Rainfall, Temperature and Dust Storm Anomalies in the African Sahel

Since dust storm frequency in the African Sahel is considered an indicator of drought phases, this paper points out basic interrelations of annual and interannual dust storm, rainfall and temperature variability in the region, as well as some large-scale patterns of atmospheric control. The normal seasonal pattern is characterized by a temperature maximum in June/July, a rainfall maximum in August, and a dust storm maximum in April which is linked to a negative soil water budget towards the end of the dry season. However, the dust storm minimum is reached as late as October, i.e. after sufficient soil water infiltration and vegetation cover increase. Thus, seasonal parameter interrelation reflects seasonal shifts in regional atmospheric circulation. Interannual variability, however, seems to occur with periodicities around 2-4 years, 6 years, 11-12 years and 25 years, a result that applies when modelling temperature, rainfall and dust storm series. Further statistical analysis provides insight into the internal structure of the short-term oscillations. Positive springtime dust storm anomalies are preceded by one or two dry years with above-normal summer temperatures. In this case, the evolution of a secondary dust storm maximum in winter is most likely. Over a sequence of 2-3 dry years, dust storm anomalies increase from spring to the following winter. When considering possible atmospheric controls, it was observed that higher summer temperatures in a Sahelian dry year are followed by increased zonal flow in the mid-latitude circumpolar vortex, and colder winters in the Sahel itself. The summer and winter relationship implies these processes to be an effect of meridional transport of excess latent heat after suppression of the West African Walker cell in early summer, and an oversteepened pressure gradient with increased surface trades during the following winter.