Large-scale effect of aerosols on precipitation in the West African Monsoon region

We used multi-year satellite observations to study aerosol effects on the large-scale variability in precipitation of the West African Monsoon (WAM) region, which is often impacted by high concentrations of desert dust and biomassburning smoke. We find a statistically significant precipitation reduction associated with high aerosol concentration near the coast of the Gulf of Guinea from late boreal autumn to winter. The largest aerosol-related precipitation reduction (∼1.5 mm d−1) is about 50% of the climatological mean precipitation in the region and occurs mainly at rain rates in the range of 2–17 mm d−1 off the northern coast of the Gulf of Guinea. This reduction cannot be linearly attributed to known climate and weather factors such as El Niño–Southern Oscillation, North Atlantic Oscillation, Atlantic sea-surface temperature, or water vapour. The fractional precipitation variance related to aerosol is about 13%, a value comparable to those related to the known climate factors. Based on the spatial pattern and seasonality of the observed precipitation reduction and its dependence on the rain rate, the observed negative correlation cannot be readily attributed to precipitation effects on aerosol by wet deposition or to rain and cloud contamination of satellite aerosol retrievals. We therefore suggest that our results can be taken as observational evidence of aerosol effects on precipitation. The aerosol associated with the observed precipitation reduction can be traced back to various African sources where large quantities of desert dust and biomass-burning smoke are emitted during much of the year. Given that the emissions of dust and smoke have varied considerably over the past several decades, in part attributable to human activities, our observed rainfall reduction may reflect an anthropogenic impact to some degree. Copyright c © 2009 Royal Meteorological Society