Upwelling features off the coast of north-western Africa in 2009-2013

Satellite data (images of sea surface temperature and chlorophyll-a), ocean surface wind products, Lagrangian observations (surface drifters) and other ancillary data (upwelling index) are used to describe the upwelling seasons off NW Africa during 2009-2013, with particular focus on the coasts of Senegal and Mauritania. The impact of the upwelling is characterised by a comparative analysis, carried out in terms of wind-induced upwelling and water/ecosystem response to this forcing, of five geographical sectors detected in the study area. The wind forcing analysis shows the most favourable upwelling conditions in the period December-June in the southern sectors (south of 16° N), and from February to October in the northern sectors (north of 18° N). Southern sectors are strongly influenced by wind forcing, whereas to the north the upwelling also occurs during the months with low Ekman transport values. The analysis of the sea surface temperature and chlorophyll-a concentration confirms the existence of an upwelling season during winter-spring in the south, and emphasizes the different behaviours between the northern and southern sectors. Drifter tracks allow the addition of details about the flow of cold water offshore and alongshore. In particular, they describe the westward transport of cold water, by means of energetic filaments rooted at specific locations along the coast, north of Cape Vert and the south-SW ward transport of the coastal water south of Cape Vert.