Contribution of Western Arabian Sea Tropical cyclones to rainfall in the
Horn of Africa
Abstract
The occurrence of tropical cyclones (TC) in the Horn of Africa and
nearby areas is for the first time examined to document their
contribution to local rainfall and their trends over the period
1990-2020. An average 1.5 TC (of any intensity) per year was observed
over the Western Arabian Sea, with two asymmetrical seasons, namely
May-June (30% of cyclonic days) and September-December (70%). Case
studies reveal that in many instances, TC-related rainfall extends
beyond 500 km from the TC center, and that substantial rains occur one
to two days after the lifecycle of the TC. Despite their rarity, in the
otherwise arid to semi-arid context characteristic of the region, TCs
contribute in both seasons to a very high percentage of total rainfall
(up to 30 to 60%) over the northwestern Arabian Sea, the Gulf of Aden
and their coastlines. Over inland northern Somalia, contributions are
much lower. TCs disproportionately contribute to some of the most
intense daily falls, which are often higher than the mean annual
rainfall. A strong increase in the number of TCs is found from 1990 to
2020, hence their enhanced contribution to local rainfall. This increase
is associated with a warmer eastern / southern Arabian Sea, a decrease
in vertical wind shear, and a strong increase in tropospheric moisture
content.