Details

Autor(en) / Beteiligte

• Funk, Chris
• Fink, Andreas H.
• Harrison, Laura
• Segele, Zewdu
• Endris, Hussen S.
• Galu, Gideon
• Korecha, Diriba
• Nicholson, Sharon

Titel

Frequent but Predictable Droughts in East Africa Driven by a Walker Circulation Intensification

Ist Teil von

• Earth's future, 2023-11, Vol.11 (11), p.n/a

Ort / Verlag

Bognor Regis: John Wiley & Sons, Inc

Erscheinungsjahr

2023

Quelle

Wiley Online Library Journals Frontfile Complete

Beschreibungen/Notizen

• During and after recent La Niña events, the decline of the eastern East African (EA) March‐April‐May (MAM) rains has set the stage for life‐threatening sequential October‐November‐December (OND) and MAM droughts. The MAM 2022 drought was the driest on record, preceded by three poor rainy seasons, and followed by widespread starvation. Connecting these dry seasons is an interaction between La Niña and climate change. This interaction provides important opportunities for long‐lead prediction and proactive disaster risk management, but needs exploration. Here, for the first time, we use observations, reanalyses, and climate change simulations to show that post‐1997 OND La Niña events are robust precursors of: (a) strong MAM “Western V sea surface temperature Gradients” in the Pacific, which (b) help produce large increases in moisture convergence and atmospheric heating near Indonesia, which in turn produce (c) regional shifts in moisture transports and vertical velocities, which (d) help explain the increased frequency of dry EA MAM rainy seasons. We also show that, at 20‐year time scales, increases in atmospheric heating in the Indo‐Pacific Warm Pool region are attributable to warming Western V SST, which is in turn largely attributable to climate change. As energy builds up in the oceans and atmosphere, during and after La Niña events, we see stronger heating and heat convergence over warm tropical waters near Indonesia. The result of this causal chain is that increased Warm Pool atmospheric heating and moisture convergence sets the stage for dangerous sequential droughts in EA. These factors link EA drying to a stronger Walker Circulation and explain the predictable risks associated with recent La Niña events. Plain Language Summary In 2022, an unprecedented sequence of five sequential failed rainy seasons, exacerbated by high global food and fuel prices, drove an exceptional food security crisis in Ethiopia, Somalia, and Kenya, pushing more than 20 million people into extreme hunger. The potential for famine loomed in some areas. Beginning in late 2020, this was the longest and most severe drought recorded in the Horn of Africa in at least 70 years, resulting in multiple failed harvests and large‐scale livestock deaths that decimated the food and income sources of rural communities. It placed increasing pressure on the cost of food among urban communities and led to rising levels of destitution and displacement. These droughts occurred against the backdrop of the “East Africa Climate Paradox”, which centers on the discrepancy between climate change model projections of increased East African March–April–May rains, and many observational studies pointing toward declines. Before the western Pacific Ocean warmed dramatically in 1998, the link between La Niña events and dry March‐April‐May (MAM) rains was weak. Since 1998, the link has been very strong. This set the stage for dangerous sequential droughts in October‐November‐December and MAM, such as in 2010/11, 2016/17, 2020/21, and 2021/22. Here, we link the decline to an important question: Why are so many recent La Niña events associated with dry March–April–May rains? La Niña events tend to reach their maximum intensity in the boreal fall and winter, often producing East African droughts during the October‐November‐December “short rains”. We explain the link between La Niña and dry MAM seasons using observations, reanalyses, and the latest (Phase 6) climate change simulations. While climate change models do not recreate the observed East African drying, they recreate the observed west Pacific warming very well. Climate change, not natural decadal variability associated with the Pacific Decadal Oscillation, has increased west Pacific sea surface temperatures. This, in turn, is increasing the “Western V Gradient”, a measure of the east‐west differences in Pacific Ocean temperatures. When this gradient is negative, there are frequent East African droughts, and this happens in a predictable way during or after recent La Niña events. This allows us to predict many dry rainy seasons approximately 8 months in advance. Such predictive capacity is important, because the frequency of strong Pacific temperature gradients is increasing. We show that climate change simulations recreate this tendency for stronger Pacific SST gradients, and project that it will continue over the coming decades. What connects East African droughts to Pacific temperature gradients? We answer this question by examining observed atmospheric heating, moisture transports, and moisture convergence patterns. In general, eastern East Africa is dry because it resides along the western edge of the Indian Ocean branch of the Indo‐Pacific “Walker Circulation”. Across eastern East Africa and the western Indian Ocean, and over the central and eastern Pacific, rainfall and moisture levels are low. In the area around Indonesia (the eastern Indian and western Pacific Oceans), winds drive moisture convergence and heavy rains. Here, building on many years of research by scientists working for the Famine Early Warning Systems Network, we show for the first time that the strength of the Walker Circulation can be quantified using atmospheric heating and moisture convergence. Since 1998, when there has been a La Niña in October–November–December, there has almost always been strong March‐April‐May heating and moisture convergence around Indonesia, and suppressed rainfall in eastern East Africa. Climate change‐enhanced La Niñas amplify the Pacific trade winds which produce strong March–April–May sea surface temperature gradients. These gradients amplify the Walker Circulation, and reduce moisture convergence and ascending atmospheric motions over the eastern Horn of Africa. We conclude with a look toward the future evolution of the Walker Circulation by relating the observed strength of the Walker Circulation to 20‐year averages of western and eastern Pacific sea surface temperatures. Both play a significant role, and together explain 96% of the observed variability. The observed Walker Circulation intensification is primarily driven by the west Pacific, which in turn is strongly related to climate change. CMIP6 projections of Pacific sea surface temperatures, combined with the observed empirical relationships, imply further strong increases in Walker Circulation intensities. Hence, additional rainfall declines appear likely, especially during or directly following La Niña events. More optimistically, the process‐based analyses presented here suggest that many of the dry seasons may be predictable at long lead times, based on Pacific sea surface temperature gradients. Key Points Human‐induced warming in the Western V region of the Pacific, combined with La Niña, has produced frequent, predictable March‐April‐May droughts Thermodynamic analyses link these droughts to a stronger Walker Circulation driven by predictable warming in the Western V region Projected CMIP6 SST increases imply a tendency for continued Walker Circulation intensification and associated drought risks in eastern East Africa