Dust aerosol impact on regional precipitation over western Africa, mechanisms and sensitivity to absorption properties

We investigate the climatic impact of shortwave and longwave radiative forcing of Saharan dust on the West African monsoon and Sahel precipitation using a regional climate model (RCM) interactively coupled to a dust model and running for the period 1996-2006. Two competing effects are found. First a reduction of monsoon intensity in the lower troposphere induced by the dust surface cooling causes a reduction of precipitation, and second an 'elevated heat pump effect' in the higher troposphere induced by the dust diabatic warming causes an increase of precipitation. In the standard model configuration, the net impact of these effects is a reduction of precipitation over most of the Sahelian region (by about 8% on average) except over a Northern Sahel - Southern Sahara band, where precipitation increases. These patterns are very sensitive to the dust absorbing properties, which modulate the intensity of the patterns and the boundary between enhanced and decreased precipitation areas. Finally we show that taking into account dust in the RCM could reduce the model bias compared to available observations. Copyright 2008 by the American Geophysical Union.