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Understanding the ocean role in the climate machinery |
/climate/ |
Contributing to a european modeling infrastructure |
forecast |
Scientific software |
nemo |
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.
The DRAKKAR simulation framework, made of a complete hierarchy of high-resolution global and basin-scale model configurations of the NEMO ocean general circulation model, is used to address this challenge. We are continuously improving the representation of fine scale dynamics in the model. This simulation framework is then used to undertake ambitious research projects as for instance OCCIPUT ANR/PRACE projet and SOBUMS ANR projet.
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?