Wind-Field Coherence and Its Variations over West Africa

Abstract

The wind-field coherence over West Africa is investigated during two contrasted periods, 1953–63 (abnormally wet) and 1963-75 (abnormally dry) in the Sahel. Zonal and meridional components of the monthly mean wind are computed with radiosonde data relative to the 950-100-hPa layer. A descriptive analysis reveals the main features of the West African atmospheric circulation, and a correlation analysis points out their dynamical coherence. The exiwnce of Hadley-type cells of Northern and Southern hemispheres are confirmed, and a mutual interaction in the lower layers is depicted. The functioning of a Walker-type cell, defined by a coupling between the zonal components of the monsoonal flow and of the tropical easterly jet (TEJ), is shown during northern summer over the whole of West Africa. There is no correlation between the TEJ velocity and the African easterly jet (AEJ) velocity over the Sahelian belt. During the dry period, the coherence of the cell circulations vanishes in concomitance with a more southward position of the intertropical convergence zone, a reduced monsoonal flow, an increased AEJ velocity, and a reduced TEJ velocity. The vertical sheer involved by the thermically enhanced AEJ is concomitant at the monthly time scale, with the decrease of the vertical coupling between the lower Layers' convergence of the monsoons flow and the lower layers' divergence associated with the TEJ.