The roles of vertical shear and topography on the eddy formation near the site of origin of the Brazil Current

The site of origin of the Brazil Current (BC) is currently one of the less explored aspects of regional circulation and mesoscale activity in the west side of the South Atlantic Subtropical gyre. The few studies that are available, based either on in situ data or on numerical modeling, seems to agree that the region is characterized by relatively weak baroclinic flow, with substantial mesoscale activity, which is quite different from other western boundary current systems (e.g. Florida Current, in the North Atlantic). We present numerical simulations that show that the main realistic mesoscale features in the eddy-rich vicinities of the BC site of origin can be successfully modeled through the dynamical interaction between parameterized versions of two opposing mean western boundary currents (BC and North Brazil Undercurrent—NBUC) and local topography, with no influence of remote dynamics or atmospheric forcing. Large BC-related anticyclones observed in previous work were reproduced and recurrently formed during the model run. Two additional sensitivity experiments were performed. When NBUC is removed from the physical setting, the BC interaction with topography is not sufficient to generate eddies similar to observations. When an idealized flat-bottom and a physiographic configuration with no Abrolhos and Royal Charlotte Banks are considered, the BC–NBUC interaction is also not capable of developing realistic mesoscale structures. Our geophysical instability analyses suggest that BC–NBUC vertical shear is promoting baroclinic energy fluxes from the mean flow to the perturbations, resulting in eddy formation and growth in the region.