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Role of the West African westerly jet in the seasonal and diurnal cycles of precipitation over West Africa

  • Weiran LiuEmail author
  • Kerry H. Cook
  • Edward K. Vizy
Article
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Abstract

The West African westerly jet (WAWJ) is a low-level jet centered near 10° N over the West African coast from June to September. This study quantifies the low-level moisture transport associated with the WAWJ on seasonal timescales, examines the relationship between the jet and West African precipitation using a moisture budget analysis, and further distinguishes the WAWJ from the West African monsoon (WAM) flow. Three reanalyses and five observational rainfall datasets are examined to build confidence. The WAWJ has a diurnal cycle with a minimum at 12 UTC and a maximum at 18 UTC, dominated by the ageostrophic wind component. According to the momentum budget analysis, the diurnal acceleration/deceleration of the WAWJ is controlled by variations in geopotential height gradient forces associated with the continental thermal low and its westward, offshore extension over the eastern Atlantic. The WAWJ wind speed and rainfall across a large region of the western Sahel (0°–10°W, 8°–18° N) are significantly and positively correlated. In this region, the moisture flux associated with the WAWJ is stronger than that associated with the southerly WAM flow from July 5 to August 20 (45 days). The moisture budget analysis reveals that the seasonal evolution of the rainfall in this analysis region is associated with zonal moisture convergence related to changes of the WAWJ. Enhanced (reduced) rainfall occurs in months with a strong (weak) WAWJ, accompanied by low-level moisture flux anomalies associated with the WAWJ instead of the southerly WAM.

Keywords

Low-level jet Sahel rainfall West African monsoon Seasonality of precipitation West African westerly jet Diurnal cycle Saharan thermal low 

Notes

Acknowledgements

This work was funded by NSF Award #1444505. The authors acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing database resources that have contributed to the research results reported within this paper. URL: http://www.tacc.utexas.edu. The Grid Analysis and Display System software (GrADS) developed at COLA/IGES was used for generating the figures.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Geological Sciences, Jackson School of GeosciencesThe University of Texas at AustinAustinUSA

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