The Southern Brazilian Shelf buoyancy-driven currents

A series of numerical simulations are conducted to investigate buoyancy-driven currents and their interaction with the tides, the wind and western boundary currents. The study area is the western South Atlantic, with emphasis on the Southern Brazilian Shelf (SBS). The buoyancy-driven circulation in this area is shown to be a complex system even in the absence of any other forcing. This complexity is basically due to the interaction between the Patos Lagoon plume and the La Plata plume which, although located further south, exerts strong influence on the SBS. Our simulations show that the amount of La Plata water transported toward the SBS is sensitive to the presence of the Patos Lagoon plume, being weaker when the Lagoon is considered in the simulations, than when it is not. It is also sensitive to the interaction with the tides, the winds, and western boundary currents.When under the influence of wind stress, the system is shown to present significant seasonal variations, with a tendency for riverborne water coastal trapping during the austral autumn, and offshore removal during the austral spring. Realistic wind stress, measured by the NASA scatterometer (NSCAT), are used in the simulations. The transport of La Plata water toward the SBS, and the interaction with the Patos Lagoon plume, is enhanced in the presence of downwelling-favorable winds and reduced in the presence of upwelling-favorable winds. This transport is also higher in the presence of the tides, suggesting the occurrence of tidal rectification, with a northward residual current being produced by the interaction of the tides with topography. A comparison with observed data show that the La Plata plume is more realistic in the presence of wind- and tidal-forcing, rather than only wind-forcing.The simulation with the western boundary currents shows basically two major effects: (1) A reduced upstream expansion of La Plata plume; (2) A significant enhancement of La Plata water transport toward the SBS. Both effects are related to a northward coastal current which appears in the southern part of the domain as a consequence of the Malvinas Current influence. Our simulations also show that the cyclonic eddies shed by the Brazil Current exert some influence in these two processes, since they favor the northward intrusion of Malvinas water. The circulation patterns obtained from the numerical simulations are compared to hydrographic and satellite observations, and are shown to reproduce the general aspects seen in the observations. These numerically reproduced circulation patterns are also shown to have strong influence on the biology of the SBS, since they control the spatial distribution of plankton