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

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 similar to 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. Designing timely and relevant societal responses to climate-related impacts and risks to humans and natural systems requires reliable information about climate variability and projected change, especially at regional scales. For this purpose, considerable efforts were devoted to the improvement of the numerical models used to represent the climate system, including better formulation of the models' physical and dynamical components and the inclusion of feedback between different components of the climate systems, such as those between the ocean and the atmosphere. In this study we aim at investigating whether the use of a regional climate model which includes an explicit representation (coupling) of the ocean is able to better simulate (i.e., adds value) the main mechanisms responsible for precipitation over Central Equatorial Africa. The results show that the coupled model is indeed able to simulate more realistically the complex physical processes and mechanisms underpinning the rainfall system. Our findings advocate for the use of the global ocean-regional atmosphere coupling approach for regional climate change projection analyses. The global ocean-regional atmosphere coupling improves the rainfall climatology compared to its atmosphere-only counterpart model The added value resulting from the coupling is plausible, as associated with improvements in the processes underpinning the rainfall system The added value is modulated by the boundary conditions, with better suitability under the imperfect forcing mode