Late 20th century attribution of drying trends in the Sahel from the Regional Climate Model (RegCM3)

The RegCM3 has been integrated for the period of 1960-2002 using initial and lateral boundary conditions from the NCEP re-analyses at 6-hour intervals. Greenhouse gas concentrations remain fixed and the land-use/ soil properties are not altered. The RegCM3 accurately portrays a trend from wetter conditions in the 1960s to very dry conditions in the 1980s. The dry 1980s temperatures in the Sahel are approximately 3°K warmer than the wet 1960s, caused by a reduction in precipitation and clouds leading to an increase in the net absorbed solar radiation at the surface. This anomalous warming maintains the steepest meridional temperature gradient and therefore the African Easterly Jet in lower latitudes. The initiation of dry conditions starts in June and persists through July and August in the model simulations. The initiation of dry conditions appears to be related to a weaker Tropical Easterly Jet which may be linked to warmer Indian Ocean temperatures over the past several decades.