An extreme Saharan dust outbreak in spring 2004 and its impact on the onset of the West African monsoon

In March 2004 a large-scale, strong, and persistent outbreak of Saharan dust onto the adjacent tropical and subtropical Atlantic Ocean was observed in satellite imagery. The dust front is initially related to a density current caused by strong evaporational cooling along a precipitating cloud band that penetrates into the northern Sahara ahead of an upper-level trough. At later stages massive upper-level convergence, sinking, low-level divergence, and an explosive anticyclogenesis over northwest Africa cause strong northerly flow and a quick spreading of the dust front to the south and west. The strong pressure gradients over North Africa are further enhanced by the formation of a cyclone ahead of the upper-trough. Surface observations show that the event was accompanied by unusual sensible weather conditions across large parts of North Africa including cold temperatures and high wind speeds in the Sahara, extreme precipitation in Libya and a heat wave at the Guinea coast. The dust outbreak pushed the Intertropical Convergence Zone to an unseasonally southerly position. The cold air over the North African continent and the large dust loadings hinder the establishment of the African heat low, resulting in an unusually late and prolonged Harmattan episode. This event is associated with a delayed northward progression of the African monsoon and widespread dry anomalies along the Guinea Coast. The described processes are documented in high-resolution surface observations of wind, temperature, dew point, and radiation at various stations in southern West Africa.