Buoyancy-driven effects on turbulent diffusivity induced by a river plume in the southern Brazilian shelf

Abstract. Freshwater plumesare important flow structures that influence the dynamics and water properties of coastal regions and continental shelves. Turbulence in plumeregions is mainly driven by shear instabilities at the interface between plumeand oceanic waters, which, in turn, depend on the geometry and outflow of a specific plumeregion. The Southern Brazilian Shelf presents a highly variable hydrographicdistribution modulated by the seasonal wind variation and the freshwater discharge from the La Plata River estuary, which has a significant impact on the continental shelfcirculation. This buoyant plumecreates strong density gradientsand interacts with local water massesresulting in a complex hydrographicpattern. In this study, high resolution hydrographyand microstructure measurements were obtained in order to verify the effect of freshwater stratification on vertical mixing in this highly dynamic continental shelf. Results show that the plumeis highly stable at southern portions of the shelf, as density displacements, or Thorpe displacements, δ T , heat diffusivity, K T , buoyancy flux, B f , and density gradientratio, R p are reduced when compared to the northern areas. Moreover, hydrographicdata suggests that the large-scale La Plata River plumehas a dynamic mid-field region due to instabilities generated when reaching the shelf break.