The seasonal upwellings in the Gulf of Guinea

Abstract

Observational as well as numerical experiments show that Ekman divergence neither at the equator nor at the coast can explain a significant part of the annual upwelling in the Gulf of Guinea. The remote forcing theory is tested by analyzing both historical data and the FOCAL hydrographic stations (1982–1984). The meteorological data base is analysed as well. Analyses of the thermostad show that waters with the same characteristics as those of the South Atlantic Central Water (SACW) are carried by the equatorial under current (EUC) and the South Equatorial Undercurrent (SEUC) from the western-central equatorial Atlantic at 35° – 29°W towards the thermostad regions in the Gulf of Guinea. The thermostad water is then carried back westward in the two geostrophic compensation flows flanking the EUC at 2° – 3°N and 2° – 3°S. The cooling of the subthermoclinal waters off Abidjan (5°N) and Pointe-Noire (5°S) is related to the advection and vertical spreading of SACW between 500m and 50m in May–June–July. An exceptional warm event occurred in early 1984 in the Gulf of Guinea. The winds collapsed nearly completely all along the equator in January–April 1984. The onset of the African monsoon was late (in July–August 1984), then the monsoon was weaker than climatology in August–October 1984. The equatorial thermocline was observed 50m deeper than climatology in February 1984, then near the surface in July 1984. Sea levels rose over the equatorial Atlantic, the surge reaching about 13cm at 6°E in February 1984, when the western-central equatorial Atlantic was nearly flat. Minimum sea levels occurred in June–July 1984 at 6°E, before the African monsoon. During this event, the maximum speed in the shallow core of the equatorial undercurrent at 4°W and 6°E was nearly insensitive to large changes in the thermal structure (downwelling or upwelling situations). In February–May 1984, we have observed distinct increasing eastward flows at the equator below 250m and minimum equatorial thermostad thickness. In aay 1984, a deep eastward jet was observed at 4°W carrying about 3 ×10 6m 3s −1 deformation and below the base of the thermostad, clearly separated from the EUC. Then, maximum thermostad development was found in July 1984, related to the shoalin of the deep jet and of the EUC. The top of the deep jet had shoaledto about 200m and its transport increased to about 4.6 × 10 6m 3s −1 within 1°30'N – 1°30'S. The spreading of the isotherms from about 300m is indicative of a geostrophic balance. Simultaneously, the equatorial thermocline was uplifted near the surface, although the base of the thermostad (13°C isotherm) remained nearly stationary. Analysis of the perturbation temperature field shows that variations of the 19°C isotherm depth as well as the thickness of the equatorial thermostad were strongly equatorially trapped, with scales associated with the second baroclinic mode. In the absence of local forcing in the Gulf of Guinea from January to July 1984, the only causal effect to explain these large perturbations in the upper 400m lies in the changes in the zonal wind stress to the west of the Gulf. The distinct semi-annual cycle observed in the thermal and salinity structures in the Gulf could be attributable to the semiannual signal in the zonal wind stress to the east of 30°W. Numerical experiments by PHILANDER and PACANOWSKI (1986a) confirm that remote forcing by changes in zonal wind stress in the western-central equatorial Atlantic is the main process in seasonal changes in the thermocline depth in the eastern equatorial Atlantic, and a secondary process along the coasts. The thinning of the thermostad in July 1984 at 3° – 4°N associated with a strong Guinea current, concommitant with its thickening at the equator associated with a deep eastward jet, suggests that the upwelling along the coast is essentially a consequence of the equatorial adjustment in response to the zonal wind stress in the equatorial zone 10°W – 35°W. Continuous observations of wind stress at St Peter and St Paul rocks (1979–1988) indicate that the trade winds relax and strengthen through a series of strong bursts superimposed on the seasonal cycle, and that an impulse forcing is approprite from intraseasonal to seasonal time scales. Since the equatorially trapped waves theory is now able to explain many aspects of both the 1984 winter warm event and the annual cycle, it is very likely that the remotely-forced equatorial dynamics govern not only the annual cycle but also the “quasi El Niño” in the eastern equatorial Atlantic Ocean.