Giannini Alessandra, Kaplan Alexey
1International Research Institute for Climate and Society, The Earth Institute at Columbia University, 61 Route 9W, Palisades, NY 10964 USA.
2Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964 USA.
Clim Change. 2019;152(3):449-466. doi: 10.1007/s10584-018-2341-9. Epub 2018 Dec 7.
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.
我们利用耦合模式比较计划（CMIP5）第5阶段产生的多模式集合，从海洋影响的角度综合当前对萨赫勒地区降水变化过去和未来外部强迫的理解。CMIP5多模式均值模拟了萨赫勒地区降水的20世纪演变，包括20世纪中叶向20世纪80年代初最干旱年份的下降以及此后的部分恢复。我们利用一个将人为排放与亚热带北大西洋相对于全球热带海洋温度变化联系起来的物理论据，来证明20世纪后期萨赫勒地区干旱可间接归因于二战后时期特有的气溶胶和温室气体的独特组合。随着全球热带变暖持续加剧，因环境立法遏制酸雨导致北大西洋周边气溶胶排放随后减少，这与当前的部分恢复以及未来变湿的预测一致。应用于上述海表面温度（SST）指数的奇异值分解（SVD）简洁地描述了海洋对萨赫勒地区降水的影响，并揭示了20世纪和21世纪排放影响的近正交性：气溶胶和温室气体的独立影响投射到SST指数的差异上并解释过去的变化，而温室气体的主导作用投射到它们的总和上并解释未来的预测。这一结果挑战了这样一种假设，即由于人为变暖在过去的萨赫勒干旱中起了作用，持续变暖将导致进一步干旱。事实上，21世纪温室气体占主导地位，未受到气溶胶的挑战，其预测结果与变暖导致的季风增强一致，与之前的多模式试验相比，这种响应在CMIP5中更加连贯。
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