Spatial and Temporal Variability of Summer Precipitation Extremes in Ethiopia and Their Possible Mechanisms

Abstract

Ethiopia is a country that is highly vulnerable to climate change and often faces extreme weather events, creating great challenges related to food security, water resources, and socio-economic development. This study investigates the spatial-temporal characteristics of extreme rainfall (>30 mm∙day − 1 ) over Ethiopia during June-September (JJAS) from 1981 to 2022 and their possible mechanisms. It is found that the climatological extreme rainfall frequency exceeds 10 - 12 days∙yr − 1 in the Ethiopian highlands but falls below 1 day∙yr − 1 in lowland regions, highlighting the important role of orography and monsoon dynamics. Trend analysis shows a clear dipole pattern, with significant increases in the western and southwestern regions (p = 0.001) and decreases in northern and northeastern areas of Ethiopia (p = 0.003 to 0.036). EOF analysis reveals a major highland-wide uniform mode (EOF1; 23% of variance) and a secondary north-south dipole mode (EOF2; 9%), which both exhibit evident interannual-to-interdecadal variabilities. While the EOF1 is closely linked to ENSO with a significant negative correlation (R = −0.42), PC2 shows a prominent upward trend with large-scale warming. After removing this trend, the interannual variability of PC2 is significantly associated with the preceding FMAM North Atlantic Tripole (NAT; R = 0.35) and the concurrent JJAS Indian Ocean Dipole (IOD; R = 0.30). Further analyses show that La Niña events can enhance the Somali low-level jet, strengthening southwesterly wind anomalies that transport abundant moisture and favor the occurrence of precipitation extremes. In contrast, the NATI and IOD induce anticyclonic circulations over eastern and northeastern Africa, respectively. These atmospheric responses lead to moisture convergence in southern Ethiopia and divergence in the north. This promotes precipitation extremes, primarily in the southern part of the country. Our results highlight the importance of incorporating the three basin-scale air-sea coupled modes into early warning and seasonal forecasting systems, thereby enhancing adaptation strategies and flood-risk management for precipitation extremes.