Examining the role of unusually warm Indo-Pacific sea-surface temperatures in recent African droughts

Southern Africa (SA) and eastern Africa (EA) experienced a sequence of severe droughts in December-February (SA DJF) 2015-2016, October-December (EA OND) 2016 and March-April-May 2017 (EA MAM). This sequence contributed to severe food insecurity. While climate variability in these regions is very complex, the goal of this study is to analyse the role played by unusually warm Indo-Pacific SSTs, where unusual is defined as a 1-in-6 year event. We use observed sea-surface temperatures (SST) and satellite-gauge rainfall observations, a 20-member ensemble of Community Atmospheric Model version 5.1 simulations (CAM5), and a 40-member ensemble of climate change simulations from the Community Earth Systems Model version 1 (CESM1) Large Ensemble Community Project (LENS) to explore climate conditions associated with warm events identified based on eastern and western Pacific SSTs. Our analysis suggests that strong El Nino's may be followed by warm western Pacific SST conditions, which can lead to conditions conducive to successive and potentially predictable droughts in SA DJF, EA OND and EA MAM. We show that different regions of warm SST are related to recent droughtsSA DJF: Nino 3.4; EA OND: western equatorial Pacific (WEP); and EA MAM: western North Pacific (WNP). For DJF and MAM, respectively, the CAM5 model driven with observed SST and the same model driven within a climate change experiment indicate that warmer El Nino's and WNP events produce more intense atmospheric responses, potentially associated with more severe droughts. OND climate seems to be strongly influenced by the Indian Ocean Dipole, which corresponds with some WEP events. Given global warming, we suggest that the extreme Nino 3.4 and west Pacific SST events responsible for 2015-2017 droughts are likely to reoccur, thus humanitarian agencies should prepare to predict and respond to multi-year drought and substantial food insecurity in SA and EA.