Frequent but Predictable Droughts in East Africa Driven by A Walker
Circulation Intensification
Abstract
The decline of the eastern East African (EA) March-April-May (MAM) rains
poses a life-threatening “enigma,” an enigma linked to sequential
droughts in the most food-insecure region of the world. The MAM 2022
drought was the driest on record, preceded by three poor rainy seasons,
and followed by widespread starvation. Connecting these droughts is an
interaction between La Niña and climate change, an interaction that
provides exciting opportunities for long-lead prediction and proactive
disaster risk management. Using observations, reanalyses, and climate
change simulations, we show here, for the first time, that post-1997 OND
La Niña events are robust precursors of: (1) strong MAM “Western V
Gradients” in the Pacific, which help produce (2) large increases in
moisture convergence and atmospheric heating near Indonesia, which
appear associated with (3) regional shifts in moisture transports and
vertical velocities, which (4) help explain more frequent dry EA rainy
seasons. Understanding this causal chain will help make long-lead
forecasts more actionable. Increased Warm Pool atmospheric heating and
moisture convergence sets the stage for dangerous sequential droughts in
EA. At 20-year time scales, we show that these Warm Pool heating
increases are attributable to observed Western V warming, which is, in
turn, largely attributable to climate change. As energy builds up in the
oceans and atmosphere, we see stronger convergence patterns, which offer
opportunities for prediction. Hence, linking EA drying to a stronger
Walker Circulation can help explain the “enigma” while underscoring
the predictable risks associated with recent La Niña events.