Dynamic and Thermodynamic Contributions to Rainfall Changes in the Congo Basin: Evaluation of the Impact of an RCM's formulation

Addressing the impacts of climate change requires, first of all, understanding the mechanisms driving changes, especially at regional scale. In particular, policymakers and other stakeholders need physically robust climate change information to drive societal responses to a changing climate. In this study, we analyse the late 21st century precipitation projections (2071-2100) over the Congo Basin under Representative Concentration Pathway (RCP) 8.5, from the Rossby Centre Regional Climate Model (RCM) RCA4. In particular, we examine the impact of the RCM formulation (reduction of turbulent mixing) on future projections, by comparing the results of the modified version (RCA4-v4) with those of the standard version (RCA4-v1) used in CORDEX (Coordinated Regional climate Downscaling EXperiment). The two RCM versions are driven by two global climate models participating in the Coupled Model Intercomparison Project phase 5 (CMIP5). Results show that seasonal precipitation is largely affected by modifications in the atmospheric column moisture convergence or divergence, in turn, associated with dynamic and thermodynamic effects. Projected decreased precipitation in the dry seasons is associated with an increased moisture divergence, driven by dynamic effects (changes in circulation). Precipitation is projected to overall increase in the wet seasons, related to both dynamic and thermodynamic effects, but with larger thermodynamic contribution (changes in specific humidity). By comparing the two model versions, we found that the formulation strongly influences precipitation projections as well as the boundary conditions (driving GCM). This result could be very informative in view to ensure models fitness for the purpose of future projections for decision-makers.