Coastal mesoscale from lagrangian analysis of altimetry and multi-sensor observations
Résumé
Oceanic mesoscale plays a key role in modulating large-scale circulation, heat fluxes and primary production enhancement. Such hydrodynamic processes are also crucial at regional scales where the associated currents are known to significantly influence water-mass mixing and exchanges between the coastal zone and the open ocean. Nevertheless, the characterization of coastal mesoscale dynamics from along-track altimetric observations is complicated from a technical and algorithmic point of view and thus requires continuous developments (refer to the CTOH-XTRACK processor, PISTACH and COASTALT initiatives). Moreover, the high spatial/temporal variability and complexity associated with coastal mesoscale processes make them difficult to be studied with sparse in-situ observations and highly-smoothed standard altimetric products which lack the high resolution often required to correctly represent regional features. Alternative options rely on generating satellite altimetric maps specifically adapted to the coastal domain and also on developing methodologies based on the fusion of multi-sensor platforms in conjunction with numerical simulations. In this respect, optimal interpolation methods for improving the coastal circulation description have been developed over the North Western Mediterranean Sea (NWM), an area marked by a relatively low signal to noise ratio compared to the global ocean. We have also used vertical Empirical Orthogonal Functions (from a numerical model and in-situ measurements) in order to rebuild realistic coastal geostrophic currents both at surface and throughout the water column. Our approach characterizes the main coastal mesoscale features which can be confirmed by comparison with several independent in-situ measurements done by the MIO and IMEDEA laboratories (drifters, ADCP, gliders...). In particular, the developed methods are able to improve the accuracy of surface geostrophic current compared to standard AVISO product. Even better results can be obtained when complementary geophysical information (from bathymetry, sea surface temperature, tide gauges) are integrated in the optimal interpolation scheme. The high resolution 3D currents were then coupled with a Lagrangian code to simulate particle trajectories and therefore better interpret the influence of coastal mesoscale circulation on 3D exchanges between the Gulf of Lion continental shelf and the NWM open sea. Finally, the long-term statistical analysis (> 10 years) of these current fields was exploited to better characterize potential inter-annual changes in the coastal mesoscale activity of the NWM.