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 (>700hPa) 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.
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The constructive comments and suggestions of the editor and two anonymous reviewers led to several key improvements of the first version of the manuscript. The first author wish to express their gratitude to very fruitful discussions with F. Guichard (CNRM). The authors would like to acknowledge support from the Swedish Government through the Swedish International Development Cooperation Agency (SIDA). This work is partially supported by the International Joint Laboratory’s research “Dynamics of Land Ecosystems in Central Africa: A Context of Global Changes” (IJL DYCOCA/LMI DYCOFAC). GNL acknowledges support by PREFACE project (EU FP7/2007-2013 under grant agreement no. 603521); National Research Foundation SARChI Chair in Ocean-Atmosphere-Land modelling and ACCESS project. We also acknowledge logistical support from the CORDEX International Project Office, the Swedish Meteorological Institute and the Climate System Analysis Group at the University of Cape Town. We are grateful to all the modeling groups that performed the simulations and made their data available.
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- Congo Basin rainfall biases
- Global warming levels
- Moisture convergence