Century-long kilometre-scale Ocean eddy-rich global climate simulation with the coupled IFS CY48R1 – FESOM 2.5 model

Abstract. We present novel centennial-scale global climate simulations at kilometre-scale resolution utilizing the coupled IFS-FESOM model, featuring a 9 km atmosphere and a minimal 5 km ocean. Following the HighResMIP protocol, a 50-year high-resolution coupled spin-up was conducted, which was followed by a 65-year historical simulation (1950–2014) and a scenario simulation (SSP2-4.5, 2015-2050). This was accompanied by a 100-year control simulation (1950–2050) employing the 1950 radiative forcing. These simulations explicitly resolve ocean mesoscale eddies within a long-term climate context. Overall, the model demonstrates an improved mean climate state compared to CMIP6 models, with a notable reduction in persistent model biases, except for the polar regions. Performance metrics reveal reduced global errors in surface temperature, winds, and cloud formations. The very high-resolution ocean captures eddy-rich dynamics and realistic boundary current variability, contributing to an improved sea surface salinity patterns and a strengthened Atlantic Meridional Overturning Circulation (peak ~20 Sv). The simulation also reproduces internal climate variability with high fidelity, notably a realistic El Niño–Southern Oscillation with the desired quasi-periodicity (~4–5 years) and realistic winter teleconnection patterns. Sea ice and high-latitude biases have been identified as the primary remaining challenges: the model overestimates the extent of Arctic sea ice, resulting in a cold bias in the Northern high latitudes, while an initialization error in Antarctic snow cover induces a warm bias over Antarctica. Furthermore, there is a warm bias over the Weddell Sea with high ocean mix layer depth, associated with a winter devoid of sea ice. Despite persistent sea-ice and high-latitude biases, the coupled system remains stable over centennial time scales with minimal long-term drift. These results demonstrate the feasibility and scientific value of global coupled climate simulations operating in the ocean eddy-rich regime at sub-10 km resolution. The IFS–FESOM kilometre-scale configuration thus represents a significant step forward in the development of next-generation Earth system models that robustly bridge global climate dynamics and regional-scale processes over multi-decadal to centennial periods.