Abstract
Future projections of the Sahel rainfall are highly uncertain, with different climate models showing widely differing rainfall trends. Moreover, the twentieth-century cross-model consensus linking Sahel rainfall to tropical sea-surface temperatures (SSTs) is no longer applicable in the twenty-first century. Here we show that the diverse future Northern Hemisphere differential warming between extratropical and tropical SSTs can explain the discrepancy in the projected Sahel rainfall. The relationship between SST and Sahel rainfall that holds for the twentieth-century persists into the twenty-first century when the differential SST warming is taken into account. A suite of SST-sensitivity experiments confirms that strong Northern Hemisphere extratropical warming induces a significant increase in Sahel rainfall, which can predominate over the drying impact of tropical SST warming. These results indicate that a trustworthy projection of Sahel rainfall requires the estimation of the most likely future Northern-hemispheric differential warming.
Highlights
Future projections of the Sahel rainfall are highly uncertain, with different climate models showing widely differing rainfall trends
In GFDLESM2M, the amplitudes of tropical and extratropical sea-surface temperatures (SSTs) warming are similar, whereas in MIROC-ESM, extratropical warming is more prominent than warming in the tropics, in the Northern Hemisphere (NH)
Similar to the atmospheric response over North Africa to the NH extratropical warming, tropical Atlantic SST warming can result in an increase in Sahel rainfall with the intensification of the Sahara heat low and the increased low-level moisture flux[32,33]
Summary
Future projections of the Sahel rainfall are highly uncertain, with different climate models showing widely differing rainfall trends. We show that the diverse future Northern Hemisphere differential warming between extratropical and tropical SSTs can explain the discrepancy in the projected Sahel rainfall. A suite of SST-sensitivity experiments confirms that strong Northern Hemisphere extratropical warming induces a significant increase in Sahel rainfall, which can predominate over the drying impact of tropical SST warming. These results indicate that a trustworthy projection of Sahel rainfall requires the estimation of the most likely future Northern-hemispheric differential warming. A multi-model analysis reveals that future Sahel rainfall change can be substantially modified by the magnitude of extratropical warming, which is confirmed by a series of SST-sensitivity experiments. A confident projection of future Sahel rainfall requires an understanding of the cause of the discrepancy in rainfall predictions in current climate models, and here, we contribute to such understanding by establishing NH differential warming as a crucial factor underlying the uncertainty regarding future Sahel rainfall
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