[1] This article discusses the results of a suite of numerical simulations of the oceanic circulation in the Southwestern Atlantic Shelf region that are aimed to characterize its mean circulation and seasonal variability and to determine the dynamical mechanisms controlling them. Our experiments indicate that south of 40°S the mean circulation is dominated by a general northeastward flow in the southern portion of the shelf, which is controlled by the discharges from the Magellan Straits, tidal mixing, wind forcing, and the offshore influence of the Malvinas Current farther north. The region from 40°S to 33°S presents the highest seasonal variability, with intrusions of cold sub-Antarctic waters and the northward expansion of mixtures of the Río de la Plata waters in late fall and a slower retraction of the plume during spring-summer. Wind stress variability seems to be the primarily forcing mechanism for the plume dynamics. These model results are in reasonable agreement with observations and previous model results. The present solutions also reveal important additional features of the shelf response. The along-shelf circulation, for example, is largely driven by the western boundary currents in the middle and outer shelf, with induced transports that are 3 times larger than in experiments forced by winds and tides. The analysis also indicates that the upstream influence of the Malvinas Current is felt well beyond its retroflection point in the form of a northward middle-shelf current and that the interaction of the Brazil Current with the Brazilian shelf topography is primarily responsible for inducing steady shelf break upwelling.

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[1] This article analyzes the barotropic circulation in the Southwestern Atlantic Shelf using a three-dimensional numerical model forced with winds and tides. South of 40°S, the shelf circulation is dominated by the propagation of the semidiurnal tides. In this region the diurnal tides are generally weak, except at the shelf edge where they resonate with northward propagating, continental shelf waves. North of 40°S, the tidal circulation is relatively weak, and the circulation is mainly driven by the winds. The wind-driven annual mean circulation is characterized by a broad northeastward flow south of approximately 40°S and is characterized by a southwestward flow farther north. The intense mixing associated with the Patagonian tides enhances the bottom friction that balances the energy input from the wind stress forcing. In contrast with previous results our simulation shows a detrainment of the northward volume transport with latitude due to an offshore flow along the edge of the Patagonian shelf break. The largest seasonal variations of the shelf circulation are observed in the region between 45°S and 25°S where, during the fall, there is a development of a clockwise gyre and a northeastward flow north of 40°S. The gyre weakens toward the winter, and the northeastward flow reverses directions.

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