Dynamics of Wet and Dry Years in West Africa

Abstract

This paper proposes a theoretical framework for describing interannual climatic variability over West Africa. The dynamical theory of zonally symmetrical thermally direct circulations suggests that a meridional monsoon circulation must develop over any tropical region (off the equator) when the absolute vorticity near the tropopause reaches a threshold value of zero. However, for a moist atmosphere that satisfies a quasi-equilibrium balance between moist convection and the radiative forcing, the absolute vorticity at upper-tropospheric levels is a function of both latitude and the meridional distribution of boundary-layer entropy. Hence, the onset of a monsoon circulation depends in a nonlinear fashion on these two factors. The theory predicts that a flat distribution of entropy does not drive any circulation and that a relatively large gradient of entropy should drive a strong monsoon circulation. The location of the region of West Africa, relatively close to the equator, dictates that the dynamics of a monsoon over that region are relatively sensitive to interannual fluctuations in the meridional gradient of boundary-layer entropy. Here, we present observations on entropy and wind over West Africa during the monsoon seasons of 1958 and 1960. The following observations were consistent with the proposed relationship between boundary-layer entropy and the monsoon circulation: a large meridional gradient of boundary-layer entropy, a healthy monsoon, and wet conditions over the Sahel region were observed in 1958; and a nearly flat distribution of entropy, very weak circulation, and relatively dry conditions were observed in 1960. Moreover, the proposed theoretical relationship between the meridional gradient of boundary-layer entropy and the monsoon circulation over West Africa is consistent with the empirical observations of sea surface temperature anomalies (SSTAs) in the tropical Atlantic and rainfall in the Sahel region. Theoretically, a cold (warm) SSTA in the region located south of the West African coast should favor a large (small) meridional gradient of entropy, a strong (weak) monsoon circulation, and wet (dry) conditions in the Sahel. A large body of observations confirms that cold (warm) SSTAs off the southern coast of West Africa are associated with wet (dry) years in the Sahel region.