Seasonal and intraseasonal surface chlorophyll-a variability along the northwest African coast

Five years of SeaWiFS ocean color data are used to characterize the variability of surface chlorophyll (SCHL) over seasonal and intraseasonal timescales along the northwest African coast (between 10 degrees N and 33 degrees N). This variability is interpreted in regards of remotely sensed wind stress and sea surface temperature and of climatological surface nitrate concentration. Three regions with fairly different behaviors are identified: the region of the subtropical gyre (24 degrees N-33 degrees N) is characterized by a weak seasonality and chlorophyll confined at the coast. The inter-gyre region off Cape Blanc (19 degrees N-24 degrees N) is characterized by a weak seasonality and a persistent large offshore extension of chlorophyll. The region of the recirculation gyre (10 degrees N-19 degrees N) is characterized by a strong seasonality and a large offshore extension of chlorophyll from February to May followed by an abrupt chlorophyll drop that propagates northward from May to June. The seasonal variability is well explained by the seasonal variability in wind-forcing. Nutrient limitation is the key factor that explains the weak offshore extension of chlorophyll in the North. The chlorophyll drop in the South is attributed to the weakening of the wind-forcing and the simultaneous advection of warm water from the South by a coastal and seasonal branch of the North Equatorial Counter Current (NECC). Intraseasonal variability is present in all regions. The cases of good correlation between the intraseasonal variability of the chlorophyll and of the wind-forcing are found to be associated with weak chlorophyll offshore extension and large nutrient limitation.