Presented at the 2015 Aquarius/SAC-D Science Team MeetingA high-resolution model is used to characterize the dominant patterns of sea surface salinity (SSS) variability in the southwestern Atlantic region. We identify three dominant modes of SSS variability. The first two represent the seasonal variations of the freshwater plumes over the continental shelf and the third mode represents the salinity exchanges between the shelf and the deep ocean. A diagnostic study using floats and passive tracers identifies the pathways taken by the freshwater plumes. During the winter the plumes leave the shelf region north of the BMC. During the summer the plumes are entrained more directly into the BMC. The vertical structure of the salinity plumes is controlled by the high frequency component of the wind stress while the horizontal component is controlled by the low-frequency component of the wind stress forcing and the inter-annual variations of the Rio de la Plata discharge. Dynamical analysis reveals that the cross-shelf flow has a dominant barotropic structure and, therefore, the SSS anomalies detected by Aquarius represent net mass exchanges between the shelf and the deep ocean. The net cross-shelf volume flux is 1.21 Sv. This outflow is largely compensated by an inflow from the Patagonian shelf.