Multiyear La Niña Events and Multiseason Drought in the Horn of Africa

Abstract

One of the primary sources of predictability for seasonal hydroclimate forecasts are sea surface temperatures (SSTs) in the tropical Pacific, including the El Niño Southern Oscillation. Multi-year La Niña events in particular may be both predictable at long lead times and favor drought in the bimodal rainfall regions of East Africa. However, SST patterns in the tropical Pacific and adjacent ocean basins often differ substantially between first- and second-year La Niñas, which can change how these events affect regional climate. Here, we demonstrate that multi-year La Niña events favor drought in the Horn of Africa in three consecutive seasons (OND-MAM-OND). But they do not tend to increase the probability of a fourth season of drought owing to the sea surface temperatures and associated atmospheric teleconnections in the MAM long rains season following second-year La Niña events. First-year La Niñas tend to have both greater subsidence over the Horn of Africa, associated with warmer waters in the West Pacific that enhance the Walker Circulation, and greater cross-continental moisture transport, associated with a warm Tropical Atlantic, as compared to second-year La Niñas. Both the increased subsidence and enhanced cross-continental moisture transport favors drought in the Horn of Africa. Our results provide physical understanding of the sources and limitations of predictability for using multi-year La Niña forecasts to predict drought in the Horn of Africa.

Tailored Forecasts Can Predict Extreme Climate Informing Proactive Interventions in East Africa

“…Extreme negative OND WPG and MAM WVG events are associated with values less than −1Z. Change in extreme event frequencies (# of events per 100 years) were calculated by taking the frequency differences between 2020-2030 and 1920-1979, and (Anderson et al, 2022). However, wet EHoA rainy seasons, associated with exceptionally warm western Indian Ocean and eastern Pacific conditions, are also expected (Abram et al, 2008;Cai et al, 2014aCai et al, , 2014bCai et al, , 2015aCai et al, , 2021Cai et al, , 2022Cheng et al, 2019;Ihara et al, 2008).…”

“…If WPG/WVG events do become even more frequent, then enhanced forecast systems will be a critical tool for managing risk. One challenge associated with improving forecasts is the difficulty in linking research-based attribution studies (Anderson et al, 2022;Funk et al, 2018Funk et al, , 2019 with the operational "consolidated" forecast system used by groups such as ICPAC (https://www.icpac.net/seasonal-forecast/). These forecasts use spatially explicit maps and are presented at seasonal Climate Outlook Fora in East Africa.…”

“…Historically, La Niña events occur every 3‐to‐5 years (Diaz, 2000; Glantz, 2001). Hence, very frequent La Niñas, a lack of a warming trend in the eastern Pacific (Seager et al., 2019, 2022), and rapid warming in the west Pacific have created a large increase in Pacific SST gradients (Figure 1e), setting the stage for sequential droughts, especially during multi‐year La Niñas (Anderson et al., 2022). However, wet EHoA rainy seasons, associated with exceptionally warm western Indian Ocean and eastern Pacific conditions, are also expected (Abram et al., 2008; Cai et al., 2014a, 2014b, 2015a, 2021, 2022; Cheng et al., 2019; Ihara et al., 2008).…”

Serengeti–Masai Mara ecosystem dynamics inferred from rainfall extremes

“…However, multiple EDE aggregated into compound extremes, unlike the EWE, which would lessen wet and vegetative conditions distribution gradient imperative for the SMME migratory ecosystem. The 1997/1998El Niño (McPhaden 1999 led to the 1998 wet season EWE preceding the compound EDE in 1999-2000, which was also coupled with the multiyear La Niña events (Shabbar and Yu 2009, Okumura et al 2017, Raj Deepak et al 2019, Anderson et al 2023, usually developing after a strong El Niño (Iwakiri and Watanabe 2021). Principally, the El Niño Southern Oscillations (ENSO) and Indian Ocean Dipole (IOD) actively modulate the East African rainfall variability by developing wetter conditions during their positive phases and drier conditions during their negative phases for both short (October-December) and long (March-April) rains (Anderson et al 2023, Palmer et al 2023.…”

Glo3DHydroClimEventSet(v1.0): A global‐scale event set of hydroclimatic extremes detected with the 3D DBSCAN‐based workflow (1951–2022)

“…Accordingly, the Glo3DHydroClimEventSet(v1.0) database employed SA ExtrePreci,1d , SA Heat,1d and SA Cold,1d to describe the anomaly degree of extreme precipitations, heatwaves and cold surges. Also, in drought communities, the Standardized Anomaly Index (SAI) recommended by the WMO and GWP (2016), also called the statistical Z‐Score Index, has been successfully used in previous drought studies (Anderson et al, 2023; Wu et al, 2001). In the present study, SPAI 90d or 30d and STAI 90d or 30d , that is, SAI of 90‐ and 30‐day precipitation and 2‐m air temperature, are used to describe meteorological droughts and concurrent near‐surface temperature anomalies at seasonal and monthly scales.…”