Abstract

We analyze hydrographic and current observations of an upwelling center off eastern South America on the Uruguayan coast (∼35S) and downshelf from the Rio de la Plata estuary. Our observations show that the buoyancy-driven subtidal alongshore circulation is modulated by winds. During the winter, strong upwelling-favorable local winds forced the Rio de la Plata buoyant plume poleward. In the summer, the plume detached from the coast with warm and saline subtropical waters intruding from the north. These waters bounded the upwelling of cold waters in the nearshore area off Uruguay. A shallow submarine canyon cutting obliquely across the shelf facilitates the advection of cold waters into the coastal upwelling domain. An upwelling jet was detected off Uruguay for summer conditions, while for winter the plume configuration generated strong horizontal shear of along-shelf currents. Cross-shelf circulation was dependent on the coastal stratification. Under low stratification (summer) a two-layer circulation developed, with thick surface and bottom mixed layers flowing in opposite directions. High stratification levels (winter) modified this pattern by allowing circulation along a thick stratified interior. The existence of upwelling was linked to the history of wind events and shelf stratification. During the winter, downwelling winds frequently restore the plume, so the upwelling efficiency is very low. In the summer, downwelling events are less frequent and intense, so that the cumulative effect of upwelling events act to export the Plata freshwaters offshore. The reduction of inner-shelf stratification increases the likelihood of a full upwelling to the surface. Analysis of the wind-induced Ekman transport suggests that the Uruguay upwelling system reflects a seasonal wind pattern modulated by significant interannual variability.

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