Abstract
We exploit the multi-model ensemble produced by phase 5 of the Coupled Model Intercomparison Project (CMIP5) to synthesize current understanding of external forcing of Sahel rainfall change, past and future, through the lens of oceanic influence. The CMIP5 multi-model mean simulates the twentieth century evolution of Sahel rainfall, including the mid-century decline toward the driest years in the early 1980s and the partial recovery since. We exploit a physical argument linking anthropogenic emissions to the change in the temperature of the sub-tropical North Atlantic Ocean relative to the global tropical oceans to demonstrate indirect attribution of late twentieth century Sahel drought to the unique combination of aerosols and greenhouse gases that characterized the post-World War II period. The subsequent reduction in aerosol emissions around the North Atlantic that resulted from environmental legislation to curb acid rain, occurring as global tropical warming continued unabated, is consistent with the current partial recovery and with projections of future wetting. Singular Value Decomposition (SVD) applied to the above-mentioned sea surface temperature (SST) indices provides a succinct description of oceanic influence on Sahel rainfall and reveals the near-orthogonality in the influence of emissions between twentieth and twenty-first centuries: the independent effects of aerosols and greenhouse gases project on the difference of SST indices and explain past variation, while the dominance of greenhouse gases projects on their sum and explains future projection. This result challenges the assumption that because anthropogenic warming had a hand in past Sahel drought, continued warming will result in further drying. In fact, the twenty-first century dominance of greenhouse gases, unchallenged by aerosols, results in projections consistent with warming-induced strengthening of the monsoon, a response that has gained in coherence in CMIP5 compared to prior multi-model exercises.
Highlights
The Sahel witnessed an outstanding climatic shift with the abrupt onset of drought in the late 1960s
In the second half of the twentieth century, as global dimming (Stanhill and Cohen 2001; Liepert 2002) opposed global warming in the northern hemisphere, a unique combination of anthropogenic emissions contributed to the late twentieth century drying of the Sahel through their effect on sea surface temperatures: aerosols by cooling the North Atlantic and greenhouse gases by warming the tropical oceans, especially the Indian Ocean
Our argument is distinct from others previously proposed, which attributed late twentieth century Sahel drought solely to aerosols, whether through cooling of the North Atlantic or of the entire northern hemisphere (Rotstayn and Lohmann 2002; Kawase et al 2010; Ackerley et al 2011; Booth et al 2012; Hwang et al 2013; Park et al 2015; Wang et al 2016), in three ways
Summary
The Sahel witnessed an outstanding climatic shift with the abrupt onset of drought in the late 1960s. Recent work attempting to explain the disagreement in the sign of projected twenty-first century rainfall change (Biasutti and Giannini 2006) with differences in projected SST change added the explicit consideration of oceanic warming, resulting in a linear regression model that describes Sahel rainfall as a function of two regional SST indices: the difference of tropical North and tropical South Atlantic SST averages and the average of tropical Indo-Pacific sector SSTs (Biasutti et al 2008) This regression model was further distilled into a single predictor, the North Atlantic Relative Index (NARI; Giannini et al 2013), computed as the difference between SST averages over the sub-tropical North Atlantic (10°N–40°N, 75°W–15°W) and the global tropical oceans (20°S–20°N). In the BConclusions: Past is not prologue^ section, we conclude with a synthesis that finds coherence in attributing past Sahel drought to anthropogenic emissions, while lending credence to projections of a wetter future
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