Evaluation of Evaporation Climatology for the Congo Basin Wet Seasons in 11 Global Climate Models

Abstract

Across the Congo, there is a wide spread in rainfall in the two wet seasons in Coupled Model Intercomparison Project 5 global climate models (GCMs). As the Congo is believed to be a moisture recycling hot spot, the evaporation of excess water from the land surface in some models could be amplifying the model spread in rainfall. This study performs an exploratory process‐based evaluation of Congo Basin evaporation in 11 Coupled Model Intercomparison Project 5 GCMs that took part in the Atmospheric Model Intercomparison Project. Our aims are to improve scientific understanding about Congo evaporation, and to determine whether there are opportunities to improve how models produce Congo evaporation. Climatologically, we find that models with “realistic” rainfall simulate higher rainfall in November, the peak of the second wet season, than March, the peak of the first. However, models with “realistic” evaporation simulate lower evaporation in November than March, because these models suppress the transpiration component of the evaporation in November relative to March. In both wet seasons, subgrid rainfall schemes make these models simulate a credible ratio of transpiration to canopy evaporation, and cause them to generate evaporation in a more realistic manner. We therefore trust how these models produce evaporation in the wet seasons, and argue that lower transpiration is likely to explain why evaporation is lower in November than March in reality. We also suggest that using subgrid rainfall schemes in all GCMs could improve how models produce Congo evaporation during the wet seasons. This might reduce the model spread in Congo rainfall.

Contrasting controls on Congo Basin evaporation at the two rainfall peaks

“…This suggests that rainfall amounts on the basin-wide average are similar in March and November. Crowhurst et al (2020) also found that the best performing GCMs with low root mean square errors against LandFlux-EVAL evaporation reference data (Mueller et al 2013) have lower evaporation in November than March on the basin-wide average, which agrees with several reanalyses (Fig. 1b, Table 1).…”

“…To find out why evaporation is lower in November than March on the basin-wide average, and to begin understanding the controls on the seasonal cycle of basin evaporation, this study will first need to identify where in the basin evaporation is lower in November than March, and then discover which evaporation components are responsible in these regions separately. Crowhurst et al (2020) found that on the basin-wide average, the best performing GCMs have similar canopy evaporation and soil evaporation in March and November, but lower transpiration in November than March. The authors therefore argued that lower transpiration is likely to explain why evaporation is lower in November than March on the basin-wide average.…”

“…This suggests that factors other than rainfall could modulate EVI during SON. Averaged across the Congo Basin, Crowhurst et al (2020) found that the best performing global climate models (GCMs) with low root mean square errors against CHIRPS2 rainfall (Funk et al 2015) have slightly higher rainfall at the November rainfall peak than the March rainfall peak. While this agrees with several studies (Haensler et al 2013;Washington et al 2013;Creese and Washington 2016), and several reanalyses ( Fig.…”

“…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 km 2 in size (Taylor et al 2018).…”

Data-driven estimates of evapotranspiration and its controls in the Congo Basin

“…While some previous studies have applied a similar technique to the Congo Basin as part of larger-scale experiments, these studies assumed terrestrial water storage was constant over their study periods (Marshall et al, 2012;Ukkola and Prentice, 2013;Weerasinghe et al, 2020) -a plausible assumption for long-term ET estimates but one that could mask a large degree of ET variability on annual and shorter timescales. Indeed, remotely sensed evidence suggests water storage anomalies within the basin do change significantly on monthly and interannual timescales (Crowley et al, 2006;Rodell et al, 2018), even if long-term trends are typically small relative to the magnitude of ET fluxes (Weerasinghe et al, 2020). Thus in order to explore seasonal cycles and variations in basin-wide ET, terrestrial water storage must be constrained in inverted-water-balance models.…”