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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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On the mechanisms that control the rainy season transition period in the southern Congo Basin
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 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.
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- Award ID(s):
- 1917781
- PAR ID:
- 10577485
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Climate Dynamics
- Volume:
- 63
- Issue:
- 3
- ISSN:
- 0930-7575
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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