The intra-seasonal variability of the West African summer monsoon is characterised by three main modes around 15 days and 40 days periodicity.

Around 15 days, two types of modes were differentiated: one quasi-stationary centre close to the equator along the Guinean coast which can be coupled with atmospheric Kelvin waves propagating eastward; and a second mode finding its origin in Central Africa and propagating westward on Sahelian latitudes similarly to atmospheric Rossby waves to the west side of the tropical Atlantic. Around 40 days, one mode modulating the whole of the west and central African convection was also depicted and shows links with the Madden-Julian Oscillation coming from the Indian Ocean.

Then the impact of these modes on the convective activity is examined using high resolution MCS data (cloud cluster identification from full-resolution Meteosat infrared channel database). These analyses show that reinforcement or weakening convection anomalies links to these modes modulates significantly the cloud cluster distribution.