Remote sensing of the Northwest African upwelling and its production dynamics

Abstract

The North Atlantic off NW Africa is characterized by the trade wind induced upwelling of cold and nutrient rich waters. As one of most strongest eastern boundary upwelling areas it is marked by low sea surface temperature and high bio-production, both subject to strong seasonal and interannual variations.Satellite measurements of sea surface temperature (SST) and chlorophyll-a (Chl-a) in the upper water-column were used to (a) recognize the upwelling of deep water and its distribution and mixing in the surface, (b) identify the development of phytoplankton blooms and (c) provide further insights in the characteristics of its variations.Measurements of surface Chl-a were compared to deep-sea fluxes of organic carbon in order to determine the relationship between both parameters and to further calculate the export of Corg and the export variation strength.The off-shore extension of SST-defined upwelling from 1988 through 1999 between 18° and 25°N shows maxima in January and May/June with an average area of 140,000 km2 (standard deviation of 20,000 km2). Minima in August exhibit an extension of 43,000 km2 (15,000 km2 standard deviation). A correlation between SST and North Atlantic Oscillation (NAO), previously found in global studies from the North Atlantic, was not discovered in this local-to-regional investigation. On a small-scale-view the upwelling development exhibits a highly variable nature. Local patches of low upwelling intensity exist even in above average years of upwelling. The time period fall 1998 - summer 1999 stand out because of unparalleled strong upwelling not correlated with strong positive NAO, although the opposite should be expected. An intense El Niño-event followed in the Pacific Ocean whose long-distance effects are assumed to have influence, for instance, on the SST in the Atlantic.