Publication: On the detection of externally forced decadal modulations of the Sahel rainfall over the whole twentieth century in the CMIP6 ensemble
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2022-11-01
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American Meteorological Society
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Abstract
The Sahel semiarid region was marked during the twentieth century by significant modulations of its rainfall regime at the decadal time scale. Part of these modulations have been associated with the internal variability of the climate system, linked to changes in oceanic sea surface temperature. More recently, several studies have highlighted the influence of external forcings during the dry period in the 1980s and the recovery around the 2000s. In this work we evaluate the internally and externally driven decadal modulations of Sahel rainfall during the entire twentieth century using a set of 12 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6). We begin by proposing a physically based definition of Sahel rainfall that takes into account the southward bias in the location of the Sahelian ITCZ simulated by all the models. Our results show that the amplitude of the decadal variability, which is underestimated by most models, is mainly produced by the internally driven component. Conversely, the external forcing tends to enhance the synchrony of the simulated and observed decadal modulations in most models, providing statistically significant correlations of the historical ensemble mean with observations in 1/3 of the models, namely IPSL-CM6A-LR, INM-CM5-0, MRI-ESM2-0, and GISS-E2-1-G. Further analysis of the detection and attribution runs of the IPSL-CM6A-LR shows that anthropogenic aerosol dominate the decadal modulations of Sahel rainfall simulated by this model, suggesting that at least a part of the impact is ocean-mediated and operated through shifts in the ITCZ and the Saharan heat low.
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© 2022 American Meteorological Society. This work was supported by the PDI/MSC (Programme Doctoral International: Modélisation des Systèmes Complexes), the scholarship of SCAC (Service de Coopération et d’Action Culturelle), and the ERASMUS1 programme through action KA107. This work was also supported by the Laboratoire Mixte International ECLAIRS2 (IRD), the UCM XVII call for cooperation and sustainable development, and the Ambassade de France en Espagne. EM also acknowledges the European Commission TRIATLAS (No. 817578), NextGEMS (No. 101003470), and the Spanish Ministry of Science and Competitiveness DISTROPIA (PID2021-125806NB-I00) projects. JM acknowledges the support of the JPI climate/JPI ocean project (ANR-19-JPOC-003).