Abstract
Decadal Sahelian rainfall variability was mainly driven by sea surface temperatures (SSTs) during the twentieth century. At the same time SSTs showed a marked long-term global warming (GW) trend. Superimposed on this long-term trend decadal and multi-decadal variability patterns are observed like the Atlantic Multidecadal Oscillation (AMO) and the inter-decadal Pacific Oscillation (IPO). Using an atmospheric general circulation model we investigate the relative contribution of each component to the Sahelian precipitation variability. To take into account the uncertainty related to the use of different SST data sets, we perform the experiments using HadISST1 and ERSSTv3 reconstructed sets. The simulations show that all three SST signals have a significant impact over West Africa: the positive phases of the GW and the IPO lead to drought over the Sahel, while a positive AMO enhances Sahel rainfall. The tropical SST warming is the main cause for the GW impact on Sahel rainfall. Regarding the AMO, the pattern of anomalous precipitation is established by the SSTs in the Atlantic and Mediterranean basins. In turn, the tropical SST anomalies control the impact of the IPO component on West Africa. Our results suggest that the low-frequency evolution of Sahel rainfall can be interpreted as the competition of three factors: the effect of the GW, the AMO and the IPO. Following this interpretation, our results show that 50% of the SST-driven Sahel drought in the 1980s is explained by the change to a negative phase of the AMO, and that the GW contribution was 10%. In addition, the partial recovery of Sahel rainfall in recent years was mainly driven by the AMO.
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Acknowledgments
We would like to thank the anonymous reviewers for their helpful comments, which greatly helped to improve the manuscript. This work was supported by the POSDEXT-MEC programme of the Spanish Ministry for Science and Innovation and by the COMPAS and DecCen project funded by the research council of Norway. Additional support was provided by the Spanish projects: MICINN CGL2009-10285 and MARM MOVAC 200800050084028. This work was carried out in the framework of the AMMA-EU project. Based on French initiative, AMMA was built by an international scientific group and is currently funded by a large number of agencies, especially form France, UK, US and Africa. It has been beneficiary of a major financial contribution from the European Community's Sixth Framework Research Programme. Detailed information on scientific coordination and funding is available on the AMMA International website http://www.amma-international.org.
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Mohino, E., Janicot, S. & Bader, J. Sahel rainfall and decadal to multi-decadal sea surface temperature variability. Clim Dyn 37, 419–440 (2011). https://doi.org/10.1007/s00382-010-0867-2
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DOI: https://doi.org/10.1007/s00382-010-0867-2