A first description of the summer upwelling off the Bay of Algeciras and its role in the northwestern Alboran Sea

Abstract

A local upwelling process is observed in the mouth of the Bay of Algeciras showing a singular pattern, partially disengaged of the neighboring coastal upwelling variability. The physical mechanisms which drive this local upwelling are not clear, but the Ekman pumping seems to be the most important forcing factor. The surface chlorophyll-a variability associated with this local upwelling is uncorrelated with the concentration registered in the inner Bay of Algeciras or in the surrounding coastal and shelf waters. Secondary, the emerged waters in the mouth of the Bay of Algeciras flow eastwards into the northwestern Alboran Sea dragged by the northern edge of the Atlantic Jet, forming a narrow and elongated nutrient-enriched area (hereafter called filament) usually detached from the coast. A sampling station located at 14.6 km from the coastline and 68 km far from the mouth of the Bay of Algeciras was employed to monitor the presence of the filament in the Alboran Sea. The filament is most frequently presented during the summer (in fact, it was detectable in more than 50% of the days of august). When the filament is distinguishable, its chlorophyll-a concentration doubles in average the values registered in neighboring locations, and in annual average also higher nitrate and phosphate surface concentrations (about 30%) are registered at the filament location. These results indicate that the filament provides nutrients to the surface waters of Alboran not only when it is optically detected. Our analyses indicate that the trajectory of the filament follows the northern edge of the western gyre of Alboran although its width and speed are variable depending on the Atlantic Jet properties and the related sub-mesoscale activity.