A Theoretical Study of the Vitória Eddy Genesis

Dante Campagnoli Napolitano, Instituto Oceanográfico - University of Sao Paulo, São Paulo, Brazil, Cesar B Rocha, Scripps Institution of Oceanography, UCSD, La Jolla, CA, United States, Ilson C Da Silveira, Instituto Oceanografico - University of São Paulo, Sao Paulo, Brazil, Iury Tercio Simoes-Sousa, Woods Hole Oceanographic Institution, Woods Hole, United States and Glenn Flierl, MIT, Cambridge, MA, United States
Abstract:
South of the Vitória-Trindade Ridge—a seamount chain off East Brazil—the Brazil Current (BC) meanders cyclonically within Tubarão Bight, eventually forming the Vitória Eddy. It was recently found that the Intermediate Western Boundary Current (IWBC), which flows equatorward below the BC, cyclonically recirculate within the same bight due to topographic constraints. We here study the role of the BC-IWBC coupling in the Vitória Eddy generation. Our theoretical approach using a simple two-layer QG model reduces the coupling of the layers to a stretching term, (Ψ21)Rd-2, where Rd is the baroclinic deformation radius. In our idealized channel, this term connects a meridional jet in the upper layer (the BC) and a stationary eddy in the lower layer (the IWBC recirculation). Based on AVISO sea-surface height data and information in the literature, we define a reference jet and the stationary eddy and vary the upper-jet speed. By comparing linear and nonlinear solutions, we find that nonlinearity is necessary, but not sufficient for the upper-layer eddy formation; the system is intrinsically dependent on the jet velocity. A weak upper-layer jet slowly meanders and develops into a cyclone above the lower layer eddy. As the jet velocity increases, the eddy becomes broader and its growth is faster. But a too-strong jet has an opposite effect: the BC decouples from the IWBC and no eddy is formed. The restrained layer coupling by the stationary eddy prevents eddy pinch-off in the model. The simulated Vitória Eddy develops locally and nonlinear effects trigger downstream meandering.