FGO : Geophysical Fluids and Oceanography

See all groups

ARSOUZE Thomas+33 1 69 33 30 54Ingénieur de recherche / Research Engineer
STEGNER Alexandre+33 1 44 32 22 66
+33 1 69 31 98 42
Chargé de recherches CNRS

The research performed within the FGO team concerns the study of the meso and submeso scale geophysical fluids, especially in oceanic environments at regional basins and their continental margin scales. The main objectives are to understand the dynamical mechanisms (instabilities, upwelling, deep convection, waves-flow and topography-flow interactions, generation and trajectories of coherent structures) that affect ocean circulation at these scales. Particular emphasis is put on the study of local oceanic phenomena, such as submeso-scale and mesoscale eddies, and their role in the transport of water mass properties in the ocean, especially in the cross-shore exchange. These studies are essential to improve our understanding and the modeling of the general circulation of the ocean basins and their ecosystems. Improving models is indeed part of the major issues of operational oceanography and forecasting of regional evolutions related to global climate change. Mediterranean Sea is the main area for these studies and is part of the issues addressed by the "Chantier Méditerranée" supported by the l'Institut National des Sciences de l'Univers of the CNRS (INSU) and other major international programs that bring the emphasis on the role of the continental margins and in-situ monitoring of the marine environment in an operational environment.

The originality of the FGO team is to exploit the complementarities between theoretical models, laboratory experiments, experiments at sea and numerical models for the study of the dynamical features at these scales. The different activities thus extend from the physical modeling of oceanic flows in a laboratory-rotating platform, to the participation in oceanographic campaigns, or to the development of a modeling platform, for applications ranging from studying the Mediterranean Sea climate or the marine ecosystems.

The FGO team is composed of members from ENSTA (Karine Beranger, Thomas Arsouze) and CNRS (Alexandre Stegner LMD), and works in close relations with laboratories from Saclay’s Plateau and from IPSL, but also with Mercator Ocean, CNRM, the MIO, the IFREMER, the LPO, and other laboratories around the Mediterranean basin.

Among the achievements of the team to serve the oceanographic community, we should include the development of a modeling platform of the Mediterranean Sea with an operational component and applications in marine biogeochemistry and ecology (SIMED, MORCE-MED, MEDICCBIO). Other actions include process studies: the wakes of islands (TIRIS), the current-topography interactions (TOPIECC, SYNBIOS), the air-sea-continental surfaces interactions (ASICS-MED, MED-AGRIF, REMEMBER).

 

SiMED (SImulation of the MEDiterranean Sea)

Numerical simulation is used in the SIMED national project to study the general circulation of the Mediterranean Sea. Different models are developed based on the numerical code NEMO (http://www.locean-ipsl.upmc.fr/NEMO). These models are eddy-permitting, i.e. they realistically simulate mesoscale eddies that are about 100km (10 radius of deformation) in the Mediterranean (Figure 1). The Mediterranean Sea is the site of a thermohaline circulation triggered by winter convection events in several sub-basins (Figure 1). Dense waters are formed, following a high evaporation in autumn and winter, sink and spread at intermediate to bottom depths. This thermohaline circulation and the water mass pathways are quite difficult to simulate realistically, especially for the Mediterranean Sea as it is poorly stratified, and the characteristics of the different water masses are difficult to simulate. It is also the site of a cyclonic coastal circulation, triggered by Atlantic Waters entering through the Strait of Gibraltar. This influx of relatively unsalty and hot water makes up for the annual water loss (0.7m/yr) due to net evaporation basin and the annual heat loss (5 W / m²). Atlantic Waters flow in a thin stream (~ 50km) along the steep coastal slope. This current is associated with meanders and separations into different branches and generates mesoscale eddies. The simulation of such structures makes possible the study of cross-shore exchange, including transport of surface and deep water mass characteristics into the interior of the sub-basins.

 

Figure 1: Bathymetry and main geographical areas of the Mediterranean Sea. Areas of dense water formation are depicted by circles (Beranger et al. 2010).

 

 

MORCE-MED (MOdélisation Régionale CoupléE en MEDitérrannée - Coupled Regional Modeling in the Maditerranean basin)

The MORCE-MED project aims to study the regional climate, all coupled processes (physics, bio-geo-chemistry) and the interactions between the different compartments of the Earth system (ocean, continent, atmosphere). We have developed a platform for modeling high-resolution coupled ocean-atmosphere in collaboration with the LMD / IPSL. Our tools are the numerical codes NEMO for the ocean, WRF for the atmosphere (http://www.wrf-model.org) and the OASIS coupler (http://pantar.cerfacs.fr/3-26568 -OASIS.php). These studies contribute to the study of the water cycle in the Mediterranean basin, which is the main objective of the HyMeX project. The physical connection and biogeochemistry is performed with the MIO and LSCE within two theses in progress.

Figure 2: Schematic of coupling the NEMO-OASIS-WRF components, made ​​for the Europe-Mediterranean zone (Lebeaupin Brossier et al 2013, http://www.gisclimat.fr/projet/morce-med.).

 

 

TIRIS (Three dimensional Instabilities at high Reynolds number around Islands)

 

The interaction of an ocean current with islands or archipelagos indicates a strong eddy activity in their wake. The structures thus created are of considerable importance in the transport and mixing of biogeochemical components. Several dynamic processes such as filamentation, upwellings and inertial instabilities (Figure 3) can greatly increase vertical and horizontal mixing in localized areas. Vertical mixing, which connects the geochemical components of the deep ocean with phyto and zooplankton of the euphotic layer, increases the life cycle. The oceanic islands or archipelagos then behave as "biological oasis" in the middle of the ocean.

 

To quantify the intensity and location of mixing zones in the wake of islands, this project combines experimental (ENSTA, LMD and LEGI-Coriolis in Grenoble) and numerical (ROMS) studies, but also in-situ measurement (Madeira Island). This project involves fluid mechanics, oceanographers and marine biology experts.

 

 

 

Figure 3: a wake downstream cylindrical island in a thin rotating fluid layer. Unstable perturbations of small-scale (Dyer et al. 2010) are visible in the anticyclonic eddies (black color).


 

 

TopIECC (Topographic Impact on Coastal Current and Eddies)

The development of high-resolution regional circulation models becomes a necessity in order to correctly reproduce oceanography coastal areas – open seas exchanges. These exchanges control the heat and salinity balance, pollutant dispersion and distribution of nutrients and biological species. From this point of view, the regional circulation models are still imperfect. The interaction of coastal current with the continental slope bathymetry, the associated dissipation mechanisms, and the role of a deep circulation along it (even of low amplitude) are difficult to model.

The purpose of this project is to improve our understanding of the processes and to propose adequate evolutions in the models (parameterization, discretization, forcing, etc...). TOPIECC tries to identify the transition between the two dynamical processes responsible for the formation of large vortex structures: barotropic and baroclinic instability of the coastal current or nonlinear meandering induced by the topography of the slope. An innovative aspect of this project is to show that physical modeling can provide quantitative data to improve numerical models for a relatively small cost in comparison with obtaining in-situ data. Data from experimental models will be used for systematic comparisons with idealized configurations (Figure 4) of the NEMO model also used in operational oceanography. These dynamic configurations will be compared with in situ data from campaigns EGYPT-EGGITO (eastern Mediterranean basin) and COUPLING (Bransfield Strait, Antarctica).

Figure 4: Evolution of the vorticity field (red = cyclonic, anticyclonic = blue) of a coastal current computed by the NEMO model. The initial conditions are identical to those of laboratory experiments. The images of the upper panel correspond to a configuration without bathymetry whereas in the lower frame, a semicircle steep embankment panel was added.