Temporal variability of mineral dust concentrations over West Africa: analyses of a pluriannual monitoring from the AMMA

The Sahelian belt is known to be a region where atmospheric levels of suspended mineral dust are among the highest observed on Earth. In the framework of the AMMA (African Monsoon Multidisciplinary Analysis) International Program, a transect of 3 ground based stations, the "Sahe- lian Dust Transect" (SDT), has been deployed in order to obtain quantitative information on the mineral dust content and its variability over the Sahel. The three stations, namely Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Sene- gal) are aligned around 14 N along the east-westward main pathway of the Saharan and Sahelian dust towards the At- lantic Ocean. We discuss data collected between January 2006 and December 2008 to investigate the main charac- teristics of the mineral dust concentration over West Africa and their connection with the dominant meteorological situ- ations. The succession of the dry season during which the Sahel is under the influence of the dry Harmattan wind and the wet season induced by the entrance of the monsoon flow is clearly identified from the basic meteorological parameters (air temperature and moisture, wind direction). Atmospheric dust concentrations at the three stations exhibit a similar sea- sonal cycle, with a monthly maximum during the dry season and a minimum occurring during the rainy season, indicating that the general pattern of dust concentration is similar at re- gional scale. This seasonal cycle of the dust concentrations is not phased with the seasonal cycle of surface wind veloc- ity locally measured, suggesting that it is mainly controlled by Saharan dust transport. Local dust emissions induced by strong surface winds are responsible for the occurrence of ex- tremely high daily concentrations observed at the beginning of the rainy season. A decrease in the dust concentration is observed when moving from Niger to Senegal.