Circulation and Hydrography in the Western South Atlantic Shelf and Export to the Deep Adjacent Ocean: 30°S to 40°S

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 degrees S and 38 degrees 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 Rio de la Plata (RDP) waters extends northward to 32 degrees 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 degrees S and is exported offshore along the BMC. A second branch of SASW reaches 33 degrees 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 3.44Sv to the open ocean between 32.1 degrees S and 37.7 degrees S. The majority of the offshore flow occurs between 34.7 degrees S and 37.7 degrees S. Plain Language Summary The western South Atlantic shelf is characterized by a northeastward flow of Subantarctic Shelf Water (SASW) that meets a southwestward flow of Subtropical Shelf Waters creating the Subtropical Shelf Front (STSF). The STSF is capped by low salinity layer derived from the Rio de la Plata. The outer shelf region is swept by intense and opposing flows: the cold-fresh Malvinas Current and the warm-salty Brazil Current. Here, we analyze data from an oceanographic survey of the western South Atlantic shelf (31-38 degrees S) in October 2013. We describe the distribution and circulation of the water masses with emphasis on the cross-shelf exchange. On-shelf subsurface intrusions of oceanic waters and river discharge supply nutrients to the shelf. We estimate an off-shelf transport of 3.44Sv. The majority of this flow is located at the Brazil-Malvinas Confluence and at 33.5 degrees S. A fraction of the exported shelf water is associated with high biological activity. This off-shelf flow might also transfer planktonic eggs and larvae to the open ocean, modulating their abundance over the shelf. Moreover, the detrainment of high-chlorophyll shelf waters may play an important role in the fertilization of the western South Atlantic. Key Points The circulation of the western South Atlantic shelf is determined based on in situ observations We estimate a net offshore flow of 3.44Sv from the shelf to the open ocean Diluted AAIW and SACW intrusions and RDP waters supply nutrients to the continental shelf