Observation of oceanic origin of Sahel precipitation from space

► Space-based moisture transport combines surface onshore flow with offshore flow aloft. ► The transport data identify two rain regimes in West Africa Monsoon region. ► Summer rainfall in south is driven by moisture from Gulf of Guinea in south. ► Rainfall in north is driven by moisture from Atlantic in west. ► August net influx from Atlantic interacts with weakened easterly jet to cause peak rainfall.