This study investigates the diurnal cycle of the West African monsoon, its seasonal modulation with particular focus on the monsoon onset period. A composite analysis around the monsoon onset date is applied to the 1979-2000 NCEP and ERA40 reanalyses at 0000, 0600, 1200 and 1800 UTC. This study points out two independent modes describing the spatio-temporal variability of the diurnal cycle of the surface wind and temperature. While the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system, the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date. The latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date.

The diurnal cycle of dry and deep convection is also investigated. A distinct diurnal cycle of deep convection in the ITCZ is evidenced with a peak at 1200 UTC before the monsoon onset, and at 1800 UTC after the monsoon onset. Strong ascending motions associated with deep convection generate gravity wave that propagates northward and reaches the Sarahan Heat Low region 12 hours later. The diurnal cycle of the dry convection in the Sarahan Heat Low is similar during the pre-onset and the post-onset periods with a peak at night (0000 UTC) consistently with the nocturnal jet intensification. This convection is localized at 15°N and 20°N before and after the monsoon onset, respectively. Despite many similarities between the results obtained using NCEP and ERA40 reanalyses, thus giving confidence on the significance of our results, there are some differences, especially in the diurnal cycle of deep convection which limit the interpretation of some of our results.