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Diagnosing Kenya Rainfall in Boreal Autumn: Further Exploration

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  • 1 Kenya Meteorological Department, Nairobi, Kenya
  • | 2 Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, Madison, Wisconsin
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Abstract

A powerful zonal vertical circulation cell along the Indian Ocean equator controls the boreal autumn rains in Kenya, with a tight negative correlation between surface westerlies (UEQ) and rainfall. UEQ is favored by a steep eastward pressure gradient (PWE) and slow winds in the downstream portion of the South Indian Ocean trade winds (SIW). The high phase of the Southern Oscillation (SO) favors weak SIW, lower pressure in the east, and thus steeper PWE, but that is also affected by pressure in the west. In 1958–97 circulation causalities were most distinct in the regime of abundant rain and slow UEQ, with the SO in the low phase. In the regime of deficient rain and fast UEQ, relationships were less distinct, although the SO was in the high phase almost throughout. In the decade 2001–10, UEQ had a weaker (stronger) correlation with PWE (SIW), the SO was more active and contributing in the proper sense to pressure in the east, but PWE was mainly controlled by pressure in the west. UEQ strongly controlled the rainfall in 1958–97 and 2001–10.

Corresponding author address: Stefan Hastenrath, Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, 1225 West Dayton Street, Madison, WI 53706. E-mail: slhasten@wisc.edu

Abstract

A powerful zonal vertical circulation cell along the Indian Ocean equator controls the boreal autumn rains in Kenya, with a tight negative correlation between surface westerlies (UEQ) and rainfall. UEQ is favored by a steep eastward pressure gradient (PWE) and slow winds in the downstream portion of the South Indian Ocean trade winds (SIW). The high phase of the Southern Oscillation (SO) favors weak SIW, lower pressure in the east, and thus steeper PWE, but that is also affected by pressure in the west. In 1958–97 circulation causalities were most distinct in the regime of abundant rain and slow UEQ, with the SO in the low phase. In the regime of deficient rain and fast UEQ, relationships were less distinct, although the SO was in the high phase almost throughout. In the decade 2001–10, UEQ had a weaker (stronger) correlation with PWE (SIW), the SO was more active and contributing in the proper sense to pressure in the east, but PWE was mainly controlled by pressure in the west. UEQ strongly controlled the rainfall in 1958–97 and 2001–10.

Corresponding author address: Stefan Hastenrath, Department of Atmospheric and Oceanic Sciences, University of Wisconsin—Madison, 1225 West Dayton Street, Madison, WI 53706. E-mail: slhasten@wisc.edu
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