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 additional low-salinity waters, primarily from the Magellan Strait. Farther north, the shelf narrows considerably and is subject to the influence of large freshwater discharges and warm-salty intrusions of subtropical waters from the Brazil Current. Intense frontal transitions at various near shore locations and along the shelf break promote vertical circulations that inject nutrients into the upper layer. This nutrient injection leads to enhanced growth of phytoplankton, and, in some regions, 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 undergoes 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 toward the transition between subantarctic and subtropical shelf waters near 34°S, which is balanced by export of shelf waters to the deep ocean. This article describes the contrasting water masses, frontal features, and circulation patterns of this region.