Intra-seasonal modulation of convection in the West African monsoon

Abstract

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.