason-3 over Lake Issykkul (Kyrgyzstan). Remote Sensing, 10(11).
Abstract: Calibration/Validation (C/V) studies using sites in the oceans have a long history and protocols are well established. Over lakes, C/V allows addressing problems such as the performance of the various retracking algorithms and evaluating the accuracy of the geophysical corrections for continental waters. This is achievable when measurements of specific and numerous field campaigns and a ground permanent network of level gauges and weather stations are processed. C/V consists of installation of permanent sites (weather stations, limnigraphs, and GPS reference points) and the organization of regular field campaigns. The lake Issykkul serves as permanent site of C/V, for a multi-mission purpose. The objective of this paper is to calculate the altimeter biases of Jason-3 and Sentinel-3A, both belonging to an operational satellite system which is used for the long-term monitoring of lake level variations. We have also determined the accuracy of the altimeters of these two satellites, through a comparison analysis with in situ data. In 2016 and 2017, three campaigns have been organized over this lake in order to estimate the absolute bias of the nadir altimeter onboard the Jason-3 and Sentinel-3A. The fieldwork consisted of measuring water height using a GPS system, carried on a boat, along the track of the altimeter satellite across the lake. It was performed at the time of the pass of the altimeter. Absolute altimeter biases were calculated by averaging the water height differences along the pass of the satellite (GPS from the boat system versus altimetry). Jason-3 operates in a Low Resolution Mode (LRM), while the Sentinel-3A operates in Synthetic Aperture Radar (SAR) mode. In this study we found that the absolute biases measured for Jason-3 were -28 +/- 40 mm with the Ocean retracker and 206 +/- 30 mm with the Ice-1 retracker. The biases for Sentinel-3A were -14 +/- 20 mm with the Samosa (Ocean like) retracker and 285 +/- 20 mm with the OCOG (Ice-1-like) retracker. We have also evaluated the accuracy of these two altimeters over Lake Issykkul which reached to 3 cm, for both the instruments, using the Ocean retracker.
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Fablet, R., Verron, J., Mourre, B., Chapron, B., & Pascual, A. (2018). Improving Mesoscale Altimetric Data From a Multitracer Convolutional Processing of Standard Satellite-Derived Products. Ieee Transactions On Geoscience And Remote Sensing, 56(5), 2518–2525.
Abstract: Multisatellite measurements of altimeter-derived sea surface height (SSH) have provided a wealth of information on the ocean. Yet, horizontal scales below 100 km remain scarcely resolved. Especially, in the Mediterranean Sea, an important fraction of the mesoscale range, characterized by a small Rossby radius of deformation of 15-20 km, is not properly retrieved by altimeter-derived gridded products. Here, we investigate a novel processing of AVISO products with a view to resolving the horizontal scales sensed by current along-track altimeter data. The key feature of our framework is the use of linear convolutional operators to model the fine-scale SSH detail as a function of different sea surface fields, especially optimally interpolated SSH and sea surface temperature (SST). The proposed model embeds the surface quasi-geostrophic SST-SSH synergy as a special case. Using an observing system simulation experiment with simulated SSH data from model outputs in the Western Mediterranean Sea, we show that the proposed approach has the potential for improving current optimal interpolations of gridded altimeter-derived SSH fields by more than 20% in terms of relative SSH and kinetic energy mean square error, as well as in terms of spectral signatures for horizontal scales ranging from 30 to 100 km. Our results also suggest that SST-SSH relationship may only play a secondary role compared with the interscale SSH cascade. We further discuss the relevance of the proposed approach in the context of future altimetric satellite missions.
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Fresnay, S., Ponte, A. L., Le Gentil, S., & Le Sommer, J. (2018). Reconstruction of the 3-D Dynamics From Surface Variables in a High-Resolution Simulation of North Atlantic. Journal Of Geophysical Research-Oceans, 123(3), 1612–1630.
Abstract: Several methods that reconstruct the three-dimensional ocean dynamics from sea level are presented and evaluated in the Gulf Stream region with a 1/60 degrees realistic numerical simulation. The use of sea level is motivated by its better correlation with interior pressure or quasi-geostrophic potential vorticity (PV) compared to sea surface temperature and sea surface salinity, and, by its observability via satellite altimetry. The simplest method of reconstruction relies on a linear estimation of pressure at depth from sea level. Another method consists in linearly estimating PV from sea level first and then performing a PV inversion. The last method considered, labeled SQG for surface quasi-geostrophy, relies on a PV inversion but assumes no PV anomalies. The first two methods show comparable skill at levels above -800 m. They moderately outperform SQG which emphasizes the difficulty of estimating interior PV from surface variables. Over the 250-1,000 m depth range, the three methods skillfully reconstruct pressure at wavelengths between 500 and 200 km whereas they exhibit a rapid loss of skill between 200 and 100 km wavelengths. Applicability to a real case scenario and leads for improvements are discussed.
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Gomez-Navarro, L., Fablet, R., Mason, E., Pascual, A., Mourre, B., Cosme, E., et al. (2018). SWOT Spatial Scales in the Western Mediterranean Sea Derived from Pseudo-Observations and an Ad Hoc Filtering. Remote Sensing, 10(4).
Abstract: The aim of this study is to assess the capacity of the Surface Water Ocean Topography (SWOT) satellite to resolve fine scale oceanic surface features in the western Mediterranean. Using as input the Sea Surface Height (SSH) fields from a high-resolution Ocean General Circulation Model (OGCM), the SWOT Simulator for Ocean Science generates SWOT-like outputs along a swath and the nadir following the orbit ground tracks. Given the characteristic temporal and spatial scales of fine scale features in the region, we examine temporal and spatial resolution of the SWOT outputs by comparing them with the original model data which are interpolated onto the SWOT grid. To further assess the satellite's performance, we derive the absolute geostrophic velocity and relative vorticity. We find that instrument noise and geophysical error mask the whole signal of the pseudo-SWOT derived dynamical variables. We therefore address the impact of removal of satellite noise from the pseudo-SWOT data using a Laplacian diffusion filter, and then focus on the spatial scales that are resolved within a swath after this filtering. To investigate sensitivity to different filtering parameters, we calculate spatial spectra and root mean square errors. Our numerical experiments show that noise patterns dominate the spectral content of the pseudo-SWOT fields at wavelengths below 60 km. Application of the Laplacian diffusion filter allows recovery of the spectral signature within a swath down to the 40-60 km wavelength range. Consequently, with the help of this filter, we are able to improve the observation of fine scale oceanic features in pseudo-SWOT data, and in the estimation of associated derived variables such as velocity and vorticity.
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Larue, F., Royer, A., De Seve, D., Roy, A., & Cosme, E. (2018). Assimilation of passive microwave AMSR-2 satellite observations in a snowpack evolution model over northeastern Canada. Hydrology And Earth System Sciences, 22(11), 5711–5734.
Abstract: Over northeastern Canada, the amount of water stored in a snowpack, estimated by its snow water equivalent (SWE) amount, is a key variable for hydrological applications. The limited number of weather stations driving snowpack models over large and remote northern areas generates great uncertainty in SWE evolution. A data assimilation (DA) scheme was developed to improve SWE estimates by updating meteorological forcing data and snowpack states with passive microwave (PMW) satellite observations and without using any surface-based data. In this DA experiment, a particle filter with a Sequential Importance Resampling algorithm (SIR) was applied and an inflation technique of the observation error matrix was developed to avoid ensemble degeneracy. Advanced Microwave Scanning Radiometer 2 (AMSR-2) brightness temperature (T-B) observations were assimilated into a chain of models composed of the Crocus multilayer snowpack model and radiative transfer models. The microwave snow emission model (Dense Media Radiative Transfer – Multi-Layer model, DMRT-ML), the vegetation transmissivity model (omega-tau(opt)), and atmospheric and soil radiative transfer models were calibrated to simulate the contributions from the snowpack, the vegetation, and the soil, respectively, at the top of the atmosphere. DA experiments were performed for 12 stations where daily continuous SWE measurements were acquired over 4 winters (2012-2016). Best SWE estimates are obtained with the assimilation of the T-Bs at 11, 19, and 37 GHz in vertical polarizations. The overall SWE bias is reduced by 68% compared to the original SWE simulations, from 23.7 kg m(-2) without assimilation to 7.5 kg m(-2) with the assimilation of the three frequencies. The overall SWE relative percentage of error (RPE) is 14.1% (19% without assimilation) for sites with a fraction of forest cover below 75 %, which is in the range of accuracy needed for hydrological applications. This research opens the way for global applications to improve SWE estimates over large and remote areas, even when vegetation contributions are up to 50% of the PMW signal.
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Leroux, S., Penduff, T., Bessieres, L., Molines, J. M., Brankart, J. M., Serazin, G., et al. (2018). Intrinsic and Atmospherically Forced Variability of the AMOC: Insights from a Large-Ensemble Ocean Hindcast. Journal Of Climate, 31(3), 1183–1203.
Abstract: This study investigates the origin and features of interannual-decadal Atlantic meridional overturning circulation (AMOC) variability from several ocean simulations, including a large (50 member) ensemble of global, eddy-permitting (1/4 degrees) ocean-sea ice hindcasts. After an initial stochastic perturbation, each member is driven by the same realistic atmospheric forcing over 1960-2015. The magnitude, spatiotemporal scales, and patterns of both the atmospherically forced and intrinsic-chaotic interannual AMOC variability are then characterized from the ensemble mean and ensemble spread, respectively. The analysis of the ensemble-mean variability shows that the AMOC fluctuations north of 40 degrees N are largely driven by the atmospheric variability, which forces meridionally coherent fluctuations reaching decadal time scales. The amplitude of the intrinsic interannual AMOC variability never exceeds the atmospherically forced contribution in the Atlantic basin, but it reaches up to 100% of the latter around 35 degrees S and 60% in the Northern Hemisphere midlatitudes. The intrinsic AMOC variability exhibits a large-scale meridional coherence, especially south of 25 degrees N. An EOF analysis over the basin shows two large-scale leading modes that together explain 60% of the interannual intrinsic variability. The first mode is likely excited by intrinsic oceanic processes at the southern end of the basin and affects latitudes up to 40 degrees N; the second mode is mostly restricted to, and excited within, the Northern Hemisphere midlatitudes. These features of the intrinsic, chaotic variability (intensity, patterns, and random phase) are barely sensitive to the atmospheric evolution, and they strongly resemble the "pure intrinsic'' interannual AMOC variability that emerges in climatological simulations under repeated seasonal-cycle forcing. These results raise questions about the attribution of observed and simulated AMOC signals and about the possible impact of intrinsic signals on the atmosphere.
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Llovel, W., Penduff, T., Meyssignac, B., Molines, J., Terray, L., Bessieres, L., et al. (2018). Contributions of Atmospheric Forcing and Chaotic Ocean Variability to Regional Sea Level Trends Over 1993-2015. Geophysical Research Letters, 45(24), 13405–13413.
Abstract: A global 1/4 degrees ocean/sea-ice 50-member ensemble simulation is analyzed to disentangle the imprints of the atmospheric forcing and the chaotic ocean variability on regional sea level trends over the satellite altimetry period. We find that the chaotic ocean variability may mask atmospherically forced regional sea level trends over 38% of the global ocean area from 1993 to 2015, and over 47% of this area from 2005 to 2015. These regions are located in the western boundary currents, in the Southern Ocean and in the subtropical gyres. While these results do not question the anthropogenic origin of global mean sea level rise, they give new insights into the intrinsically oceanic versus atmospheric forcing of regional sea level trends and provide new constraints on the measurement time required to attribute regional sea level trends to the atmospheric forcing or to climate change. Plain Language Summary As a direct consequence of anthropogenic influences, global mean sea level rises in response to ocean warming and land ice melting. Since the early 1990s, satellite altimetry has revealed large regional contrasts in sea level trends, controlled by temperature and salinity changes, oceanic processes and atmospheric forcing. Using an ensemble of forced eddying ocean simulations, we show that regional sea level trends over the altimetric period are only partly determined by the atmospheric evolution (both natural and anthropogenic): nonlinear ocean processes produce additional sea level trends that are inherently random, which can compete in certain regions with the externally forced trends. These results do not question the existence of global and regional sea level trends, but suggest that sea level trends may not be unambiguously attributed to external causes in certain regions.
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Markina, M., Gavrikov, A., Gulev, S., & Barnier, B. (2018). Developing configuration of WRF model for long-term high-resolution wind wave hindcast over the North Atlantic with WAVEWATCH III. Ocean Dynamics, 68(11), 1593–1604.
Abstract: The spatial resolution of wind forcing fields is critical for modeling ocean surface waves. We analyze here the performance of the non-hydrostatic numerical weather prediction system WRF-ARW (Weather Research and Forecasting) run witha 14-km resolution for hindcasting wind waves in the North Atlantic. The regional atmospheric model was run in the domain from 20 degrees N to 70 degrees N in the North Atlantic and was forced with ERA-Interim reanalysis as initial and boundary conditions in a spectral nudging mode. Here, we present the analysis of the impact of spectral nudging formulation (cutoff wavelengths and depth through which full weighting from reanalysis data is applied) onto the performance of the modeled 10-m wind speed and wind wave fields for 1year (2010). For modeling waves, we use the third-generation spectral wave model WAVEWATCH III. The sensitivity of the atmospheric and wave models to the spectral nudging formulation is investigated via the comparison with reanalysis and observational data. The results reveal strong and persistent agreement with reanalysis data during all seasons within the year with well-simulated annual cycle and regional patterns independently of the nudging parameters that were tested. Thus, the proposed formulation of the nudging provides a reliable framework for future long-term experiments aiming at hindcasting climate variability in the North Atlantic wave field. At the same time, dynamical downscaling allows for simulation of higher waves in coastal regions, specifically near the Greenland east coast likely due to a better representation of the mesoscale atmospheric dynamics in this area.
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Marson, J. M., Myers, P. G., Hu, X. M., & Le Sommer, J. (2018). Using Vertically Integrated Ocean Fields to Characterize Greenland Icebergs' Distribution and Lifetime. Geophysical Research Letters, 45(9), 4208–4217.
Abstract: Icebergs represent approximately half of Greenland's yearly mass loss, having important implications for biological productivity, freshwater fluxes in the ocean, and navigation. This study applies an iceberg model that uses integrated ocean fields (from surface to iceberg keel) to simulate the drift and decay of Greenland icebergs. This version of iceberg model (VERT) is compared with a more widely adopted version (SURF) which only uses surface ocean fields in its equations. We show that icebergs in VERT tend to drift along the shelf break, while in SURF they concentrate along the coastline. Additionally, we show that Greenland's southeast coast is the source of similar to 60% of the icebergs that cross the interior of the Labrador Seaa region that stages buoyancy-driven convection and is, therefore, sensitive to freshwater input. Plain Language Summary Thousands of icebergs break off from Greenland every year, threatening navigation along North America's east coast. Since it is difficult to monitor individual icebergs, computer simulations are useful to help us understand their common pathways and, potentially, to predict when and from where icebergs come from. In this study, we use an improved iceberg model (one that uses the variations of ocean currents and temperature with depth to interact with icebergs) to simulate Greenland icebergs' distribution and their persistence in different regions of the North Atlantic. We show that this improved version better reproduces iceberg pathways observed in the past. Moreover, we find that icebergs breaking off from the southeast part of Greenland compose most icebergs reaching the middle of the Labrador Seaa region where iceberg melt may affect ocean circulation and, consequently, heat distribution from tropics to poles. This means that an increasing volume of icebergs coming out of this particular region in a warmer world might have an effect back on the climate.
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Merino, N., Jourdain, N. C., Le Sommer, J., Goosse, H., Mathiot, P., & Durand, G. (2018). Impact of increasing antarctic glacial freshwater release on regional sea-ice cover in the Southern Ocean. Ocean Modelling, 121, 76–89.
Abstract: The sensitivity of Antarctic sea-ice to increasing glacial freshwater release into the Southern Ocean is studied in a series of 31-year ocean/sea-ice/iceberg model simulations. Glaciological estimates of ice-shelf melting and iceberg calving are used to better constrain the spatial distribution and magnitude of freshwater forcing around Antarctica. Two scenarios of glacial freshwater forcing have been designed to account for a decadal perturbation in glacial freshwater release to the Southern Ocean. For the first time, this perturbation explicitly takes into consideration the spatial distribution of changes in the volume of Antarctic ice shelves, which is found to be a key component of changes in freshwater release. In addition, glacial freshwater-induced changes in sea ice are compared to typical changes induced by the decadal evolution of atmospheric states. Our results show that, in general, the increase in glacial freshwater release increases Antarctic sea ice extent. But the response is opposite in some regions like the coastal Amundsen Sea, implying that distinct physical mechanisms are involved in the response. We also show that changes in freshwater forcing may induce large changes in sea-ice thickness, explaining about one half of the total change due to the combination of atmospheric and freshwater changes. The regional contrasts in our results suggest a need for improving the representation of freshwater sources and their evolution in climate models.
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Penduff, T., Serazin, G., Leroux, S., Close, S., Molines, J., Barnier, B., et al. (2018). Chaotic Variability of Ocean Heat Content CLIMATE-RELEVANT FEATURES AND OBSERVATIONAL IMPLICATIONS. Oceanography, 31(2), 63–71.
Abstract: Global ocean models that admit mesoscale turbulence spontaneously generate a substantial interannual-to-multidecadal chaotic intrinsic variability in the absence of atmospheric forcing variability at these timescales. This phenomenon is substantially weaker in non-turbulent ocean models but provides a marked stochastic flavor to the low-frequency variability in eddying ocean models, which are being coupled to the atmosphere for next-generation climate projections. In order to disentangle the atmospherically forced and intrinsic ocean variabilities, the OCCIPUT (OceaniC Chaos – ImPacts, strUcture, predicTability) project performed a long (1960-2015), large ensemble (50 members) of global ocean/sea ice 1/4 degrees simulations driven by the same atmospheric reanalysis, but with perturbed initial conditions. Subsequent ensemble statistics show that the ocean variability can be seen as a broadband “noise,” with characteristic scales reaching multiple decades and basin sizes, locally modulated by the atmospheric variability. In several mid-latitude regions, chaotic processes have more impact than atmospheric variability on both the low-frequency variability and the long-term trends of regional ocean heat content. Consequently, certain climate-relevant oceanic signals cannot be unambiguously attributed to atmospheric variability, raising new issues for the detection, attribution, and interpretation of oceanic heat variability and trends in the presence of mesoscale turbulence.
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Revuelto, J., Lecourt, G., Lafaysse, M., Zin, I., Charrois, L., Vionnet, V., et al. (2018). Multi-Criteria Evaluation of Snowpack Simulations in Complex Alpine Terrain Using Satellite and In Situ Observations. Remote Sensing, 10(8).
Abstract: This work presents an extensive evaluation of the Crocus snowpack model over a rugged and highly glacierized mountain catchment (Arve valley, Western Alps, France) from 1989 to 2015. The simulations were compared and evaluated using in-situ point snow depth measurements, in-situ seasonal and annual glacier surface mass balance, snow covered area evolution based on optical satellite imagery at 250 m resolution (MODIS sensor), and the annual equilibrium-line altitude of glaciers, derived from satellite images (Landsat, SPOT, and ASTER). The snowpack simulations were obtained using the Crocus snowpack model driven by the same, originally semi-distributed, meteorological forcing (SAFRAN) reanalysis using the native semi-distributed configuration, but also a fully distributed configuration. The semi-distributed approach addresses land surface simulations for discrete topographic classes characterized by elevation range, aspect, and slope. The distributed approach operates on a 250-m grid, enabling inclusion of terrain shadowing effects, based on the same original meteorological dataset. Despite the fact that the two simulations use the same snowpack model, being potentially subjected to same potential deviation from the parametrization of certain physical processes, the results showed that both approaches accurately reproduced the snowpack distribution over the study period. Slightly (although statistically significantly) better results were obtained by using the distributed approach. The evaluation of the snow cover area with MODIS sensor has shown, on average, a reduction of the Root Mean Squared Error (RMSE) from 15.2% with the semi-distributed approach to 12.6% with the distributed one. Similarly, surface glacier mass balance RMSE decreased from 1.475 m of water equivalent (W.E.) for the semi-distributed simulation to 1.375 m W.E. for the distribution. The improvement, observed with a much higher computational time, does not justify the recommendation of this approach for all applications; however, for simulations that require a precise representation of snowpack distribution, the distributed approach is suggested.
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Serazin, G., Penduff, T., Barnier, B., Molines, J. M., Arbic, B. K., Muller, M., et al. (2018). Inverse Cascades of Kinetic Energy as a Source of Intrinsic Variability: A Global OGCM Study. Journal Of Physical Oceanography, 48(6), 1385–1408.
Abstract: A seasonally forced 1/12 degrees global ocean/sea ice simulation is used to characterize the spatiotemporal inverse cascade of kinetic energy (KE). Nonlinear scale interactions associated with relative vorticity advection are evaluated using cross-spectral analysis in the frequency-wavenumber domain from sea level anomaly (SLA) time series. This analysis is applied within four eddy-active midlatitude regions having large intrinsic variability spread over a wide range of scales. Over these four regions, mesoscale surface KE is shown to spontaneously cascade toward larger spatial scales-between the deformation scale and the Rhines scale- and longer time scales (possibly exceeding 10 years). Other nonlinear processes might have to be invoked to explain the longer time scales of intrinsic variability, which have a substantial surface imprint at midlatitudes. The analysis of a fully forced 1/12 degrees hindcast shows that low-frequency and synoptic atmospheric forcing barely affects this inverse KE cascade. The inverse cascade is also at work in a 1/4 degrees simulation, albeit with a weaker intensity, consistent with the weaker intrinsic variability found at this coarser resolution. In the midlatitude North Pacific, the spatiotemporal cascade transfers KE from high-frequency frontal Rossby waves (FRWs), probably generated by baroclinic instability, toward the lower-frequency, westward-propagating mesoscale eddy (WME) field. The WMEs provide local gradients of potential vorticity that support these short Doppler-shifted FRWs. FRWs have periods shorter than 2 months and might be subsampled by altimetric observations, perhaps explaining why the temporal inverse cascade deduced from high-resolution models and mapped altimeter products can be quite different. The nature of the nonlinear interactions between FRWs and WMEs remains unclear but might involve wave turbulence processes.
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Sokolovskiy, M. A., Verron, J., & Carton, X. J. (2018). The formation of new quasi-stationary vortex patterns from the interaction of two identical vortices in a rotating fluid. Ocean Dynamics, 68(6), 723–733.
Abstract: Within the framework of the quasi-geostrophic approximation, the interactions of two identical initially circular vortex patches are studied using the contour dynamics/surgery method. The cases of barotropic vortices and of vortices in the upper layer of a two-layer fluid are considered. Diagrams showing the end states of vortex interactions and, in particular, the new regime of vortex triplet formation are constructed for a wide range of external parameters. This paper shows that, in the nonlinear evolution of two such (like-signed) vortices, the filaments and vorticity fragments surrounding the merged vortex often collapse into satellite vortices. Therefore, the conditions for the formation and the quasi-steady motions of a new type of triplet-shaped vortex structure are obtained.
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Tsujino, H., Urakawa, S., Nakano, H., Small, R., Kim, W., Yeager, S., et al. (2018). JRA-55 based surface dataset for driving ocean-sea-ice models (JRA55-do). Ocean Modelling, 130, 79–139.
Abstract: We present a new surface-atmospheric dataset for driving ocean-sea-ice models based on Japanese 55-year atmospheric reanalysis (JRA-55), referred to here as JRA55-do. The JRA55-do dataset aims to replace the CORE interannual forcing version 2 (hereafter called the CORE dataset), which is currently used in the framework of the Coordinated Ocean-ice Reference Experiments (COREs) and the Ocean Model Intercomparison Project (OMIP). A major improvement in JRA55-do is the refined horizontal grid spacing (similar to 55 km) and temporal interval (3 hr). The data production method for JRA55-do essentially follows that of the CORE dataset, whereby the surface fields from an atmospheric reanalysis are adjusted relative to reference datasets. To improve the adjustment method, we use high-quality products derived from satellites and from several other atmospheric reanalysis projects, as well as feedback on the CORE dataset from the ocean modelling community. Notably, the surface air temperature and specific humidity are adjusted using multi-reanalysis ensemble means. In JRA55-do, the downwelling radiative fluxes and precipitation, which are affected by an ambiguous cloud parameterisation employed in the atmospheric model used for the reanalysis, are based on the reanalysis products. This approach represents a notable change from the CORE dataset, which imported independent observational products. Consequently, the JRA55-do dataset is more self-contained than the CORE dataset, and thus can be continually updated in near real-time. The JRA55-do dataset extends from 1958 to the present, with updates expected at least annually. This paper details the adjustments to the original JRA-55 fields, the scientific rationale for these adjustments, and the evaluation of JRA55-do. The adjustments successfully corrected the biases in the original JRA-55 fields. The globally averaged features are similar between the JRA55-do and CORE datasets, implying that JRA55-do can suitably replace the CORE dataset for use in driving global ocean-sea-ice models.
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Verron, J., Bonnefond, P., Aouf, L., Birol, F., Bhowmick, S. A., Calmant, S., et al. (2018). The Benefits of the Ka-Band as Evidenced from the SARAL/AltiKa Altimetric Mission: Scientific Applications. Remote Sensing, 10(2).
Abstract: The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedicated primarily to oceanography. The mission objectives were firstly the observation of the oceanic mesoscales but also global and regional sea level monitoring, including the coastal zone, data assimilation, and operational oceanography. SARAL/AltiKa proved also to be a great opportunity for inland waters applications, for observing ice sheet or icebergs, as well as for geodetic investigations. The mission ended its nominal phase after three years in orbit and began a new phase (drifting orbit) in July 2016. The objective of this paper is to highlight some of the most remarkable achievements of the SARAL/AltiKa mission in terms of scientific applications. Compared to the standard Ku-band altimetry measurements, the Ka-band provides substantial improvements in terms of spatial resolution and data accuracy. We show here that this leads to remarkable advances in terms of observation of the mesoscale and coastal ocean, waves, river water levels, ice sheets, icebergs, fine scale bathymetry features as well as for the many related applications.
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Vetra-Carvalho, S., Van Leeuwen, P. J., Nerger, L., Barth, A., Altaf, M. U., Brasseur, P., et al. (2018). State-of-the-art stochastic data assimilation methods for high-dimensional non-Gaussian problems. Tellus Series A-Dynamic Meteorology And Oceanography, 70.
Abstract: This paper compares several commonly used state-of-the-art ensemble-based data assimilation methods in a coherent mathematical notation. The study encompasses different methods that are applicable to high-dimensional geophysical systems, like ocean and atmosphere and provide an uncertainty estimate. Most variants of Ensemble Kalman Filters, Particle Filters and second-order exact methods are discussed, including Gaussian Mixture Filters, while methods that require an adjoint model or a tangent linear formulation of the model are excluded. The detailed description of all the methods in a mathematically coherent way provides both novices and experienced researchers with a unique overview and new insight in the workings and relative advantages of each method, theoretically and algorithmically, even leading to new filters. Furthermore, the practical implementation details of all ensemble and particle filter methods are discussed to show similarities and differences in the filters aiding the users in what to use when. Finally, pseudo-codes are provided for all of the methods presented in this paper.
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von Schuckmann, K., Le Traon, P. Y., Smith, N., Pascual, A., Brasseur, P., Fennel, K., et al. (2018). Copernicus Marine Service Ocean State Report. J. Oper. Oceanogr., 11, S1–S142.
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Bessieres, L., Leroux, S., Brankart, J. M., Molines, J. M., Moine, M. P., Bouttier, P. A., et al. (2017). Development of a probabilistic ocean modelling system based on NEMO 3.5: application at eddying resolution. Geoscientific Model Development, 10(3), 1091–1106.
Abstract: This paper presents the technical implementation of a new, probabilistic version of the NEMO ocean-sea-ice modelling system. Ensemble simulations with N members running simultaneously within a single executable, and interacting mutually if needed, are made possible through an enhanced message-passing interface (MPI) strategy including a double parallelization in the spatial and ensemble dimensions. An example application is then given to illustrate the implementation, performances, and potential use of this novel probabilistic modelling tool. A large ensemble of 50 global ocean-sea-ice hindcasts has been performed over the period 1960-2015 at eddy-permitting resolution (1/4 degrees) for the OCCIPUT (oceanic chaos – impacts, structure, predictability) project. This application aims to simultaneously simulate the intrinsic/chaotic and the atmospherically forced contributions to the ocean variability, from mesoscale turbulence to interannual-to-multidecadal timescales. Such an ensemble indeed provides a unique way to disentangle and study both contributions, as the forced variability may be estimated through the ensemble mean, and the intrinsic chaotic variability may be estimated through the ensemble spread.
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Brodeau, L., Barnier, B., Gulev, S. K., & Woods, C. (2017). Climatologically Significant Effects of Some Approximations in the Bulk Parameterizations of Turbulent Air-Sea Fluxes. Journal Of Physical Oceanography, 47(1), 5–28.
Abstract: This paper quantifies the impacts of approximations and assumptions in the parameterization of bulk formulas on the exchange of momentum, heat, and freshwater computed between the ocean and atmosphere. An ensemble of sensitivity experiments is examined. Climatologies of wind stress, turbulent heat flux, and evaporation for the period 1982-2014 are computed using SST and surface meteorological state variables from ERA-Interim. Each experiment differs from the defined control experiment in only one aspect of the parameterization of the bulk formulas. The wind stress is most sensitive to the closure used to relate the neutral drag coefficient to the wind speed in the bulk algorithm, which mainly involves the value of the Charnock parameter. The disagreement between the state-of-the-art algorithms examined is typically on the order of 10%. The largest uncertainties in turbulent heat flux and evaporation are also related to the choice of the algorithm (typically 15%) but also emerge in experiments examining approximations related to the surface temperature and saturation humidity. Thus, approximations for the skin temperature and the saltrelated reduction of saturation humidity have a substantial impact on the heat flux and evaporation (typically 10%). Approximations such as the use of a fixed air density, sea level pressure, or simplified formula for the saturation humidity lead to errors no larger than 4% when tested individually. The impacts of these approximations combine linearly when implemented together, yielding errors up to 20% over mid-and subpolar latitudes.
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Donat-Magnin, M., Jourdain, N. C., Spence, P., Le Sommer, J., Gallee, H., & Durand, G. (2017). Ice-Shelf Melt Response to Changing Winds and Glacier Dynamics in the Amundsen Sea Sector, Antarctica. Journal Of Geophysical Research-Oceans, 122(12), 10206–10224.
Abstract: It has been suggested that the coastal Southern Ocean subsurface may warm over the 21st century in response to strengthening and poleward shifting winds, with potential adverse effects on West Antarctic glaciers. However, using a 1/12 degrees ocean regional model that includes ice-shelf cavities, we find a more complex response to changing winds in the Amundsen Sea. Simulated offshore subsurface waters get colder under strengthened and poleward shifted winds representative of the SAM projected trend. The buoyancy-driven circulation induced by ice-shelf melt transports this cold offshore anomaly onto the continental shelf, leading to cooling and decreased melt below 450 m. In the vicinity of ice-shelf fronts, Ekman pumping contributes to raise the isotherms in response to changing winds. This effect overwhelms the horizontal transport of colder offshore waters at intermediate depths (between 200 and 450 m), and therefore increases melt rates in the upper part of the ice-shelf cavities, which reinforces the buoyancy-driven circulation and further contributes to raise the isotherms. Then, prescribing an extreme grounding line retreat projected for 2100, the total melt rates simulated underneath Thwaites and Pine Island are multiplied by 2.5. Such increase is explained by a larger ocean/ice interface exposed to CDW, which is then amplified by a stronger melt-induced circulation along the ice draft. Our main conclusions are that (1) outputs from ocean models that do not represent ice shelf cavities (e.g., CMIP5 models) should not be directly used to predict the thermal forcing of future ice shelf cavities; (2) coupled ocean/ice sheet models with a velocity-dependent melt formulation are needed for future projections of glaciers experiencing a significant grounding line retreat.
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Ducousso, N., Le Sommer, J., Molines, J. M., & Bell, M. (2017). Impact of the “Symmetric Instability of the Computational Kind” at mesoscale- and submesoscale-permitting resolutions. Ocean Modelling, 120, 18–26.
Abstract: The energy-and enstrophy-conserving momentum advection scheme (EEN) used over the last 10 years in NEMO is subject to a spurious numerical instability. This instability, referred to as the Symmetric Instability of the Computational Kind (SICK), arises from a discrete imbalance between the two components of the vector-invariant form of momentum advection. The properties and the method for removing this instability have been documented by Hollingsworth et al. (1983), but the extent to which the SICK may interfere with processes of interest at mesoscale-and submesoscale-permitting resolutions is still unkown. In this paper, the impact of the SICK in realistic ocean model simulations is assessed by comparing model integrations with different versions of the EEN momentum advection scheme. Investigations are undertaken with a global mesoscale-permitting resolution (1/4 degrees) configuration and with a regional North Atlantic Ocean submesoscale-permitting resolution (1/60 degrees) configuration. At both resolutions, the instability is found to alter primarily the most energetic current systems, such as equatorial jets, western boundary currents and coherent vortices. The impact of the SICK is found to increase with model resolution with a noticeable impact at mesoscale-permitting resolution and a dramatic impact at submesoscale-permitting resolution. The SICK is shown to distort the normal functioning of current systems, by redirecting the slow energy transfer between balanced motions to a spurious energy transfer to internal inertia-gravity waves and to dissipation. Our results indicate that the SICK is likely to have significantly corrupted NEMO solutions (when run with the EEN scheme) at mesocale-permitting and finer resolutions over the last 10 years.
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Durán Moro, M., Brankart, J. - M., Brasseur, P., & Verron, J. (2017). Exploring image data assimilation in the prospect of high-resolution satellite oceanic observations. Ocean Dynamics, 67(7), 875–895.
Abstract: Satellite sensors increasingly provide high-resolution (HR) observations of the ocean. They supply observations of sea surface height (SSH) and of tracers of the dynamics such as sea surface salinity (SSS) and sea surface temperature (SST). In particular, the Surface Water Ocean Topography (SWOT) mission will provide measurements of the surface ocean topography at very high-resolution (HR) delivering unprecedented information on the meso-scale and submeso-scale dynamics. This study investigates the feasibility to use these measurements to reconstruct meso-scale features simulated by numerical models, in particular on the vertical dimension. A methodology to reconstruct three-dimensional (3D) multivariate meso-scale scenes is developed by using a HR numerical model of the Solomon Sea region. An inverse problem is defined in the framework of a twin experiment where synthetic observations are used. A true state is chosen among the 3D multivariate states which is considered as a reference state. In order to correct a first guess of this true state, a two-step analysis is carried out. A probability distribution of the first guess is defined and updated at each step of the analysis: (i) the first step applies the analysis scheme of a reduced-order Kalman filter to update the first guess probability distribution using SSH observation; (ii) the second step minimizes a cost function using observations of HR image structure and a new probability distribution is estimated. The analysis is extended to the vertical dimension using 3D multivariate empirical orthogonal functions (EOFs) and the probabilistic approach allows the update of the probability distribution through the two-step analysis. Experiments show that the proposed technique succeeds in correcting a multivariate state using meso-scale and submeso-scale information contained in HR SSH and image structure observations. It also demonstrates how the surface information can be used to reconstruct the ocean state below the surface.
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Gourdeau, L., Djath, B., Ganachaud, A., Nino, F., Birol, F., Verron, J., et al. (2017). Altimetry in a Regional Tropical Sea. Ieee Geoscience And Remote Sensing Magazine, 5(3), 44–52.
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Gourdeau, L., Verron, J., Chaigneau, A., Cravatte, S., & Kessler, W. (2017). Complementary Use of Glider Data, Altimetry, and Model for Exploring Mesoscale Eddies in the Tropical Pacific Solomon Sea. Journal Of Geophysical Research-Oceans, 122(11), 9209–9229.
Abstract: Mesoscale activity is an important component of the Solomon Sea circulation that interacts with the energetic low-latitude western boundary currents of the South Tropical Pacific Ocean carrying waters of subtropical origin before joining the equatorial Pacific. Mixing associated with mesoscale activity could explain water mass transformation observed in the Solomon Sea that likely impacts El Nino Southern Oscillation dynamics. This study makes synergetic use of glider data, altimetry, and high-resolution model for exploring mesoscale eddies, especially their vertical structures, and their role on the Solomon Sea circulation. The description of individual eddies observed by altimetry and gliders provides the first elements to characterize the 3-D structure of these tropical eddies, and confirms the usefulness of the model to access a more universal view of such eddies. Mesoscale eddies appear to have a vertical extension limited to the Surface Waters (SW) and the Upper Thermocline Water (UTW), i.e., the first 140-150 m depth. Most of the eddies are nonlinear, meaning that eddies can trap and transport water properties. But they weakly interact with the deep New Guinea Coastal Undercurrent that is a key piece of the equatorial circulation. Anticyclonic eddies are particularly efficient to advect salty and warm SW coming from the intrusion of equatorial Pacific waters at Solomon Strait, and to impact the characteristics of the New Guinea Coastal Current. Cyclonic eddies are particularly efficient to transport South Pacific Tropical Water (SPTW) anomalies from the North Vanuatu Jet and to erode by diapycnal mixing the high SPTW salinity.
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Groisman, P., Shugart, H., Kicklighter, D., Henebry, G., Tchebakova, N., Maksyutov, S., et al. (2017). Northern Eurasia Future Initiative (NEFI): facing the challenges and pathways of global change in the twenty-first century. Progress In Earth And Planetary Science, 4.
Abstract: During the past several decades, the Earth system has changed significantly, especially across Northern Eurasia. Changes in the socio-economic conditions of the larger countries in the region have also resulted in a variety of regional environmental changes that can have global consequences. The Northern Eurasia Future Initiative (NEFI) has been designed as an essential continuation of the Northern Eurasia Earth Science Partnership Initiative (NEESPI), which was launched in 2004. NEESPI sought to elucidate all aspects of ongoing environmental change, to inform societies and, thus, to better prepare societies for future developments. A key principle of NEFI is that these developments must now be secured through science-based strategies co-designed with regional decision-makers to lead their societies to prosperity in the face of environmental and institutional challenges. NEESPI scientific research, data, and models have created a solid knowledge base to support the NEFI program. This paper presents the NEFI research vision consensus based on that knowledge. It provides the reader with samples of recent accomplishments in regional studies and formulates new NEFI science questions. To address these questions, nine research foci are identified and their selections are briefly justified. These foci include warming of the Arctic; changing frequency, pattern, and intensity of extreme and inclement environmental conditions; retreat of the cryosphere; changes in terrestrial water cycles; changes in the biosphere; pressures on land use; changes in infrastructure; societal actions in response to environmental change; and quantification of Northern Eurasia's role in the global Earth system. Powerful feedbacks between the Earth and human systems in Northern Eurasia (e.g., mega-fires, droughts, depletion of the cryosphere essential for water supply, retreat of sea ice) result from past and current human activities (e.g., large-scale water withdrawals, land use, and governance change) and potentially restrict or provide new opportunities for future human activities. Therefore, we propose that integrated assessment models are needed as the final stage of global change assessment. The overarching goal of this NEFI modeling effort will enable evaluation of economic decisions in response to changing environmental conditions and justification of mitigation and adaptation efforts.
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Jourdain, N. C., Mathiot, P., Merino, N., Durand, G., Le Sommer, J., Spence, P., et al. (2017). Ocean circulation and sea-ice thinning induced by melting ice shelves in the Amundsen Sea. Journal Of Geophysical Research-Oceans, 122(3), 2550–2573.
Abstract: A 1/128 ocean model configuration of the Amundsen Sea sector is developed to better understand the circulation induced by ice-shelf melt and the impacts on the surrounding ocean and sea ice. Eighteen sensitivity experiments to drag and heat exchange coefficients at the ice shelf/ocean interface are performed. The total melt rate simulated in each cavity is function of the thermal Stanton number, and for a given thermal Stanton number, melt is slightly higher for lower values of the drag coefficient. Sub-ice-shelf melt induces a thermohaline circulation that pumps warm circumpolar deep water into the cavity. The related volume flux into a cavity is 100-500 times stronger than the melt volume flux itself. Ice-shelf melt also induces a coastal barotropic current that contributes 45612% of the total simulated coastal transport. Due to the presence of warm circumpolar deep waters, the melt-induced inflow typically brings 4-20 times more heat into the cavities than the latent heat required for melt. For currently observed melt rates, approximately 6-31% of the heat that enters a cavity with melting potential is actually used to melt ice shelves. For increasing sub-ice-shelf melt rates, the transport in the cavity becomes stronger, and more heat is pumped from the deep layers to the upper part of the cavity then advected toward the ocean surface in front of the ice shelf. Therefore, more ice-shelf melt induces less sea-ice volume near the ice sheet margins. Plain Language Summary The ice-shelf cavities of the Amundsen Sea, Antarctica, act as very powerful pumps that create strong inflows of warm water under the ice-shelves, as well as significant circulation changes in the entire region. Such warm inflows bring more heat than required to melt ice, so that a large part of that heat exits ice-shelf cavities without being used. Due to mixing between warm deep waters and melt freshwater, melt-induced flows are warm and buoyant when they leave cavities. Therefore, they reach the ocean surface near ice-shelf fronts and can melt significant amounts of sea ice. It is thus suggested that climatic trends in sub ice-shelf melt could partly explain sea ice trends near the ice-sheet margins in the Amundsen Sea region.
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Moro, M. D., Brankart, J. M., Brasseur, P., & Verron, J. (2017). Exploring image data assimilation in the prospect of high-resolution satellite oceanic observations. Ocean Dynamics, 67(7), 875–895.
Abstract: Satellite sensors increasingly provide high-resolution (HR) observations of the ocean. They supply observations of sea surface height (SSH) and of tracers of the dynamics such as sea surface salinity (SSS) and sea surface temperature (SST). In particular, the Surface Water Ocean Topography (SWOT) mission will provide measurements of the surface ocean topography at very high-resolution (HR) delivering unprecedented information on the meso-scale and submeso-scale dynamics. This study investigates the feasibility to use these measurements to reconstruct meso-scale features simulated by numerical models, in particular on the vertical dimension. A methodology to reconstruct three-dimensional (3D) multivariate meso-scale scenes is developed by using a HR numerical model of the Solomon Sea region. An inverse problem is defined in the framework of a twin experiment where synthetic observations are used. A true state is chosen among the 3D multivariate states which is considered as a reference state. In order to correct a first guess of this true state, a two-step analysis is carried out. A probability distribution of the first guess is defined and updated at each step of the analysis: (i) the first step applies the analysis scheme of a reduced-order Kalman filter to update the first guess probability distribution using SSH observation; (ii) the second step minimizes a cost function using observations of HR image structure and a new probability distribution is estimated. The analysis is extended to the vertical dimension using 3D multivariate empirical orthogonal functions (EOFs) and the probabilistic approach allows the update of the probability distribution through the two-step analysis. Experiments show that the proposed technique succeeds in correcting a multivariate state using meso-scale and submeso-scale information contained in HR SSH and image structure observations. It also demonstrates how the surface information can be used to reconstruct the ocean state below the surface.
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Roman-Cascon, C., Pellarin, T., Gibon, F., Brocca, L., Cosme, E., Crow, W., et al. (2017). Correcting satellite-based precipitation products through SMOS soil moisture data assimilation in two land-surface models of different complexity: API and SURFEX. Remote Sensing Of Environment, 200, 295–310.
Abstract: Global rainfall information is useful for many applications. However, real-time versions of satellite-based rainfall products are known to contain errors. Recent studies have demonstrated how the information about rainfall intrinsically contained in soil moisture data can be utilised for improving rainfall estimates. That is, soil moisture dynamics are impacted for several days by the accumulated amount of rainfall following within a particular event. In this context, soil moisture data from the Soil Moisture Ocean Salinity (SMOS) satellite is used in this study to correct rainfall accumulation estimates provided by satellite-based real-time precipitation products such as CMORPH, TRMM-3B42RT or PERSIANN. An algorithm based on the SMOS measurements data assimilation is tested in two land-surface models of different complexity: a simple hydrological model (Antecedent Precipitation Index (API)) and a more sophisticated state-of-the-art land-surface model (SURFEX (Surface Extemalisee)). We show how the assimilation technique, based on a particle filter method, generally leads to a significant improvement in rainfall estimates, with slightly better results for the simpler (and less computationally demanding) API model. This methodology has been evaluated for six years at ten sites around the world with different land use and climatological features. The results also show the limitations of the methodology in regions highly affected by mountainous terrain, forest or intense radio-frequency interference (RFI), which can notably affect the quality of the retrievals. The satisfactory results shown here invite the future operational application of the methodology in near-real time on a global scale.
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Serazin, G., Jaymond, A., Leroux, S., Penduff, T., Bessieres, L., Llovel, W., et al. (2017). A global probabilistic study of the ocean heat content low-frequency variability: Atmospheric forcing versus oceanic chaos. Geophysical Research Letters, 44(11), 5580–5589.
Abstract: A global 1/4 degrees ocean/sea ice 50-member ensemble simulation is used to disentangle the low-frequency imprints of the atmospherically forced oceanic variability and of the chaotic intrinsic oceanic variability (IOV) on the large-scale (10 degrees x10 degrees) ocean heat content (OHC) between 1980 and 2010. The IOV explains most of the interannual-to-decadal large-scale OHC variance over substantial fractions of the global ocean area that increase with depth: 9%, 22%, and 31% in the 0-700m, 700-2000m and 2000m bottom layers, respectively. Such areas concern principally eddy-active regions, mostly found in the Southern Ocean and in western boundary current extensions, and also concern the subtropical gyres at intermediate and deep levels. The oceanic chaos may also induce random multidecadal fluctuations so that large-scale regional OHC trends computed on the 1980-2010 period cannot be unambiguously attributed to the atmospheric forcing in several oceanic basins at various depths. These results are likely to raise detection and attribution issues from real observations.
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Storto, A., Masina, S., Balmaseda, M., Guinehut, S., Xue, Y., Szekely, T., et al. (2017). Steric sea level variability (1993-2010) in an ensemble of ocean reanalyses and objective analyses. Climate Dynamics, 49(3), 709–729.
Abstract: Quantifying the effect of the seawater density changes on sea level variability is of crucial importance for climate change studies, as the sea level cumulative rise can be regarded as both an important climate change indicator and a possible danger for human activities in coastal areas. In this work, as part of the Ocean Reanalysis Intercomparison Project, the global and regional steric sea level changes are estimated and compared from an ensemble of 16 ocean reanalyses and 4 objective analyses. These estimates are initially compared with a satellite-derived (altimetry minus gravimetry) dataset for a short period (2003-2010). The ensemble mean exhibits a significant high correlation at both global and regional scale, and the ensemble of ocean reanalyses outperforms that of objective analyses, in particular in the Southern Ocean. The reanalysis ensemble mean thus represents a valuable tool for further analyses, although large uncertainties remain for the inter-annual trends. Within the extended intercomparison period that spans the altimetry era (1993-2010), we find that the ensemble of reanalyses and objective analyses are in good agreement, and both detect a trend of the global steric sea level of 1.0 and 1.1 ± 0.05 mm/year, respectively. However, the spread among the products of the halosteric component trend exceeds the mean trend itself, questioning the reliability of its estimate. This is related to the scarcity of salinity observations before the Argo era. Furthermore, the impact of deep ocean layers is non-negligible on the steric sea level variability (22 and 12 % for the layers below 700 and 1500 m of depth, respectively), although the small deep ocean trends are not significant with respect to the products spread.
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Treguier, A. M., Lique, C., Deshayes, J., & Molines, J. M. (2017). The North Atlantic Eddy Heat Transport and Its Relation with the Vertical Tilting of the Gulf Stream Axis. Journal Of Physical Oceanography, 47(6), 1281–1289.
Abstract: Correlations between temperature and velocity fluctuations are a significant contribution to the North Atlantic meridional heat transport, especially at the northern boundary of the subtropical gyre. In satellite observations and in a numerical model at 1/12 degrees resolution, a localized pattern of positive eddy heat flux is found northwest of the Gulf Stream, downstream of its separation at Cape Hatteras. It is confined to the upper 500 m. A simple kinematic model of a meandering jet can explain the surface eddy flux, taking into account a spatial shift between the maximum velocity of the jet and the maximum cross-jet temperature gradient. In the Gulf Stream such a spatial shift results from the nonlinear temperature profile and the vertical tilting of the velocity profile with depth. The numerical model suggests that the meandering of the Gulf Stream could account, at least in part, for the large eddy heat transport (of order 0.3 PW) near 36 degrees N in the North Atlantic and for its compensation by the mean flow.
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Tuzet, F., Dumont, M., Lafaysse, M., Picard, G., Arnaud, L., Voisin, D., et al. (2017). A multilayer physically based snowpack model simulating direct and indirect radiative impacts of light-absorbing impurities in snow. Cryosphere, 11(6), 2633–2653.
Abstract: Light-absorbing impurities (LAIs) decrease snow albedo, increasing the amount of solar energy absorbed by the snowpack. Its most intuitive and direct impact is to accelerate snowmelt. Enhanced energy absorption in snow also modifies snow metamorphism, which can indirectly drive further variations of snow albedo in the near-infrared part of the solar spectrum because of the evolution of the near-surface snow microstructure. New capabilities have been implemented in the detailed snowpack model SURFEX/ISBACrocus (referred to as Crocus) to account for impurities' deposition and evolution within the snowpack and their direct and indirect impacts. Once deposited, the model computes impurities' mass evolution until snow melts out, accounting for scavenging by meltwater. Taking advantage of the recent inclusion of the spectral radiative transfer model TARTES (Two-stream Analytical Radiative TransfEr in Snow model) in Crocus, the model explicitly represents the radiative impacts of light-absorbing impurities in snow. The model was evaluated at the Col de Porte experimental site (French Alps) during the 2013-2014 snow season against in situ standard snow measurements and spectral albedo measurements. In situ meteorological measurements were used to drive the snowpack model, except for aerosol deposition fluxes. Black carbon (BC) and dust deposition fluxes used to drive the model were extracted from simulations of the atmospheric model ALADIN-Climate. The model simulates snowpack evolution reasonably, providing similar performances to our reference Crocus version in terms of snow depth, snow water equivalent (SWE), near-surface specific surface area (SSA) and shortwave albedo. Since the reference empirical albedo scheme was calibrated at the Col de Porte, improvements were not expected to be significant in this study. We show that the deposition fluxes from the ALADIN-Climate model provide a reasonable estimate of the amount of light-absorbing impurities deposited on the snowpack except for extreme deposition events which are greatly underestimated. For this particular season, the simulated melt-out date advances by 6 to 9 days due to the presence of light-absorbing impurities. The model makes it possible to apportion the relative importance of direct and indirect impacts of light-absorbing impurities on energy absorption in snow. For the snow season considered, the direct impact in the visible part of the solar spectrum accounts for 85% of the total impact, while the indirect impact related to accelerated snow metamorphi
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Valdivieso, M., Haines, K., Balmaseda, M., Chang, Y. S., Drevillon, M., Ferry, N., et al. (2017). An assessment of air-sea heat fluxes from ocean and coupled reanalyses. Climate Dynamics, 49(3), 983–1008.
Abstract: Sixteen monthly air-sea heat flux products from global ocean/coupled reanalyses are compared over 19932009 as part of the Ocean Reanalysis Intercomparison Project (ORA-IP). Objectives include assessing the global heat closure, the consistency of temporal variability, comparison with other flux products, and documenting errors against in situ flux measurements at a number of OceanSITES moorings. The ensemble of 16 ORA-IP flux estimates has a global positive bias over 1993-2009 of 4.2 +/- 1.1 W m(-2). Residual heat gain (i. e., surface flux + assimilation increments) is reduced to a small positive imbalance (typically, + 1-2 W m(-2)). This compensation between surface fluxes and assimilation increments is concentrated in the upper 100 m. Implied steady meridional heat transports also improve by including assimilation sources, except near the equator. The ensemble spread in surface heat fluxes is dominated by turbulent fluxes (>40 W m(-2) over the western boundary currents). The mean seasonal cycle is highly consistent, with variability between products mostly <10 W m-2. The interannual variability has consistent signal-to-noise ratio (similar to 2) throughout the equatorial Pacific, reflecting ENSO variability. Comparisons at tropical buoy sites (10 degrees S-15 degrees N) over 2007-2009 showed too little ocean heat gain (i. e., flux into the ocean) in ORA-IP (up to 1/3 smaller than buoy measurements) primarily due to latent heat flux errors in ORA-IP. Comparisons with the Stratus buoy (20 degrees S, 85 degrees W) over a longer period, 2001-2009, also show the ORA-IP ensemble has 16 W m(-2) smaller net heat gain, nearly all of which is due to too much latent cooling caused by differences in surface winds imposed in ORA-IP.
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Yan, Y., Barth, A., Beckers, J. M., Brankart, J. M., Brasseur, P., & Candille, G. (2017). Comparison of different incremental analysis update schemes in a realistic assimilation system with Ensemble Kalman Filter. Ocean Modelling, 115, 27–41.
Abstract: In this paper, three incremental analysis update schemes (IAU 0, IAU 50 and IAU 100) are compared in the same assimilation experiments with a realistic eddy permitting primitive equation model of the North Atlantic Ocean using the Ensemble Kalman Filter. The difference between the three IAU schemes lies on the position of the increment update window. The relevance of each IAU scheme is evaluated through analyses on both thermohaline and dynamical variables. The validation of the assimilation results is performed according to both deterministic and probabilistic metrics against different sources of observations. For deterministic validation, the ensemble mean and the ensemble spread are compared to the observations. For probabilistic validation, the continuous ranked probability score (CRPS) is used to evaluate the ensemble forecast system according to reliability and resolution. The reliability is further decomposed into bias and dispersion by the reduced centred random variable (RCRV) score. The obtained results show that 1) the IAU 50 scheme has the same performance as the IAU 100 scheme 2) the IAU 50/100 schemes outperform the IAU 0 scheme in error covariance propagation for thermohaline variables in relatively stable region, while the IAU 0 scheme outperforms the IAU 50/100 schemes in dynamical variables estimation in dynamically active region 3) in case with sufficient number of observations and good error specification, the impact of IAU schemes is negligible. The differences between the IAU 0 scheme and the IAU 50/100 schemes are mainly due to different model integration time and different instability (density inversion, large vertical velocity, etc.) induced by the increment update. The longer model integration time with the IAU 50/100 schemes, especially the free model integration, on one hand, allows for better re-establishment of the equilibrium model state, on the other hand, smooths the strong gradients in dynamically active region. (C) 2017 Elsevier Ltd. All rights reserved.
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Zolina, O., Dufour, A., Gulev, S. K., & Stenchikov, G. (2017). Regional Hydrological Cycle over the Red Sea in ERA-Interim. Journal Of Hydrometeorology, 18(1), 65–83.
Abstract: The major sources of atmospheric moisture over the Red Sea are analyzed using ERA-Interim for the 1979-2013 period. The vertical structure of moisture transports across the coastlines has been computed separately for the western and eastern coasts of the Red Sea. The vertical structure of the moisture transport from the Red Sea to the continents is dominated by a breeze-like circulation in the near-surface layer and the Arabian high above 850 hPa. The lower-layer, breeze-like circulation is acting to export the moisture to the northwest of Africa and to the Arabian Peninsula and contributes about 80% of the moisture exports from the Red Sea, dominating over the transport in the upper layer, where the moisture is advected to the Arabian Peninsula in the northern part of the sea and to the African continent in the southern part. Integrated moisture divergence over the Red Sea decreased from the early 1980s to 1997 and then increased until the 2010s. Associated changes in the moisture export were provided primarily by the increasing intensity of the breeze-associated transports. The transports above the boundary layer, while being strong across the western and the eastern coasts, have a smaller effect on the net moisture export. The interannual variability of the moisture export in the near-surface layer was found to be closely correlated with the variability in sea surface temperature, especially in summer. Implications of the observed changes in the moisture advection for the hydrological cycle of the Middle East are discussed.
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Akuetevi, C. Q. C., Barnier, B., Verron, J., Molines, J. M., & Lecointre, A. (2016). Interactions between the Somali Current eddies during the summer monsoon: insights from a numerical study. Ocean Science, 12(1), 185–205.
Abstract: Three hindcast simulations of the global ocean circulation differing by resolution (1/4 or 1/12 degrees) or parametrization or atmospheric forcing are used to describe the interactions between the large anticyclonic eddies generated by the Somali Current system during the Southwest Monsoon. The present investigation of the Somalian coherent eddy structures allows us to identify the origin and the subsequent development of the cyclones flanked upon the Great Whirl (GW) previously identified by Beal and Donohue (2013) in satellite observations and to establish that similar cyclones are also flanked upon the Southern Gyre (SG). These cyclones are identified as potential actors in mixing water masses within the large eddies and offshore the coast of Somalia. All three simulations bring to light that during the period when the Southwest Monsoon is well established, the SG moves northward along the Somali coast and encounters the GW. The interaction between the SG and the GW is a collision without merging, in a way that has not been described in observations up to now. During the collision the GW is pushed to the east of Socotra Island, sheds several smaller patches of anticyclonic vorticity, and often reforms into the Socotra Eddy, thus proposing a formation mechanism for that eddy. During this process the GW gives up its place to the SG. This process is robust throughout the three simulations.
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Bichet, A., Kushner, P. J., & Mudryk, L. (2016). Estimating the Continental Response to Global Warming Using Pattern-Scaled Sea Surface Temperatures and Sea Ice. Journal Of Climate, 29(24), 9125–9139.
Abstract: Better constraining the continental climate response to anthropogenic forcing is essential to improve climate projections. In this study, pattern scaling is used to extract, from observations, the patterned response of sea surface temperature (SST) and sea ice concentration (SICE) to anthropogenically dominated long-term global warming. The SST response pattern includes a warming of the tropical Indian Ocean, the high northern latitudes, and the western boundary currents. The SICE pattern shows seasonal variations of the main locations of sea ice loss. These SST-SICE response patterns are used to drive an ensemble of an atmospheric general circulation model, the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5 (CAM5), over the period 1980-2010 along with a standard AMIP ensemble using observed SST-SICE. The simulations enable attribution of a variety of observed trends of continental climate to global warming. On the one hand, the warming trends observed in all seasons across the entire Northern Hemisphere extratropics result from global warming, as does the snow loss observed over the northern midlatitudes and northwestern Eurasia. On the other hand, 1980-2010 precipitation trends observed in winter over North America and in summer over Africa result from the recent decreasing phase of the Pacific decadal oscillation and the recent increasing phase of the Atlantic multidecadal oscillation, respectively, which are not part of the global warming signal. The method holds promise for near-term decadal climate prediction but as currently framed cannot distinguish regional signals associated with oceanic internal variability from aerosol forcing and other sources of short-term forcing.
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Carton, X., Ciani, D., Verron, J., Reinaud, J., & Sokolovskiy, M. (2016). Vortex merger in surface quasi-geostrophy. Geophysical And Astrophysical Fluid Dynamics, 110(1), 1–22.
Abstract: The merger of two identical surface temperature vortices is studied in the surface quasi-geostrophic model. The motivation for this study is the observation of the merger of submesoscale vortices in the ocean. Firstly, the interaction between two point vortices, in the absence or in the presence of an external deformation field, is investigated. The rotation rate of the vortices, their stationary positions and the stability of these positions are determined. Then, a numerical model provides the steady states of two finite-area, constant-temperature, vortices. Such states are less deformed than their counterparts in two-dimensional incompressible flows. Finally, numerical simulations of the nonlinear surface quasi-geostrophic equations are used to investigate the finite-time evolution of initially identical and symmetric, constant temperature vortices. The critical merger distance is obtained and the deformation of the vortices before or after merger is determined. The addition of external deformation is shown to favor or to oppose merger depending on the orientation of the vortex pair with respect to the strain axes. An explanation for this observation is proposed. Conclusions are drawn towards an application of this study to oceanic vortices.
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Charrois, L., Cosme, E., Dumont, M., Lafaysse, M., Morin, S., Libois, Q., et al. (2016). On the assimilation of optical reflectances and snow depth observations into a detailed snowpack model. Cryosphere, 10(3), 1021–1038.
Abstract: This paper examines the ability of optical reflectance data assimilation to improve snow depth and snow water equivalent simulations from a chain of models with the SAFRAN meteorological model driving the detailed multilayer snowpack model Crocus now including a two-stream radiative transfer model for snow, TARTES. The direct use of reflectance data, allowed by TARTES, instead of higher level snow products, mitigates uncertainties due to commonly used retrieval algorithms.Data assimilation is performed with an ensemble-based method, the Sequential Importance Resampling Particle filter, to represent simulation uncertainties. In snowpack modeling, uncertainties of simulations are primarily assigned to meteorological forcings. Here, a method of stochastic perturbation based on an autoregressive model is implemented to explicitly simulate the consequences of these uncertainties on the snowpack estimates.Through twin experiments, the assimilation of synthetic spectral reflectances matching the MODerate resolution Imaging Spectroradiometer (MODIS) spectral bands is examined over five seasons at the Col du Lautaret, located in the French Alps. Overall, the assimilation of MODIS-like data reduces by 45aEuro-% the root mean square errors (RMSE) on snow depth and snow water equivalent. At this study site, the lack of MODIS data on cloudy days does not affect the assimilation performance significantly. The combined assimilation of MODIS-like reflectances and a few snow depth measurements throughout the 2010/2011 season further reduces RMSEs by roughly 70aEuro-%. This work suggests that the assimilation of optical reflectances has the potential to become an essential component of spatialized snowpack simulation and forecast systems. The assimilation of real MODIS data will be investigated in future works.
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Ciani, D., Carton, X., & Verron, J. (2016). On the merger of subsurface isolated vortices. Geophysical And Astrophysical Fluid Dynamics, 110(1), 23–49.
Abstract: Vortex merger is a phenomenon characterizing the whole class of geophysical vortices, from atmospheric storms and large oceanic eddies up to small scale turbulence. Here we focus on the merger of subsurface oceanic anticyclones in an idealized primitive equations model. This study has been motivated by past and recent observations of colliding lens-like anticyclones off of Gibraltar Strait. The critical conditions for merger (critical merger distance and time needed for merger) are determined. We will show that the predictions of classical two-dimensional merger are not verified for subsurface isolated vortices. For instance, critical merger distances will be reduced because of the vortex potential vorticity (PV) structure. The post-merger characteristics of the vortex (radius, extension and PV), are also determined. Merger-related effects, like production of peripheral filaments and small-scale eddies are also investigated and suggest the contribution of merger in both direct and inverse energy cascades.
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de Lavergne, C., Madec, G., Le Sommer, J., Nurser, A. J. G., & Garabato, A. C. N. (2016). On the Consumption of Antarctic Bottom Water in the Abyssal Ocean. Journal Of Physical Oceanography, 46(2), 635–661.
Abstract: The abyssal ocean is primarily filled by cold, dense waters formed around Antarctica and collectively referred to as Antarctic Bottom Water (AABW). At steady state, AABW must be consumed in the ocean interior at the same rate it is produced, but how and where this consumption is achieved remains poorly understood. Here, estimates of abyssal water mass transformation by geothermal heating and parameterized internal wave-driven mixing are presented. This study uses maps of the energy input to internal waves by tidal and geostrophic motions interacting with topography combined with assumptions about the distribution of energy dissipation to evaluate dianeutral transports induced by breaking internal tides and lee waves. Geothermal transformation is assessed based on a map of geothermal heat fluxes. Under the hypotheses underlying the constructed climatologies of buoyancy fluxes, the authors calculate that locally dissipating internal tides and geothermal heating contribute, respectively, about 8 and 5 Sverdrups (Sv; 1 Sv equivalent to 10(6) m(3) s(-1)) of AABW consumption (upwelling), mostly north of 30 degrees S. In contrast, parameterized lee wave-driven mixing causes significant transformation only in the Southern Ocean, where it forms about 3 Sv of AABW, decreasing the mean density but enhancing the northward flow of abyssal waters. The possible role of remotely dissipating internal tides in complementing AABW consumption is explored based on idealized distributions of mixing energy. Depending mostly on the chosen vertical structure, such mixing could drive 1 to 28 Sv of additional AABW upwelling, highlighting the need to better constrain the spatial distribution of remote dissipation. Though they carry large uncertainties, these climatological transformation estimates shed light on the qualitative functioning and key unknowns of the diabatic overturning.
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de Lavergne, C., Madec, G., Le Sommer, J., Nurser, A. J. G., & Garabato, A. C. N. (2016). The Impact of a Variable Mixing Efficiency on the Abyssal Overturning. Journal Of Physical Oceanography, 46(2), 663–681.
Abstract: In studies of ocean mixing, it is generally assumed that small-scale turbulent overturns lose 15%-20% of their energy in eroding the background stratification. Accumulating evidence that this energy fraction, or mixing efficiency R-f, significantly varies depending on flow properties challenges this assumption, however. Here, the authors examine the implications of a varying mixing efficiency for ocean energetics and deep-water mass transformation. Combining current parameterizations of internal wave-driven mixing with a recent model expressing R-f as a function of a turbulence intensity parameter Re-b = epsilon(nu)/nu N-2, the ratio of dissipation epsilon(nu) to stratification N-2 and molecular viscosity nu, it is shown that accounting for reduced mixing efficiencies in regions of weak stratification or energetic turbulence (high Re-b) strongly limits the ability of breaking internal waves to supply oceanic potential energy and drive abyssal upwelling. Moving from a fixed R-f = 1/6 to a variable efficiency R-f(Re-b) causes Antarctic Bottom Water upwelling induced by locally dissipating internal tides and lee waves to fall from 9 to 4 Sverdrups (Sv; 1 Sv equivalent to 10(6) m(3) s(-1)) and the corresponding potential energy source to plunge from 97 to 44 GW. When adding the contribution of remotely dissipating internal tides under idealized distributions of energy dissipation, the total rate of Antarctic Bottom Water upwelling is reduced by about a factor of 2, reaching 5-15 Sv, compared to 10-33 Sv for a fixed efficiency. The results suggest that distributed mixing, overflow-related boundary processes, and geothermal heating are more effective in consuming abyssal waters than topographically enhanced mixing by breaking internal waves. These calculations also point to the importance of accurately constraining R-f (Re-b) and including the effect in ocean models.
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Escudier, R., Renault, L., Pascual, A., Brasseur, P., Chelton, D., & Beuvier, J. (2016). Eddy properties in the Western Mediterranean Sea from satellite altimetry and a numerical simulation. Journal Of Geophysical Research-Oceans, 121(6), 3990–4006.
Abstract: Three different eddy detection and tracking methods are applied to the outputs of a high-resolution simulation in the Western Mediterranean Sea in order to extract mesoscale eddy characteristics. The results are compared with the same eddy statistics derived from satellite altimetry maps over the same period. Eddy radii are around 30 km in altimetry maps whereas, in the model, they are around 20 km. This is probably due to the inability of altimetry maps to resolve the smaller mesoscale in the region. About 30 eddies are detected per day in the basin with a very heterogeneous spatial distribution and relatively short lifespans (median life around 13 days). Unlike other areas of the open ocean, they do not have a preferred direction of propagation but appear to be advected by mean currents. The number of detected eddies seems to present an annual cycle when separated according to their lifespan. With the numerical simulation, we show that anticyclones extend deeper in the water column and have a more conic shape than cyclones.
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Garnier, F., Brankart, J. M., Brasseur, P., & Cosme, E. (2016). Stochastic parameterizations of biogeochemical uncertainties in a 1/4 degrees NEMO/PISCES model for probabilistic comparisons with ocean color data. Journal Of Marine Systems, 155, 59–72.
Abstract: In spite of recent advances, biogeochemical models are still unable to represent the full complexity of natural ecosystems. Their formulations are mainly based on empirical laws involving many parameters. Improving biogeochemical models therefore requires to properly characterize model uncertainties and their consequences. Subsequently, this paper investigates the potential of using random processes to simulate some uncertainties of the 1/4 degrees coupled Physical-Biogeochemical NEMO/PISCES model of the North Atlantic ocean. Starting from a deterministic simulation performed with the original PISCES formulation, we propose a generic method based on AR(1) random processes to generate perturbations with temporal and spatial correlations. These perturbations are introduced into the model formulations to simulate 2 classes of uncertainties: the uncertainties on biogeochemical parameters and the uncertainties induced by unresolved scales in the presence of non-linear processes. Using these stochastic parameterizations, a probabilistic version of PISCES is designed and a 60-member ensemble simulation is performed. With respect to the simulation of chlorophyll, the relevance of the probabilistic configuration and the impacts of these stochastic parameterizations are assessed. In particular, it is shown that the ensemble simulation is in good agreement with the SeaWIFS ocean color data. Using these observations, the statistical consistency (reliability) of the ensemble is evaluated with rank histograms. Finally, the benefits expected from the probabilistic description of uncertainties (model error) are discussed in the context of future ocean color data assimilation. (C) 2015 Elsevier B.V. All rights reserved.
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Jouanno, J., Ochoa, J., Pallas-Sanz, E., Sheinbaum, J., Andrade-Canto, F., Candela, J., et al. (2016). Loop Current Frontal Eddies: Formation along the Campeche Bank and Impact of Coastally Trapped Waves. Journal Of Physical Oceanography, 46(11), 3339–3363.
Abstract: Velocity data from a mooring array deployed northeast of the Campeche Bank (CB) show the presence of subinertial, high-frequency (below 15 days) velocity fluctuations within the core of the northward flowing Loop Current. These fluctuations are associated with the presence of surface-intensified Loop Current frontal eddies (LCFEs), with cyclonic vorticity and diameter, 100 km. These eddies are well reproduced by a high-resolution numerical simulation of the Gulf of Mexico, and the model analysis suggests that they originate along and north of the CB, their main energy source being the mixed baroclinic-barotropic instability of the northward flow along the shelf break. There is no indication that these high-frequency LCFEs contribute to the LC eddy detachment in contrast to the low-frequency LCFEs (periods. 30 days) that have been linked to Caribbean eddies and the LC separation process. Model results show that wind variability associated with winter cold surges are responsible for the emergence of high-frequency LCFEs in a narrow band of periods (6-10 day) in the region of the CB. The dynamical link between the formation of these LCFEs and the wind variability is not direct: (i) the large-scale wind perturbations generate sea level anomalies on the CB as well as first baroclinic mode, coastally trapped waves in the western Gulf of Mexico; (ii) these waves propagate cyclonically along the coast; and (iii) the interaction of these anomalies with the Loop Current triggers cyclonic vorticity perturbations that grow in intensity as they propagate downstream and develop into cyclonic eddies when they flow north of the Yucatan shelf.
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Leroux, S., Bellon, G., Roehrig, R., Caian, M., Klingaman, N. P., Lafore, J. P., et al. (2016). Inter-model comparison of subseasonal tropical variability in aquaplanet experiments: Effect of a warm pool. Journal of Advances in Modeling Earth Systems, 8(4), 1526–1551.
Abstract: This study compares the simulation of subseasonal tropical variability by a set of six state-of-the-art AGCMs in two experiments in aquaplanet configuration: a zonally symmetric experiment, and an experiment with a warm pool centered on the equator. In all six models, the presence of the warm pool generates zonal asymmetries in the simulated mean states in the form of a "Gill-type'' response, made more complex by feedbacks between moisture, convective heating and circulation. Noticeable differences appear from one model to another. Only half the models simulate mean low-level equatorial westerlies over the warm pool area. The presence of the warm pool can also favor the development of large-scale variability consistent with observed Madden-Julian Oscillation (MJO) characteristics, but this happens only in half the models. Our results do not support the idea that the presence of the warm pool and/or of mean low-level equatorial westerlies are sufficient conditions for MJO-like variability to arise in the models. Comparing spectral characteristics of the simulated Convectively Coupled Equatorial Waves (CCEWs) in the aquaplanet experiments and the corresponding coupled atmosphere-ocean (i. e., CMIP) and atmosphere-only (i. e., AMIP) simulations, we also show that there is more consistency for a given model across its configurations, than for a given configuration across the six models. Overall, our results confirm that the simulation of subseasonal variability by given model is significantly influenced by the parameterization of subgrid physical processes (most-likely cloud processes), both directly and through modulation of the mean state.
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Merino, N., Le Sommer, J., Durand, G., Jourdain, N. C., Madec, G., Mathiot, P., et al. (2016). Antarctic icebergs melt over the Southern Ocean: Climatology and impact on sea ice. Ocean Modelling, 104, 99–110.
Abstract: Recent increase in Antarctic freshwater release to the Southern Ocean is suggested to contribute to change in water masses and sea ice. However, climate models differ in their representation of the freshwater sources. Recent improvements in altimetry-based detection of small icebergs and in estimates of the mass loss of Antarctica may help better constrain the values of Antarctic freshwater releases. We propose a model-based seasonal climatology of iceberg melt over the Southern Ocean using state-of-the-art observed glaciological estimates of the Antarctic mass loss. An improved version of a Lagrangian iceberg model is coupled with a global, eddy-permitting ocean/sea ice model and compared to small icebergs observations. Iceberg melt increases sea ice cover, about 10% in annual mean sea ice volume, and decreases sea surface temperature over most of the Southern Ocean, but with distinctive regional patterns. Our results underline the importance of improving the representation of Antarctic freshwater sources. This can be achieved by forcing ocean/sea ice models with a climatological iceberg fresh-water flux. (C) 2016 Elsevier Ltd. All rights reserved.
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Ruggiero, G. A., Cosme, E., Brankart, J. M., Le Sommer, J., & Ubelmann, C. (2016). An Efficient Way to Account for Observation Error Correlations in the Assimilation of Data from the Future SWOT High-Resolution Altimeter Mission. Journal Of Atmospheric And Oceanic Technology, 33(12), 2755–2768.
Abstract: Most data assimilation algorithms require the inverse of the covariance matrix of the observation errors. In practical applications, the cost of computing this inverse matrix with spatially correlated observation errors is prohibitive. Common practices are therefore to subsample or combine the observations so that the errors of the assimilated observations can be considered uncorrelated. As a consequence, a large fraction of the available observational information is not used in practical applications. In this study, a method is developed to account for the correlations of the errors that will be present in the wide-swath sea surface height measurements, for example, the Surface Water and Ocean Topography (SWOT) mission. It basically consists of the transformation of the observation vector so that the inverse of the corresponding covariance matrix can be replaced by a diagonal matrix, thus allowing to genuinely take into account errors that are spatially correlated in physical space. Numerical experiments of ensemble Kalman filter analysis of SWOT-like observations are conducted with three different observation error covariance matrices. Results suggest that the proposed method provides an effective way to account for error correlations in the assimilation of the future SWOT data. The transformation of the observation vector proposed herein yields both a significant reduction of the root-mean-square errors and a good consistency between the filter analysis error statistics and the true error statistics.
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Sanchez, S., Fournier, A., Aubert, J., Cosme, E., & Gallet, Y. (2016). Modelling the archaeomagnetic field under spatial constraints from dynamo simulations: a resolution analysis. Geophysical Journal International, 207(2), 983–1002.
Abstract: Archaeomagnetic observations are key to recovering the behaviour of the geomagnetic field over the past few millennia. The corresponding data set presents a highly heterogeneous distribution in both space and time. Furthermore, the data are affected by substantial age and experimental uncertainties. In order to mitigate these detrimental properties, time-dependent global archaeomagnetic field models are usually constructed under spatial and temporal regularization constraints, with the use of bootstrap techniques to account for data uncertainties. The models so obtained are the product of an adjustable trade-off between goodness-of-fit and model complexity. The spatial complexity is penalized by means of a norm reflecting the minimization of Ohmic dissipation within the core. We propose in this study to resort to alternative spatial constraints relying on the statistics of a numerical dynamo simulation with Earth-like features. To that end, we introduce a dynamo norm in an ensemble least-squares iterative framework, the goal of which is to produce single-epoch models of the archaeomagnetic field. We first validate this approach using synthetic data. We next construct a redistributed archaeomagnetic data set between 1200 BC and 2000 AD by binning the data in windows of 40-yr width. Since the dynamo norm is not adjustable, we can legitimately calculate a resolution matrix to quantify the resolving power of the available archaeomagnetic data set. Gauss coefficients are resolved up to spherical harmonic degree 3 for the first thousand years of the interval, to degree 4 for the next thousand years and to degree 5 during the last millennium. These conclusions are based on the distribution and uncertainties that characterize the data set, and do not take into account the possible presence of outliers. Comparison between our model, called AmR, and previously published archaeomagnetic field models confirms the archaeomagnetic resolution analysis: it highlights the dichotomy between data-driven coefficients for which model predictions coincide (within their respective uncertainties), and prior-driven coefficients. This study opens the way to physics-based models of the archaeomagnetic field; future work will be devoted to integrating the framework here introduced into a time-dependent ensemble assimilation scheme.
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Serazin, G., Meyssignac, B., Penduff, T., Terray, L., Barnier, B., & Molines, J. M. (2016). Quantifying uncertainties on regional sea level change induced by multidecadal intrinsic oceanic variability. Geophysical Research Letters, 43(15), 8151–8159.
Abstract: A global eddy-permitting (1/4 degrees resolution) ocean general circulation model is shown to spontaneously generate intrinsic oceanic variability (IOV) up to multidecadal timescales (T > 20years) under a repeated seasonal atmospheric forcing. In eddy-active regions, the signature of this multidecadal eddy-driven IOV on sea level is substantial, weakly autocorrelated, and is comparable to (and may clearly exceed) the corresponding signature of internal climate variability (ICV) produced by current coupled climate modelswhose laminar ocean components may strongly underestimate IOV. Deriving sea level trends from finite-length time series in eddy-active regions yields uncertainties induced by this multidecadal IOV, which are of the same order of magnitude as those due to ICV. A white noise model is proposed to approximate the low-frequency tail of the IOV spectra and could be used to update ICV estimates from current climate simulations and projections.
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Akuetevi, C. Q. C., & Wirth, A. (2015). Dynamics of turbulent western-boundary currents at low latitude in a shallow-water model. Ocean Science, 11(3), 471–481.
Abstract: The dynamics of low latitude turbulent western-boundary currents (WBCs) crossing the Equator are considered using numerical results from integrations of a reduced-gravity shallow-water model. For viscosity values of 1000 m(2)s(-1) and greater, the boundary layer dynamics compares well to the analytical Munk-layer solution. When the viscosity is reduced, the boundary layer becomes turbulent and coherent structures in the form of anticyclonic eddies, bursts (violent detachments of the viscous sub-layer, VSL) and dipoles appear. Three distinct boundary layers emerge, the VSL, the advective boundary layer and the extended boundary layer. The first is characterized by a dominant vorticity balance between the viscous transport and the advective transport of vorticity; the second by a balance between the advection of planetary vorticity and the advective transport of relative vorticity. The extended boundary layer is the area to which turbulent motion from the boundary extends. The scaling of the three boundary layer thicknesses with viscosity is evaluated. Characteristic scales of the dynamics and dissipation are determined. A pragmatic approach to determine the eddy viscosity diagnostically for coarse-resolution numerical models is proposed.
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Balmaseda, M. A., Hernandez, F., Storto, A., Palmer, M. D., Alves, O., Shi, L., et al. (2015). The Ocean Reanalyses Intercomparison Project (ORA-IP). Journal Of Operational Oceanography, 8, S80–S97.
Abstract: Uncertainty in ocean analysis methods and deficiencies in the observing system are major obstacles for the reliable reconstruction of the past ocean climate. The variety of existing ocean reanalyses is exploited in a multi-reanalysis ensemble to improve the ocean state estimation and to gauge uncertainty levels. The ensemble-based analysis of signal-to-noise ratio allows the identification of ocean characteristics for which the estimation is robust (such as tropical mixedlayer-depth, upper ocean heat content), and where large uncertainty exists (deep ocean, Southern Ocean, sea ice thickness, salinity), providing guidance for future enhancement of the observing and data assimilation systems.
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Brankart, J. M., Candille, G., Garnier, F., Calone, C., Melet, A., Bouttier, P. A., et al. (2015). A generic approach to explicit simulation of uncertainty in the NEMO ocean model. Geoscientific Model Development, 8(5), 1285–1297.
Abstract: In this paper, a generic implementation approach is presented, with the aim of transforming a deterministic ocean model (like NEMO) into a probabilistic model. With this approach, several kinds of stochastic parameterizations are implemented to simulate the non-deterministic effect of unresolved processes, unresolved scales and unresolved diversity. The method is illustrated with three applications, showing that uncertainties can produce a major effect in the circulation model, in the ecosystem model, and in the sea ice model. These examples show that uncertainties can produce an important effect in the simulations, strongly modifying the dynamical behaviour of these three components of ocean systems.
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Candille, G., Brankart, J. M., & Brasseur, P. (2015). Assessment of an ensemble system that assimilates Jason-1/Envisat altimeter data in a probabilistic model of the North Atlantic ocean circulation. Ocean Science, 11(3), 425–438.
Abstract: A realistic circulation model of the North Atlantic ocean at 0.25 degrees resolution (NATL025 NEMO configuration) has been adapted to explicitly simulate model uncertainties. This is achieved by introducing stochastic perturbations in the equation of state to represent the effect of unresolved scales on the model dynamics. The main motivation for this work is to develop ensemble data assimilation methods, assimilating altimetric data from past missions Jason-1 and Envisat. The assimilation experiment is designed to provide a description of the uncertainty associated with the Gulf Stream circulation for years 2005/2006, focusing on frontal regions which are predominantly affected by unresolved dynamical scales. An ensemble based on such stochastic perturbations is first produced and evaluated using along-track altimetry observations. Then each ensemble member is updated by a square root algorithm based on the SEEK (singular evolutive extended Kalman) filter (Brasseur and Verron, 2006). These three elements – stochastic parameterization, ensemble simulation and 4-D observation operator – are then used together to perform a 4-D analysis of along-track altimetry over 10-day windows. Finally, the results of this experiment are objectively evaluated using the standard probabilistic approach developed for meteorological applications (Toth et al., 2003; Candille et al., 2007). The results show that the free ensemble – before starting the assimilation process – correctly reproduces the statistical variability over the Gulf Stream area: the system is then pretty reliable but not informative (null probabilistic resolution). Updating the free ensemble with altimetric data leads to a better reliability with an information gain of around 30% (for 10-day forecasts of the SSH variable). Diagnoses on fully independent data (i.e. data that are not assimilated, like temperature and salinity profiles) provide more contrasted results when the free and updated ensembles are compared.
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Gehlen, M., Barciela, R., Bertino, L., Brasseur, P., Butenschon, M., Chai, F., et al. (2015). Building the capacity for forecasting marine biogeochemistry and ecosystems: recent advances and future developments. Journal Of Operational Oceanography, 8, S168–S187.
Abstract: Building the capacity for monitoring and forecasting marine biogeochemistry and ecosystem dynamics is a scientific challenge of strategic importance in the context of rapid environmental change and growing public awareness of its potential impacts on marine ecosystems and resources. National Operational Oceanography centres have started to take up this challenge by integrating biogeochemistry in operational systems. Ongoing activities are illustrated in this paper by presenting examples of (pre-) operational biogeochemical systems active in Europe and North America for global to regional applications. First-order principles underlying biogeochemical modelling are briefly introduced along with the description of biogeochemical components implemented in these systems. Applications are illustrated with examples from the fields of hindcasting and monitoring ocean primary production, the assessment of the ocean carbon cycle and the management of living resources. Despite significant progress over the past 5 years in integrating biogeochemistry into (pre-) operational data-assimilation systems, a sustained research effort is still needed to assess these systems and their products with respect to their usefulness to the management of marine systems.
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Gregoire, M., Levy, M., Marra, J., Borges, A. V., & Brasseur, P. (2015). The variability of primary production in the ocean: From the synoptic to the global scale. The 45th International Liege Colloquium on Ocean Dynamics, Liege, Belgium, May 13-17, 2013 Preface. Journal Of Marine Systems, 147, 1–2.
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Gregorio, S., Penduff, T., Serazin, G., Molines, J. M., Barnier, B., & Hirschi, J. (2015). Intrinsic Variability of the Atlantic Meridional Overturning Circulation at Interannual-to-Multidecadal Time Scales. Journal Of Physical Oceanography, 45(7), 1929–1946.
Abstract: The low-frequency variability of the Atlantic meridional overturning circulation (AMOC) is investigated from 2, 1/4 degrees, and 1/12 degrees global ocean-sea ice imulations, with a specific focus on its internally generated (i.e., intrinsic) component. A 327-yr climatological 1/4 degrees simulation, driven by a repeated seasonal cycle (i.e., a forcing devoid of interannual time scales), is shown to spontaneously generate a significant fraction R of the interannual-to-decadal AMOC variance obtained in a 50-yr fully forced hindcast (with reanalyzed atmospheric forcing including interannual time scales). This intrinsic variance fraction R slightly depends on whether AMOCs are computed in geopotential or density coordinates, and on the period considered in the climatological simulation, but the following features are quite robust when mesoscale eddies are simulated (at both 1/4 degrees and 1/12 degrees resolutions); R barely exceeds 5%-10% in the subpolar gyre but reaches 30%-50% at 34 degrees S, up to 20%-40% near 25 degrees N, and 40%-60% near the Gulf Stream. About 25% of the meridional heat transport interannual variability is attributed to intrinsic processes at 34 degrees S and near the Gulf Stream. Fourier and wavelet spectra, built from the 327-yr 1/4 degrees climatological simulation, further indicate that spectral peaks of intrinsic AMOC variability (i) are found at specific frequencies ranging from interannual to multidecadal, (ii) often extend over the whole meridional scale of gyres, (iii) stochastically change throughout these 327 yr, and (iv) sometimes match the spectral peaks found in the fully forced hindcast in the North Atlantic. Intrinsic AMOC variability is also detected at multidecadal time scales, with a marked meridional coherence between 35 degrees S and 25 degrees N (15-30 yr periods) and throughout the whole basin (50-90-yr periods).
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Heuze, C., Vivier, F., Le Sommer, J., Molines, J. M., & Penduff, T. (2015). Can we map the interannual variability of the whole upper Southern Ocean with the current database of hydrographic observations? Journal Of Geophysical Research-Oceans, 120(12), 7960–7978.
Abstract: With the advent of Argo floats, it now seems feasible to study the interannual variations of upper ocean hydrographic properties of the historically undersampled Southern Ocean. To do so, scattered hydrographic profiles often first need to be mapped. To investigate biases and errors associated both with the limited space-time distribution of the profiles and with the mapping methods, we colocate the mixed-layer depth (MLD) output from a state-of-the-art 1/12 degrees DRAKKAR simulation onto the latitude, longitude, and date of actual in situ profiles from 2005 to 2014. We compare the results obtained after remapping using a nearest neighbor (NN) interpolation and an objective analysis (OA) with different spatiotemporal grid resolutions and decorrelation scales. NN is improved with a coarser resolution. OA performs best with low decorrelation scales, avoiding too strong a smoothing, but returns values over larger areas with large decorrelation scales and low temporal resolution, as more points are available. For all resolutions OA represents better the annual extreme values than NN. Both methods underestimate the seasonal cycle in MLD. MLD biases are lower than 10 m on average but can exceed 250 m locally in winter. We argue that current Argo data should not be mapped to infer decadal trends in MLD, as all methods are unable to reproduce existing trends without creating unrealistic extra ones. We also show that regions of the subtropical Atlantic, Indian, and Pacific Oceans, and the whole ice-covered Southern Ocean, still cannot be mapped even by the best method because of the lack of observational data.
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Lemarie, F., Debreu, L., Madec, G., Demange, J., Molines, J. M., & Honnorat, M. (2015). Stability constraints for oceanic numerical models: implications for the formulation of time and space discretizations. Ocean Modelling, 92, 124–148.
Abstract: Except for vertical diffusion (and possibly the external mode and bottom drag), oceanic models usually rely on explicit time-stepping algorithms subject to Courant-Friedrichs-Lewy (CFL) stability criteria. Implicit methods could be unconditionally stable, but an algebraic system must be solved at each time step and other considerations such as accuracy and efficiency are less straightforward to achieve. Depending on the target application, the process limiting the maximum allowed time-step is generally different. In this paper, we introduce offline diagnostics to predict stability limits associated with internal gravity waves, advection, diffusion, and rotation. This suite of diagnostics is applied to a set of global, regional and coastal numerical simulations with several horizontal/vertical resolutions and different numerical models. We show that, for resolutions finer that 1/2 degrees, models with an Eulerian vertical coordinate are generally constrained by vertical advection in a few hot spots and that numerics must be extremely robust to changes in Courant number. Based on those results, we review the stability and accuracy of existing numerical kernels in vogue in primitive equations oceanic models with a focus on advective processes and the dynamics of internal waves. We emphasize the additional value of studying the numerical kernel of oceanic models in the light of coupled space-tune approaches instead of studying the time schemes independently from spatial discretizations. From this study, we suggest some guidelines for the development of temporal schemes in future generation multi-purpose oceanic models. (C) 2015 Elsevier Ltd. All rights reserved.
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Marsh, R., Ivchenko, V. O., Skliris, N., Alderson, S., Bigg, G. R., Madec, G., et al. (2015). NEMO-ICB (v1.0): interactive icebergs in the NEMO ocean model globally configured at eddy-permitting resolution. Geoscientific Model Development, 8(5), 1547–1562.
Abstract: An established iceberg module, ICB, is used interactively with the Nucleus for European Modelling of the Ocean (NEMO) ocean model in a new implementation, NEMO-ICB (v1.0). A 30-year hindcast (1976-2005) simulation with an eddy-permitting (0.25 degrees) global configuration of NEMO-ICB is undertaken to evaluate the influence of icebergs on sea ice, hydrography, mixed layer depths (MLDs), and ocean currents, through comparison with a control simulation in which the equivalent iceberg mass flux is applied as coastal runoff, a common forcing in ocean models. In the Southern Hemisphere (SH), drift and melting of icebergs are in balance after around 5 years, whereas the equilibration timescale for the Northern Hemisphere (NH) is 15-20 years. Iceberg drift patterns, and Southern Ocean iceberg mass, compare favourably with available observations. Freshwater forcing due to iceberg melting is most pronounced very locally, in the coastal zone around much of Antarctica, where it often exceeds in magnitude and opposes the negative freshwater fluxes associated with sea ice freezing. However, at most locations in the polar Southern Ocean, the annual-mean freshwater flux due to icebergs, if present, is typically an order of magnitude smaller than the contribution of sea ice melting and precipitation. A notable exception is the southwest Atlantic sector of the Southern Ocean, where iceberg melting reaches around 50% of net precipitation over a large area. Including icebergs in place of coastal runoff, sea ice concentration and thickness are notably decreased at most locations around Antarctica, by up to similar to 20% in the eastern Weddell Sea, with more limited increases, of up to similar to 10% in the Bellingshausen Sea. Antarctic sea ice mass decreases by 2.9 %, overall. As a consequence of changes in net freshwater forcing and sea ice, salinity and temperature distributions are also substantially altered. Surface salinity increases by similar to 0.1 psu around much of Antarctica, due to suppressed coastal runoff, with extensive freshening at depth, extending to the greatest depths in the polar Southern Ocean where discernible effects on both salinity and temperature reach 2500 m in the Weddell Sea by the last pentad of the simulation. Substantial physical and dynamical responses to icebergs, throughout the global ocean, are explained by rapid propagation of density anomalies from high-to-low latitudes. Complementary to the baseline model used here, three prototype modifications to NEMO-ICB are also introduced and discussed.
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Martin, M. J., Balmaseda, M., Bertino, L., Brasseur, P., Brassington, G., Cummings, J., et al. (2015). Status and future of data assimilation in operational oceanography. Journal Of Operational Oceanography, 8, S28–S48.
Abstract: The GODAE OceanView systems use various data assimilation algorithms, including 3DVar, EnOI, EnKF and the SEEK filter with a fixed basis, using different time windows. The main outputs of the operational data assimilation systems, the increments, have been compared for February 2014 in various regions. The eddy-permitting systems' increments are similar in a number of the regions, indicating similar forecast errors are being corrected, while the eddy-resolving systems represent smaller-scale structures in the mid-latitude regions investigated and appear to have smaller biases. Monthly average temperature increments show significant SST biases, particularly in the systems which assimilate swath satellite SST data, indicating systematic errors in the surface heat fluxes and the way in which they are propagated vertically by the ocean models. On-going developments to the data assimilation systems include improvements to the specification of error covariances, improving assimilation of data near the equator, and understanding the effect of assimilation on the Atlantic Meridional Overturning Circulation. Longer term developments are expected to include the implementation of more advanced algorithms to make use of flow-dependent error covariance information. Assimilation of new data sources over the coming years, such as wide-swath altimetry, is also expected to improve the accuracy of ocean state estimates and forecasts provided by the GODAE OceanView systems.
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Reffray, G., Bourdalle-Badie, R., & Calone, C. (2015). Modelling turbulent vertical mixing sensitivity using a 1-D version of NEMO. Geoscientific Model Development, 8(1), 69–86.
Abstract: Through two numerical experiments, a 1-D vertical model called NEMO1D was used to investigate physical and numerical turbulent-mixing behaviour. The results show that all the turbulent closures tested (k + l from Blanke and Delecluse, 1993, and two equation models: generic length scale closures from Umlauf and Burchard, 2003) are able to correctly reproduce the classical test of Kato and Phillips (1969) under favourable numerical conditions while some solutions may diverge depending on the degradation of the spatial and time discretization. The performances of turbulence models were then compared with data measured over a 1-year period (mid-2010 to mid-2011) at the PAPA station, located in the North Pacific Ocean. The modelled temperature and salinity were in good agreement with the observations, with a maximum temperature error between 2 and 2 degrees C during the stratified period (June to October). However, the results also depend on the numerical conditions. The vertical RMSE varied, for different turbulent closures, from 0.1 to 0.3 degrees C during the stratified period and from 0.03 to 0.15 degrees C during the homogeneous period. This 1-D configuration at the PAPA station (called PAPA1D) is now available in NEMO as a reference configuration including the input files and atmospheric forcing set described in this paper. Thus, all the results described can be recovered by downloading and launching PAPA1D. The configuration is described on the NEMO site (http://www.nemo-ocean.eu/Using-NEMO/Configurations/C1D_PAPA). This package is a good starting point for further investigation of vertical processes.
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Ruggiero, G. A., Ourmieres, Y., Cosme, E., Blum, J., Auroux, D., & Verron, J. (2015). Data assimilation experiments using diffusive back-and-forth nudging for the NEMO ocean model. Nonlinear Processes In Geophysics, 22(2), 233–248.
Abstract: The diffusive back-and-forth nudging (DBFN) is an easy-to-implement iterative data assimilation method based on the well-known nudging method. It consists of a sequence of forward and backward model integrations, within a given time window, both of them using a feedback term to the observations. Therefore, in the DBFN, the nudging asymptotic behaviour is translated into an infinite number of iterations within a bounded time domain. In this method, the backward integration is carried out thanks to what is called backward model, which is basically the forward model with reversed time step sign. To maintain numeral stability, the diffusion terms also have their sign reversed, giving a diffusive character to the algorithm. In this article the DBFN performance to control a primitive equation ocean model is investigated. In this kind of model non-resolved scales are modelled by diffusion operators which dissipate energy that cascade from large to small scales. Thus, in this article, the DBFN approximations and their consequences for the data assimilation system set-up are analysed. Our main result is that the DBFN may provide results which are comparable to those produced by a 4Dvar implementation with a much simpler implementation and a shorter CPU time for convergence. The conducted sensitivity tests show that the 4Dvar profits of long assimilation windows to propagate surface information downwards, and that for the DBFN, it is worth using short assimilation windows to reduce the impact of diffusion-induced errors. Moreover, the DBFN is less sensitive to the first guess than the 4Dvar.
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Schiller, A., Bell, M., Brassington, G., Brasseur, P., Barciela, R., De Mey, P., et al. (2015). Synthesis of new scientific challenges for GODAE OceanView. Journal Of Operational Oceanography, 8, S259–S271.
Abstract: The marine environment plays an increasingly important role in shaping economies and infrastructures, and touches upon many aspects of our lives, including food supplies, energy resources, national security and recreational activities. Global Ocean Data Assimilation Experiment (GODAE) and GODAE OceanView have provided platforms for international collaboration that significantly contribute to the scientific development and increasing uptake of ocean forecasting products by end users who address societal issues such as those listed above. Many scientific challenges and opportunities remain to be tackled in the ever-changing field of operational oceanography, from the observing system to modelling, data assimilation and product dissemination. This paper provides a brief overview of past achievements in GODAE OceanView, but subsequently concentrates on the future scientific foci of GODAE OceanView and its Task Teams, and provides a vision for the future of ocean forecasting.
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Serazin, G., Penduff, T., Gregorio, S., Barnier, B., Molines, J. M., & Terray, L. (2015). Intrinsic Variability of Sea Level from Global 1/12 degrees Ocean Simulations: Spatiotemporal Scales. Journal Of Climate, 28(10), 4279–4292.
Abstract: In high-resolution ocean general circulation models (OGCMs), as in process-oriented models, a substantial amount of interannual to decadal variability is generated spontaneously by oceanic nonlinearities: that is, without any variability in the atmospheric forcing at these time scales. The authors investigate the temporal and spatial scales at which this intrinsic oceanic variability has the strongest imprints on sea level anomalies (SLAs) using a 1/12 degrees global OGCM, by comparing a "hindcast'' driven by the full range of atmospheric time scales with its counterpart forced by a repeated climatological atmospheric seasonal cycle. Outputs from both simulations are compared within distinct frequency-wavenumber bins. The fully forced hindcast is shown to reproduce the observed distribution and magnitude of low-frequency SLA variability very accurately. The small-scale (L<6 degrees) SLA variance is, at all time scales, barely sensitive to atmospheric variability and is almost entirely of intrinsic origin. The high-frequency (mesoscale) part and the low-frequency part of this small-scale variability have almost identical geographical distributions, supporting the hypothesis of a nonlinear temporal inverse cascade spontaneously transferring kinetic energy from high to low frequencies. The large-scale (L<12 degrees) low-frequency variability is mostly related to the atmospheric variability over most of the global ocean, but it is shown to remain largely intrinsic in three eddy-active regions: the Gulf Stream, Kuroshio, and Antarctic Circumpolar Current (ACC). Compared to its 1/4 degrees predecessor, the authors' 1/12 degrees OGCM is shown to yield a stronger intrinsic SLA variability, at both mesoscale and low frequencies.
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Stewart, K. D., Spence, P., Waterman, S., Sommer, J., Molines, J. M., Lilly, J. M., et al. (2015). Anisotropy of eddy variability in the global ocean. Ocean Modelling, 95, 53–65.
Abstract: The anisotropy of eddy variability in the global ocean is examined in geostrophic surface velocities derived from satellite observations and in the horizontal velocities of a 1/12 degrees global ocean model. Eddy anisotropy is of oceanographic interest as it is through anisotropic velocity fluctuations that the eddy and mean-flow fields interact dynamically. This study is timely because improved observational estimates of eddy anisotropy will soon be available with Surface Water and Ocean Topography (SWOT) altimetry data. We find there to be good agreement between the characteristics and distributions of eddy anisotropy from the present satellite observations and model ocean surface. In the model, eddy anisotropy is found to have significant vertical structure and is largest close to the ocean bottom, where the anisotropy aligns with the underlying isobaths. The highly anisotropic bottom signal is almost entirely contained in the barotropic variability. Upper-ocean variability is predominantly baroclinic and the alignment is less sensitive to the underlying bathymetry. These findings offer guidance for introducing a parameterization of eddy feedbacks, based on the eddy kinetic energy and underlying bathymetry, to operate on the barotropic flow and better account for the effects of barotropic Reynolds stresses unresolved in coarse-resolution ocean models. (C) 2015 Elsevier Ltd. All rights reserved.
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Verron, J., Sengenes, P., Lambin, J., Noubel, J., Steunou, N., Guillot, A., et al. (2015). The SARAL/AltiKa Altimetry Satellite Mission. Marine Geodesy, 38, 2–21.
Abstract: The India-France SARAL/AltiKa mission is the first Ka-band altimetric mission dedi-cated to oceanography. The mission objectives are primarily the observation of the oceanic mesoscales but also include coastal oceanography, global and regional sea level monitoring, data assimilation, and operational oceanography. Secondary objectives include ice sheet and inland waters monitoring. One year after launch, the results widely confirm the nominal expectations in terms of accuracy, data quality and data availability in general.Today's performances are compliant with specifications with an overall observed performance for the Sea Surface Height RMS of 3.4cm to be compared to a 4cm requirement. Some scientific examples are provided that illustrate some salient features of today's SARAL/AltiKa data with regard to standard altimetry: data availability, data accuracy at the mesoscales, data usefulness in costal area, over ice sheet, and for inland waters.
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Vidard, A., Bouttier, P. A., & Vigilant, F. (2015). NEMOTAM: tangent and adjoint models for the ocean modelling platform NEMO. Geoscientific Model Development, 8(4), 1245–1257.
Abstract: Tangent linear and adjoint models (TAMs) are efficient tools to analyse and to control dynamical systems such as NEMO. They can be involved in a large range of applications such as sensitivity analysis, parameter estimation or the computation of characteristic vectors. A TAM is also required by the 4D-Var algorithm, which is one of the major methods in data assimilation. This paper describes the development and the validation of the tangent linear and adjoint model for the NEMO ocean modelling platform (NEMOTAM). The diagnostic tools that are available alongside NEMOTAM are detailed and discussed, and several applications are also presented.
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Vivier, F., Park, Y. H., Sekma, H., & Le Sommer, J. (2015). Variability of the Antarctic Circumpolar Current transport through the Fawn Trough, Kerguelen Plateau. Deep-Sea Research Part Ii-Topical Studies In Oceanography, 114, 12–26.
Abstract: The Kerguelen Plateau is a major topographic obstacle to the eastward flowing Antarctic Circumpolar Current (ACC). While approximately two-third of the ACC transport is diverted to the North, most of the remaining flow engulfs in the Fawn Trough, the only deep passage across the plateau. As part of the TRACK (TRansport ACross the Kerguelen plateau) project, three mooring lines of current meters were deployed in the Fawn Trough for one year in February 2009, underneath ground-track 94 of the Jason-2 satellite altimeter. Full depth CTD-LADCP casts carried out during the deployment cruise were previously analyzed to provide a comprehensive description of the regional circulation, featuring in particular a volume transport of similar to 43 Sv across the Fawn Trough (Park et al., 2009). Here we present a time series of the transport in the Fawn Trough estimated from current meter observations, featuring a mean eastward transport of 34 Sv (possibly biased low by at most 5 Sv) and a root mean squared variability of 6 Sv, consistent with LADCP estimates (43 Sv in February 2009 and 38 Sv in January 2010). In addition, we analyze to what extent the transport can be directly monitored from along-track satellite altimeter data, which would enable study of the variability of the Fawn Trough Current from a now 20-year long archive. The ability to reconstruct the flow from a limited set of moored instruments as well as from altimeter-derived surface geostrophic velocity is further assessed from synthetic data extracted from a high-resolution pen-Antarctic simulation. While a canonical method to derive transport from altimetry, previously applied to the Malvinas Current, gives here unsatisfactory comparisons with in situ estimates, an ad hoc approach using only the two northernmost mooring lines yields an estimate well correlated (similar to 0.8) with in situ transport at subseasonal time scales during the one year period of observations. At interannual time scales, however, both methods provide significantly correlated (0.7) transport estimates, suggesting that long-term transport fluctuations across the Kerguelen Plateau can be confidently estimated from altimetry. These consistently indicate a measurable impact of the outstanding 1997-1998 El Nifio Southern Oscillation (ENSO) event, yielding an increase of the annual mean transport of similar to 3 Sv, possibly with a one year lag. The transport estimate based on the ad hoc approach is significantly correlated (0.6) with the Southern Annular Mode (SAM) index at interannual time scales, suggesting that an intensification of the circumpolar winds drives an increase in the transport across the Kerguelen Plateau. (C) 2014 Elsevier Ltd. All rights reserved.
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Yan, Y., Barth, A., Beckers, J. M., Candille, G., Brankart, J. M., & Brasseur, P. (2015). Ensemble assimilation of ARGO temperature profile, sea surface temperature, and altimetric satellite data into an eddy permitting primitive equation model of the North Atlantic Ocean. Journal Of Geophysical Research-Oceans, 120(7), 5134–5157.
Abstract: Sea surface height, sea surface temperature, and temperature profiles at depth collected between January and December 2005 are assimilated into a realistic eddy permitting primitive equation model of the North Atlantic Ocean using the Ensemble Kalman Filter. Sixty ensemble members are generated by adding realistic noise to the forcing parameters related to the temperature. The ensemble is diagnosed and validated by comparison between the ensemble spread and the model/ observation difference, as well as by rank histogram before the assimilation experiments. An incremental analysis update scheme is applied in order to reduce spurious oscillations due to the model state correction. The results of the assimilation are assessed according to both deterministic and probabilistic metrics with independent/semiindependent observations. For deterministic validation, the ensemble means, together with the ensemble spreads are compared to the observations, in order to diagnose the ensemble distribution properties in a deterministic way. For probabilistic validation, the continuous ranked probability score (CRPS) is used to evaluate the ensemble forecast system according to reliability and resolution. The reliability is further decomposed into bias and dispersion by the reduced centered random variable (RCRV) score in order to investigate the reliability properties of the ensemble forecast system. The improvement of the assimilation is demonstrated using these validation metrics. Finally, the deterministic validation and the probabilistic validation are analyzed jointly. The consistency and complementarity between both validations are highlighted.
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Arbic, B. K., Muller, M., Richman, J. G., Shriver, J. F., Morten, A. J., Scott, R. B., et al. (2014). Geostrophic Turbulence in the Frequency-Wavenumber Domain: Eddy-Driven Low-Frequency Variability. Journal Of Physical Oceanography, 44(8), 2050–2069.
Abstract: Motivated by the potential of oceanic mesoscale eddies to drive intrinsic low-frequency variability, this paper examines geostrophic turbulence in the frequency-wavenumber domain. Frequency-wavenumber spectra, spectral fluxes, and spectral transfers are computed from an idealized two-layer quasigeostrophic (QG) turbulence model, a realistic high-resolution global ocean general circulation model, and gridded satellite altimeter products. In the idealized QG model, energy in low wavenumbers, arising from nonlinear interactions via the well-known inverse cascade, is associated with energy in low frequencies and vice versa, although not in a simple way. The range of frequencies that are highly energized and engaged in nonlinear transfer is much greater than the range of highly energized and engaged wavenumbers. Low-frequency, low-wavenumber energy is maintained primarily by nonlinearities in the QG model, with forcing and friction playing important but secondary roles. In the high-resolution ocean model, nonlinearities also generally drive kinetic energy to low frequencies as well as to low wavenumbers. Implications for the maintenance of low-frequency oceanic variability are discussed. The cascade of surface kinetic energy to low frequencies that predominates in idealized and realistic models is seen in some regions of the gridded altimeter product, but not in others. Exercises conducted with the general circulation model suggest that the spatial and temporal filtering inherent in the construction of gridded satellite altimeter maps may contribute to the discrepancies between the direction of the frequency cascade in models versus gridded altimeter maps seen in some regions. Of course, another potential reason for the discrepancy is missing physics in the models utilized here.
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Djath, B., Melet, A., Verron, J., Molines, J. M., Barnier, B., Gourdeau, L., et al. (2014). A 1/36 degrees model of the Solomon Sea embedded into a global ocean model: On the setting up of an interactive open boundary nested model system. Journal Of Operational Oceanography, 7(1), 34–46.
Abstract: The implementation of a regional 1/36 degrees numerical model of a key sub region of the southwestern Pacific Ocean: the Solomon Sea is discussed.This model is two-way embedded into a 1/12 degrees resolution basin-scale model, itself one-way nested in a global 1/12 degrees resolution ocean model.The three main questions discussed in this study concern (i) the bathymetry, (ii) the setting up of adequate forcing functions, especially regarding the wind stress parameterization, and (iii) the strategy used to embed and conned the model configurations together Such a system, exemplified here for the Solomon Sea, represents a prototype of embedded model systems that are considered in operational oceanography.
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Djath, B., Verron, J., Melet, A., Gourdeau, L., Barnier, B., & Molines, J. M. (2014). Multiscale dynamical analysis of a high-resolution numerical model simulation of the Solomon Sea circulation. Journal Of Geophysical Research-Oceans, 119(9), 6286–6304.
Abstract: A high 1/36 degrees resolution numerical model is used to study the ocean circulation in the Solomon Sea. An evaluation of the model with (the few) available observation shows that the 1/36 degrees resolution model realistically simulates the Solomon Sea circulations. The model notably reproduces the high levels of mesoscale eddy activity observed in the Solomon Sea. With regard to previous simulations at 1/12 degrees resolution, the average eddy kinetic energy levels are increased by up to approximate to 30-40% in the present 1/36 degrees simulation, and the enhancement extends at depth. At the surface, the eddy kinetic energy level is maximum in March-April-May and is minimum in December-January-February. The high subsurface variability is related to the variability of the western boundary current (New Guinea Coastal Undercurrent). Moreover, the emergence of submesoscales is clearly apparent in the present simulations. A spectral analysis is conducted in order to evidence and characterize the modeled submesoscale dynamics and to provide a spectral view of scales interactions. The corresponding spectral slopes show a strong consistency with the Surface Quasi-Geostrophic turbulence theory.
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Ganachaud, A., Cravatte, S., Melet, A., Schiller, A., Holbrook, N. J., Sloyan, B. M., et al. (2014). The Southwest Pacific Ocean circulation and climate experiment (SPICE). Journal Of Geophysical Research-Oceans, 119(11), 7660–7686.
Abstract: The Southwest Pacific Ocean Circulation and Climate Experiment (SPICE) is an international research program under the auspices of CLIVAR. The key objectives are to understand the Southwest Pacific Ocean circulation and the South Pacific Convergence Zone (SPCZ) dynamics, as well as their influence on regional and basin-scale climate patterns. South Pacific thermocline waters are transported in the westward flowing South Equatorial Current (SEC) toward Australia and Papua-New Guinea. On its way, the SEC encounters the numerous islands and straits of the Southwest Pacific and forms boundary currents and jets that eventually redistribute water to the equator and high latitudes. The transit in the Coral, Solomon, and Tasman Seas is of great importance to the climate system because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Nino-Southern Oscillation, while the southward transports influence the climate and biodiversity in the Tasman Sea. After 7 years of substantial in situ oceanic observational and modeling efforts, our understanding of the region has much improved. We have a refined description of the SPCZ behavior, boundary currents, pathways, and water mass transformation, including the previously undocumented Solomon Sea. The transports are large and vary substantially in a counter-intuitive way, with asymmetries and gating effects that depend on time scales. This paper provides a review of recent advancements and discusses our current knowledge gaps and important emerging research directions. Key Points <list id=“jgrc20950-list-0001” list-type=“bulleted”> <list-item id=“jgrc20950-li-0001”>Southwest Pacific WBCs transport large volumes toward the equator and the pole <list-item id=“jgrc20950-li-0002”>Pathways are complex; water properties tend to erode during the transit <list-item id=“jgrc20950-li-0003”>Variations due to seasons, ENSO and the SPCZ modulate the relative WBC strengths
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Gaultier, L., Djath, B., Verron, J., Brankart, J. M., Brasseur, P., & Melet, A. (2014). Inversion of submesoscale patterns from a high-resolution Solomon Sea model: Feasibility assessment. Journal Of Geophysical Research-Oceans, 119(7), 4520–4541.
Abstract: A high-resolution realistic numerical model of the Solomon Sea, which exhibits a high level of variability at mesoscales and submesoscales, is used to explore new avenues for data assimilation. Image data assimilation represents a powerful methodology to integrate information from high-resolution observations such as satellite sea surface temperature or chlorophyll, or high-resolution altimetric sea surface height that will be observed in the forthcoming SWOT mission. The present study investigates the feasibility and accuracy of the inversion of the dynamical submesoscale information contained in high-resolution images of sea surface temperature (SST) or salinity (SSS) to improve the estimation of oceanic surface currents. The inversion method is tested in the context of twin experiments, with SST and SSS data provided by a model of the Solomon Sea. For that purpose, synthetic tracer images are built by binarizing the norm of the gradient of SST, SSS or spiciness. The binarized tracer images are compared to the dynamical image which is derived from the Finite-Size Lyapunov Exponents. The adjustment of the dynamical image to the tracer image provides the optimal correction to be applied on the surface velocity field. The method is evaluated by comparing the result of the inversion to the reference model solution. The feasibility of the inversion of various images (SST, SSS, both SST and SSS or spiciness) is explored on two small areas of the Solomon Sea. We show that errors in the surface velocity field can be substantially reduced through the inversion of tracer images.
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Gourdeau, L., Verron, J., Melet, A., Kessler, W., Marin, F., & Djath, B. (2014). Exploring the mesoscale activity in the Solomon Sea: A complementary approach with a numerical model and altimetric data. Journal Of Geophysical Research-Oceans, 119(4), 2290–2311.
Abstract: The Solomon Sea is an area of high level of eddy kinetic energy (EKE), and represents a transit area for the low-latitude western boundary currents (LLWBCs) connecting the subtropics to the equatorial Pacific and playing a major role in ENSO dynamics. This study aims at documenting the surface mesoscale activity in the Solomon Sea for the first time. Our analysis is based on the joint analysis of altimetric data and outputs from a 1/12 degrees model simulation. The highest surface EKE is observed in the northern part of the basin and extends southward to the central basin. An eddy tracking algorithm is used to document the characteristics and trajectories of coherent mesoscale vortices. Cyclonic eddies, generated in the south basin, are advected to the north by the LLWBCs before merging with stationary mesoscale structures present in the mean circulation. Anticyclonic eddies are less numerous. They are generated in the southeastern basin, propagate westward, reach the LLWBCs, and dissipate. The seasonal and interannual modulations of the mesoscale activity are well marked. At seasonal time scale, maximum (minimum) activity is in May-June (September). At interannual time scale, the mesoscale activity is particularly enhanced during La Nina conditions. If instabilities of the regional circulations seem to explain the generation of mesoscale features, the modulation of the mesoscale activity seems to be rather related with the intrusion at Solomon Strait of the surface South Equatorial Current, rather than to the LLWBCs, by modulating the horizontal and vertical shears suitable for instabilities. Key Points <list id=“jgrc20632-list-0001” list-type=“bulleted”> <list-item id=“jgrc20632-li-0001”>A first analysis of the surface mesoscale activity in the Solomon Sea <list-item id=“jgrc20632-li-0002”>Mesoscale is related with the SEC inflow at Solomon Strait rather than to LLWBC <list-item id=“jgrc20632-li-0003”>Maximum activity is in May-June, it is enhanced during La Nina conditions
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Josey, S. A., Yu, L., Gulev, S., Jin, X., Tilinina, N., Barnier, B., et al. (2014). Unexpected impacts of the Tropical Pacific array on reanalysis surface meteorology and heat fluxes. Geophysical Research Letters, 41(17), 6213–6220.
Abstract: The Tropical Pacific mooring array has been a key component of the climate observing system since the early 1990s. We identify a pattern of strong near surface humidity anomalies, colocated with the array, in the widely used European Center for Medium Range Weather Forecasting Interim atmospheric reanalysis. The pattern generates large, previously unrecognized latent and net air-sea heat flux anomalies, up to 50Wm(-2) in the annual mean, in reanalysis derived data sets employed for climate studies (TropFlux) and ocean model forcing (the Drakkar Forcing Set). As a consequence, uncertainty in Tropical Pacific ocean heat uptake between the 1990s and early 2000s at the mooring sites is significant with mooring colocated differences in decadally averaged ocean heat uptake as large as 20Wm(-2). Furthermore, these results have major implications for the dual use of air-sea flux buoys as reference sites and sources of assimilation data that are discussed.
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Jourdain, N. C., Barnier, B., Ferry, N., Vialard, J., Menkes, C. E., Lengaigne, M., et al. (2014). Tropical cyclones in two atmospheric (re)analyses and their response in two oceanic reanalyses. Ocean Modelling, 73, 108–122.
Abstract: In this paper, we first evaluate the ability of the European Centre for Medium Range Forecast operational analysis and the ERA-Interim reanalysis to capture the surface wind signature of tropical cyclones (TCs). In those products, the error on the TC position is typically similar to 150 km, cyclones are too big (similar to 250 km in ERA-Interim and >100 km in the operational analysis against similar to 50 km in observations) and the maximum wind speed is on average underestimated by 15-27 m.s(-1) for strong TCs. These biases are generally reduced with the increase of horizontal resolution in the operational analysis, but remain significant at T1279 (similar to 16 km). We then assess the TCs oceanic temperature signature in two global eddy-permitting ocean reanalyses (GLORYS1 and GLORYS2) forced by the above atmospheric products. The resulting cold wake is on average underestimated by similar to 50% in the two oceanic reanalyses. This bias is largely linked to the underestimated TCs strength in the surface forcing, and the resulting underestimated vertical mixing. The overestimated TC radius also tends to overemphasize the Ekman pumping response to the cyclone. Underestimating vertical mixing without underestimating Ekman pumping results in the absence of the observed subsurface warming away from the TC tracks in the two reanalyses. Data assimilation only marginally contributes to reducing these errors, partly because cyclone signatures are not well resolved by the ocean observing system. Based on these results, we propose some assimilation and forcing strategies in order to improve the restitution of TC signatures in oceanic reanalyses. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.
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Metref, S., Cosme, E., Snyder, C., & Brasseur, P. (2014). A non-Gaussian analysis scheme using rank histograms for ensemble data assimilation. Nonlinear Processes In Geophysics, 21(4), 869–885.
Abstract: One challenge of geophysical data assimilation is to address the issue of non-Gaussianities in the distributions of the physical variables ensuing, in many cases, from nonlinear dynamical models. Non-Gaussian ensemble analysis methods fall into two categories, those remapping the ensemble particles by approximating the best linear unbiased estimate, for example, the ensemble Kalman filter (EnKF), and those resampling the particles by directly applying Bayes' rule, like particle filters. In this article, it is suggested that the most common remapping methods can only handle weakly non-Gaussian distributions, while the others suffer from sampling issues. In between those two categories, a new remapping method directly applying Bayes' rule, the multivariate rank histogram filter (MRHF), is introduced as an extension of the rank histogram filter (RHF) first introduced by Anderson (2010). Its performance is evaluated and compared with several data assimilation methods, on different levels of non-Gaussianity with the Lorenz 63 model. The method's behavior is then illustrated on a simple density estimation problem using ensemble simulations from a coupled physical-biogeochemical model of the North Atlantic ocean. The MRHF performs well with low-dimensional systems in strongly non-Gaussian regimes.
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Remy, F., Flament, T., Michel, A., & Verron, J. (2014). Ice sheet survey over Antarctica using satellite altimetry: ERS-2, Envisat, SARAL/AltiKa, the key importance of continuous observations along the same repeat orbit. International Journal Of Remote Sensing, 35(14), 5497–5512.
Abstract: From September 2002 to October 2010, the Envisat radar altimeter surveyed Greenland and Antarctica ice sheets on a 35 day repeat orbit, providing a unique data set for ice sheet mass balance studies. Up to 85 repeat cycles are available and the whole Envisat data set may be along-track processed in order to provide height variability and trend with a good spatial resolution for the objectives of ice sheet survey. Soon, a joint Centre National d'Etudes Spatiales/Indian Space Research Organisation mission, SARAL (Satellite with Argos and AltiKa), with the AltiKa payload on board, will be launched on exactly the same orbit (less than 1 km of the nomimal orbit in the across-track direction). This will allow an extension of previous European Remote Sensing (ERS) satellite, ERS-1 and ERS-2, and Envisat missions of the European Space Agency (ESA), in particular from the point of view of ice altimetry. However, AltiKa operates in the Ka band (36.8 GHz), a higher frequency than the classical Ku band (13.6 GHz), leading to important modifications and potential improvements in the interactions between radar wave and snow-pack. In this paper, a synthesis is presented of all available information relevant to ice altimetry scientific purposes as derived from the Envisat mission: mean and temporal derivatives of the height -but also of the backscatter and of the two waveform parameters -snow-pack change corrections, across-track surface slope at 1 km scale, etc. The spatial and temporal variability of ice sheet surface elevation is investigated with the help of the high-resolution Envisat along-track observations. We show that at least 50 repeat cycles are needed to reach the required accuracy for the elevation trend. It is thus advocated as potentially highly beneficial for SARAL/AltiKa as a follow-on mission. Moreover, the novel characteristics of SARAL/AltiKa are promising in improving our understanding of snow penetration impact.
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Talandier, C., Deshayes, J., Treguier, A. M., Capet, X., Benshila, R., Debreu, L., et al. (2014). Improvements of simulated Western North Atlantic current system and impacts on the AMOC. Ocean Modelling, 76, 1–19.
Abstract: Previous studies have shown that low horizontal resolution (of the order of 1 degrees) ocean models, hence climate models, are not able to adequately represent boundary currents nor mesoscale processes which affect the dynamics and thermohaline circulation of the ocean. While the effect of mesoscale eddies can be parameterized in low resolution models, boundary currents require relatively high horizontal resolution. We clarify the impact of increasing the resolution on the North Atlantic circulation, with emphasis on the Atlantic Meridional Overturning Circulation (AMOC), by embedding a 1/8 degrees nest covering the North Atlantic into a global 1/2 degrees model. Increasing the resolution in the nest leads to regional improvements of the circulation and thermohaline properties in the Gulf Stream area, for the North Atlantic Current, in the subpolar gyre and the Nordic Seas, consistent with those of previous studies. In addition, we show that the Deep Western Boundary Current dense water transport increases with the nest, from the overflows down to Flemish Cap, due to an increase in the Denmark Strait overflow as well as dense water formation in the subpolar gyre. This increases the Atlantic Meridional Overturning Circulation in density space by about 8 Sv in the subpolar gyre in the nested configuration. When exiting the Labrador Sea around 53 degrees N we illustrate that the Deep Western Boundary Current successively interacts with the upper ocean circulation composed with the North Atlantic Current in the intergyre region, the Northern Recirculation Gyre, and the Gulf Stream near Cape Hatteras. This surface/deep current interaction seems to induce an increase of the AMOC intensity in depth-space, giving rise to an AMOC maximum near 35 degrees N. This process is missing in the configuration without nesting. At 26.5 degrees N, the AMOC is 4 Sv larger in the nested configuration and is in good agreement with observations. Finally, beyond the nest imprint (i.e. in the low resolution area) in the South Atlantic the AMOC maximum at 40 degrees S is 3 Sv larger at the end of the simulation meaning that information is able to propagate outside the nest without being fully damped. This underlines the benefit of using the nest for a reasonable computing time compared to a fully global higher resolution configuration. (C) 2014 Elsevier Ltd. All rights reserved.
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Treguier, A. M., Deshayes, J., Le Sommer, J., Lique, C., Madec, G., Penduff, T., et al. (2014). Meridional transport of salt in the global ocean from an eddy-resolving model. Ocean Science, 10(2), 243–255.
Abstract: The meridional transport of salt is computed in a global eddy-resolving numerical model (1/12 degrees resolution) in order to improve our understanding of the ocean salinity budget. A methodology is proposed that allows a global analysis of the salinity balance in relation to surface water fluxes, without defining a “freshwater anomaly” based on an arbitrary reference salinity. The method consists of a decomposition of the meridional transport into (i) the transport by the time-longitude-depth mean velocity, (ii) time-mean velocity recirculations and (iii) transient eddy perturbations. Water is added (rainfall and rivers) or removed (evaporation) at the ocean surface at different latitudes, which creates convergences and divergences of mass transport with maximum and minimum values close to +/- 1 Sv. The resulting meridional velocity effects a net transport of salt at each latitude (+/- 30 Sv PSU), which is balanced by the time-mean recirculations and by the net effect of eddy salinity-velocity correlations. This balance ensures that the total meridional transport of salt is close to zero, a necessary condition for maintaining a quasi-stationary salinity distribution. Our model confirms that the eddy salt transport cannot be neglected: it is comparable to the transport by the time-mean recirculation (up to 15 Sv PSU) at the poleward and equatorial boundaries of the subtropical gyres. Two different mechanisms are found: eddy contributions are localized in intense currents such as the Kuroshio at the poleward boundary of the subtropical gyres, while they are distributed across the basins at the equatorward boundaries. Closer to the Equator, salinity-velocity correlations are mainly due to the seasonal cycle and large-scale perturbations such as tropical instability waves.
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