Impact of Dust Aerosols on the Sahel Climate: A GCM Investigation of Aerosol-Radiation-Cloud-Precipitation Interactions

Dust is one of the most important aerosol sources, which cover the sky over Africa, the equatorial Atlantic Ocean, Middle East, and many other areas over the world. The objective of this study is to investigate the impact of dust aerosols on regional climate, with a focus on the Sahel region where dust events frequently occur, by examining responses of the regional climate system to direct, semi-direct, and indirect aerosol radiative forcings in the UCLA AGCM in terms of the cloud, radiation, temperature, precipitation, and general circulation patterns. The 3D monthly GOCART dust data have been employed in two scenarios, representing more severe dust loadings during 1980s and less dust during 2000s, respectively. Offline simulations reveal that 1) dust direct effect could overshadow cloud forcing under heavily polluted case; 2) semi-direct effect may change the sign of dust radiative forcing, indicating that cloud forcing associated with aerosol semi-direct effect can exceed direct aerosol forcing; and 3) changes in radiative forcing associated with indirect effect are not very significant with increasing AOD. AGCM simulations show that aerosol indirect effect tends to act oppositely to the aerosol direct and semi-direct effects with a much smaller magnitude, and hence the direct and semi-direct effects play a dominant role in the overall climatic effect of dust aerosols over the Sahel region. Increased dust aerosols in the AGCM simulations in 1980s tend to draw the precipitation further in land, which reduces the precipitation in the south part of the rainfall band over Sahel Savanna area centered at 10 N, where precipitation is shifted to the north over Sahel and produced a dipole anomaly. Dust AOD seems to play an important role in the distribution of the precipitation over North Africa since aerosol is the only forcing in these two experiments. To examine the uncertainty regarding the role of dust in Sahel climate variability, the 3D monthly climatology of the GOCART and MATCH dust data has been employed, respectively, in another two experiments. Differences in the simulated precipitation due to the use of different aerosol data reveal that the responses of the regional climate to the aerosol forcing are sensitive to the aerosol optical depth and properties.