Where Does Moisture Come From Over the Congo Basin?

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 (delta D), 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 with delta D in 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. Observed delta D relative to a range of observationally constrained, isotopic mixing models representative of water vapor coming from land suggests that 83% +/- 9% of the free tropospheric moisture come from ET in February, and 45% +/- 13% in April, versus 59% +/- 12% in August and 31% +/- 12% in 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. Plain Language Summary The Congo Basin hosts the world's second largest rainforest and is a major rainfall center. It has been unclear whether moisture from the adjacent oceans or from evapotranspiration of the rainforests is its main moisture source, especially for initiating the rainy seasons. Using a suite of satellite data, we show that evapotranspiration, especially transpiration, is important for atmospheric moisture throughout the year. However, the relative contribution of ocean moisture from transport increases prior to fall rainy season and not prior to the spring rainy season, indicating that the relative contribution of ET to atmospheric moisture is higher in spring than in fall. Our finding highlights the importance of the rainforests in maintaining and modulating atmospheric humidity.

Automated summary

The study found that in the Congo Basin, evapotranspiration (especially transpiration) plays an important role in atmospheric moisture throughout the year. The relative contribution of ET to atmospheric moisture is higher in spring compared to fall, as the transport of moisture from the oceans increases before the fall rainy season.