The West African monsoon onset and intra-seasonal variability

The daily values of raingauge amounts and NCEP/NCAR reanalyses have been used on the period 1968-1990 to investigate the monsoon onset over West Africa. The onset can be characterized by an abrupt latitudinal shift from a quasi-stationary stage at 5N in May-June to another quasi-stationary stage at 10N in July-August. The date of each onset has been defined by examining the daily time series of the two first EOF components computed on rainfall Hovmoeller diagrams centered on 10W-10E. Then NCEP/NCAR mean composite wind fields have been computed by using the date of the onset for each year as a reference date. Moreover we computed these composite fields both for intra-seasonal and seasonal variability by filtering the data with a cut-off at 60 days. We have shown that the abrupt shift of the rainbelt is concomitant to the westward propagation along 17.5N and at 925 hPa of a cyclonic circulation belonging to a 20-40-day atmospheric pattern. We have defined an index based on the relative vorticity field which can help to a medium-range forecast of this onset.