Uncertainty in simulating twentieth century West African precipitation trends: the role of anthropogenic aerosol emissions

Abstract

Anthropogenic aerosol emissions from North America and Europe have strong effects on the decadal variability of the West African monsoon (WAM). Anthropogenic aerosol effective radiative forcing is model dependent, but the impact of such uncertainty on the simulation of long-term WAM variability is unknown. We use an ensemble of simulations with HadGEM3-GC3.1 that span the most recent estimates in simulated anthropogenic aerosol effective radiative forcing. We show that uncertainty in anthropogenic aerosol radiative forcing leads to significant uncertainty at simulating multi-decadal trends in West African precipitation. At the large scale, larger forcing leads to a larger decrease in the interhemispheric temperature gradients, in temperature over both the North Atlantic Ocean and northern Sahara. There are also differences in dynamic changes specific to the WAM (locations of the Saharan heat low and African Easterly Jet, of the strength of the West African westerly jet, and of African Easterly Wave activity). We also assess effects on monsoon precipitation characteristics and temperature. We show that larger aerosol forcing results in a decrease of the number of rainy days and of heavy and extreme precipitation events and warm spells. However, simulated changes in onset and demise dates do not appear to be sensitive to the magnitude of aerosol forcing. Our results demonstrate the importance of reducing the uncertainty in anthropogenic aerosol forcing for understanding and predicting multi-decadal variability in the WAM. Plain Language SummaryThe Sahelian drought of the 1970s and 1980s had consequences on agriculture, economy, and population migration, among others. The Sahelian drought is known to be partly caused by emissions of aerosol pollution from North America and Europe, leading to a reduction in rainfall for West Africa. However, the effect of aerosol pollution on atmospheric radiation-the light and heat that passes through the atmosphere-is uncertain, and the models we use to examine past and future climate change show a wide range of responses to these effects. We use a novel collection of simulations to assess the range of different outcomes for the West Africa monsoon based on this uncertainty in the effects of aerosol pollution. We show that simulations in which the atmosphere has a weak response to aerosol pollution do not reproduce the observed drying trend over West Africa, while simulations with a stronger atmospheric response to pollution feature a larger drought. This uncertainty in the effects of aerosol pollution leads to uncertain changes in the West African monsoon winds and rainfall and in extremes of rainfall and temperature.