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1. On the Mechanisms that Control the Rainy Season Transition Period in the Southern Congo Basin

   https://doi.org/10.21203/rs.3.rs-3851954/v1  

   Worden, Sarah; Fu, Rong (January 2024, Research Square)

   Abstract 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 and surface specific humidity increase due to increased oceanic moisture. Meanwhile, ET starts increasing for the first time 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 the 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 and vegetation response to the seasonal change of insolation. 

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2. On the mechanisms that control the rainy season transition periods in the equatorial Congo Basin

   Worden, Sarah; Fu, Rong (June 2026, ACP)

   The Congo Basin equatorial region (2°S-2°N, within the watershed), experiences semi-annual rainy seasons (RSs) in boreal spring and fall. Previously, the mechanisms driving the transitions to these RSs have not been investigated systematically. We show that both RS transitions begin with increases in the low-level atmospheric moisture transport from 10 the Atlantic Ocean into the region, ~1.5 months prior to the spring RS and ~2 months prior to the fall RS. Evapotranspiration contributes the most to atmospheric moisture but does not change significantly throughout both transition periods. Sharp precipitation jumps 10 days before the start of the spring and fall RSs result from boundary layer moisture increases orographically uplifted to the lower troposphere by the East African Rift and Congo Basin Cell ascending branch. This destabilizes the lower free troposphere, lowering the level of free convection and decreasing convective inhibitive energy. 15 Meanwhile, the African Easterly Jet-North and westerly return flow of the Congo Basin Cell induce vertical shear for the spring and fall RSs, respectively. Mid-tropospheric convergence strengthens due to return flow at ~700 hPa from shallow meridional cells that direct low-level moisture towards the West African Heat Low prior to the spring RS, and to the Congo Air Boundary prior to the fall RS. Therefore, the RS onsets over the equatorial region are driven by seasonal changes in large-scale atmospheric circulation, contrasting with the pivotal role of increasing evapotranspiration in driving the transition to the RS onset over the Southern Congo Basin. 

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3. Dynamical and Thermodynamic Elements of Modeled Climate Change at the East African Margin of Convection

   https://doi.org/10.1002/2017GL075486  

   Giannini, Alessandra; Lyon, Bradfield; Seager, Richard; Vigaud, Nicolas (January 2018, Geophysical Research Letters)

   We propose a dynamical interpretation of model projections for an end-of-century wetting in equatorial East Africa. In the current generation of global climate models, increased atmospheric moisture content associated with warming is not the dominant process explaining the increase in rainfall, as the regional circulation is only weakly convergent even during the rainy seasons. Instead, projected wetter future conditions are generally consistent with the El Niño-like trend in tropical Pacific sea surface temperatures in climate models. In addition, a weakening in moisture convergence over the adjacent Congo Basin and Maritime Continent cores of convection results in the weakening of near-surface winds, which increases moisture advection from the Congo Basin core toward the East African margin. Overall confidence in the projections is limited by the significant biases in simulation of the regional climatology and disagreement between observed and modeled tropical Pacific sea surface temperature trends to date. 

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4. Where Does Moisture Come From Over the Congo Basin?

   https://doi.org/10.1029/2020JG006024  

   Worden, Sarah; Fu, Rong; Chakraborty, Sudip; Liu, Junjie; Worden, John (August 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract The Congo Basin hosts the world's second largest rainforest and is a major rainfall center. However, the primary sources of moisture needed to maintain this forest, either from evapotranspiration (ET) or advection from the ocean, remain unclear. We use satellite observations of the deuterium content of water vapor (), solar induced fluorescence (SIF), precipitation, and atmospheric reanalysis to examine the relative contribution of ET to moisture in the free troposphere. We find that SIF, an indicator of photosynthesis, covaries within early rainy seasons, suggesting that ET is an important contributor to atmospheric moisture in both the spring and fall rainy seasons. However, the relative contribution of ET to the free tropospheric moisture varies between the two rainy seasons. Observedrelative to a range of observationally constrained, isotopic mixing models representative of water vapor coming from land suggests thatof the free tropospheric moisture come from ET in February, andin April, versusin August andin October. Reanalysis indicate that this difference between seasons is due to increased advection of ocean air during the fall season, thus reducing the relative contribution of ET to the Congo Basin in the fall. In addition, ET is the primary atmospheric moisture source in the winter and summer dry seasons, consistent with estimates reported in literature. Our results highlight the importance of ET from the Congo rainforest as an important source of moisture for initiating the rainy seasons. 

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5. Hydrodynamics of regional and seasonal variations in Congo Basin precipitation

   https://doi.org/10.1007/s00382-021-06066-3  

   Cook, Kerry H.; Vizy, Edward K. (January 2021, Climate Dynamics)

   The processes that determine the seasonality of precipitation in the Congo Basin are examined using the atmospheric column moisture budget. Studying the fundamental determinants of Congo Basin precipitation seasonality supports process-based studies of variations on all time scales, including those associated with greenhouse gas-induced global warming. Precipitation distributions produced by the ERA5 reanalysis provide sufficient accuracy for this analysis, which requires a consistent dataset to relate the atmospheric dynamics and moisture distribution to the precipitation field. The Northern and Southern Hemisphere regions of the Congo Basin are examined separately to avoid the misconception that Congo Basin rainfall is primarily bimodal. While evapotranspiration is indispensable for providing moisture to the atmospheric column to support precipitation in the Congo Basin, its seasonal variations are small and it does not drive precipitation seasonality. During the equinoctial seasons, precipitation is primarily supported by meridional wind convergence in the moist environment in the 800–500 hPa layer where moist air flows into the equatorial trough. Boreal fall rains are stronger than boreal spring rains in both hemispheres because low-level moisture divergence develops in boreal spring in association with the developing Saharan thermal low. The moisture convergence term also dominates the moisture budget during the summer season in both hemispheres, with meridional convergence in the 850–500 hPa layer as cross-equatorial flow interacts with the cyclonic flow about the Angola and Sahara thermal lows. Winter precipitation is low because of dry air advection from the winter hemisphere subtropical highs over the continent. 

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