Rainfall Events during the West African Dry-Season: Forcing from the Extratropics and Predictability

The rare rainfall events of the West African dry season in boreal winter can have substantial impacts on the local hydrology and human activities. A better understanding of their dynamics and predictability can therefore have the benefit of an effective warning of the people and mitigation of the impacts. This study comprises both an illustrative example of widespread and intense dry-season precipitation in West Africa on 20 and 21 January 2004 and a statistical evaluation of forecasts of such events made by the European Centre for Medium-Range Weather Forecasts (ECMWF).

The dynamical analysis of the January 2004 event reveals that the unusual rainfalls are related to the inland penetration of moist southerlies from the Gulf of Guinea associated with a northward shift and intensification of the normally weak wintertime heat low over West Africa. The falling pressure north of about 10°N during the preceding 4–5 days is caused by two different mechanisms: (a) Between 15 and 18 January a weak upper-level trough moves slowly eastward across West Africa. The formation of thin high clouds and the high moisture content on the eastern side of the disturbance enhance the local Greenhouse effect. The associated anomalous warming causes the first slow pressure fall. (b) On 19 January a more intense upper-disturbance penetrates into Algeria and merges with the prior system. This creates a band of negative pressure tendencies across West Africa, mainly caused by subsidence to the southwest of the trough and by warm advection to the southeast.

Examination of operational precipitation forecasts by the ECMWF indicates some skill in predicting such events several days in advance. Most likely the comparably large influence of the usually well-predicted extratropical circulation on the Tropics leads to higher predictability than for ‘ordinary' summertime convection.