Diurnal Cycle of West African Westerly Jet and Mesoscale Convective Systems over the Sahel

Agriculture in Africa depends heavily on seasonal variations of precipitation, including the onset and length of the rainy seasons and the seasonal amount of rainfall. Seasonal study of precipitation over the Sahel is therefore necessary.

The West African westerly jet (WAWJ) is a low-level westerly jet near 10°N, directed from the eastern Atlantic onto the West African coast. It forms in June and dissipates in October. Most studies did not distinguish it with the monsoon westerlies. However, the WAWJ is distinct from the monsoon flow in location, vertical structure, dynamics, and association with precipitation variability (Pu and Cook 2010, 2012).

Past study of the WAWJ indicates that it has a weak semidiurnal cycle in August, with maxima at 06 UTC and 18UTC, while the southwesterly monsoon flow has one maximum at 06 LT and one minimum at 18LT. The seasonal variations of the diurnal cycles of the WAWJ and its relationship with Sahel rainfall are not well understood.

One purpose of this study is to examine the diurnal cycle of the WAWJ and its variation from June to October. In addition, we examine the relationship between the WAWJ and mesoscale convective systems (MCSs) over the Sahel. More specific questions include:

• What is the dynamics of two WAWJ diurnal peaks? Why are the diurnal cycles of the WAWJ and the monsoon flow different, especially in late afternoon (18 UTC)?
• Does the morning peak of the WAWJ precondition the lower atmosphere for strong MCSs activity over the Sahel?
• Does the WAWJ have a higher correlation with MCSs than the monsoon flow in the late afternoon since the WAWJ has a maximum at 18 UTC when monsoon flow is at a minimum?
• Does the WAWJ influence the locations of MCSs over Sahel and how?

ECMWF ERA-Interim reanalysis and the Japanese 55-Year reanalysis are chosen as the primary resources. To understand variations of the WAWJ, the horizontal momentum balance is analyzed. Understanding the sources of moisture supporting MCSs is helpful to answer the questions above. In the analysis of moisture budget, WAWJ and monsoon flow are distinguished further. The vertical profiles of moist static energy as the measure of atmospheric stability are considered. The results provide insights of the relationship between the WAWJ and Sahel MCSs and help to improve the forecasts of extreme events.