Climate variability in an Earth system Model of Intermediate Complexity: from interannual to centennial timescales

This thesis explores the climate mean state and climate variability repro- duced by atmosphere-ocean coupled configurations of the Planet Simulator (PlaSim), an Earth-system Model of Intermediate Complexity (EMIC). In particular, the sensitivity to variations in oceanic parameters is explored in three atmosphere-ocean coupled configurations: using a simple mixed-layer (ML) ocean at two horizontal resolutions (T21 - 600 km and T42 - 300 km) or a more complex dynamical ocean, the Large Scale Geostrophic (LSG) ocean, at T21 atmospheric horizontal resolution. Sensitivity experiments allow to identify a reference oceanic diffusion coefficient in the ML ocean and a vertical oceanic diffusion profile in LSG, which ensure a simulated climate in good agreement with the present climate. For each model configuration, the Equilibrium Climate Sensitivity (ECS) is estimated from simulations with an increased CO2 concentration compared to pre-industrial simulations. The resulting ECS values are higher than values estimated in other EMICs or models of the Coupled Intercomparison Project Phase 5 (CMIP5) and Phase 6 (CMIP6), especially in the PlaSim-ML configurations. The climate variability of the model is then explored on different timescales, from the centennial to the interannual one.