Upwelling filaments off Cap Blanc: Interaction of the NW African upwelling current and the Cape Verde frontal zone eddy field?
Résumé
The hydrographical and dynamical properties of the upwelling filaments forming off Cap Blanc (Mauritania) are investigated using remotely sensed and in situ data collected in April/May 2009 during the strongest upwelling season. The area is situated at the southern edge of the NW African upwelling system, where the Cape Verde Frontal Zone (CVFZ) separates warmer, saltier North Atlantic Central Water (NACW) and cooler, fresher South Atlantic Central Water (SACW). Sea surface temperature images indicated the presence of an upwelling filament extending >280 km offshore, rooted over the Cap Blanc promontory and entrained around a warm-core anticyclonic eddy. After this filament started to decay, a new cold filament developed at the approximate same location. High resolution Moving Vessel Profiler (MVP) and Acoustic Doppler Current Profiler (ADCP) surveys of these mesoscale structures revealed that both filaments were carrying South Atlantic Central Water (SACW) offshore through an intense jet-like flow. Similarity of the relative vorticity structure across the filament with that of the tangent eddy suggested that the latter was responsible for the offshore current. Tracking of this eddy in altimetric data demonstrated that it originated from the CVFZ, as implied by its hydrographic structure. Altimetric data also revealed that another anticyclonic structure centered over the Cap Blanc promontory was responsible for the northwestward advection of SACW into the base of the filament. The results support the idea that some upwelling filaments can be produced by the interaction of an external eddy field, including topographic eddies, with the upwelled water.