Variability of InterTropical Front (ITF) and Mesoscale Convective Weather Systems (MCWSs) over West African Soudano-Sahel Zone

During the April-October rainy season over West Africa, the InterTropical Front (ITF) is the leading edge of the wedge of moist monsoon air that advances northward across the Soudano-Sahel, after which it retreats southward. Several hundred kilometers to the south of the migrating ITF, where the monsoon flow is sufficiently deep, distinctive westward propagating mesoscale convective weather systems (MCWSs) deliver the monsoon rainfall. Daily dew point temperature (rainfall) data for 3-5 decades are used to document ITF (MCWS) variability on seasonal, interannual, and decadal time-scales. The northward ITF advance and MCWS development (size, intensity) proceed slowly from April through mid-August, and only half as fast as the subsequent southward ITF retreat and MCWS decay through October. A strong positive concurrent relationship occurs between anomalies of ITF latitude (degrees north) and rainfall amount on all time-scales. This relationship is weakest (correlation of +0.50) during August, when the ITF is furthest north, the monsoon layer is thickest, and rainfall maximizes. However, the April ITF latitude anomaly has promising rainfall predictive potential for all months before and after August. On the multidecadal scale, the well known post-1960s Sahel drought conditions stemmed from a striking southward ITF displacement and associated reduction in MCWS size and intensity, rather than from the complete absence of MCWSs. Several of the key observational results can focus numerical model experimentation designed to elucide their causation.