Upwelling Patterns off Portugal

Abstract

Remotely sensed sea surface temperatures are used to characterize the Portuguese coastal upwelling from the distribution and evolution of its surface thermal signature. During the summer, upwelled waters occupy the surface layers over the whole western shelf and part of the upper slope of Portugal, and their areal extent pulsates onshore-offshore in response to cycles of northerly winds, reaching 30–50 km from the coast under calm conditions and extending to 100–200 km during and shortly after strong north winds.

Three regions are examined in detail, each showing a different upwelling pattern related to characteristic topographical constraints. On the wide, flat meridional shelf north of the Nazaré Canyon, upwelling is fairly homogeneous alongshore. Off the southern half of the Portuguese west coast, three-dimensionality is induced on the upwelling distribution by a mesoscale protrusion of the coastline which, associated with the nearshore deep features of the Lisboa and Setúbal canyons, apparently causes an offshore separation of the waters upwelled north of the Bay of Setfabal. South of Cape Sines, isotherms again tend to follow the depth contours, but thermal gradients are closer to the shore in relation to the steep shelf of this region. The zonal south coast of Portugal, the Algarve, is affected directly by upwelling only under locally favorable, westerly winds which blow only occasionally. However, during cycles of moderate to strong north winds over the west coast, even with calm wind conditions in the Algarve, the cold upwelled waters of the west coast apparently turn around Cape São Vicente and then seem to flow eastward along the shelf break, although the same pattern could be generated by shelf edge upwelling. During a particularly strong northwest wind event, upwelled waters were observed to cover the whole Algarve shelf and then proceed offshore from near the mouth of the Guadiana River at the Spanish-Portuguese border, again following the shelf edge of the Gulf of Cadiz. During the upwelling spin-down, a coastal countercurrent seemed to carry warm surface waters to the west, eventually reaching São Vicente, and even proceeding northward along the west coast when winds reversed to southerly. The decoupling between the shelf and the upper slope waters of Algarve is attributed to a strong (≃500 m high) bathymetric “step” extending along the whole Algarve shelf edge.

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