Dynamics controlling the seasonal cycle of Congo Basin evaporation
Keywords: seasonal cycle
global climate models
tropical climate
evaporation
reanalyses
Congo Basin

Evaporation is an integral component of Congo Basin climate and a crucial moisture source for basin rainfall. However, the dynamics controlling the climatological seasonal cycle of basin evaporation are unclear. Therefore, this thesis aims to make progress towards a complete understanding of the dynamics controlling the seasonal cycle of Congo Basin evaporation. Knowledge of these dynamics will provide a theoretical framework useful for advancing basin hydrological science, and a baseline against which global climate models can be evaluated.
The LandFlux-EVAL evaporation benchmark product is used in this thesis as a point of reference for Congo Basin evaporation. On the basin-wide average, LandFlux-EVAL evaporation increases from January to its one peak in March, is stable between March and April, decreases from April to a trough in July, and recovers between July and January. Evaporation is also lower at the November rainfall peak than the March rainfall peak, despite similar rainfall in the CHIRPS2 rainfall product. Using the ERA5-Land reanalysis, which effectively reproduces the pattern of lower evaporation in November than March in LandFlux-EVAL, this thesis derives plausible dynamics responsible for this feature of the seasonal cycle. Lower surface downward shortwave radiation (DSR) is a plausible driver of lower leaf area index (LAI) in the rainforest and northeastern savannah, which is the main reason for lower transpiration, and therefore lower evaporation, in November.
As evaporation differs substantially between the basin’s evergreen rainforest and deciduous savannah, to expand beyond the difference between the peaks a regional approach is used based on land cover type. Collectively, the dynamics that explain why evaporation is suppressed at the November rainfall peak relative to the March rainfall peak, and the dynamics controlling the seasonal cycles of evaporation in the rainforest and savannah, will explain what controls the seasonal cycle of evaporation on the basin-wide average. Therefore, this thesis seeks to derive plausible dynamics controlling the seasonal cycle of evaporation in these two ecoregions.
However, the thesis finds that ERA5-Land effectively reproduces the seasonal cycle of evaporation from LandFlux-EVAL in the savannah only. Therefore, the following plausible dynamics are derived from ERA5-Land controlling the savannah’s seasonal cycle of evaporation. These are considered from November to November to account for evaporation in one wet and one dry season. Between November and March, an increase in surface DSR explains an increase in LAI, transpiration and evaporation. Between March and April, the effects of a decrease in rainfall and increase in surface DSR offset one another, leading to stable evaporation. Between April and July, a decrease in rainfall explains why evaporation falls. Between July and November, early green-up and an increase in rainfall explain why evaporation recovers.
The lack of agreement between ERA5-Land and LandFlux-EVAL in the basin’s rainforest precludes the use of ERA5-Land to derive the dynamics controlling the seasonal cycle of evaporation here. However, this thesis suggests that using a sub-grid rainfall scheme in the land surface model of the next version of ERA5-Land could bring the seasonal cycle of evaporation from ERA5-Land into good agreement with LandFlux-EVAL in the rainforest and allow the next version of this reanalysis to be used to derive the dynamics controlling the seasonal cycle of evaporation here. This could then achieve a complete understanding on the basin-wide average of why the seasonal cycle of Congo Basin evaporation behaves as it does.