UCLA

The Congo Basin contributes a disproportionately large amount to water, carbon, and energy in Africa and globally. However, this region has been least studied among all tropical regions, in part due to a lack of well-constrained data available on the high resolution needed to address their spatial and temporal heterogeneities. We thus cannot yet provide a creditable assessment of changes to the Congo Basin water cycle under the influence of both climate and land cover and land use changes. My dissertation aims at advancing our understanding of land-atmosphere interactions and the mechanisms controlling the rainy season onsets within the basin, both keys towards better assessing its resilience. I first investigated the sources of moisture in the atmosphere, a necessary condition for rainfall in Chapter 3. I used remotely-sensed water vapor isotopes, in conjunction with a suite of other satellite, in-situ, and reanalysis estimates and isotope mixing model simulations, to disentangle the relative contributions of evapotranspiration (ET) versus advected oceanic moisture to atmospheric moisture towards rainfall. I reveal that ET provides the most atmospheric moisture throughout the year, and especially for the onset of the spring rainy season. This suggests that the Congo Basin is especially vulnerable to land-use rapid expansion, which can reduce moisture availability and potentially exacerbate climatic drying in that region. I then evaluated the water fluxes at the interface between the atmosphere and land surface in Chapter 4. My research demonstrated that water vapor isotopes, when normalized to reduce their sensitivity to large-scale changes in atmospheric moisture, co-vary with the water balance, a metric for examining the net flux of moisture into the surface. I show that on basin and sub-basin scales, the Congo Basin displays limited water deficits with insignificant variations over the 21st century, despite observed rainfall variability and other changes to its water cycle. We also confirm high ET on basin and sub-basin scales, thus providing an additional constraint on existing ET model estimates. Built on the foundation laid in chapters 3 and 4, I next explore the fundamental mechanisms that drive the transition from dry to the rainy season over the Congo basin, a far more complex problem that those in the previous chapters. In Chapter 5, I first focus on the southern Congo Basin with one rainy season in boreal fall. I show that this transition is initiated by a decrease in moisture export towards the Sahel. Then, ET increases due to increases in surface radiation and vegetation photosynthesis. Using water vapor isotopes, I show that ET becomes the main source of atmospheric moisture prior to the start of the rainy season. I additionally show that the African Easterly Jet South and the Congo Air Boundary are key for inducing atmospheric conditions amenable for deep convection. Overall, I show that the rainy season onset is a result of combined large-scale atmospheric circulation changes and vegetation responses to the seasonal change of insolation. In Chapter 6, I explore the mechanisms of the transition periods to the boreal spring and fall rainy seasons in the equatorial Congo, which is mostly covered by tropical rainforests. I show that the transition to both rainy seasons is initiated by changes in atmospheric moisture transport across its western boundary. While ET contributes the most to atmospheric moisture to rainfall prior to both rainy seasons, it does not change significantly during the transition periods and instead provides background moisture. Generally, thermodynamic conditions indicate an unstable atmosphere, but changes in the level of free convection (LFC) and convective inhibitive energy (CIN) must happen for deep convection to initiate. This is done via increases in boundary layer moisture orographically lifted by the African Easterly Rift, decreasing the LFC and hence CIN. Meanwhile, the African Easterly Jet North and the return branch of the Congo Basin Cell provides shear prior to the spring and fall rainy seasons, respectively. Together, this creates atmospheric conditions favorable for the initiation of deep convection. Through the above discussed works, I have made an important first step toward a systemic and holistic understanding of the Congo Basin water cycle, setting up future studies of understanding the mechanisms controlling its change and its impacts on the vegetation and nations that lie within, an aspirational goal of my career.