Small Scale Ocean Dynamics in the Cape Basin, South of Africa, and the Impact on the Ocean Circulation.

We investigate the role of meso-submesoscale features in the intensely turbulent activity characterizing the Cape Basin region, south-west of Africa, and its effect on the water mass exchanges and transformations, between the upper and intermediate layers, with a particular focus on the fate of Antarctic Intermediate Water. With this aim, realistic numerical simulations of the ocean circulation around Southern Africa have been run, at increasing horizontal and vertical resolutions, using different configurations of the Regional Ocean Model System. Validation of the runs output against observations (satellite and in-situ data) show that the model is able to reproduce the mean structure of the regional circulation, as well as the main characteristics, in the vertical and horizontal extents, of the water masses and currents present in the area (AAIW, NADW, Agulhas Current). Analysis of the major components of the Ertel Potential Vorticity budget, carried out with the output of the runs at highest resolutions (at 1/24° and at 1/48°), suggest that at the surface small scale instabilities (vortices, filaments and meanders) drive a highly ageostrophic flow, especially along the most prominent topographic features of the study region, while more quasi-geostrophic dynamics dominate at intermediate levels. The difference between these 2 dynamical regimes have been further explored looking at slopes of Kinetic Energy power spectra, which seem to confirm the preliminary results of the EPV analysis. Additional diagnostics have been set up to look at the nature of the meso-submesoscales processes, like the surface estimation of the Eddy Available Potential Energy, used as an indicator of the interior turbulence, and the Lagrangian Integration of Water particles, to statistically assess the impact of these instabilities on the pathways and mixing rates of the local watermasses.