Physical-biological interactions in the Brazil Current meanders

This doctoral dissertation discusses the basic dynamics of the mesoscale cyclonic quasi-stationary meanders of the Brazil Current and their impact on plankton distribution off SE Brazil. The wind-driven coastal upwelling systems in the region promote intrusions of nutrient-rich waters onto the continental shelf, supported by coastal divergence induced by the meanders. We perform experiments in a 1½-layer quasi-geostrophic (QG) model based on contour dynamics coupled to a Nutrients-Phytoplankton-Zooplankton (NPZ) model. The results suggest that mesoscale activity impacts the planktonic populations off SE Brazil. In situ observations of these phenomena are scarce. We present an unprecedented sampling effort that revealed sub-mesoscale processes in a meander’s rim. We show the biota responds to different dynamic regimes in distinct parts of the feature. Inspired by the observations, we choose to build a hierarchy of idealized models coupled to an NPZ model to investigate physical-biological interactions at mesoscale and submesoscale. We use a 2-layer QG model also based on contour dynamics, showing the meanders’ stationarity is due to the almost pure baroclinic nature of this system. The lateral stirring transports enriched waters offshore, impacting the NPZ fields in addition to the eddy-induced upwelling. Then, a continuously stratified model is used to investigate submesoscale dynamics. The results suggest that submesoscale processes increase the heterogeneity in the NPZ distribution, with preferred areas for enhanced biological activity shifting from the eddy center in a 10 km resolution experiment to the eddy rim in a 1 km resolution experiment. Lastly, we briefly investigate strong biases in sea surface temperature(SST) during coastal upwelling events.