Variations in southern African precipitation have a major impact on local communities, increasing climate-related risks and affecting water and food security, as well as natural ecosystems. However, future changes in southern African precipitation are uncertain, with climate models showing a wide range of responses from near-term projections (2020-2040) to the end of the 21st century (2080-2100). Here, we assess the uncertainty in southern African precipitation change using five Ocean-Atmosphere General Circulation single model initial-condition large ensembles (30 to 50 ensemble members) and four emissions scenarios. We show that the main source of uncertainty in 21st Century projections of southern African precipitation is the internal climate variability. In addition, we find that differences between ensemble members in simulating future changes in the location of the Angola Low explain a large proportion (~60%) of the uncertainty in precipitation change. Together, the internal variations in the large-scale circulation over the Pacific Ocean and the Angola Low explain ~64% of the uncertainty in southern African precipitation change. We suggest that a better understanding of the future evolutions of the southern African precipitation may be achieved by understanding better the model’s ability to simulate the Angola Low and its effects on precipitation.

Future changes in Sahel precipitation are uncertain because of large differences among projections from various models. In order to explore this uncertainty, we use a storyline approach which seeks to identify alternative plausible evolutions of Sahel precipitation and their driving factors. By analysing projections from the CMIP6 climate models, we show that changes in North Atlantic and in Euro-Mediterranean temperatures explain up to 50% of the central Sahel precipitation change uncertainty. We then construct several storylines of Sahel precipitation change based on future plausible changes in North Atlantic and Euro-Mediterranean temperatures. In one storyline, an amplified warming of both the North Atlantic and the Euro-Mediterranean areas promotes a northward shift of the West African monsoon, increasing precipitation over the central Sahel, while, in another storyline, a moderate warming in both regions is associated with a small change in precipitation over the central Sahel and a decrease in precipitation over the western Sahel, at the end of the 21st century. These results indicate that reducing uncertainty in future changes in Sahel precipitation could be achieved by reducing uncertainty in the future warming of the North Atlantic and the Euro-Mediterranean areas.

This paper expands on work showing that the winter North Atlantic Oscillation (NAO) is predictable on decadal timescales to quantify the skill in capturing the North Atlantic eddy-driven jet’s location and speed. By focussing on decadal predictions made for years 2-9 from the 6th Coupled Model Intercomparison Project over 1960-2005 we find that there is significant skill in both jet latitude and speed associated with the skill in the NAO. However, the skill in all three metrics appears to be sensitive to the period over which it is assessed. In particular, the skill drops considerably when evaluating hindcasts up to the present day as models fail to capture the latest observed northern shift and strengthening of the winter eddy-driven jet and more positive NAO. We suggest the drop in atmospheric circulation skill is related to reduced skill in North Atlantic Sea surface temperature.

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.