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Communication incl. Poster: BibTeX citation key:  Zhangb
Zhang, C. & Hagos, S. M. 2009. Double Overturning Meridional Circulations in West Africa: Their Causes and Roles in Monsoon Development. Work presented at Third International AMMA Conference, July 20—24, at Ouagadougou, Burkina Faso.
Added by: Devic 2009-09-23 11:34:06    Last Edited by: roussot 2009-10-16 16:38:39
Categories: General, Monsoon system and its variability
Keywords: Atmospheric Boundary Layer
Creators: Hagos, Zhang
Publisher: African Monsoon Multidisciplinary Analyses (Ouagadougou, Burkina Faso)
Collection: Third International AMMA Conference

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In situ sounding observations and global reanalysis products both reveal a double-cell structure in the meridional overturning circulation over West Africa. In addition to the classic deep, Hadley-type overturning circulation, there is a shallow overturning circulation that extends from the surface only to the lower troposphere. This study explores the causes of these two circulations and their roles in meridional moisture transport from the ocean to the West African Monsoon (WAM) region. Divergent wind fields from global reanalysis products are decomposed into a component that is directly related to precipitation and another component that is not. The meridional overturning circulation directly associated with precipitation is a classic Hadley-like deep cell extending from the surface to the tropopause. The circulation not directly associated with precipitation is shallow, which is mostly driven by meridional gradients in surface temperature and sensible heat flux due partially to the Saharan heat low. The lower branch southerly flows of both circulations transport moisture from the ocean (Gulf of Guinea) into the West African continent. Near-surface moisture convergence by the precipitation-driven deep circulation coincides with the rainfall maximum and therefore favors the enhancement of local monsoon rainfall. In contrast, the maximum of near-surface moisture convergence by the surface-flux driven shallow circulation is always north of the rainfall maximum and thereby facilitates the northward migration of the monsoon before its peak season. It is argued that a complete understanding of the WAM dynamics needs to take into consideration of the combined and balanced effects of both circulations.
Last Edited by: roussot