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

Abstract

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 Niño'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 droughts—SA DJF: Niño 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 Niño'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 Niño 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.

Pacific sea surface temperature linkages with Tanzania's multi‐season drying trends

“…As discussed in Funk et al . (, ), warm north Pacific SST anomalies produce ridging aloft that accentuates the influence of the equatorial SST gradient. Eastern Pacific cool SST induce upper‐level equatorially trapped cyclones; these combine with upper‐level highs associated with Western V SST to induce a vigorous response over the Warm Pool and the Eastern Africa/Indian Ocean region.…”

“…MAMJ SST composites are insignificant over the east Pacific. Rather, the area that appears to be most significant is the north Pacific, which on decadal time scales covaries strongly as part of a “Western V” structure that stretches from near the maritime continent into the extratropical Pacific (Funk and Hoell, ; Funk and Hoell, ; Funk et al ., , ). Composites of 5‐day OLR and CHIRPS data have also highlighted SST in this same general region.…”

“…The SST anomaly pattern appears strongly related to northern and southern West Pacific SST (Figure b) as discussed in Funk et al . (; ), ridging over the northern and southern Pacific Ocean and disruptions of the sub‐tropical Westerly Jets (Figure b). Relatively higher heights and lower velocity potentials over the equatorial west Pacific (Figures b and b) drive enhanced convergence aloft near the equatorial dateline.…”

“…This study and Funk et al . (; ) indicate that warming in the eastern Indian Ocean (~90°E, 0°N as shown in Figure 8d of Manatsa et al .) helps drive baroclinic increases in upper level heights over the warm pool, which helps turn the Pacific subtropical westerly jet anomalies shown in our Figure b eastward, to subside near the dateline.…”

“…Near term precipitation projections from the CMIP5 ensemble are considered unreliable in many regions (van Oldenborgh et al ., ), which can pose challenges for adaptation planning. A prime example is East Africa, where predictions for a wetter boreal spring directly oppose substantial drying trends that have occurred (Rowell et al ., ; Funk et al ., ; ). As noted earlier, the SST‐based statistical projections reported here are ripe with uncertainty, but they do illustrate a potential for very different precipitation trajectories in the DJF and MAMJ Tanzania areas if Pacific teleconnections of the past several decades were to persist.…”