Presenter: Filipe Pereira (University of São Paulo)

Description:
The circulation of the continental margin off SE Brazil is dominated by an intricate system of western boundary (WB) currents. The Brazil Current flows poleward, and together with the equatorward flow of the Intermediate Western Boundary Current underneath it, forms a 1st-baroclinic-mode jet over the slope. This system presents strong mesoscale activity between 20ºS-24ºS, with meanders growing quasi-stationarily, primarily due to baroclinic instability. Using in situ observations, satellite imagery, and previous studies, we investigate these boundary-jet dynamics by building and exploring a set of idealized models. The first model is a 2-layer QG model based on contour dynamics to examine the basic mechanisms involved in the development of the meanders. A second model is a similar setup but continuously stratified and run using the MITgcm. Both models are coupled to a simple NPZ model to investigate the effect of the meanders on the plankton distribution. For the QG model, an initial disturbance in the mean jet grows quasi-stationarily. PV anomalies lead to layer coupling and baroclinic conversion with upwelling (downwelling) along the meander's southern (northern) portion. The presence of the WB inhibits the development of anticyclones in the upper layer. The NPZ results show that the WB acts as a source of material offshore. For the MITgcm results, a 10-km resolution experiment presents overall behavior similar to the QG one. The Rossby Number, Ro, is generally small so that the numerical experiment is evolving mesoscale processes. A 1-km experiment presents a profusion of smaller-scale eddies along the meander edges with submesoscale identified by spots of high Ro in the mixed layer. Our results suggest that the meander's dynamics strongly influence the NPZ tracers' distribution. The smaller-scale processes may be responsible for the observed chlorophyll patterns seen in satellite images.