The Seasonal Cycle of Wind-Stress Curl in Subtropical Eastern Boundary Current Regions

Historical surface marine wind reports have been processed to define characteristic seasonal distributions of wind-stress curl over the four major eastern boundary current regions of the World Ocean (i.e., the California, Canary, Benguela, and Peru/Humboldt systems) on smaller scales than previously available. Interregional comparisons show that these “classical” coastal upwelling systems are characterized by cyclonic wind-stress curl near the continental boundaries and anticyclonic curl offshore, in association with predominantly equatorward (upwelling favorable) alongshore wind stress. The cross-shore profile of alongshore stress typically has an offshore maximum located some 200 to 300 km from the coast. The decay of the wind stress toward the coast defines a region of cyclonic wind-stress curl, where coastal upwelling is enhanced by curl-induced oceanic upwelling (Ekman pumping). During summer in the respective hemispheres, the region of cyclonic curl expands poleward in each boundary current system. Particularly intense cyclonic curl occurs adjacent to capes during summer coastal upwelling maxima. During fall and winter when the upwelling system weakens, the latitudinal extent of cyclonic curl adjacent to the coast contracts, and the moist intense cyclonic curl is usually associated with coastal bights.