Unraveling the Origin of Rainfall over Horn of Africa Drylands

The Horn of Africa drylands (HAD) are highly vulnerable to hydroclimatic extremes, with droughts and floods frequently leading to famines, crop losses, and significant humanitarian crises. However, development of robust mitigation measures has been hindered by the lack of understanding of the drivers of the two main rainfall seasons in the region: the long (March–May) and short (October–December) rains. In particular, the inter-annual variability of the long rains has been subject of much debate; a significant amount of research has attempted to diagnose the drivers of the observed decline in the long rains. Given the ecological and socio-economic importance of the two rain seasons for the HAD region, understanding the major moisture sources and their variability in both space and time is essential. Such an analysis can help disentangle the causes of temporal variability in rainfall, especially the long rains, improve forecasts, and build ecosystem and community resilience against hydroclimatic extremes.

To trace the origin of rainfall over the HAD region, we use global simulations of the FLEXPART version 9.01, forced with the ERA-Interim reanalysis for a period of 37 years (1980–2016). The FLEXPART outputs include the properties of the air parcels at 3-hourly time steps, which are then post-processed to identify the source regions of rainfall using the Heat and Moisture Tracking Framework (HAMSTER v1.2.0) described by Keune et al. (2021). Using this framework, we first trace the rainfall occurring over the HAD region during the long and short rain seasons to their terrestrial and oceanic sources spatially. Then, we track the changes in the contributions of ocean and land evaporation to HAD rainfall in time over the 37-year period. 

Preliminary results show that around 80% of HAD rainfall originates from Indian Ocean evaporation, for both seasons. For both seasons the contribution of evaporation from land is relatively low compared to the oceanic contribution. For the long rains, a similar amount of moisture originates from recycling (local) and remote sources (10.9% and 10.5% respectively). On the other hand the short rains show a larger proportion of local recycling (13.8%) relative to remote land evaporation (9.4%). The larger contribution of remote land sources for the long rains arises from the Indian subcontinent and Southeast Asia. Further, we shed light on the trends and anomalies in source regions for the two rain seasons, with particular focus on the anomalies in moisture sources that are characteristic of extreme dry and wet conditions.

References:

Keune, J., Schumacher, D. L., and Miralles, D. G.: A holistic framework to estimate the origins of atmospheric moisture and heat using a Lagrangian model, Geosci. Model Dev. Discuss. [preprint], in review, 2021.