This summary is derived from the chapter on the Eastern Tropical Atlantic Environment as described by Y. Gouriou (in Ressources, Pêche et Biologie des Thonidés, Gouriou, 1988). The author points out that within the overall oceanographic context of the Eastern Tropical Atlantic, the Côte d'Ivoire/Ghana upwelling is to be considered as a rather minor source of primary production (Figure 1). However, the causes of its clearly seasonal occurrence are not precisely understood.
The mechanism of intertropical Atlantic upwelling occurring along the Senegalese coastline is well known (Ekman divergence). In the case of the Côte d'Ivoire/Ghana upwelling, recent theoretical propositions concerning the Gulf of Guinea have been put forth as possible explanations of the phenomenon. They are summarized hereafter.
It must be remembered that the annual cycle of sea surface temperature along the coast in the Gulf of Guinea is divided into:
a cold season from June to September;
a warm season from October to May, with a short cold season between January and February).
Up to now, and because of sea surface thermal variations limited in amplitude (1° to 3° C) and in space (coastal area), the shortest cold season has attracted far less attention than the longest one, despite its sharp inter-annual variations which may lead to its complete disappearance; the shortest cold season, which is unknown in the equatorial zone, seems to be a strictly coastal phenomenon.
In fact, this succession of a minor upwelling, of limited amplitude in certain years, and of a major upwelling is observed in Tema and Abidjan. But with a few exceptions, no regular cartography showing the extent of these upwellings was made (Figure 2) and, therefore, knowledge on these upwelling is derived mainly from oceanographic measurements of coastal surface temperatures.
From the adequate series of coastal temperature data recorded between 1962 and 1982 it has been demonstrated (Picaut, 1983) that the propagation of the surface temperature signal was running east to west from the Togo/Benin area up to Cape Palmas; this concerns the longest cold season (major upwelling) of June through September. According to Roy (1982), propagation of the short cold season minimum sea surface temperature signal runs from east to west along the coast of Ghana and Côte d'Ivoire and in a direction opposite to the Guinea current; Roy, however, could not identify the mechanism which explained this phenomenon.
Therefore, the conclusion is that the two upwellings, that is, the minor upwelling of January-February, and the major upwelling of June-September, are of totally different origin. The cause of the major upwelling could be a Kelvin wave moving eastward along the Equator, which is divided into two coastal waves once it reaches the African coastal belt, each propagating along the coastline, one southward, the other northward from the Gulf of Guinea. The effect of these two waves is a rise of the surface thermal structure allowing for the sub-thermocline waters to reach the surface.
As proposed by Moore et. al., this explanation based on the assumption of a sudden increase in winds off Brazil, is rather schematic, given the complexity of the actual wind structure (in space and time) and the continuous pattern of wind intensification and abatement.
It has been possible, with numerical models, to determine the mechanisms of the ocean's response to wind action. The winds' continuous variations generate multiple waves which propagate and are reflected on the eastern and western boundaries of the basin. The superposition (reflection + interferences) of all these waves contributes to the equilibrium of the basin. It is therefore difficult to distinguish the propagation of individual waves. Schematically, the ocean at the Equator would respond to wind tension intergrated through the basin and not only to tension off Brazil.
As yet, there is no definite explanation for the minor upwelling which can be attributed to three potential mechanisms (Gouriou, 1987).
Ekman's divergence: - Along the northern coast (Côte d'Ivoire, Ghana) the monsoon wind blows almost parallel to the coast; however, because of only slight variations of winds in the Gulf of Guinea, it has not been possible to correlate wind with upwelling (Bakun, 1978).
Advection by currents: - A subsurface countercurrent flows westward along the northern coast, but it is too weak to create any upwelling by advection of cool waters originating from the Bay of Biafra. Along the southern coast, the cool water transported by the Behguela current which flows westward as from 18°S cannot explain the upwelling occurring north of 130°S.
Current-induced upwelling: - It has been suggested by Ingham (1970) that a rise in the isotherms induced by a stronger Guinea current when in geostrophic equilibrium may cause water cooling as observed along the Côte d'Ivoire and Ghana coastlines; but the numerical models have demonstrated that any cooling thus induced would be minimal.
Two methods have been selected for calculating upwelling indices:
the first method consists in fortnightly calculations of the difference between temperatures recorded and the selected reference temperature of 26°C (similar results are obtained with different reference temperatures, i.e., 25°, 24°, 23°C; Figures 3a, 3b and 3c).
the second consists in fortnightly summations of anomalies in temperatures calculated in relation to an average cycle for the whole period (i.e., 1966 to 1986). This approach gives a better understanding of the impact of the minor upwelling.
examination of Figures 4 and 5 shows that there is correlation between the indices of upwelling calculated for the major upwelling season at Tema and Abidjan but not for those of the shortest cold season. Furthermore, the assumption that minor and major upwellings have different origins is corroborated by the fact that neither in Abidjan nor in Tema is there any apparent correlation between short and long cold season upwelling (Figures 6 and 7). In view of the above, the working group insisted that upwelling indices should be calculated for both cold seasons and for Ghana as well as for Côte d'Ivoire, because they are independent and that their impact is different in terms of the dynamics of the fishery-climate relationship (i.e., recruitment-upwelling index).
an index has been calculated for the Côte d'Ivoire upwelling in the major cold season; it is based on surface temperature values recorded in Port Bouët. The index obtained shows a good correlation (to the nearest scale factor) with the longest cold season (Figure 8). The upwelling index trend relates to values of the Ghanaian series (Table 1). It was noted that this index, which remained at low levels between 1979 and 1983, increased significantly as from 1982 to subsequently reach levels as high as those of 1974–78. It was further noted that according to available information for 1987, the upwelling of the main cold season was met in Ghana and Côte d'Ivoire.
