Dry-Season Precipitation in Tropical West Africa and Its Relation to Forcing from the Extratropics

Abstract

Precipitation is a major socioeconomic factor in the Guineo-Soudanian zone of tropical West Africa with its distinct summer rainy season from May to October. Albeit rare, precipitation during the dry season can have substantial impacts on the local hydrology and human activities reaching from the rotting of harvests to improved grazing conditions. This study provides an observationally based synoptic and dynamical analysis of an abundant rainfall event during the dry season of 2003/04 that affected the countries of Nigeria, Benin, Togo, and Ghana. The results point to a forcing of the rainfalls from the extratropics in the following ways: 1) Upper-level clouds and moisture to the east of a weak, quasi-stationary extratropical disturbance enhance the greenhouse effect over the Sahel and the adjacent Sahara, and thereby cause a net-column warm anomaly and falling surface pressure. 2) One day before the precipitation event, negative pressure tendencies are further enhanced through warm advection and subsidence associated with the penetration of a more intense upper-trough into Algeria. 3) The resulting northward shift and intensification of the weak wintertime heat low allows low-level moist southerlies from the Gulf of Guinea to penetrate into the Soudanian zone. 4) Finally, daytime heating of the land surface and convective dynamics initiate heavy rainfalls. Operational forecasts of this event were promising, pointing to a strong control by the comparatively well-predicted extratropical upper-level circulation.