Effective drought prediction can be enhanced by a clear understanding of the drivers of drought. How we conceptualize climate change influences our ability to identify the fingerprints of change. Together with Simon Wang, Jin-Ho Yoon, and Robert R. Gillies, I have helped edit a new AGU book on ‘Climate Extremes: Patterns and Mechanisms’, examining how climate change may be bringing more extreme events. Recognizing these influences, can improve our ability to anticipate climate extremes. In the chapter I wrote for this book, I discuss how increases in the intensity of both the El Niño-Southern Oscillation (ENSO) and the West Pacific Warming Mode may be making both El Niño-like and La Niña-like extremes more severe. In 2015 and 2016, an anthropogenic enhancement of the 2014-16 El Niño event may have exacerbated the severe droughts that struck northern Ethiopia and Southern Africa. CHG scientists have a BAMS article examining this topic. In 2016 and 2017, extreme West Pacific warming likely contributed to the severe East African droughts that struck in October-November-December (2016) and March-April-May (2017), resulting in current near-famine conditions in Somalia (Figure 1) and eastern Ethiopia (Figure 2). Recent Famine Early Warning Systems Network (FEWS NET) reporting finds that in Somalia 2016 and 2017 harvests were very poor (~25% and ~50% of normal, respectively). In many areas of Eastern Ethiopia and Central Somalia, the twelve month June 2016-May 2017 rainfall was the lowest on record (since 1981). Many Pastoralists have lost more than 60% of their herds – a huge loss in livelihood and accumulated wealth. Both Ethiopia and Somalia suffer from outbreaks of cholera, with Somalia experiencing more than 50,000 cases since January 2017.
“Improved humanitarian access in Somalia, and urgent, sustained assistance in Somalia and southeastern Ethiopia, is needed to mitigate very high levels of acute malnutrition and the threat of loss of life.” (FEWS NET Alert)
The situation is dire, but would likely have been worse without humanitarian assistance. In Somalia, US humanitarian assistance doubled between December and January (from assistance for 0.5 to 1 million people), and then doubled again between January and February, reaching 2.4 million in June. While the distribution and quantity of aid could be increased, humanitarian relief is providing life-saving assistance to millions of people; current UN estimates indicate that food aid is reaching about 2.5 million people out of a targeted 3.3 million. An important corollary of this assistance is the stabilization of cereal prices. Between October of 2010 and May of 2011, the prices of red sorghum in the Somali city of Baidoa climbed by 300%. These price increases made it extremely difficult for poor households to purchase food, contributing to ~205,000 drought-related deaths between January and June of 2011 (link). In 2017, by comparison, sorghum prices in Bay increased by 70%, and mortality rates have not increased by the large amounts seen in 2011. Ethiopia and Somalia continue to face very dangerous near-famine levels of insecurity. Water supplies and rangeland conditions are likely to deteriorate as we enter the dry season (July-September). Levels of international assistance remain below the required levels. Nonetheless, timely assistance in 2017, guided by effective early warning, has helped millions of people. The region, however, will almost certainly continue to face severe water and fodder shortages, since the next likely chance of rain will not come until October.
Effective predictions of the 2016 and 2017 East African Droughts
Here at the Climate Hazards Group, we believe that climate change is making sea surface temperatures more extreme, with hotter Eastern Pacific conditions during El Niños, and warmer Western Pacific conditions during La Niña-like time periods. We also believe that such extreme sea surface temperatures can provide opportunities for prediction. This approach led to our successful prediction of both the 2016 and 2017 East African droughts (Figure 3), as reported here on this blog. In our first posting, from October 9th, we noted that very warm Western Pacific and moderately cool Eastern Pacific sea surface temperatures would likely result in below normal October-November-December rains. This was expressed as a statistical forecast for dry (-1 standardized anomaly) conditions. We also noted that ‘we should be concerned about the possibility of two poor rainy seasons in the spring and fall of 2016’ in Eastern Kenya and Southern Somalia. In the next blog, on November 9th, we included October rainfall in our predictions, noting that Eastern Kenya and Southern Somalia October rains are very highly correlated (r=0.91) with October-November-December rainfall totals.
In December of 2016 we turned our attention to the 2017 March-April-May season. Our concern was that we might see yet another drought, driven by a combination of persistently warm Western/Northern Pacific sea surface temperatures and cool La Niña-like Eastern Pacific conditions. A statistical model based on observed sea surface conditions performed well, predicting six out of seven of the most recent droughts using that model. We predicted a substantial (-1 standardized anomaly) East African drought. In December of 2016, East Pacific sea surface temperatures were near neutral, while Western/Northern Pacific sea surface temperatures were exceptionally hot. These conditions were quite different than in 2010 (the last severe drought) when both the West Pacific and East Pacific were cooler. In January and February we updated our forecasts, while also engaging in many discussions with our fellow early warning counterparts in the US, Europe and Africa. In January FEWS NET issued an alert suggesting that severe drought, rising prices, limited access and dry forecasts might produce famine in Somalia in 2017. The drought monitoring and climate predictions produced by the East African IGAD Climate Prediction and Applications Centre (ICPAC) during this time period were excellent and accurate; rapidly identifying the severe October-November-December dryness while also predicting below normal 2017 spring rains based on statistically recalibrated global climate model forecasts. In February of 2017, a joint alert was issued by FEWS NET, the World Food Programme, the European Commission, and the UN Food and Agriculture Organization identifying the elevated risk of Somali drought (based in part on a CHG forecast) and calling for ‘urgent and substantial’ provision of food aid and ‘resource mobilization to address the impact of an extended post-2016 lean season’.
In late April of 2017 we analyzed empirical relationships between March-April rainfall and Somali ‘Gu’ sorghum harvests, suggesting that the data indicated that April was by far the most important month for grain filling, and predicting that 2017 ‘Gu’ harvests were going to be very poor (about 50% of normal) based on poor March-April 2017 rainfall. The 2016 and 2017 forecasts have verified. The 2016 and 2017 rainy seasons were poor. Vegetation/pasture were very heavily degraded, as predicted, and the 2017 Gu harvests were low, as we estimated using March-April rainfall observations.
Can we resolve the East African Climate Change paradox?
FEWS NET climate change research began in 2003 when in the course of routine analysis we came across severe declines in annual precipitation in agriculturally productive and heavily populated regions of eastern Ethiopia. Figure 4 shows an updated time series of March-June rainfall for this region, through 2017. We see a severe decline in rainfall in a densely populated food insecure area; in the 20 years since 1998; only 5 years have been above normal, based on a 1900-2017 baseline. Our recent papers have also documented increased crop water stress, reduced soil moisture and stream runoff, and declines in vegetation. This drying is part of a wide-spread drying tendency associated with a strong Walker Circulation (a, b, c, d, e), which we believe is related to anthropogenic warming in the Indo-Pacific. The Walker Circulation is the world’s largest atmospheric circulation feature, and is made up of contrasting cells of ascending air and heavy rainfall near Indonesia and dry descending air over the Eastern Pacific and East Africa/Western Indian Ocean. Steve Baragona’s Voice of America story on the current East Africa drought (here) provides a great animation showing how the Walker circulation contributes to drying over East Africa. Pete Peterson has also produced a nice animation showing how increased rainfall near Indonesia is associated with declining East Africa precipitation (here).
The relationship between climate change and the March-May East African ‘long’ rains has been a topic of considerable debate, largely because climate change models predict that East Africa should already be getting wetter, while observations show that it this region is drying, resulting in the ‘East African Climate Paradox’. This has engendered two basic explanations for the East African drought. According to the first explanation the climate models are wrong, and East African March-May drying is due to low frequency (anthropogenic) warming in the Western Pacific and Indian Ocean, probably exacerbated by natural La Niña-like climate tendencies. According to the second explanation, the models are right, and East African drying is primarily due to an extreme expression of natural decadal variability. Studies focused on observed rainfall (1, 2, 3, 4) and paleo-climate indicators (5, 6) tend to support hypothesis 1. These studies note that the CMIP climate change models fail to represent well the March-May rains (6) while also over-estimating El Niño-related sea surface temperature increases in the Eastern Pacific (7).
We think that climate models are great, but not perfect. They have trouble representing (‘parameterizing’) the exceptionally complex processes associated with tropical precipitation, cloud formation and coupled ocean-atmosphere phenomena like the El Niño-Southern Oscillation. The models tend to overemphasize ENSO-related warming in the Eastern Pacific, leading, we believe to a spurious weakening of the Walker Circulation and increased precipitation over Eastern Africa during March-April-May. Here, we present a data-driven analysis based from a paper (link) that we have just submitted to the Quarterly Journal of the Royal Meteorological Society for a special issue focusing on the research of the International Precipitation Working Group.
We start by simply plotting global standardized March-May sea surface temperature anomalies during the six driest (1981-2016) eastern East African rainy seasons: 2011, 1984, 2000, 2009, 1999 and 2004. When eastern East Africa is dry, this region of the Western North Pacific tends to be very warm, and these warm sea surface temperature conditions are associated with circulation patterns that intensify the Walker Circulation (Figure 5E,F in link), increasing the Pacific trade winds, increasing rainfall near Indonesia, and bringing dry air down over Eastern Africa.
In 2016 and 2017, we used the negative relationship between Western and Northern Pacific sea surface temperatures and East African rainfall to produce our successful forecasts. Figure 6 shows the relationship between standardized 1998-2017 East African March-May rainfall and rainfall estimates based on sea surface temperatures in the yellow box in Figure 5. While not a perfect predictor, this is a strong teleconnection that correctly predicts all the recent droughts. 2017 is shown with a red dot. Since Nino 3.4 sea surface temperature conditions were actually slightly positive (El Niño-like) in March-May of 2017 (usually associated with wetter than average conditions), the very warm Western North Pacific ocean conditions seem largely responsible for the 2017 East African drought. We refer to this region as a ‘longcycle’ crop growing area because it focuses on high cool areas of eastern Ethiopian highlands, where crop have a long growing cycle, but can produce much higher yields than quicker maturing ‘shortcycle’ varieties.
Figure 7 shows a long time series of standardized March-May Western Pacific sea surface temperatures, along with the corresponding ensemble average standardized sea surface temperatures from a large (53 member) set of climate change simulations from the climate explorer. There is a strong relationship between climate change and sea surface temperatures that explains 40% of the season-to-season variance. The time series has a large (>+1 standardized anomaly) climate change influence, as well as a step-like increase after the 1997/98 El Niño, when East Africa transitioned to drier condi tions.
Interestingly, we can see the cooling influence of the Agung, El Chichon, and Mount Pinatubo volcanic eruptions in 1965, 1982 and 1991-92. These dips indicate that radiation plays an important role in determining Western North Pacific Ocean temperatures. There is also an interannual El Niño influence, with Western North Pacific sea surface temperatures being cooler and warmer during El Niño and La Niña years. We often see recent El Niño events followed La Niña-like climate conditions and increases in West Pacific sea surface temperatures. The 1997/98, 2002/03, 2009/10, 2006/07 and 2015/16 El Niño events have all been followed by warm West Pacific sea surface temperature conditions. We then experienced East African droughts in 1999, 2000, 2001, 2004, 2008, 2009, 2011 and 2017. While more research on this is needed, it seems that El Niño events release energy from the lower ocean that ends up warming the Western Pacific, creating opportunities for prediction.
The substantial post-1997 warming of the Western North Pacific (Figure 7) has been associated with a concomitant decline in East African March-May rainfall. Figure 8 shows 15-year averages of standardized eastern East African March-May rainfall (blue line) along the with regression estimates of eastern East African March-May rainfall based on 15-year averages of observed Western North Pacific sea surface temperatures (blue line) and Western North Pacific sea surface temperatures from a climate change ensemble (purple line) . Low frequency (15 year average) variations in March-May East African rainfall time series track closely (r=0.7) with estimates based on sea surface temperatures. As the Western North Pacific has warmed, the Walker Circulation has intensified and East African rainfall has declined substantially. Neither East African March-May rainfall nor Western North Pacific sea surface temperatures track closely with the Pacific Decadal Oscillation or smoothed El Niño (Eastern Pacific) sea surface temperatures (Fig. 4 in link). While natural decadal variability probably helped enhance East African precipitation in the 1980s and 1900s, the current substantial decline and low 2017 rainfall outcome appears largely due to anthropogenic warming of the Western North Pacific.
To resolve the East African Climate Paradox, I would suggest that we can explain most large scale sea surface temperature changes in the Pacific as arising from two patterns of climate variability – the El Niño-Southern Oscillation (ENSO) pattern and the ‘West Pacific Warming Mode’ pattern (JCLIM paper; Chapter in AGU Extremes Book). Both modes of variability are associated with warming, but in different places. ENSO-related warming appears in equatorial Eastern Pacific, associated with strong El Niño events. We have recently argued that anthropogenic warming enhanced the extreme 2015/16 El Niño event, increasing the severity of the 2015 and 2015/16 Ethiopian and Southern African droughts (here). Following El Niño events, we then tend to see large increases in Western Pacific sea surface temperatures, contributing to the 1999, 2000, 2001, 2004, 2008, 2009, 2011 and 2017 East African droughts.
When we focus on how mean sea surface temperatures and precipitation averages are changing in the models, we find substantial discord. The models are predicting a shift towards an El Niño-like climate and increases in East African precipitation.
When we instead focus on how sea surface temperature extremes, and associated precipitation anomalies, are changing in the climate change models, we find substantial consilience. Ironically, ensemble averages of climate change simulations may actually be more prone to biases. Small problems, like the tendency to overestimate the strength of El Niños (cf. Figure 3b JCLIM paper), may strongly influence the ensemble average. Focusing how the models represent extreme events may be more informative, especially in the context of drought early warning.
The climate change models are predicting that we will experience both more extreme El Niño events and more extreme Western North Pacific events. We explore this in depth in our new paper. In this study, we examine a large (40 member) ensemble of climate change simulations from the Community Earth System Model, and explore the change in precipitation responses associated with very warm versus just warm Western North Pacific sea surface temperatures (Figure 9). The climate change models predict that we will experience more frequent very warm Western North Pacific sea surface temperature conditions (Figure 7), and that when these conditions arise we will see a stronger Walker circulation (more rainfall near Indonesia), and less rainfall over Eastern Africa and the southwestern Arabian peninsula (Figure 9).
In conclusion, it seems likely that the recent increased frequency of East African March-May droughts are related to warmer Western Pacific sea surface temperatures (Figures 5, 6, 8 and 9), which have warmed substantially due to a combination of anthropogenic climate change and ENSO influences (Figure 7). These droughts appear to be largely predictable, and associated with severe human impacts which can be partially mitigated though humanitarian assistance.
In Somalia in 2010/11, drought, political instability, violent conflict, and global food price volatility resulted in 258,000 deaths, with 133,000 of these deaths being children under five years old (link). In 2017, Somalia and eastern Ethiopia again face one of the most severe droughts on record (here).
In Eastern Ethiopia millions of people of people face severe hunger. Herd sizes have been dramatically reduced; “households have few, if any, livestock to sell and … milk availability will remain very low in 2017”. Some 1.7 million people are estimated to be facing severe food shortages, associated with 20-50% caloric deficiencies. Unless food aid allocations are increased, caloric deficits may exceed 50%, and “poor households in the worst-affected pastoral areas will begin to move into Catastrophe (IPC Phase 5) and acute malnutrition and mortality may rise further” (FEWS NET alert, July 19th). In Somalia, the most recent assessments by the Food Security and Nutrition Analysis Unit – Somalia (FSNAU report) identify more than 50,000 cases of Acute Watery Diarrhea/Cholera, and very high levels of physical wasting among children in camps holding internally displaced persons (IDPs). July 2017 FSNAU estimates indicate that some 3.24 million people face crisis or emergency levels of food insecurity. Since February, emergency food aid assistance has been scaled up from aid for 1 million people to aid for 2.4 million in June of 2017. This assistance has helped, but more aid is needed, and access to humanitarian assistance in many areas in central and southern Somalia remains a challenge.
While the food security crises in eastern Ethiopia and Somalia have been caused by many factors in addition to drought, the long term decline of the March-May East African long rains (Figure 10) has certainly contributed to food insecurity in this region. The 1900-2017 time series shown in Figure 10 has been produced by combining 1900-1980 Centennial Trends precipitation data with the 1981-2017 CHIRPS archive. Both of these data sets benefit from a high quality collection of rainfall gauge observations, and there is a high level of agreement between these observations during their period of overlap (1981-2014), with a correlation of 0.94. Figure 10 shows standardized March-May rainfall anomalies, with a value of -1 indicating a poor season. Of the nineteen years since 1999, eight seasons have been poor (1999, 2000, 2001, 2004, 2008, 2009, 2011 and 2017), and fourteen seasons have been below normal, based on a 1900-2017 baseline. These eight dry seasons have been associated with very warm Western North Pacific sea surface temperatures (Figure 6). Anthropogenic climate change has helped produce these warm sea surface temperature conditions (Figure 7). Estimating the influence of Western North Pacific warming on rainfall (Figure 9) indicates a strong negative (~-0.7 standardized anomalies) rainfall response, similar in magnitude to results from a previous experiment using the CAM5 atmospheric model (here). Anthropogenic climate change has likely contributed to the current March-May drought and the increased frequency of poor March-May rainfall outcomes. Many of these droughts, however, appear to be predictable.