The variability of the monsoon intensity over West Africa at intra-seasonal time scales has been studied mostly in regard to synoptic variability associated to easterly waves. By using the daily frequency of rainfall data set from the IRD for the period 1968-1990, as well as NCEP/NCAR and ERA reanalyses, we extend the study of the intra-seasonal variability to time scales between 10 and 60 days through spectral analysis.

Wavelet analysis applied each year to daily Sahelian rainfall indexes highlights the occurrences of intermittent intra-seasonal signal in this spectral window which determine enhanced and weakened phases in the West African monsoon. The mean composite spatial pattern associated to these phases points out a westward propagation of both rainfall and wind fields anomalies, characterized by a main period about 10-15 days. A weaker intra-seasonal signal at lower frequency (around 35 days) appears to influence strongly the seasonal cycle of the ITCZ, by controlling its abrupt northward shift at the end of June, which signs the beginning of the summer monsoon over West Africa.