Dynamics of Wet and Dry Years in West Africa

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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 dynam... 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 dynam...

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