High sea surface chlorophyll concentration on the Argentine Continental Shelf frequently extends to the deep ocean in the vicinity of the Brazil/Malvinas Confluence (BMC). The offshore transport of shelf waters likely plays a key role in the biogeochemical balance of the western South Atlantic and promotes the offshore transport of planktonic species. We analyze data from an oceanographic survey carried out in the western South Atlantic shelf between 31°S and 38°S in October 2013. We describe the distribution and circulation of the water masses and focus on the exchanges with the open ocean. On-shelf subsurface intrusions of oceanic waters and river discharge supply nutrients to the shelf. A low-salinity tongue of Río de la Plata (RDP) waters extends northward to 32°S. Below these waters Subantarctic and Subtropical Shelf Waters (SASW and STSW) meet to form the Subtropical Shelf Front. The main SASW branch, oversaturated in oxygen and with high-fluorescence mixes with a detachment of Brazil Current waters at 38°S and is exported offshore along the BMC. A second branch of SASW reaches 33°S mixing along its way with RDP and STSW and returns southward after splitting into an onshore and an offshore branch. The offshore branch is exported to the open ocean through the BMC. These export routes are in overall qualitative agreement with those indicated by a high-resolution reanalysis. We estimate a net off-shelf transport (geostrophic [96.2%] plus Ekman [3.8%]) of 3.44 Sv to the open ocean between 32.1°S and 37.7°S. The majority of the offshore flow occurs between 34.7°S and 37.7°S.

The variability and drivers of the cross-shelf exchanges between the Southwestern Atlantic shelf and the open ocean from 30 to 40°S are analyzed using a high-resolution ocean model reanalysis at daily resolution. The model's performance was first evaluated using altimetry data, and independent mooring and hydrographic data collected in the study area. Model transports are in overall good agreement with all other estimates. The record-mean (1993–2018) cross-shore transport is offshore, 2.09 ± 1.60 Sv. 73% of the shelf-open ocean exchange occurs in the vicinity of Brazil-Malvinas Confluence (~38°S) and 20% near 32°S. This outflow is mostly contributed by northward alongshore transport through 40°S (63%) and the remaining by southward transport through 30°S (37%). The cross-shore flow presents weak seasonal variations, with a maximum in austral summer, and high variability at subannual and weekly time scales. The latter is mainly associated with abrupt wind changes generated by synoptic atmospheric systems. Alongshore wind variations set up sea-level changes in the inner shelf which in turn drive large anomalies in the associated geostrophic alongshore flow. The difference in inner shelf sea-level anomalies at 30 and 40°S is a good indicator of cross-shelf exchange at seasonal and shorter time scales. Episodes of extreme offshore transport that reach up to 9.45 Sv and last about 2 days are driven by convergence of these alongshore flows over the shelf. Large exports of shelf waters lead to freshening of the upper open ocean as revealed by in-situ and satellite observations. In contrast, onshore extreme events drive open ocean water intrusions of up to 6.53 Sv and last <4 days. These inflows, particularly the subtropical waters from the Brazil Current, induce a substantial salinification of the outer shelf.