The transition zone of the Canary Current upwelling region

Like all the major upwelling regions, the Canary Current is characterised b y intense mesoscale structure in the transition zone between the cool, nutr ient-rich waters of the coastal upwelling regime and the warmer, oligotroph ic waters of the open ocean. The Canary Island archipelago, which straddles the transition, introduces a second source of Variability by per turbing t he general southwestward flow of both ocean currents and Trade winds. The c ombined effects of the flow disturbance and the eddying and meandering of t he boundary between upwelled and oceanic waters produce a complex pattern o f regional variability. On the basis of historical data and a series of int erdisciplinary field studies, the principal features of the region are desc ribed. These include a prominent upwelling filament originating near 28 deg rees N off the African coast, cyclonic and anti-cyclonic eddies downstream of the archipelago, and warm wake regions protected from the Trade winds by the high Volcanic peaks of the islands. The filament is shown to be a recu rrent feature, apparently arising from the interaction of a topographically trapped cyclonic eddy with the outer edge of the coastal upwelling zone. I ts role in the transport and exchange of biogenic material, including fish larvae, is considered. Strong cyclonic eddies, observed throughout the year , drift slowly southwestward from Gran Canaria. One sampled in late summer was characterised by large vertical isopycnal displacements, apparent surfa ce divergence and strong upwelling, producing a fourfold increase in chloro phyll concentrations over background values. Such intense eddies can be res ponsible for a major contribution to the vertical flux of nitrogen. The lee region of Gran Canaria is shown to be a location of strong pycnocline defo rmation resulting from Ekman pumping on the wind shear boundaries, which ma y contribute to the eddy formation process. (C) 1998 Elsevier Science Ltd. All rights reserved.