IDENTIFICATION AND CHARACTERIZATION OF UPWELLING ZONES

Using data gathered by oceanography surveys during the 1994-1999period, in cold and warm seasons, and thanks to remote sensing, it was possible to locate different upwelling zones in the Moroccan Atlantic coast which are drived by coastal resurgences and determine their main characteristics. During the winter season, the evolution of superficial isotherms reveals three cold water spots between Cape Cantin and Cape Ghir (zone 1), between Cape Draa and Cape Juby (zone 2) and between Cape Boujdor and Dakhla (zone 3). The recorded temperature was of the order of 15 ° C in the three zones with a very marked thermal front at 26 ° N. The resurgence in these areas revealed significant concentrations of surface phosphates (0.8 and 0.7 μM) respectively at Cape Cantin and Cape Boujdor. Similarly, high concentrations exceeding 1 μM were recorded between Cape Blanc and Cape Barbas (zone 4) and are probably due to the influence of the South Atlantic Central Waters, known for their richness in mineral matter. Generally, the extent of the cold zone corresponds to that of the continental shelf, ranging from 15 to 25 miles (Makaoui and al., 2005).

The situation in winter during 1997 and 1998 is characterized by a decline of the resurgence activity, which was expressed by an increase in sea surface temperature (19 ° C) and a very marked depletion of phosphates. This situation coincides with water height anomalies of more than 5 cm compared to the seasonal average cycle detected by Topex / Poseidon in the Gulf of Guinea and the Moroccan Atlantic coast since the beginning of 1995 (Lodyc, 1995), which hides the usual development of upwelling. According to Hagen (2001), the 1997 phenomenon of the ENSO (El Nino-Southern Oscillation) in the Pacific Ocean has an influence on the upwelling regime of both North West Africa and South West Africa.

 

 

During the summer season, the increase in surface temperature in the rising areas and the shrinkage of their surface have characterized this season. The activity of the resurgence is manifested at the level of zones 1 and 2 by an important contribution of the mineral matter to the level of the continental shelf; Whereas at zone 4 the coast / wide thermal gradient is greater than 2 ° C and the phosphate contents are high. Zone 3 is characterized by a regression of resurgence activity, as in winter.

 

The surveys carried out in April 1999 illustrates the spring season coverage and shows normal upwelling activity throughout the Moroccan Atlantic coast with a less intensity than the summer season and more marked than the summer season. These resurgences are demonstrated through high levels of phosphates which exceed 0.5 μM. The resurgences at Cape Boujdor are accompanied by a cold water which drifts southwards to Dakhla (zone 3), forming a coastal filament favored by the Canarycurrent flow in this zone along the coast that get away from the limit of this zone towards the south.

 

 

Using Russian surveys data collected during the autumn season, from 2003 to 2005, the cold North Atlantic waters, brought by the Canary cold current, influence upwelling activity north of Cape Juby (zones 1 and 2). While at zone 3 there is a southward shift of upwelling south of Dakhla to Cape Blanc by surface appearance of phosphate-rich (0.8 μM) and low  dissolved oxygen (4 ml.l-1) water. This richness of phosphates is the result of both upwellingactivity between Cape Blanc and Cape Barbas and the contributions of the Southern Atlantic Central Waters, featured by weak salinities from the south and which causes filament formation at Dakhla, which drifts offshore. During this season, the Cape Boujdor upwelling itself is weak clearly marked by the drift of the thermal front south of 25 ° N and the appearance of saline and less rich in phosphateswaters. This is practically the only season where there has been very little activity in zone 3.

 

 

Thus, four zones, characterized by the rise of cold waters, have been identified along the Moroccan Atlantic coast. These areas are: Cap Cantin-Cap Ghir (Zone 1), Cap Draa-Cap Juby (Zone 2), Cape Boujdor-Dakhla (Zone 3) and Cap Barbas-Cap Blanc (Zone 4) .

 

 

Extract from the National Report on Physical Oceanography, INRH- 2012