Seasonal Variability of the Mauritania Current and Hydrography at 18°N

Extensive field campaigns in the Mauritanian upwelling region between 2005 and 2016 provide the database for analyzing the seasonal variability of the eastern boundary circulation (EBC) and associated water mass distribution at 18 degrees N. The data set includes shipboard upper ocean current, hydrographic, and oxygen measurements from nine research cruises conducted during upwelling (December to April) and relaxation (May to July) seasons. During the upwelling season, the EBC closely resembles a classical eastern boundary current regime, with a poleward undercurrent flowing beneath an equatorward coastal jet. In contrast, elevated poleward flow exceeding 30cm/s and extending from the surface down to 250-m depth is observed during the relaxation season. The pronounced seasonal variability of the across-shore structure of the EBC can be related to local wind forcing and is in general agreement with Sverdrup balance. The EBC transport is correlated to the wind stress curl leading the transport by 7days. The short lead time suggests a fast response of locally forced waves adjusting the EBC to wind forcing. The seasonal and vertical water mass distribution is presented based on hydrographic observations. The meridional oxygen distribution and corresponding water mass partitioning into South and North Atlantic Central Water masses reveal a possible northerly ventilation pathway in the deeper layers of the central water stratum. Our results suggest that the poleward surface flow and the poleward undercurrent both are a consequence of the cyclonic wind stress curl forcing and thus propose to name it the Mauritania Current. Plain Language Summary In this study we use a multiyear data set of shipboard current measurements from nine research cruises conducted between 2005 and 2016. We provide a description of the boundary circulation and its seasonality in the Mauritanian upwelling region off Northwest Africa, which is poorly understood owing to the lack of sufficient observations so far. We find poleward flow in the Mauritania Current to be enhanced and extending from the surface to about 250-m depth during the relaxation season between May and July. During the upwelling season between December and April, weaker poleward flow is confined to the subsurface layers while equatorward flow dominates at the surface. We relate the observed seasonal variability to local wind forcing and find general agreement with theoretical considerations. Water masses in the Mauritanian upwelling region are of both North and South Atlantic origin. Using measurements of temperature and salinity, we present the seasonal and vertical water mass distribution in relation to the boundary circulation. Meridional oxygen and water mass distributions reveal a possible ventilation pathway for oxygenated waters from the north between 250- and 500-m depth.