2.1 Mechanism of the Coastal Upwelling
2.2 Oceanographic Conditions observed since 1975
The availability of S. aurita depends upon the intensity of the coastal upwelling (Mensah, 1973; ORSTOM, 1976).
Off Ghana and Ivory Coast, in addition to the major upwelling (from July to October) a minor upwelling occurs between January and March each year; it persists for varying periods ranging from two to five weeks. It is of a lower intensity than the major upwelling; the surface temperatures vary between 25° and 27° C.
Data resulting from oceanographic research on the continental shelf of Ghana since 1976 have been used to confirm the present theory that the causes of the coastal upwelling in the area are non-local (ORSTOM, 1976).
Even though the mechanism of the coastal upwelling has not been determined yet, the latest work by Houghton (1974), Houghton and Mensah (1978) and Moore et al. (1978) has confirmed that the upwelling event and changes in the hydrography and water circulation do not correlate with the changes in the coastal wind. There are important differences between the Ghana regime and that observed in other coastal upwelling areas, and no simple driving mechanism is apparent. Existing theoretical models may not be applicable here. It is suggested that waves of oceanic origin might play an important role in the evolution of the coastal hydrography.
Surface current records and hydrographic observations suggest an offshore displacement in the surface layer, but this and the changes in sea level do not correlate with the surface wind. Changes in the circulation on the shelf do not seem to cause the upwellings. However, observation of low frequency waves over the shelf and on the equator in the Gulf of Guinea suggest that they may provide a non-local driving mechanism for the Ghanaian coastal upwelling. The sudden changes in the coastal hydrography during the upwelling could be the effect of a superposition of these waves on the hydrographic structure of the Gulf of Guinea. Since the waves can be generated non-locally, this would account for the lack of correlation between the upwelling and the local wind. Current active research in the Equatorial Atlantic should provide more information about this problem in the near future.
The series of upwelling indices obtained during the last working group (ORSTOM, 1976) was updated using surface temperatures recorded in Abidjan and Tema from 1976 to 1978 (Table 1, Figure 1). Examination of the results indicates that the upwelling has remained at a high and relatively constant level since 1975. However, the initial data for 1979 indicate a very low level, probably roughly equal to that in 1968.
Data collected at the Abidjan coastal station in 1966 to 1978 (Prive and Corre, 1979) were used to calculate the annual mean salinity anomalies during this period (Figure 2). The diagram indicates a period of low salinity before 1970 but high levels from 1971 to 1974. It will nevertheless be noted that the variations are only moderate, and that such variations do not necessarily directly affect the biology of S. aurita.
