The Rio de la Plata plume dynamics over the Southwestern Atlantic Continental Shelf and its link with the large scale atmospheric variability on interannual timescales

The interannual variability of the Rio de la Plata plume over the Southwestern Atlantic Continental Shelf is analyzed based on the global ocean reanalysis ORAP5.0 and the global atmospheric reanalysis Era-Interim for the 35-years period 1979-2013. A regional validation of the dataset is performed to evaluate the ocean reanalysis ability to solve the main features associated with the plume dynamics. Following the satisfactory evaluation, an Empirical Orthogonal Function analysis is performed to extract characteristic modes of variability of the sea surface salinity. Three leading modes explain 35%, 30% and 15% of the total variance and are associated with large scale atmospheric variability patterns characteristic of the Southern Hemisphere circulation. Mode 1 is characterized by marked salinity anomalies to the south and north of the Rd1P Estuary mouth. Its particular imprint is associated with an organized along-shelf wind structure modulated by the Southern Annular Mode (SAM). Mode 2 presents salinity anomalies that extend from the Rd1P Estuary to the Uruguayan and southern Brazilian coasts and is related to interannual variability of the Rd1P discharge and to regional wind forcing over the plume. Both the river outflow and the atmospheric forcing over the shelf are strongly associated with the Pacific South American pattern (PSA1). Finally, mode 3 presents large salinity anomalies at 38 degrees S, south of the mouth of the Rd1P Estuary and is associated with displacements of the freshwater plume due to local wind stress anomalies over the estuary, which are modulated by the PSA2.