IFS-FESOM

IFS-FESOM integrates ECMWF’s Integrated Forecasting System (IFS model description) with AWI’s Finite-volumE Sea-ice Ocean Model (FESOM2, [Danilov et al., 2017], FESOM repository) in a single-executable, sequential coupling framework [Rackow et al., 2025]. The atmosphere advances for one hour and passes fluxes to the ocean, which then integrates forward to the same checkpoint. The updated ocean surface fields are used for the subsequent atmospheric step. Grid-to-grid coupling uses Gaussian distance-weighted remapping. ecWAM waves ([ECMWF, 2023], repository) are coupled indirectly via the atmosphere. The IFS runoff is directly coupled with FESOM. In addition to the core Climate DT simulations, so-called storyline simulations were performed with IFS-FESOM at 10km resolution in the atmosphere. This page provides an introduction to the ocean and sea-ice model FESOM.

Simulations and IFS description

• Control Simulation
• Historical and Projection Simulations
• Storyline Simulations
• IFS model description

Ocean and Sea Ice (FESOM2)

FESOM uses the NG5 unstructured mesh, which provides ~3-4 km resolution in high latitudes and western boundary current regions and ~13 km resolution in the tropics. For ocean vertical mixing, a turbulent kinetic energy (TKE) closure scheme is used [Gaspar et al., 1990]. The ocean physics further use the resolution dependent Redi isopycnal diffusion [Redi, 1982], and biharmonic, flow-aware viscosity. For the sea-ice physics the following parameterisations were used: elastic–viscous–plastic (EVP) rheology (Hibler, 1979; Hunke & Dukowicz, 1997), standard zero-layer thermodynamics (Parkinson & Washington, 1979), and a prognostic snow layer (Owens & Lemke, 1990). More details can be found in [Rackow et al., 2025].

In contrast to the generation 1 runs in Climate DT, the sea-ice coupling was changed from fractional coupling (which assumes a fixed ice thickness of 1.5m) to thermodynamical coupling, where the IFS model computes the atmosphere–sea ice fluxes using sea-ice properties (temperature, albedo) as input directly coupled from FESOM. This is a much more physically realistic parametrization that tightens the coupling between the atmosphere and sea-ice (via additional feedback mechanisms).

The finite-element sea ice model (FESIM) is described in [Danilov et al., 2015].

Initial conditions and forcing

The forcing assumes the SSP3-7.0 scenario for greenhouse gases and prognostic ozone computation, where the CAMS-based aerosol composition has been extended into the future by using the MACv2-SP scheme.