A kinematic analysis of the Indian/Atlantic interocean exchange

We analyze the Indian/Atlantic interocean exchanges in an eddy-permitting numerical simulation focusing on the Cape Basin, a region of the southeastern Atlantic bounded on the northwest by the Walvis Ridge and on the south by the Agulhas Ridge (near 40°S). To quantify the relative importance of the different dynamical mechanisms involved in the interocean exchange, we separated the climatological mean circulation from the transients. The analysis indicates that Agulhas eddies influence not only the transient but also fluxes associated with the mean circulation (eddy fluxes, for example, supply most of the energy of the Benguela Current). A distinctive characteristic of the eddy variability within the Cape Basin is the co-existence of cyclonic and anticyclonic vortices in dipole structures that resemble the Heton model of Hogg and Stommel (Proc. R. Soc. London. A 397 (1985) 1). Anticyclones are surface-intensified vortices that, in spite of their baroclinic structure, reach to deep layers. Cyclones are bottom-intensified vortices with dominant barotropic structure that projects into the upper layer. The propagation of both cyclones and anticyclones is strongly affected by the bottom topography. Our analysis shows that the Walvis Ridge and the Vema Seamount block the passage of bottom-intensified cyclones and changes the trajectories of the upper-intensified anticyclones. Although most anticyclones are able to escape the basin, the deep compensation generated by the ridge generates an energy loss of approximately 30%, and a change of the eddy trajectory to a more westward direction.