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OSUG - Terre Univers Environnement

Research activities

by Herve Denis - 14 November 2016 ( maj : 15 December 2016 )

MEOM’s research is currently organized along the following axes :

  • Exploring ocean fine scales motions
  • Quantifying chaos and uncertainties in the ocean evolution
  • Understanding the ocean role in the climate machinery

Exploring the ocean fine scale dynamics

Fine scale ocean dynamics plays a key role in the climate machinery through various scale interaction mechanisms. Fine-scale ocean dynamics refer to ocean physical processes occuring at scales smaller than 200km, in the open ocean. This includes a fraction of mesoscale processes, with scales close to the first internal Rossby radius (20 to 200km). At mesoscales, oceans are populated by waves motions and coherent eddies, that contribute actively to transporting matter across ocean basins and from equator to poles. Fine scale processes also comprise submesoscale processes, with scale smaller than the first internal Rossby radius (1-20km). At submesoscales, oceans are populated by intense vortices, fronts and filaments, all of which are contributing to vertical exchanges between the ocean interior and the surface, fluxing in particular nutrients from the ocean sub-surface to the sun-lit surface layers where primary production occurs.

Overall, fine-scale dynamics is also key because the energy injected by winds in the large ocean circulation is fluxed through fine-scale processes on its way toward dissipation at smaller scale. Knowing how and and where energy is being exchanged in the ocean provides a key constrain for projecting the future evolution of the oceans.

Over the past 20 years, the combination of satellite remote sensing and ocean models have shown the ubiquity of ocean fine-scale processes and the key role they are playing in the climate machinery, but numerous essential key questions are still unanswered and fine-scale processes are still challenging to represent realistically in ocean models. The MEOM team contributes to unravel the rich variety of ocean processes occuring at fine scales by improving the representation of fine-scale dynamics in ocean models and developping innovative procedures for revealing the signature of fine scale dynamics in in-situ and satellite observations. Because we believe in the complementarity of observational and modelling efforts we use realistics ocean models, as for instance our new North Atlantic 1/60° NEMO model configuration to study how new observational plateforms may inform about fine scale processes.

On-going activities on fine-scales dynamics in our group are accompagnying and preparing new satellite ocean observation missions, such as the CNES/ISRO SARAL/AltiKA mission (launched in 2013), the future CNES/NASA Surface Water and Ocean Topography (SWOT) mission to be launched in 2021 and the geostationary ocean color OCAPI mission. We contribute actively to the scientific definition of these missions and to the preparation of innovative inversion and data assimilation methods that will help improve the observability of fine scales with these instruments. We also pursue the development of image assimilation techniques for sea surface temperature and ocean color data, that are available at a kilometric resolution from existing satellites.

Quantifying chaos and uncertainties in the ocean evolution

Ocean dynamics is described by non-linear equations, which connect the evolution of oceanic state variables (velocity, temperature, salinity, etc). Some theories predict that because of nonlinearity, the ocean evolution is not totally determined by external drivers such as the atmospheric variability: MEOM’s high-resolution simulations confirm that the ocean evolution also depends substantially on small uncertainties in initial conditions, even over long (decadal and longer) periods. The ocean evolution is thus not fully deterministic in the turbulent regime, but partly chaotic over a wide range of timescales: it should be simulated from ensembles of multiple realizations, described and studied from probability distribution functions that represent this inherent uncertainty.

The ocean models are also uncertain by construction due to e.g. unresolved (e.g. subgrid-scale) processes, or to the unperfect representation of the real dynamics. The MEOM team is working on methods to explicitly include these modelling uncertainties into ocean simulations by adding stochastic parameterizations : ensembles of such simulations represent the range of possible oceanic evolutions given these uncertainties. They may then be consistently corrected through data assimilation techniques to better match the observations and provide fully probabilistic forecasts.

Understanding the ocean role in the climate machinery

Ocean physical properties and their ongoing changes due to anthropogenic influences have huge consequences. Ocean circulation controls to a large extent ocean biomass and biogeochemical cycles, ocean acidification, ocean heat and carbon uptake and sequestration, and the patterns of regional sea level changes.

This role of ocean circulation in the climate machinery is tied to a range of scale interaction mechanisms. Ocean currents indeed vary on spatial scales that are much smaller than the size of the ocean basins: mesoscale turbulence (meanders and eddies of 10 km to 100 km size) and boundary currents flowing along continental slopes and shelves contribute to a large part of the oceanic transports of heat and freshwater between the subtropics and the polar latitudes. In the upper-ocean, the sub-mesoscale filaments and coherent vortices ( 1-10 km) involve particularly intense vertical exchanges, driving high frequency energy and biogeochemistry fluxes.

These fine-scale eddies and narrow boundary currents are poorly observed and are not adequately represented in the climate prediction models used for the IPCC assessment. Their associated non- linear contributions on larger scales are consequently unknown, raising issues concerning the possible role of the fine-scale ocean turbulence in climate. Our challenge is to understand the interaction mechanisms between the ocean fine-scale ocean processes and ocean variability at larger-scale.

In the MEOM team, we tackle this question with comprehensive ocean circulation models. Why a modelling approach? Since observations at eddy scales are much too sparse to address these complex issues, high-resolution ocean simulations are the only possible way to proceed, but such an approach requires a consistent framework in which the scale interactions can be quantified, allowing a true assessment and improvement of the parameterizations in climate prediction models.

Among the questions that we adress, we in particular tackle the following issues :

  • How well can we map and quantify the processes of generation and decay of ocean eddies?
  • What are the mechanisms that drive the short term, interannual and decadal oceanic variability?
  • How changes in physical properties of the ocean can affect ocean biogeochemical cycles ?
  • What are the impact of boundary currents, eddies and small scale processes on the large scale circulation and its variability?
  • What fraction of the low-frequency oceanic variance is directly constrained by the atmospheric forcing?
  • How subsatantial are the integrated impacts of the submesoscale turbulence on air-sea exchanges of energy, on the upper ocean stratification, or on vertical fluxes of matter be substantial?