A Lagrangian analysis of the sources of rainfall over the Horn of Africa
Drylands
Abstract
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 forecast and
climate models, revealing key gaps in our understanding of the drivers
of rainfall in the region. In contrast to existing climate modelling and
observation-based studies, here we analyze the HAD rainfall from an
observationally-constrained Lagrangian perspective. We quantify and map
the major oceanic and terrestrial sources of moisture driving the
variability in the long and short rains. 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, i.e., 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, which has important implications for (sub-)seasonal
forecasts and long-term projections of the HAD rainfall.