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Role of the West African Westerly Jet in Sahel Rainfall Variations

Journal Article · · Journal of Climate
 [1];  [2]
  1. Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York; The University of Texas at Austin, Austin, TX
  2. Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, and Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas
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The West African westerly jet is a low-level feature of the summer climatology that transports moisture from the eastern Atlantic onto the African continent at 8N-11N. Here we examine the relationship between the jet and Sahel precipitation variability in August, when both the jet and rainfall reach their seasonal maxima. Variations of the West African westerly jet are significantly positively correlated with precipitation variations over the Sahel on both interannual and decadal time scales. Three periods are identified (1958-1971, 1972-1987, and 1988-2009) corresponding to times with a wet Sahel/strong jet, dry Sahel/weak jet, and relatively wet Sahel/strong jet. In wet (dry) periods, enhanced (decreased) westerly moisture fluxes associated with a strong (weak) jet increase (decrease) the low-level moisture content over the Sahel, decreasing (enhancing) the stability of the atmosphere. This association between the jet and Sahel rainfall is also found in case studies of 1964, 1984, 1999, and 2007. The southerly moisture flux associated with the West African monsoon has less pronounced decadal variability than the westerly moisture flux of the West African westerly jet, and weaker correlations with Sahel rainfall. When the monsoon flow is weak, e.g., 1999 and 2007, the Sahel may still experience positive precipitation anomalies in association with strong westerly moisture transport by the jet. The West African westerly jet is also important for stabilizing the regional vorticity balance by introducing strong relative vorticity gradients. Northward flow advects low relative vorticity south of the jet to balance positive vorticity tendencies generated by mid-tropospheric condensation.
Research Organization:
Univ. of Texas, Austin, TX (United States)
Sponsoring Organization:
USDOE SC Office of Biological and Environmental Research (SC-23); USDOE Advanced Research Projects Agency - Energy (ARPA-E)
DOE Contract Number:
FG02-08ER64610
OSTI ID:
1038903
Report Number(s):
DOE/ER/64610-4
Journal Information:
Journal of Climate, Journal Name: Journal of Climate Journal Issue: 8 Vol. 25; ISSN 0894-8755
Publisher:
American Meteorological Society
Country of Publication:
United States
Language:
English

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Related Subjects

54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
Africa
Sahel
West African monsoon
West African westerly jet
climate variability
jets
precipitation
rainfall