Process-oriented assessment of RCA4 regional climate model projections over the Congo Basin under 1.5. C and 2. C global warming levels: influence of regional moisture fluxes

Understanding the processes responsible for precipitation and its future change is important to develop plausible and sustainable climate change adaptation strategies, especially in regions with few available observed data like Congo Basin ( CB). This paper investigates the atmospheric circulation processes associated with climate model biases in CB rainfall, and explores drivers of projected rainfall changes. Here we use an ensemble of simulations from the Swedish Regional Climate Model ( RCM) RCA4, driven by eight General Circulation Models ( GCMs) from the Coupled Model Intercomparison Project Phase 5 ( CMIP5), for the 1.5. C and 2. C global warming levels ( GWLs), and under the representative concentration pathways ( RCPs) 4.5 and 8.5. RCA4 captures reasonably well the observed patterns of CB rainfall seasonality, but shows dry biases independent of seasons and large scale driving atmospheric conditions. While simulations mimic observed peaks in transition seasons ( March- May and September- November), the rain- belt is misplaced southward ( northward) in December- February ( June- August), reducing the latitudinal extent of rainfall. Moreover, ERA- Interim reanalysis driven RCM simulation and RCM- GCM combinations show similar results, indicating the dominance of systematic biases. Modelled dry biases are associated with dry upper- tropospheric layers, resulting from a western outflow stronger than the eastern inflow and related to the northern component of African Easterly Jet. From the analysis of the climate change signal, we found that regional scale responses to anthropogenic forcings vary across GWLs and seasons. Changes of rainfall and moisture divergence are correlated, with values higher in March- May than in September- November, and larger for global warming of 2.0. C than at 1.5. C. There is an increase of zonal moisture divergence fluxes in upper atmospheric layers ( > 700 hPa) under RCP8.5 compared to RCP4.5. Moreover, it is found that additional warming of 0.5. C will change the hydrological cycle and water availability in the CB, with potential to cause challenges to water resource management, agriculture, hydro- power generation, sanitation and ecosystems.