Synoptic variability associated with wet and dry spells in the Western Sahel

Abstract  

Differences in vertical and horizontal structures of the atmospheric wind circulation and its diurnal variation between wet and dry spells over West Africa are studied. CHIRPS products are used to detect the wet and dry spells and Era-Interim reanalysis data to characterize the synoptic conditions during wet and dry spells. The analysis of the zonal wind shows a stronger monsoon flux over the Northern Sahel and the Guinea region during wet spells than that during dry spells, which coincides with a weaker (stronger) and northward (southward) displaced African easterly jet (AEJ) during wet (dry) spells. This strengthening of the monsoon is consistent with the intensification of the Saharan heat low (SHL) observed during wet spells. The vertical velocity analysis also shows an intensification of the southern updraft core associated with deep convection during wet spells than during dry spells. At higher levels, the TEJ intensity is stronger (weaker) during wet (dry) spells. The analysis also showed that the activity of the African easterly waves (AEWs) and the relative humidity are stronger during wet spells than those during dry spells, suggesting that the monsoon flux brings enough moisture to promote the convective activity during wet sequences. This finding is consistent with the weakening of the low-level geopotential height during wet spells over the Sahel region. The diurnal variation analysis shows that the monsoon flux magnitude strongly intensifies during the nighttime and reaches its maximum value at 06:00 during both spells. When considering the difference between the spells, the monsoon flow is stronger (weaker) over the Gulf of Guinea and the Northern Sahel for all synoptic hours during wet (dry) spells. At mid-levels, the AEJ strongly decreases over the Sahel region at 18:00 and 00:00 in both spells. This jet is stronger and southward during dry spells. As for the TEJ, its intensity increases during nighttime and reaches its maximum at 00:00 during both periods. However, the upper levels’ jet magnitude is stronger during wet spells than that during dry spells, especially at 00:00 and 12:00. The analysis of the activity of 3–5-day AEWs shows a more vigorous activity over the Western Sahel at 00:00 during both spells, especially during wet spells.