DYNAMICS OF WET AND DRY YEARS IN WEST-AFRICA

This paper proposes a theoretical framework for describing interannual climatic variability over West Africa. The dynamical theory of zonall y symmetrical thermally direct circulations suggests that a meridional monsoon circulation must develop over any tropical region (off the eq uator) when the absolute vorticity near the tropopause reaches a thres hold value of zero. However, for a moist atmosphere that satisfies a q uasi-equilibrium balance between moist convection and the radiative fo rcing, the absolute vorticity at upper-tropospheric levels, is a funct ion of both latitude and the meridional distribution of boundary-layer entropy. Hence, the onset of a monsoon circulation depends in a nonli near fashion on these two factors. The theory predicts that a flat dis tribution of entropy does not drive any circulation and that a relativ ely large gradient of entropy should drive a strong monsoon circulatio n. 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 gra dient of boundary-layer entropy. Hence, we present observations on ent ropy and wind over West Africa during the monsoon seasons of 1958 and 1960. The following observations were consistent with the proposed rel ationship between boundary-layer entropy and the monsoon circulation: a large meridional gradient of boundary-layer entropy, a healthy monso on, and wet conditions over the Sahel region were observed in 1958; an d a nearly flat distribution of entropy, very weak circulation, and re latively dry conditions were observed in 1960. Moreover, the proposed theoretical relationship between the meridional gradient of boundary-l ayer entropy and the monsoon circulation over West Africa is consisten t with the empirical observations of sea surface temperature anomalies (SSTAs) int he tropical Atlantic and rainfall in the Sahel region. Th eoretically, a cold (warm) SSTA in the region located south of the Wes t African coast should favor a large (small) meridional gradient of en tropy, a strong (weak) monsoon circulation, and wet (dry) conditions i n the Sahel. A large body of observations confirms that cold (warm) SS TAs off the southern coast of West Africa are associated with wet (dry ) years in the Sahel region.