Physical Oceanography of the SW Atlantic Shelf: A review

Abstract

The continental shelf of the western South Atlantic is characterized by three regions subject to distinct oceanographic regimes. The wide subantarctic shelf, south of approximately 35°S is occupied by cold, low salinity waters derived from the Subantarctic Zone and further diluted by the inflow of low salinity waters, primarily from the Straits of Magellan. Farther north, the shelf narrows considerably and is subject to the influence of large freshwater discharges, and of warm-salty intrusions of subtropical waters from the Brazil Current. Intense frontal transitions are present near-shore at various locations, and along the shelfbreak. The fronts promote vertical circulations that inject nutrients into the upper layer, leading to enhanced growth of phytoplankton and in some regions, lead to a significant uptake of atmospheric CO2. While the subantarctic shelf is under the influence of strong westerlies and high amplitude tides, most of the subtropical shelf is under the influence of seasonally reversing winds and a micro tidal regime. The shelf characteristics are also influenced by the offshore circulation, which is dominated by the equatorward flow of cold, nutrient rich waters of the Malvinas Current in the south, and the poleward flow of warm, salty and oligotrophic waters of the Brazil Current in the north. There is a convergent large-scale mean circulation towards the transition between subantarctic and subtropical shelf waters near 34°S, which is compensated by export of shelf waters to the deep ocean. This article describes the contrasting water masses, frontal features and circulation patterns of this region.