Towards understanding the occurrence of both wet and dry Sahel seasons during El Niño: The modulating role of the global ocean

Prior research has shown that dry conditions tend to persist in the Sahel when El Nino develops. Yet, during the historic 2015 El Nino, Sahel summer precipitation was anomalously high, particularly in the second half of the season. This seeming inconsistency motivates a reexamination of the variability of precipitation during recent El Nino years. We identify and composite around two different outcomes for Sahel summer season: an anomalously wet season or an anomalously dry season as El Nino develops to its peak conditions over the observational record spanning 1950-2015. We find consistently cool temperatures across the global tropics outside the Nino-3.4 region when the Sahel is anomalously wet during El Nino years and a lack of cooling throughout the tropics when the Sahel is anomalously dry. The striking differences in oceanic surface temperatures between wet years and dry years are consistent with a rearrangement of the entire global circulation in favor of increased rainfall in West Africa despite the presence of El Nino.