Echanges indo-atlantiques d'eau subtropicale en relation aux structures frontales et de méso-échelle: Utilisation de la série temporelle altimétrique de niveau de la mer

Indo-Atlantic exchanges of subtropical water play a crucial role in controlling global climate. The intense mesoscale activity south of Africa contributes to these exchanges, yet remains poorly described. A commonly accepted circulation scheme of those waters assmues that the subtropical gyres of the neighbouring oceans are partially connected, forming a super-gyre. Using an altimeter-derived sea-level time series spanning over 12 years, we study the exchanges through the "northern branch" of the super-gyre (Indian to Atlantic ocean), and through the "southern branch" (opposite direction). Regarding the northern branch, an initial study of the Agulhas Current Retroflection yields a statistical description of its behaviour, and shows the disrupting role of local bathymetry. Then, a thorough tracking of all Agulhas Rings trajectories shed by the Retroflection yields 3 typical paths, linked to regional bathymetry, whose contributions to inter-ocean exchanges are evaluated. Applying the same methods to regional simulations shows inadequate behaviour of the Retroflection and ring trajectories. Exchanges through the southern branch of the super-gyre could be associated with the a jet-like flow of the Subtropical Front, the question of its continuity thus being crucial. If the front seems continuous in the mean sea level field, applying the method developed to locate the mean front to weekly SSH fields shows that mesoscale features west of the retroflection strongly disturb its path, making it discontinuous. Exchanges mechanisms through this branch are thus probably more complex.