Contrasting controls on Congo Basin evaporation at the two rainfall peaks

Abstract

Evaporation is a crucial driver of Congo Basin climate, but the dynamics controlling the seasonality of basin evaporation are not well understood. This study aims to discover why evaporation on the basin-wide average is lower at the November rainfall peak than the March rainfall peak, despite similar rainfall. Using 16-year mean LandFlux-EVAL data, we find that evaporation is lower in November than March in the rainforest and the eastern savannah. The ERA5-Land reanalysis, which effectively reproduces this pattern, shows that transpiration is the main component responsible for lower evaporation in these regions. Using ERA5-Land, we find the following contrasting controls on transpiration, and therefore evaporation, at the two rainfall peaks: (a) In the northern rainforest, there is lower leaf area index (LAI) in November, driven by lower surface downward shortwave radiation (DSR), and lower vapour pressure deficit (VPD) in November, driven by lower sensible heat flux that results from lower net radiation. The combination of lower LAI and VPD explains lower transpiration, and therefore lower evaporation, in November. (b) In the southern rainforest, and in the north-eastern savannah, there is lower LAI in November, driven by lower surface DSR, and this explains lower transpiration, and therefore lower evaporation, in November. (c) In the south-eastern savannah, there is lower LAI in November, driven by lower volumetric water content (VWC), and this explains lower transpiration, and therefore lower evaporation, in November. Collectively, these contrasting controls at the two rainfall peaks explain why the basin-wide average evaporation is lower in November than March.

Where Does Moisture Come From Over the Congo Basin?

“…On the other hand, Burnett et al (2020) calculated basin-scale ET in the Congo using water-balance methods, positing that increased radiation, as well as the availability of soil moisture, can explain increased ET in the spring: Higher direct photosynthetically active radiation (PAR) fractions combined with higher net solar radiation increase water use efficiency, while increased terrestrial water storage increases the amount of water available for transpiration. Crowhurst et al (2020), using global climate models, also calculated ET over a similar domain and found that it was higher in the spring rainy seasons compared to the fall rainy season. They attributed changes in leaf area index and vapor pressure deficit to changes in transpiration between the spring and fall rainy season.…”

“…10.1029/2020JG006024 7 of 14 Figure 4b shows the contributions of ET and MFC to precipitation within our Congo domain, as indicated by ET/P and MFC/P. ET/P shows consistently high values throughout the year and MFC/P is always lower than that of ET/P (e.g., Burnett et al, 2020;Crowhurst et al, 2020). These data support our conclusion that there is a larger relative contribution of ET to the moisture for the spring versus fall rainy seasons.…”

High Mountains Becoming Wetter While Deserts Getting Drier in Xinjiang, China since the 1980s

“…The ERA5-Land reanalysis data set, containing 50 climate variables and covering all land areas of the world, can accurately reflect climate changes of the past few decades with a relatively high temporal and spatial resolution. Therefore, this dataset has been used increasingly to study long-term climate change in areas where observational data is scarce [18]. Here, we retrieved temperature and total precipitation data with a monthly temporal resolution and a time range of 1981-2020 in ERA5-Land from the European Centre for Medium-Range Weather Forecasts (ECMWF) (https://www.ecmwf.int/, accessed on 3 October 2020).…”

Hydrodynamics of regional and seasonal variations in Congo Basin precipitation

“…Recycling ratios have been estimated in the 25-50% range (Pokam et al 2012;Dyer et al 2017;Sori et al 2017). Crowhurst et al (2021) note that basin-wide evaporation is lower in boreal fall than in boreal spring despite a similarity in precipitation due to lower leaf area indices and vapor pressure deficits in the southern rainforest.…”