The Mechanisms Controlling the Rainy Season Transition Period in the Southern Congo Basin

The Congo Basin hosts one of the largest terrestrial precipitation centers in the tropics. Yet, the mechanisms that start the rainy seasons in the 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 an increase of moisture from the Indian Ocean due to increases in the land-ocean surface temperature gradient during the late dry season, three to four months before the rainy season onset, 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 biomass burning aerosols and low-level clouds. In the early transition period, about one to three months before the rainy season onset, column water vapor and surface specific humidity increase, due to a combination of increased oceanic moisture and increases in ET for the first time. This is due to increases in shallow convection that “clear-out” the sky in between rainfall periods and increase surface radiation. Finally, in the late transition period, within one month before the rainy season onset, MFC is net convergent, and ET contributes 51.4 % of atmospheric moisture for rainfall, thus providing the atmospheric moisture needed for increasing deep convection. This increase in ET is likely due to increases in local ET and ET from the northern Congo transported by boundary-layer winds towards the Congo Air Boundary. 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. Therefore, the frequency of deep convection begins to increase, indicating the start of the rainy season. We also use a random forest model to show that surface radiation, over soil moisture and vapor pressure deficit, is the most important factor in predicting local ET during the transition period. Overall, our study provides insights into the complex mechanisms controlling the transition to the rainy season in the Southern Congo Basin and highlights the importance of ET, and hence land-atmosphere coupling, in this transition.