A Lagrangian Analysis of the Sources of Rainfall Over the Horn of Africa Drylands.

The Horn of Africa drylands (HAD) are among the most vulnerable regions to hydroclimatic extremes. The two rainfall seasons—long and short rains—exhibit high intraseasonal and interannual variability. Accurately simulating the long and short rains has proven to be a significant challenge for the current generation of weather and climate models, revealing key gaps in our understanding of the drivers of rainfall in the region. In contrast to existing climate modeling and observation‐based studies, here we analyze the HAD rainfall from an observationally‐constrained Lagrangian perspective. We quantify and map the region's major oceanic and terrestrial sources of moisture. Specifically, our results show that the Arabian Sea (through its influence on the northeast monsoon circulation) and the southern Indian Ocean (via the Somali low‐level jet) contribute ∼80% of the HAD rainfall. We see that moisture contributions from land sources are very low at the beginning of each season, but supply up to ∼20% from the second month onwards, that is, when the oceanic‐origin rainfall has already increased water availability over land. Further, our findings suggest that the interannual variability in the long and short rains is driven by changes in circulation patterns and regional thermodynamic processes rather than changes in ocean evaporation. Our results can be used to better evaluate, and potentially improve, numerical weather prediction and climate models, and have important implications for (sub‐)seasonal forecasts and long‐term projections of the HAD rainfall. Plain Language Summary: The drylands in the Horn of Africa (HAD) have suffered a series of droughts in recent years with the consecutive failure of both the major rainfall seasons. So far, the rainfall in this region has been studied using observations and outputs from global climate models. In contrast, here, we employ a moisture tracking model, which traces individual parcels of moisture‐carrying air parcels from their destination (the HAD region) to their sources (land areas within and outside the HAD, and the oceans). We find that the majority of the moisture which causes rainfall in the HAD region comes from the oceans. However, in recent years, we see that land contributions are gaining importance in affecting water availability in the region. Key Points: Indian Ocean contributes majority of the moisture to rainfall in the Horn of Africa drylandsThe role of vegetation and land contribution to rainfall during the short rains is increasingMoisture contributions are driven more by atmospheric circulation changes rather than increased evaporation from source regions [ABSTRACT FROM AUTHOR]