Robust Sahel Drying In Response To Late 20th Century Forcings - GRL

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<1> The African Sahel experienced severe drying between the 1950s and the 1980s, with partial recovery since. We compare Sahel rainfall in the 20th century, pre-industrial, and increased greenhouse gases (GHG) simulations produced for the Intergovernmental Panel on Climate Change (IPCC). The simulations forced by 20th century concentrations of aerosol and GHG reproduce (i) a global change in SST akin to that associated with Sahel drought and (ii) a correspondent drying of the Sahel. We conclude that late 20th century Sahel climate was significantly dryer than pre-industrial, and at least 30% of the drying was externally forced. Comparison between 20th century runs and runs forced by GHG alone reveals the key role of reflective aerosols: they force a gradient in SST that excites robust drying in the northern edge of the Atlantic Inter-Topical Convergence Zone (ITCZ) and in the Sahel.
Citation: Biasutti, M., and A. Giannini (2006), Robust Sahel drying in response to late 20th century forcings, Geophys. Res.
Lett., 33, L11706, doi:10.1029/2006GL026067.

1. Introduction
<2> The semi-arid African Sahel, the southern edge of the Sahara, experienced severe drying between the 1950s and the 1980s, with partial recovery since . Recent studies showed that the long-term drying must be attributed to global sea-surface temperatures (SST), rather than anthropogenic land-surface modification: general circulation models (GCMs) of the atmosphere forced with the historic time series of SST reproduce, albeit often with reduced amplitude , the time series of Sahel rainfall . The concern for human influence on Sahel climate has thus shifted from local land-use practices to the effects of anthropogenic emissions on global SST.

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<12> It is not surprising that coupled models would not faithfully and consistently reproduce the timing and magnitude of the observed trend. Even with prescribed SST, models reproduce only about 2/3 of the trend , and a growing body of literature relates internal, decadal variations in the MOC, the Atlantic gradient, and African drought. It is plausible that the abruptness and magnitude of the 1950–1999 trend were in part a consequence of internal variability in the atmosphere-ocean-land system. The forced part of the signal emerges not in a sharp 1950–1999 trend, but in the overall change in mean rainfall, a less stringent test. We claim that an internally generated, natural drying was superimposed on an externally forced, likely anthropogenic signal. A rough estimate for the role of external forcings is derived by comparing the observed 1930–1999 Sahel drying with the XX-PI difference. The former is
1.1 mm day
1 (25% over 70 years), the latter is, on average,
0.3 mm day
1 (8%), indicating that perhaps a third of the longterm drying was externally forced (or more, considering possible amplifications by vegetation and dust dynamics).

<13> The GCMs forced exclusively by anthropogenic forcings consistently reproduce the XX-PI change in global SST associated with Sahel drought and a correspondent Sahel drying, suggesting that the important external forcings are anthropogenic. Specifically, Northern Hemisphere sulfate aerosols are key for a robust drying signal in the Sahel, as they force a meridional gradient in Atlantic SST, hence reduced moisture flux by anomalous winds. In simulations where the asymmetric cooling effect of aerosols is absent or overwhelmed by the effect of increased GHG, an acrosstropics uniform warming becomes the dominant signal in SST, and the precipitation response becomes model-dependent. Hence, the cross-model consensus in interpreting the history of Sahel rainfall breaks down in forecasting its future . We hypothesize that different precipitation responses might arise from different balances between competing forcings. First, a warmer Indo-Pacific—implying a warmer, more stable free troposphere Chiang and Sobel, 2002], an upped-ante for convection , and a drier Sahel. Second, a warmer Atlantic and increased land-ocean temperature contrast—implying stronger moisture advection by the monsoon , and a wetter Sahel. Meaningful climate predictions for this vulnerable region require an understanding of what tips the balance toward a dry or wet
Sahel in different models.

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