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Sharp Downward Branch of the Walker Circulation above the Indian Ocean
  • Tsubasa Kohyama,
  • Tamaki Suematsu,
  • Hiroaki Miura
Tsubasa Kohyama
Ochanomizu University

Corresponding Author:tsubasa@is.ocha.ac.jp

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Tamaki Suematsu
University of Tokyo
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Hiroaki Miura
University of Tokyo
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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.