Mechanisms of Added Value of a Coupled Global Ocean‐Regional Atmosphere Climate Model Over Central Equatorial Africa

Abstract

There is an urgent need to enhance climate projections for Central Equatorial Africa (CEA), given the region's high vulnerability to climatic hazards and its economy's heavy dependence on climate‐sensitive sectors. This study aims to evaluate the performance of the regional earth system model ROM, composed of the atmosphere‐only regional climate model (RCM) REMO coupled with the global Max Planck Institute for Meteorology Ocean Model (MPIOM), in reproducing the precipitation climatology over CEA. ROM results are compared to those of REMO in two sets of experiments, one driven by the ERA‐Interim reanalysis and the other by the MPI‐ESM‐LR earth system model (ESM), both at ∼25‐km horizontal resolution. Results show that ocean coupling improves rainfall climatology thanks to a better representation of the physical processes and mechanisms underlying the rainfall system. In particular, an improved sea surface temperature (SST) results in a more realistic simulation of land‐atmosphere‐ocean interactions, and subsequently the atmospheric baroclinicity. Specifically, the coupling reduces the positive SST bias inherited by the driving ESM across the entire Guinea Gulf and Benguela‐Angola coastal seas. This leads to better simulated land‐ocean thermal and pressure contrasts. Improvements in land‐ocean contrasts, in turn, enhance the representation of the regional atmospheric circulation, and thus precipitation. Interestingly, the coupling is more beneficial when ROM is driven by the ESM than the reanalysis. This study emphasizes the advantage of dynamically downscaling ESMs using regional earth system models rather than atmosphere‐only RCMs, with the potential to enhance confidence in future climate projections.