Mesoscale eddies in the western mediterranean sea: characterization and understanding from satellite observations and model simulations

Abstract

Mesoscale eddies are relatively small structures that dominate the ocean variability and have large impact on large scale circulation, heat fluxes and biological processes.In the western Mediterranean Sea, a high number of eddies has been observed and studied in the past with in-situ observations. Yet, a systematic characterization of these eddies is still lacking due to the small scales involved in these processes in this region where the Rossby deformation radius that characterizes the horizontal scales of the eddies is small (10-15 km). The objective of this thesis is to perform a characterization of mesoscale eddies in the western Mediterranean. For this purpose, we propose to develop tools to study the fine scales of the basin. First, we develop an eddy resolving simulation of the region for the last 20 years. This simulation shows that existing altimetry maps underestimate the mesoscale signal. Therefore, we attempt to improve existing satellite altimetry products to better resolve mesoscale eddies. We show that this improvement is possible but at the cost of the homogeneity of the fields; the resolution can only be improved at times and locations where altimetric observations are densely distributed. In a second part, we apply three different eddy detection and tracking methods to extract eddy characteristics from the outputs of the high-resolution simulation, a coarser simulation and altimetry maps. The results allow the determination of some characteristics of the detected eddies. The size of the eddies can greatly vary but is around 25-30 km. About 30 eddies are detected per day in the region with a very heterogeneous spatial distribution. Unlike other areas of the open ocean, they are mainly advected by currents of the region. Eddies can be separated according to their lifespan. Long-lived eddies are larger in amplitude and scale and have a seasonal cycle with a peak in late summer, while short-lived eddies are smaller and more present in winter. The penetration depth of detected eddies has also a large variance but the mean depth is around 300 meters. Anticyclones extend deeper in the water column and have a more conic shape than cyclones.