<p>The Congo basin hosts one of the largest terrestrial precipitation centers. Yet, the mechanisms that start the rainy seasons in Congo have not been studied systematically. We show that the transition from the dry to the rainy season over the southern Congo is initiated by a decrease in moisture export towards the Sahel, about three to four months before the rainy season onset (RSO), referred to as the pre-transition period. During this period, evapotranspiration (ET) is low due to low surface solar radiation, resulting from low insolation and high amounts of low-level clouds. In the early transition period, one to three months before the RSO, column water vapor increases due to increased oceanic moisture transport. Meanwhile, ET starts increasing due to increases in surface radiation and vegetation photosynthesis, despite a lack of soil moisture increases. Finally, in the late transition period, about one month before the RSO, ET continues to increase, contributing equally to atmospheric moisture needed for deep convection as advected oceanic moisture. Additionally, the formation of the African Easterly-Jet South and the southward movement of the Congo Air Boundary increase vertical wind shear and provide large-scale dynamic lifting of the warm and humid air from Congo. The frequency of deep convection increases rapidly, leading to the start of the rainy season. Therefore, the RSO over southern-hemispheric Congo basin is a result of combined large-scale atmospheric circulation change driven by increasing land–ocean surface temperature gradient and vegetation response to the seasonal change of insolation.</p>