On the dynamical structure of the midshelf water column off northwest Africa

Abstract

K undu (1977 , Bottom turbulence, pp. 187–207) showed that the upwelling system off northwest Africa is dominated by turbulent friction and suggested that over such broad and shallow continental shelves, the surface and bottom frictional boundary layers overlap in the water column. Different estimates for the thickness of the boundary layers have been proposed more recently, which take into account, amongst other things, the effects of stratification. The application of these various estimates and the direct computation of the transports within the resulting layers provide a qualitatively consistent picture of the time-dependent structure of the water column on the northwest African continental shelf, with the observed cross-shelf transports in reasonable agreement with the expected Ekman transports. The only exceptions are two instances when the passage of upwelling fronts by the mooring appear to introduce an appreciable difference between the observed and the expected transports. Our results confirm those of K undu (1977) , but only for times when the wind is strongly upwelling favourable. For periods of weak winds, our calculations suggest that a non-turbulent interior region develops. Thus, the structure of the water column is dynamically non-stationary, fluctuating between non-turbulent and frictionally dominated regimes, as a function of the intensity of the wind. This implies that the study of currents through a stationary time-series approach at individual depths may not be applicable in these regions.