Intra‐seasonal modulation of convection in the West African Monsoon

By using daily gridded rainfall data and NCEP/NCAR 925 hPa wind reanalyses over the period 1968–1990, it is shown the evidence of large and coherent fluctuations in the rainfall and wind fields at intra‐seasonal time scale (10–60‐day period) over West Africa. A composite analysis based on the dates of maximum (minimum) of a regional rainfall index in wet (dry) sequences shows that these sequences belong to a quasi‐periodic signal of about 15 days. These enhanced (weakened) phases in the West African monsoon are characterized by a westward propagation of positive (negative) rainfall anomalies north of 10° N associated with a more cyclonic (anticyclonic) circulation over the Sahel consistent with a stronger (weaker) moisture advection over West Africa.