Publications

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

Abstract: The Antarctic Ice Sheet Represents The Largest Source Of Uncertainty In Future Sea Level Rise Projections, With A Contribution To Sea Level By 2100 Ranging From – 5 To 43 Cm Of Sea Level Equivalent Under High Carbon Emission Scenarios Estimated By The Recent Ice Sheet Model Intercomparison For Cmip6 (Ismip6). Ismip6 Highlighted The Different Behaviors Of The East And West Antarctic Ice Sheets, As Well As The Possible Role Of Increased Surface Mass Balance In Offsetting The Dynamic Ice Loss In Response To Changing Oceanic Conditions In Ice Shelf Cavities. However, The Detailed Contribution Of Individual Glaciers, As Well As The Partitioning Of Uncertainty Associated With This Ensemble, Have Not Yet Been Investigated. Here, We Analyze The Ismip6 Results For High Carbon Emission Scenarios, Focusing On Key Glaciers Around The Antarctic Ice Sheet, And We Quantify Their Projected Dynamic Mass Loss, Defined Here As Mass Loss Through Increased Ice Discharge Into The Ocean In Response To Changing Oceanic Conditions. We Highlight Glaciers Contributing The Most To Sea Level Rise, As Well As Their Vulnerability To Changes In Oceanic Conditions. We Then Investigate The Different Sources Of Uncertainty And Their Relative Role In Projections, For The Entire Continent And For Key Individual Glaciers. We Show That, In Addition To Thwaites And Pine Island Glaciers In West Antarctica, Totten And Moscow University Glaciers In East Antarctica Present Comparable Future Dynamic Mass Loss And High Sensitivity To Ice Shelf Basal Melt. The Overall Uncertainty In Additional Dynamic Mass Loss In Response To Changing Oceanic Conditions, Compared To A Scenario With Constant Oceanic Conditions, Is Dominated By The Choice Of Ice Sheet Model, Accounting For 52 % Of The Total Uncertainty Of The Antarctic Dynamic Mass Loss In 2100. Its Relative Role For The Most Dynamic Glaciers Varies Between 14 % For Macayeal And Whillans Ice Streams And 56 % For Pine Island Glacier At The End Of The Century. The Uncertainty Associated With The Choice Of Climate Model Increases Over Time And Reaches 13 % Of The Uncertainty By 2100 For The Antarctic Ice Sheet But Varies Between 4 % For Thwaites Glacier And 53 % For Whillans Ice Stream. The Uncertainty Associated With The Ice-Climate Interaction, Which Captures Different Treatments Of Oceanic Forcings Such As The Choice Of Melt Parameterization, Its Calibration, And Simulated Ice Shelf Geometries, Accounts For 22 % Of The Uncertainty At The Ice Sheet Scale But Reaches 36 % And 39 % For Institute Ice Stream And Thwaites Glacier, Respectively, By 2100. Overall, This Study Helps Inform Future Research By Highlighting The Sectors Of The Ice Sheet Most Vulnerable To Oceanic Warming Over The 21St Century And By Quantifying The Main Sources Of Uncertainty.

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.

Abstract: Heat and momentum exchanges at the Southern Ocean surface are crucial for the Earth's Climate, but the importance of the small-scale spatial variability of these surface fluxes is poorly understood. Here, we explore how small-scale heterogeneities of the surface conditions due in particular to ocean eddies affect the atmosphere-sea ice-ocean interactions off Adelie Land, in East Antarctica. To this end, we use a high-resolution regional atmosphere-sea ice-ocean coupled model based on the NEMO-LIM and MAR models. We explore how the atmosphere responds to small-scale heterogeneity of the ocean or sea ice surface conditions, how eddies affect the sea ice and atmosphere, and how the eddy-driven surface fluxes impact the heat, freshwater, and momentum budget of the ocean. The atmosphere is found to be more sensitive to small-scale surface temperature gradients above the ice-covered than above the ice-free ocean. Sea ice concentration is found to be weaker above anticyclonic than cyclonic eddies due to increased sea ice melting or freezing (0.8 cm/day) partly compensated by sea ice convergence or divergence. The imprint of ice-free eddies on the atmosphere is weak, but in the presence of sea ice, air warming (+ 0.3 degrees C) and wind intensification (+ 0.1 m/s) are found above anticyclonic eddies, while cyclonic eddies have the opposite effects. Removing the interactions of eddies with the sea ice or atmosphere does not affect the total sea ice volume, but increases the ocean kinetic energy by 8% and weakens northward advection of sea ice, leading to a 15% decrease in freshwater flux north of 62.5 degrees S and weaker ocean restratification.

Abstract: Recent warm atmospheric conditions have damaged the ice shelves of the Antarctic Peninsula through surface melt and hydrofracturing and could potentially initiate future collapse of other Antarctic ice shelves. However, model projections with similar greenhouse gas scenarios suggest large differences in cumulative 21st-century surface melting. So far it remains unclear whether these differences are due to variations in warming rates in individual models or whether local feedback mechanisms of the surface energy budget could also play a notable role. Here we use the polar-oriented regional climate model MAR (Modele Atmospherique Regional) to study the physical mechanisms that would control future surface melt over the Antarctic ice shelves in high-emission scenarios RCP8.5 and SSP5-8.5. We show that clouds enhance future surface melt by increasing the atmospheric emissivity and longwave radiation towards the surface. Furthermore, we highlight that differences in meltwater production for the same climate warming rate depend on cloud properties and particularly cloud phase. Clouds containing a larger amount of supercooled liquid water lead to stronger melt, subsequently favouring the absorption of solar radiation due to the snowmelt-albedo feedback. As liquid-containing clouds are projected to increase the melt spread associated with a given warming rate, they could bea major source of uncertainties in projections of the future Antarctic contribution to sea level rise.

Abstract: Recent work on the Filchner-Ronne Ice Shelf (FRIS) system has shown that a redirection of the coastal current in the southeastern Weddell Sea could lead to a regime change in which an intrusion of warm Modified Circumpolar Deep Water results in large increases in the basal melt rate. Work to date has mostly focused on how increases in the Modified Circumpolar Deep Water crossing the continental shelf break leads directly to heat driven changes in melting in the ice-shelf cavity. In this study, we introduce a Weddell Sea regional ocean model configuration with static ice shelves. We evaluate a reference simulation against radar observations of melting, and find good agreement between the simulated and observed mean melt rates. We analyze 28 sensitivity experiments that simulate the influence of changes in remote water properties of the Antarctic Slope Current on basal melting in the FRIS. We find that remote changes in salinity quasi-linearly modulate the mean FRIS net melt rate. Changes in remote temperature quadratically vary the FRIS net melt rate. In both salinity and temperature perturbations, the response is rapid and transient, with a recovery time-scale of 5-15 years dependent on the size/type of perturbation. We show that the two types of perturbations lead to different changes on the continental shelf, and that ultimately different factors modulate the melt rates in the FRIS cavity. We discuss how these results, are relevant for ocean hindcast simulations, sea level, and melt rate projections of the FRIS.

Abstract: Efficient statistical emulation of melting land ice under various climate scenarios to 2100 indicates a contribution from melting land ice to sea level increase of at least 13 centimetres sea level equivalent. The land ice contribution to global mean sea level rise has not yet been predicted(1) using ice sheet and glacier models for the latest set of socio-economic scenarios, nor using coordinated exploration of uncertainties arising from the various computer models involved. Two recent international projects generated a large suite of projections using multiple models(2-8), but primarily used previous-generation scenarios(9) and climate models(10), and could not fully explore known uncertainties. Here we estimate probability distributions for these projections under the new scenarios(11,12) using statistical emulation of the ice sheet and glacier models. We find that limiting global warming to 1.5 degrees Celsius would halve the land ice contribution to twenty-first-century sea level rise, relative to current emissions pledges. The median decreases from 25 to 13 centimetres sea level equivalent (SLE) by 2100, with glaciers responsible for half the sea level contribution. The projected Antarctic contribution does not show a clear response to the emissions scenario, owing to uncertainties in the competing processes of increasing ice loss and snowfall accumulation in a warming climate. However, under risk-averse (pessimistic) assumptions, Antarctic ice loss could be five times higher, increasing the median land ice contribution to 42 centimetres SLE under current policies and pledges, with the 95th percentile projection exceeding half a metre even under 1.5 degrees Celsius warming. This would severely limit the possibility of mitigating future coastal flooding. Given this large range (between 13 centimetres SLE using the main projections under 1.5 degrees Celsius warming and 42 centimetres SLE using risk-averse projections under current pledges), adaptation planning for twenty-first-century sea level rise must account for a factor-of-three uncertainty in the land ice contribution until climate policies and the Antarctic response are further constrained.

Abstract: Coastal polynyas of the Southern Ocean play a central role in the ventilation of the deep ocean and affect the stability of ice shelves. It appears crucial to incorporate them into climate models, but it is unclear how to adequately simulate them. In particular, there is no consensus on the atmospheric forcing resolution needed to appropriately model the sea ice in coastal Antarctica. A high resolution might be required to represent the local winds such as katabatic winds which are key drivers of coastal polynyas. To fill in this gap, we have tested the sensitivity of sea ice and air-sea-ice interactions to the resolution of the atmospheric forcing in a high-resolution ocean-sea ice model. A set of regional atmospheric simulations at horizontal resolutions of 20, 10, and 5 km are performed with an atmospheric regional model and used to force three ocean-sea ice simulations in the Adelie Land sector, East Antarctica. Due to the better representation of topography with a refined grid, the offshore component of coastal winds becomes stronger at increased resolution. The wind intensification is particularly strong down valleys channelizing the katabatic flow, with increase in wind speed ranging between 1 and 3 m/s. Under a higher resolution forcing, polynyas open more frequently and are wider. This fosters the growth rate of sea ice in polynyas, while landfast ice and pack ice are weakly affected. In polynyas, the production of sea ice is increased by up to 30% at 5 km resolution compared to 20 km resolution. Polynyas downstream of the katabatic wind pathway are more affected than the ones driven by easterly winds, highlighting the importance of the local wind conditions. Brine rejection associated with these higher sea ice production rates affects the salinity budget of the ocean and enhances both the volume and density of the dense Shelf Water produced off Adelie Land. These results underpin the need to better account for local coastal winds and polynyas in ocean and climate models.

Abstract: The future retreat rate for marine-based regions of the Antarctic Ice Sheet is one of the largest uncertainties in sea-level projections. The Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6) aims to improve projections and quantify uncertainties by running an ensemble of ice sheet models with atmosphere and ocean forcing derived from global climate models. Here, the Community Ice Sheet Model (CISM) is used to run ISMIP6-based projections of ocean-forced Antarctic Ice Sheet evolution. Using multiple combinations of sub-ice-shelf melt parameterizations and calibrations, CISM is spun up to steady state over many millennia. During the spin-up, basal friction parameters and basin-scale thermal forcing corrections are adjusted to optimize agreement with the observed ice thickness. The model is then run forward for 550 years, from 1950-2500, applying ocean thermal forcing anomalies from six climate models. In all simulations, the ocean forcing triggers long-term retreat of the West Antarctic Ice Sheet, especially in the Filchner-Ronne and Ross sectors. Mass loss accelerates late in the 21st century and then rises steadily for several centuries without leveling off. The resulting ocean-forced sea-level rise at year 2500 varies from about 150 to 1300 mm, depending on the melt scheme and ocean forcing. Further experiments show relatively high sensitivity to the basal friction law, moderate sensitivity to grid resolution and the prescribed collapse of small ice shelves, and low sensitivity to the stress-balance approximation. The Amundsen sector exhibits threshold behavior, with modest retreat under many parameter settings but complete collapse under some combinations of low basal friction and high thermal forcing anomalies. Large uncertainties remain, as a result of parameterized sub-shelf melt rates, simplified treatments of calving and basal friction, and the lack of ice-ocean coupling.

Abstract: The importance of resolving mesoscale air-sea interactions to represent cyclones impacting the East Coast of Australia, the so-called East Coast Lows (ECLs), is investigated using the Australian Regional Coupled Model based on NEMO-OASIS-WRF (NOW) at 1/4 degrees resolution. The fully coupled model is shown to be capable of reproducing correctly relevant features such as the seasonality, spatial distribution and intensity of ECLs while it partially resolves mesoscale processes, such as air-sea feedbacks over ocean eddies and fronts. The mesoscale thermal feedback (TFB) and the current feedback (CFB) are shown to influence the intensity of northern ECLs (north of 30 degrees S), with the TFB modulating the pre-storm sea surface temperature (SST) by shifting ECL locations eastwards and the CFB modulating the wind stress. By fully uncoupling the atmospheric model of NOW, the intensity of northern ECLs is increased due to the absence of the cold wake that provides a negative feedback to the cyclone. The number of ECLs might also be affected by the air-sea feedbacks but large interannual variability hampers significant results with short-term simulations. The TFB and CFB modify the climatology of SST (mean and variability) but no direct link is found between these changes and those noticed in ECL properties. These results show that the representation of ECLs, mainly north of 30 degrees S, depend on how air-sea feedbacks are simulated. This is particularly important for atmospheric downscaling of climate projections as small-scale SST interactions and the effects of ocean currents are not accounted for.

Abstract: Like many western boundary currents, the East Australian Current (EAC) extension is projected to get stronger and warmer in the future. The CMIP5 multimodel mean (MMM) projection suggests up to 5 degrees C of warming under an RCP85 scenario by 2100. Previous studies employed Sverdrup balance to associate a trend in basin wide zonally integrated wind stress curl (resulting from the multidecadal poleward intensification in the westerly winds over the Southern Ocean) with enhanced transport in the EAC extension. Possible regional drivers are yet to be considered. Here we introduce the NEMO-OASIS-WRF coupled regional climate model as a framework to improve our understanding of CMIP5 projections. We analyze a hierarchy of simulations in which the regional atmosphere and ocean circulations are allowed to freely evolve subject to boundary conditions that represent present-day and CMIP5 RCP8.5 climate change anomalies. Evaluation of the historical simulation shows an EAC extension that is stronger than similar ocean-only models and observations. This bias is not explained by a linear response to differences in wind stress. The climate change simulations show that regional atmospheric CMIP5 MMM anomalies drive 73% of the projected 12 Sv increase in EAC extension transport whereas the remote ocean boundary conditions and regional radiative forcing (greenhouse gases within the domain) play a smaller role. The importance of regional changes in wind stress curl in driving the enhanced EAC extension is consistent with linear theory where the NEMO-OASIS-WRF response is closer to linear transport estimates compared to the CMIP5 MMM.

Abstract: Climate model projections generally indicate fewer but more intense tropical cyclones (TCs) in response to increasing anthropogenic emissions. However these simulations suffer from long-standing biases in their Sea Surface Temperature (SST). While most studies investigating future changes in TC activity using high-resolution atmospheric models correct for the present-day SST bias, they do not consider the reliability of the projected SST changes from global climate models. The present study illustrates that future South Pacific TC activity changes are strongly sensitive to correcting the projected SST changes using an emergent constraint method. This additional correction indeed leads to a strong reduction of the cyclogenesis (-55%) over the South Pacific basin, while no statistically significant change arises in the uncorrected simulations. Cyclogenesis indices suggest that this strong reduction in the corrected experiment is caused by stronger vertical wind shear in response to a South Pacific Convergence Zone equatorward shift. We thus find that uncertainty in the projected SST patterns could strongly hamper the reliability of South Pacific TC projections. The strong sensitivity found in the current study will need to be investigated with other models, observational constraint methods and in other TC basins in order to assess the reliability of regional TC projections.

Abstract: Climate model projections have previously been used to compute ice shelf basal melt rates in ice sheet models, but the strategies employed – e.g., ocean input, parameterization, calibration technique, and corrections – have varied widely and are often ad hoc. Here, a methodology is proposed for the calculation of circum-Antarctic basal melt rates for floating ice, based on climate models, that is suitable for ISMIP6, the Ice Sheet Model Intercomparison Project for CMIP6 (6th Coupled Model Intercomparison Project). The past and future evolution of ocean temperature and salinity is derived from a climate model by estimating anomalies with respect to the modern day, which are added to a present-day climatology constructed from existing observational datasets. Temperature and salinity are extrapolated to any position potentially occupied by a simulated ice shelf. A simple formulation is proposed for a basal melt parameterization in ISMIP6, constrained by the observed temperature climatology, with a quadratic dependency on either the nonlocal or local thermal forcing. Two calibration methods are proposed: (1) based on the mean Antarctic melt rate (MeanAnt) and (2) based on melt rates near Pine Island's deep grounding line (PIGL). Future Antarctic mean melt rates are an order of magnitude greater in PIGL than in MeanAnt. The PIGL calibration and the local parameterization result in more realistic melt rates near grounding lines. PIGL is also more consistent with observations of interannual melt rate variability underneath Pine Island and Dotson ice shelves. This work stresses the need for more physics and less calibration in the parameterizations and for more observations of hydrographic properties and melt rates at interannual and decadal timescales.

Abstract: Changes in precipitation over the European Alps are investigated with the regional climate model MAR (Modele Atmospherique Regional) applied with a 7 km resolution over the period 1903-2010 using the reanalysis ERA-20C as forcing. A comparison with several observational datasets demonstrates that the model is able to reproduce the climatology as well as both the interannual variability and the seasonal cycle of precipitation over the European Alps. The relatively high resolution allows us to estimate precipitation at high elevations. The vertical gradient of precipitation simulated by MAR over the European Alps reaches 33% km(-1) (1.21mm d(-1) km(-1)) in summer and 38% km(-1) (1.15 mm d(-1) km(-1)) in winter, on average, over 1971-2008 and shows a large spatial variability. A significant (p value < 0.05) increase in mean winter precipitation is simulated in the northwestern Alps over 1903-2010, with changes typically reaching 20% to 40% per century. This increase is mainly explained by a stronger simple daily intensity index (SDII) and is associated with less-frequent but longer wet spells. A general drying is found in summer over the same period, exceeding 20% to 30% per century in the western plains and 40% to 50% per century in the southern plains surrounding the Alps but remaining much smaller (< 10 %) and not significant above 1500 ma.s.l. Below this level, the summer drying is explained by a reduction in the number of wet days, reaching 20% per century over the northwestern part of the Alps and 30% to 50% per century in the southern part of the Alps. It is associated with shorter but more-frequent wet spells. The centennial trends are modulated over the last decades, with the drying occurring in the plains in winter also affecting high-altitude areas during this season and with a positive trend of autumn precipitation occurring only over the last decades all over the Alps. Maximum daily precipitation index (Rx1day) takes its highest values in autumn in both the western and the eastern parts of the southern Alps, locally reaching 50 to 70 mm d(-1) on average over 1903-2010. Centennial maxima up to 250 to 300 mm d(-1) are simulated in the southern Alps, in France and Italy, as well as in the Ticino valley in Switzerland. Over 1903-2010, seasonal Rx1day shows a general and significant increase at the annual timescale and also during the four seasons, reaching local values between 20% and 40% per century over large parts of the Alps and the Apennines. Trends of Rx1day are significant (p value < 0.05) only when considering long time series, typically 50 to 80 years depending on the area considered. Some of these trends are nonetheless significant when computed over 1970-2010, suggesting a recent acceleration of the increase in extreme precipitation, whereas earlier periods with strong precipitation also occurred, in particular during the 1950s and 1960s.

Abstract: Ice flow models of the Antarctic ice sheet are commonly used to simulate its future evolution in response to different climate scenarios and assess the mass loss that would contribute to future sea level rise. However, there is currently no consensus on estimates of the future mass balance of the ice sheet, primarily because of differences in the representation of physical processes, forcings employed and initial states of ice sheet models. This study presents results from ice flow model simulations from 13 international groups focusing on the evolution of the Antarctic ice sheet during the period 2015-2100 as part of the Ice Sheet Model Intercomparison for CMIP6 (ISMIP6). They are forced with outputs from a subset of models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), representative of the spread in climate model results. Simulations of the Antarctic ice sheet contribution to sea level rise in response to increased warming during this period varies between 7:8 and 30.0 cm of sea level equivalent (SLE) under Representative Concentration Pathway (RCP) 8.5 scenario forcing. These numbers are relative to a control experiment with constant climate conditions and should therefore be added to the mass loss contribution under climate conditions similar to present-day conditions over the same period. The simulated evolution of the West Antarctic ice sheet varies widely among models, with an overall mass loss, up to 18.0 cm SLE, in response to changes in oceanic conditions. East Antarctica mass change varies between 6 :1 and 8.3 cm SLE in the simulations, with a significant increase in surface mass balance outweighing the increased ice discharge under most RCP 8.5 scenario forcings. The inclusion of ice shelf collapse, here assumed to be caused by large amounts of liquid water ponding at the surface of ice shelves, yields an additional simulated mass loss of 28mm compared to simulations without ice shelf collapse. The largest sources of uncertainty come from the climate forcing, the ocean-induced melt rates, the calibration of these melt rates based on oceanic conditions taken outside of ice shelf cavities and the ice sheet dynamic response to these oceanic changes. Results under RCP 2.6 scenario based on two CMIP5 climate models show an additional mass loss of 0 and 3 cm of SLE on average compared to simulations done under present-day conditions for the two CMIP5 forcings used and display limited mass gain in East Antarctica.

Abstract: Oceanic melting beneath ice shelves is the main driver of the current mass loss of the Antarctic ice sheet and is mostly parameterised in stand-alone ice-sheet modelling. Parameterisations are crude representations of reality, and their response to ocean warming has not been compared to 3D ocean-ice-sheet coupled models. Here, we assess various melting parameterisations ranging from simple scalings with far-field thermal driving to emulators of box and plume models, using a new coupling framework combining the ocean model NEMO and the ice-sheet model Elmer/Ice. We define six idealised one-century scenarios for the far-field ocean ranging from cold to warm, and representative of potential futures for typical Antarctic ice shelves. The scenarios are used to constrain an idealised geometry of the Pine Island glacier representative of a relatively small cavity. Melt rates and sea-level contributions obtained with the parameterised stand-alone ice-sheet model are compared to the coupled model results. The plume parameterisations give good results for cold scenarios but fail and underestimate sea level contribution by tens of percent for warm(ing) scenarios, which may be improved by adapting its empirical scaling. The box parameterisation with five boxes compares fairly well to the coupled results for almost all scenarios, but further work is needed to grasp the correct number of boxes. For simple scalings, the comparison to the coupled framework shows that a quadratic as opposed to linear dependency on thermal forcing is required. In addition, the quadratic dependency is improved when melting depends on both local and non-local, i.e. averaged over the ice shelf, thermal forcing. The results of both the box and the two quadratic parameterisations fall within or close to the coupled model uncertainty. All parameterisations overestimate melting for thin ice shelves while underestimating melting in deep water near the grounding line. Further work is therefore needed to assess the validity of these melting parameteriations in more realistic set-ups.

Abstract: The East Australian Current (EAC) plays a major role in regional climate, circulation, and ecosystems, but predicting future changes is hampered by limited understanding of the factors controlling EAC separation. While there has been speculation that the presence of New Zealand may be important for the EAC separation, the prevailing view is that the time-mean partial separation is set by the ocean's response to gradients in the wind stress curl. This study focuses on the role of New Zealand, and the associated adjacent bathymetry, in the partial separation of the EAC and ocean circulation in the Tasman Sea. Here utilizing an eddy-permitting ocean model (NEMO), we find that the complete removal of the New Zealand plateau leads to a smaller fraction of EAC transport heading east and more heading south, with the mean separation latitude shifting >100 km southward. To examine the underlying dynamics, we remove New Zealand with two linear models: the Sverdrup/Godfrey Island Rule and NEMO in linear mode. We find that linear processes and deep bathymetry play a major role in the mean Tasman Front position, whereas nonlinear processes are crucial for the extent of the EAC retroflection. Contrary to past work, we find that meridional gradients in the basin-wide wind stress curl are not the sole factor determining the latitude of EAC separation. We suggest that the Tasman Front location is set by either the maximum meridional gradient in the wind stress curl or the northern tip of New Zealand, whichever is furthest north.

Abstract: Recent advances in both ocean modeling and melt parameterization in ice-sheet models point the way toward coupled ice sheetÂ&#65533;ocean modeling, which is needed to quantify Antarctic mass loss and the resulting sea-level rise. The latest Antarctic ocean modeling shows that complex interactions between the atmosphere, sea ice, icebergs, bathymetric features, and ocean circulation on many scales determine which water masses reach ice-shelf cavities and how much heat is available to melt ice. Meanwhile, parameterizations of basal melting in standalone ice-sheet models have evolved from simplified, depth-dependent functions to more sophisticated models, accounting for ice-shelf basal topography, and the evolution of the sub-ice-shelf buoyant flow. The focus of recent work has been on better understanding processes or adding new model capabilities, but a broader community effort is needed in validating models against observations and producing melt-rate projections. Given time, community efforts in coupled ice sheetÂ&#65533;ocean modeling, already underway, will tackle the considerable challenges involved in building, initializing, constraining, and performing projections with coupled models, leading to reduced uncertainties in Antarcticaâ&#65533;&#65533;s contribution to future sea-level rise.

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.

Abstract: The Maritime Continent is a region of intense rainfall characterised by a strong diurnal cycle. This study investigates the sensitivity of rainfall characteristics to resolution in a coupled regional climate model configuration, in particular focusing on processes that modulate the diurnal cycle. Model biases are resolution dependent. Increasing resolution from 3/4A degrees to 1/4A degrees improves the mean state sea surface temperature and precipitation biases. However, at higher resolutions (1/12A degrees) rainfall becomes too strong in most areas. Daily maximum rainfall is simulated about 2-4 h earlier than in observations over both the land and the ocean, with only small improvements over high topography at higher resolution. We develop a technique to examine cross-coastal processes associated with the rainfall diurnal cycle along all coastlines. This is used to investigate the sensitivity of the diurnal cycle to resolution and to the direction of the prevailing wind. During offshore prevailing winds, most land rainfall is confined near the coastline and associated with a shallow land-sea breeze circulation at all resolution (though rainfall partly develops directly inland at 1/12A degrees). During onshore prevailing winds, rainfall propagates from the coastline to the island interior at 1/4A degrees and 1/12A degrees, whereas it appears directly over the island interior at 3/4A degrees, and this is associated with a deep convective cell across the coastline for all resolutions. Oceanic rainfall propagates far offshore during offshore prevailing winds at all resolutions, whereas it tends to remain confined near the coastline under onshore prevailing winds condition, particularly at higher resolution.

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.

Abstract: This study presents the first multidecadal and coupled regional simulation of cyclonic activity in the South Pacific. The long-term integration of state-of the art models provides reliable statistics, missing in usual event studies, of air-sea coupling processes controlling tropical cyclone (TC) intensity. The coupling effect is analyzed through comparison of the coupled model with a companion forced experiment. Cyclogenesis patterns in the coupled model are closer to observations with reduced cyclogenesis in the Coral Sea. This provides novel evidence of air-sea coupling impacting not only intensity but also spatial cyclogenesis distribution. Storm-induced cooling and consequent negative feedback is stronger for regions of shallow mixed layers and thin or absent barrier layers as in the Coral Sea. The statistical effect of oceanic mesoscale eddies on TC intensity (crossing over them 20 % of the time) is also evidenced. Anticyclonic eddies provide an insulating effect againststorm-induced upwelling and mixing and appear to reduce sea surface temperature (SST) cooling. Cyclonic eddies on the contrary tend to promote strong cooling, particularly through storm-induced upwelling. Air-sea coupling is shown to have a significant role on the intensification process but the sensitivity of TCs to SST cooling is nonlinear and generally lower than predicted by thermodynamic theories: about 15 rather than over 30 hPa degrees C-1 and only for strong cooling. The reason is that the cooling effect is not instantaneous but accumulated over time within the TC inner-core. These results thus contradict the classical evaporation-wind feedback process as being essential to intensification and rather emphasize the role of macro-scale dynamics.

Abstract: The southern hemisphere westerly winds have been strengthening and shifting poleward since the 1950s. This wind trend is projected to persist under continued anthropogenic forcing, but the impact of the changing winds on Antarctic coastal heat distribution remains poorly understood. Here we show that a poleward wind shift at the latitudes of the Antarctic Peninsula can produce an intense warming of subsurface coastal waters that exceeds 2 degrees C at 200-700 m depth. The model simulated warming results from a rapid advective heat flux induced by weakened near-shore Ekman pumping and is associated with weakened coastal currents. This analysis shows that anthropogenically induced wind changes can dramatically increase the temperature of ocean water at ice sheet grounding lines and at the base of floating ice shelves around Antarctica, with potentially significant ramifications for global sea level rise.

Abstract: We present a simple and analytical ocean boundary layer-sea ice coupled dynamical model that we apply to the modeling of Arctic sea ice motion in the frequency domain, and particularly in the inertial range. This study further complements our related work in an unpublished paper where the sea ice cover response to the Coriolis forcing has been studied. This analytical model allows interpretation of the spatial, seasonal and pluriannual dependence of the magnitude of the inertial oscillations detailed in terms of mechanical behavior of the ice cover. In this model, the sea ice mechanical response is simplified through the introduction of a linear internal friction term K. A dependence of K allows us to explain the associated dependence of the seasonal and regional Arctic sea ice inertial motion. In addition, a significant decrease of K, i.e., a mechanical weakening of the sea ice cover, is observed for the period 2002-2008 compared to 1979-2001, for the entire Arctic in both seasons. These results show that the regional, seasonal and pluriannual variations of sea ice inertial motion are not only the trivial consequence of simultaneous variations of thickness and concentration (and so of ice mass per unit area). Instead, the shrinking and thinning of the Arctic sea ice cover over the last few decades has induced a mechanical weakening, which in turns has favored sea ice fracturing and deformation.

Abstract: Coastal polynyas around Antarctica are the place of intense air-sea exchanges which eventually lead to the formation of high-salinity shelf waters (HSSW) over continental shelves. Here, the influence of atmospheric forcing on coastal polynyas in the Ross Sea is studied by contrasting the response of a regional ocean/sea-ice circulation model to two different atmospheric forcing sets. A first forcing (DFS3) is based on ERA40 atmospheric surface variables and satellite products. A second forcing (MAR) is produced on the basis of ERA40 with a dynamical downscaling procedure. As compared to DFS3, MAR forcing is shown to improve substantially the representation of small-scale patterns of coastal winds with stronger katabatic winds along the coast. The response of the ocean/sea-ice model to the two forcing sets shows that the MAR forcing improves substantially the geographical distribution of polynyas in the Ross Sea. With the MAR forcing, the polynya season is also shown to last longer with a greater ice-production rate. As a consequence, a greater flow of dense water out of the polynyas is found with the MAR forcing and the properties of HSSW are notably improved as compared to the DFS3 forcing. The factors contributing to the activity of Terra Nova Bay and Ross Ice Shelf polynyas in the model are studied in detail. The general picture that emerges from our simulations is that the properties of HSSW are mostly set by brine rejection when the polynya season resume. We found that coastal polynyas in the Ross Sea export about 0.4 Sv of HSSW which then flows along three separate channels over the Ross Shelf. A 6-month time lag is observed between the peak of activity of polynyas and the maximum transport across the sills in the channels with a maximum transport of about 1 Sv in February. This lag corresponds to the time it takes to the newly formed HSSW to spread from the polynya to the sills (at a speed of nearly 2 cm s(-1)).

Abstract: Air-sea ice-ocean interactions in the Ross Sea sector form dense waters that feed the global thermohaline circulation. In this paper, we develop the new limited-area ocean-sea ice-atmosphere coupled model TANGO to simulate the Ross Sea sector. TANGO is built up by coupling the atmospheric limited-area model MAR to a regional configuration of the ocean-sea ice model NEMO. A method is then developed to identify the mechanisms by which local coupling affects the simulations. TANGO is shown to simulate realistic sea ice properties and atmospheric surface temperatures. These skills are mostly related to the skills of the stand alone atmospheric and oceanic models used to build TANGO. Nonetheless, air temperatures over ocean and winter sea ice thickness are found to be slightly improved in coupled simulations as compared to standard stand alone ones. Local atmosphere ocean feedbacks over the open ocean are found to significantly influence ocean temperature and salinity. In a stand alone ocean configuration, the dry and cold air produces an ocean cooling through sensible and latent heat loss. In a coupled configuration, the atmosphere is in turn moistened and warmed by the ocean; sensible and latent heat loss is therefore reduced as compared to the stand alone simulations. The atmosphere is found to be less sensitive to local feedbacks than the ocean. Effects of local feedbacks are increased in the coastal area because of the presence of sea ice. It is suggested that slow heat conduction within sea ice could amplify the feedbacks. These local feedbacks result in less sea ice production in polynyas in coupled mode, with a subsequent reduction in deep water formation.