Abstract
Climatological features regarding the sharp downward branch (SDB) of the
Walker circulation above the Indian Ocean are comprehensively
investigated. Compared to the Pacific downward branch, SDB has two
distinctive features: two-peak seasonality and deep subsidence
extension. The two weak phases of SDB in boreal spring and fall
correspond well to the two rainy seasons at the Eastern Horn of Africa,
which is not reproduced well by state-of-the-art global climate models.
Unlike the Pacific counterpart, the annual-mean subsidence of SDB
extends to the surface, and is supported by horizontal cold advection
associated with the Asian Summer Monsoon. Two experiments using a
convection-permitting atmospheric general circulation model show that
mountains in East Africa, particularly the Ethiopian Highlands, is
necessary for the existence of SDB. The dry and clear climate in the
Northeast Africa, which is imprinted as a discontinuity of the
Intertropical Convergence Zone, is sustained by the East African
topography.