Contrasting controls on Congo Basin evaporation at the two rainfall peaks
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Contrasting controls on Congo Basin evaporation at the two rainfall peaks David Crowhurst1 · Simon Dadson1 · Jian Peng1,2 · Richard Washington1 Received: 9 September 2020 / Accepted: 17 November 2020 © The Author(s) 2020
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. Keywords Congo Basin · Evaporation · Transpiration · LandFlux-EVAL · ERA5-Land
1 Introduction The Congo Basin is one of the most convectively active regions of the world, receiving around 1500–2000 mm of rainfall per year (Dezfuli 2017). Over 60% of the rainfall occurs during the wet seasons (Hua et al. 2019), and the basin-wide average seasonal cycle has two rainfall peaks, one in March and the other in November (Crowhurst et al. 2020). Approximately 75% of the rainfall is delivered by mesoscale convective systems (Jackson et al. 2009), which are contiguous areas of cold cloud that exceed 25,000 km2 in size (Taylor et al. 2018). A maximum of thunderstorm activity occurs over the eastern Congo Basin, and this region has * David Crowhurst [email protected] 1
School of Geography and the Environment, University of Oxford, Oxford OX1 3QY, UK
Present Address: Department of Remote Sensing, Helmholtz Centre forEnvironmental Research−UFZ, 04318 Leipzig, Germany
2
one of the highest rainfall totals in the tropics, with an average of 10 mm day−1 delivered during boreal winter (Sandjon et al. 2012). Knowledge of
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