Abstract
AbstractSubstantial low‐frequency rainfall fluctuations occurred in the Sahel throughout the twentieth century, causing devastating drought. Modeling these low‐frequency rainfall fluctuations has remained problematic for climate models for many years. Here we show using a combination of state‐of‐the‐art rainfall observations and high‐resolution global climate models that changes in organized heavy rainfall events carry most of the rainfall variability in the Sahel at multiannual to decadal time scales. Ability to produce intense, organized convection allows climate models to correctly simulate the magnitude of late‐twentieth century rainfall change, underlining the importance of model resolution. Increasing model resolution allows a better coupling between large‐scale circulation changes and regional rainfall processes over the Sahel. These results provide a strong basis for developing more reliable and skilful long‐term predictions of rainfall (seasons to years) which could benefit many sectors in the region by allowing early adaptation to impending extremes.
Highlights
The Sahel is prone to substantial fluctuations in monsoon (July–September mean) rainfall, at interannual to decadal timescales
We show using a combination of state-of-the-art rainfall observations and high-resolution global climate models that changes in organized heavy rainfall events carry most of the rainfall variability in the Sahel at multiannual to decadal time scales
There is a strong dependence on resolution: HG130 underestimates the trend whereas the HG60 and HG25 show stronger trends that are within the observed range
Summary
The Sahel is prone to substantial fluctuations in monsoon (July–September mean) rainfall, at interannual to decadal timescales. Climate models have successfully demonstrated the importance of sea surface temperature (SST) in driving Sahel rainfall variability at these timescales [Folland et al, 1986; Giannini et al, 2003; Mohino et al, 2011]. A long-standing problem has been that most global climate models underestimate the magnitude of decadal variability in the twentieth century simulations [Biasutti, 2013; Roehrig et al, 2013]. This weak magnitude of decadal rainfall variability in models is not well understood and has received little attention, in spite of obvious implications for multiannual predictions or detection and attribution of observed Sahel rainfall change. In global atmosphere-only models in which SST is prescribed the problem widely occurs [Scaife et al, 2008] which points to an unrealistically weak teleconnection between SST change and Sahel rainfall
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