The aerosol- and water vapor-related variability of precipitation in the West Africa Monsoon

The precipitation variability in the West Africa Monsoon (WAM) is affected by many factors. Close to the largest aerosol emission sources (mineral dust from Sahara and Sahel, and biomass burning smokes from Sahel and southern Africa), the WAM provides a natural laboratory to study the possible connections between aerosol, water vapor and precipitation. Possible aerosol and water vapor effects have, however, not been thoroughly investigated. Additionally how aerosol effect is distinguished from environmental water vapor effect is hardly visited. Using long term multi-satellite observations of aerosol (TOMS), water vapor (SSM/I) and precipitation (GPCP), this study elucidates possible large-scale effects from the absorbing aerosol (primarily mineral dust and carbonaceous aerosol) and atmospheric water vapor on precipitation in the WAM. After removing seasonal cycle, ENSO, tropical Atlantic SST and North Atlantic Oscillation (NAO) effects, precipitation difference composite between anomalously high and low aerosol (or water vapor) was made. It is found that in the WAM both aerosol and water vapor effects on precipitation feature strong seasonality and spatial variability. However, aerosol and water vapor may explain precipitation variability independently to each other: when aerosol is anomalously high, the precipitation is generally suppressed in boreal cold season (September to March). This occurs significantly over central WAM; In contrast, when water vapor is anomalously high, precipitation over the southern part of the WAM is enhanced in most part of the year, which essentially leads to a southward shift of the WAM. The Kolmogorov-Smirnov test proved that the precipitation changes related to aerosol and water vapor are significantly different from those due to random sampling at 95% confidence level. Averaged over the WAM [10S~20N, 20W~20E], the aerosol and water vapor related precipitation variability are comparable and both explain ~5-20% of the residual variance of precipitation after known climatic effects (e.g., ENSO, tropical Atlantic SST modes, and NAO) are removed. Aerosol therefore might play an important role in affecting precipitation in the WAM in addition to these known climatic factors and this role is not strongly associated with the precipitation change attributed to environmental dry air.