Forced Sahel rainfall trends in the CMIP5 archive

Abstract

[1] The simulations of the fifth Coupled Models Intercomparison Project (CMIP5) strengthen previous assessments of a substantial role of anthropogenic emissions in driving precipitation changes in the Sahel, the semiarid region at the southern edge of the Sahara. Historical simulations can capture the magnitude of the centennial Sahel drying over the span of the 20th century and confirm that anthropogenic forcings have contributed substantially to it. Yet, the models do not reproduce the amplitude of observed oscillations at multidecadal timescales, suggesting that either oscillations in the forcing or the strength of natural variability are underestimated. Projections for Sahel rainfall are less robust than the 20th century hindcast and outlier projections persist, but overall the CMIP5 models confirm the CMIP3 results in many details and reaffirm the prediction of a rainy season that is more feeble at its start, especially in West Africa, and more abundant at its core across the entire Sahel. Out of 20 models, four buck this consensus. Idealized simulations from a subset of the CMIP5 ensemble-simulations designed to separate the fast land-atmosphere response to increased greenhouse gases (GHGs) from the slow response mediated through changes in sea surface temperature (SST)-confirm that the direct effect of CO 2 is to enhance the monsoon, while warmer SST induce drying over the Sahel. At the same time, these simulations suggest that the seasonal evolution of the rainfall trends in the scenario simulations, spring drying and fall wetting, is an inherently coupled response, not captured by the linear superposition of the fast and slow response to CO 2 .

On the spatial coherence of rainfall over the Saloum delta (Senegal) from seasonal to decadal time scales

“…In any case, knowledge of the interannual to decadal variability in the monsoon circulation is still quite limited (Afiesimama et al, 2006;Maloney and Shaman, 2008). The simulations of the fifth Coupled Model Intercomparison Project (CMIP5) confirm results from previous generations of models attributing a substantial role of anthropogenic emissions in driving precipitation changes in the Sahel (Biasutti, 2013). They also suggest that in a warming world, the WAM season may be shifted toward the end of the year, as they report a drying spring and wetting fall.…”

“…Fontaine and Janicot (1996) later simplified this classification as they found that the western and continental Sahel rainfall anomalies were of the same sign for the majority of stations they analyzed. More recently, east-west contrasts have also been noted at decadal timescales (Dieppois et al, 2013) as well as at longer timescales, related to the long-term trends in response to climatic forcing (e.g., Lebel et al, 2009;Monerie et al, 2012;Biasutti, 2013).…”

Northern-hemispheric differential warming is the key to understanding the discrepancies in the projected Sahel rainfall

“…For example, as shown in the future projection from 'gfdl-cm2.1' and 'miroc3_2_medres' in CMIP3 (refs 1,2), two CMIP5 models from the same family, GFDL-ESM2M and MIROC-ESM, still project a substantially dryer and wetter Sahel, respectively, under a medium future emission scenario (Representative Concentration Pathways 4.5; RCP4.5). A recent study using the strongest radiatively forced future scenario for CMIP5 also supports the model variability of future Sahel rainfall trends 16 . We cannot ignore those two extreme simulations because either the drying or wetting of the Sahel is a physically plausible response to increased radiative forcing, and these two models provide relatively accurate simulations of the West African summer rainfall in the historical period 3 .…”

Recent climatological trend of the Saharan heat low and its impact on the West African climate

“…A second significant increase is observed at the end of the rainy season (end of Augustbeginning of September). Biasutti (2013) found modifications in the seasonal cycle of the Sahelian precipitation in the twentyfirst century, as a response to greenhouse gas forcing, observing a negative (positive) trend of precipitation in early (late) monsoonal season, whereas Cook and Vizy (2015) found no difference. The SHL intensity trends observed in both early and late monsoonal season suggest a significant modulation of the global forcing by the SHL regional signal.…”