The focus of this paper is the role of meridional distribution of vegetation in the dynamics of monsoons and rainfall over West Africa. A moist zonally symmetric atmospheric model coupled with a simple land surface scheme is developed to investigate these processes. Four primary experiments have been carried out to examine the sensitivity of West African monsoons to perturbations in the meridional distribution of vegetation. In the control experiment, the authors assume a distribution of vegetation that resembles the natural vegetation cover in West Africa. Each perturbation experiment is identical to the control experiment except that a change in vegetation cover is imposed for a latitudinal belt that is 10° in width. The results of the numerical experiments demonstrate that West African monsoons and therefore rainfall distribution depend critically on the location of the vegetation perturbations. Changes in vegetation cover along the border between the Sahara desert and West Africa (desertification) may have a minor impact on the simulated monsoon circulation. However, coastal deforestation may cause the collapse of the monsoon circulation and have a dramatic impact on the regional rainfall. The observed deforestation in West Africa is then likely to be a significant contributor to the observed drought.

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Abstract. Here we develop a numerical model to investigate the hypothesis proposed by a companion paper [Eltahir, this issue], which describes a soil moisture-rainfall feedback mechanism. The model is designed to describe the seasonal evolution of the West African monsoon rainfall and is used to perform numerical experiments that elucidate the mechanisms of the response of rainfall to soil moisture anomalies. A significant rainfall anomaly is simulated by the model in response to a hypothetical soil moisture anomaly that has been imposed during early summer. However, the magnitude of this anomaly almost vanishes when the net radiation at the surface is not allowed to respond to the soil moisture anomaly. Hence the results of the numerical experiments support the proposed hypothesis and highlight the crucial importance of the radiative and dynamical feedbacks in regulating the rainfall anomalies that result from the soil moisture anomalies.