East Africa has two rainfall seasons, the main season, the long rains, runs from March to May. There is currently little understanding of what controls the amount of rainfall during this season. Recent drying, causing many areas to suffer from droughts and food shortages, contrasts with the projections from most climate models of a wetter future (the “East African climate paradox”).

This apparent disagreement does not appear to be simply due to natural variability, but may be because the drying was not driven by greenhouse gas emissions. Although rainfall has recovered somewhat recently, and 2019/2020 was exceptionally wet, it remains important to better understand the variability in the long rains in order to both improve model predictions across time-scales and explain the apparent paradox.

The new research shows that variability in rainfall during the long rains is found to be connected to the strength of easterly winds over the Congo basin and Gulf of Guinea (correlation of 0.73), with the same mechanism controlling variability on both inter‐annual and decadal timescales. This is consistent with the recently identified role of westerlies in generating rainfall. From 1998 to 2011 the winds had been getting stronger, with reduced rainfall over East Africa. The cause of the stronger wind is investigated, and is partly explained by relatively faster warming in the Sahel than over the Congo, whilst variation in Madden‐Julian Oscillation (a large scale tropical wave) activity, explains around 18% of the decadal drying. Thus the work shows that predictions of East African rainfall across timescales require robust prediction of both zonal winds and Madden Julian Oscillation activity.

Download the paper: Common mechanism for inter‐annual and decadal variability in the East African long rains.