Observed Monsoon Precipitation Suppression Caused by Anomalous Interhemispheric Aerosol Transport over the Gulf of Guinea

This study uses observations and atmospheric reanalysis products in order to understand the impacts of smoke aerosols advected from the Southern Hemisphere on the dynamics of the West African Monsoon. Seasonal biomass burning and resulting aerosol emissions have been well documented to affect regional weather patterns, especially low-level convection. Out of all monsoon months, the biggest impacts occur during July, in which anomalous smoke aerosol values can increase (decrease) by 25% (27%) in the Gulf of Guinea and precipitation can decrease (increase) by up to ~5 mm day^-1 (~3 mm day^-1) along the West African coastline. Smoke aerosols produced by biomass burning occurring near Angola and Congo are advected towards the Gulf of Guinea at elevations around the 850 hPa level. These aerosol particles absorb solar radiation and thus stabilize the lower troposphere. Satellite observations show a decrease (increase) in clouds below (above) the 500-hPa level in the Gulf of Guinea and along the West African coastline. Additional observations of shortwave radiation quantify changes in cloud coverage and monsoon dynamics. On average, we find reductions in surface shortwave radiation of ~10-15 W m^-2 over the Gulf of Guinea during increased aerosol transport, with aerosols accounting for ~ 33%-50% of that reduction. Reductions in shortwave radiation are associated with reduced sea surface temperature and increased lower-level monsoon winds. This demonstrates that increased transport of aerosols influences stability in the lower troposphere and eventually cloud coverage, potentially leading to monsoon precipitation suppression. In a broader social context, this region houses 200 million people and thus understanding these climate patterns may carry great importance.