The EU project European Eddy RIch Earth System Models (EERIE) is developing a new generation of Earth System Models (ESMs) that explicitly resolve ocean mesoscale dynamics, an essential but still poorly explored part of the climate system. By using recent advances in computing and model design, EERIE aims to improve long-term climate simulations, including variability, extremes, and potential tipping points influenced by mesoscale ocean processes. ICON in Sapphire configuration is one of these new models. Developed at the Max Planck Institute for Meteorology, ICON couples the atmosphere, land, ocean, and sea ice at kilometer-scale resolution. It resolves deep atmospheric convection and captures mesoscale to sub-mesoscale ocean eddies, with the option to refine the global ocean grid locally as a “computational telescope.” The atmospheric component uses a nonhydrostatic icosahedral C grid with a hybrid sigma-z vertical coordinate and parameterizes only unresolved processes (radiation, microphysics, turbulence). The ocean component shares the same grid and solves the hydrostatic Boussinesq equations, using only a subset of parameterizations such as vertical mixing and velocity dissipation. Sea ice is included via FESIM dynamics and a simplified thermodynamic scheme. Ocean biogeochemistry is represented by HAMOCC6, simulating more than 20 tracers. The land component, JSBACH 4, provides surface fluxes and simplified hydrology with prescribed vegetation. All components are coupled through the YAC coupler (v2.4.2). The main simulations were preceded by a 40-year spin-up period using 1950 CMIP6 forcing. From the spin-up’s final state, two parallel simulations were started: a 100-year control run and a historical run. The control run is used to identify and quantify model drift, ensuring that any long-term changes in the historical simulation could be attributed to variations in radiative forcing rather than internal drift.

The MPI-ESM1.2-ER configuration is designed to facilitate research on ocean eddies that are explicitly resolved. The 1950-control simulation, spans 200 years for ocean and 100 years for the atmosphere, forced by fixed-1950 GHG-forcing. This includes fixed orbital parameters, solar irradiance averaged over a solar cycle, greenhouse gas concentrations, and ozone concentrations. The simulation excludes volcanic aerosol forcing and focuses on tropospheric natural aerosols. The MPI-ESM1.2-ER climate model includes the following components: aerosol: none, prescribed MACv2-SP, atmos: ECHAM6.3 (spectral T127/L95; 384 × 192 longitude/latitude; 95 levels; top level 0.01 hPa), land: none, landIce: none/prescribed, ocean: MPIOM1.63 (TP6M, approximately 0.1deg; 3602x2394 longitude/latitude; 40 levels; top grid cell 0-6 m), ocnBgchem: none, seaIce: unnamed (thermodynamic (Semtner zero-layer) dynamic (Hibler 79) sea ice model). The model was run by the Max Planck Institute for Meteorology, Hamburg 20146, Germany (MPI-M) in native nominal resolutions: aerosol: 100 km, atmos: 100 km, land: none, landIce: none, ocean: 10 km, ocnBgchem: none, seaIce: 10 km. MPI-ESM-ER is a coupled climate model that is conducted following the HighResMIP protocol. Atmospheric component (ECHAM6.3) and ocean component (MPIOM) of the MPI-ESM-ER are coupled with a coupling frequency of 1hr (via OASIS3 coupler). MPI-ESM-ER includes dynamic vegetation, and interactive carbon cycle, as well as advanced representations of atmosphere and ocean processes (Stevens et al., 2013).