Impact of carbonaceous aerosols on precipitation in tropical Africa during the austral summer in the twentieth century

Abstract

We investigate the impacts of anthropogenic aerosols on a precipitation trend observed over tropical Africa during the austral summer in the twentieth century using an atmosphere-ocean general circulation model (medium-resolution version of the Model for Interdisciplinary Research on Climate (MIROC)). We conducted several numerical experiments forced with various combinations of natural and anthropogenic forcings. These experiments indicate that increased carbonaceous aerosols, especially black carbon (BC) aerosols, have played a vital role in the drying trend over tropical Africa, although increased sulfate aerosols contributed to the drying trend at the northern edge of the Intertropical Convergence Zone over tropical Africa. An analysis using an approximated moisture budget equation indicates that the increased carbonaceous aerosols cause the drying trend through an evaporation reduction and a descending anomaly over tropical Africa. The increases in BC and organic carbon aerosols enhance the absorption and scattering of solar radiation, respectively, resulting in reductions of the incident solar radiation, temperature, and evaporation at the surface. On the other hand, the absorption of solar radiation that is due to BC aerosols causes surrounding atmospheric heating in the lower troposphere, leading to an ascending anomaly over the tropical Atlantic Ocean. The ascending anomaly modulates the zonal atmospheric circulation in the Atlantic Ocean, tropical Africa, and the Indian Ocean, which drives a descending anomaly over tropical Africa. Similar atmospheric heating is observed over tropical Africa by atmospheric soundings during the austral summer in the late twentieth century, which supports our results. © 2011 by the American Geophysical Union.