Abrupt shift of the ITCZ over West Africa and intra‐seasonal variability

The onset of the monsoon system over West Africa is linked to the northward migration of the Inter‐Tropical Convergence Zone (ITCZ) during the northern spring and summer. By using daily gridded rainfall data and NCEP/NCAR wind reanalyses over the period 1968–1990, we show that this migration is characterised by an abrupt latitudinal shift of the ITCZ in late June from a quasi‐stationary location at 5N in May–June to another quasi‐stationary location at 10N in July–August. A composite analysis based on the shift dates shows that this northward shift is associated with the occurrence of a westward‐travelling monsoon depression pattern over the Sahel with characteristic periodicities of 20–40 days.