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.
Anthropogenic aerosol effective radiative forcing is model dependent,
but the impact of such uncertainty on the simulation of long-term West
African monsoon 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 West
African monsoon (locations of the Saharan heat low and African Easterly
Jet, of the strength of the west African westerly jet, and of African
Easterly Waves 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 does 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 West African monsoon.