Seasonal and inter-annual variability of chlorophyll-a concentration in the Mauritanian upwelling: Observation of an anomalous event during 1998-1999

Monthly chlorophyll-a (Chl-a) concentrations derived from SeaWiFS data for 1997-2005 and chlorophyll measurements from the Atlantic Meridional Transect for 1995-2001 have been analysed to describe seasonal and inter-annual variability of surface Chl-a in the Mauritanian upwelling. There was a moderate to strong correspondence between the seasonal cycles of surface Chl-a and the seasonal cycles of ocean physical and meteorological fields (such as sea-surface temperature, sea-surface height, and prevailing wind), with a noticeable exception in 1998 that corresponded to a strong anomalous Chl-a event (ˊ250\% increase) in the Mauritanian upwelling. Alongshore wind-stress and wind-stress curl were found to be the most significant factors controlling the variability of Chl-a (jointly explaining more than 50\% of total variance). The biological response to the alongshore wind-stress was immediate, but it lagged the wind-stress curl by 1-2 months (each explaining more than 40\% of the total Chl-a variability). These observations also demonstrate a link, hitherto unreported, between the Pacific El-Niño Southern Oscillation (ENSO) and anomalous Chl-a field in the Mauritanian upwelling. The multivariate ENSO index was shown to account for a significant part of the variability of the autumn-winter Chl-a anomaly (r=-0.52, p0.01). A cold event, following an intense El Niño in the Pacific during summer, was found to mirror the intensity of wind forcing and phytoplankton concentration in the Mauritanian upwelling a few months later. Therefore, ENSO-related changes in the local atmospheric fields are considered as the preferred candidates for explaining the observed biological changes in the Mauritanian upwelling during 1998-1999