Finite-Element Sea Ice Model is formulated on triangular meshes and is generally used as a part of FESOM. In this case it relies on FESOM infrastructure and mesh partitioning. It also exists as a stand-alone version, but differs in parallelization and the presence of auxiliary routines.

FESIM uses the simplest standard 0-layer thermodynamics. It places variables at vertices of triangular mesh for both FESOM1.4 and 2.0. In its dynamical part, it relies on the viscous-plastic (VP) rheology and offers several implementations of the elastic-viscous-plastic (EVP) solver. The versions of EVP are the original one (as presented in Hunke and Dukowicz (1997); we recommend that it is run with some changes as described in Danilov et al. 2015), modified and adaptive. The last two converge to the solution of VP algorithm if iterated sufficiently long. The stand-alone version also contains an implicit VP solver, but up to present it is not used in FESOM.

Because the EVP approach is explicit, it is subject to time step limitations, which define the number of subcycles needed for stability in the standard EVP. In the modified and adaptive EVP, stability is set independently of the number of subcycles which governs the convergence (Danilov et al, 2015, Koldunov et al, 2019). Generally, the number of subcycles is increasing with the mesh elements decreasing, and can be as large as 800 on 4.5 km meshes. The failure to observe these limitation entails noise in the the field of ice shear and divergence. It was demonstrated that FESIM can be used to simulate sea ice on resolutions at least down to 1km in the Arctic Ocean.

If FESOM is run as a part of AWI-CM, some adjustments are made to the functioning of ice thermodynamics, with some fast processes treated on the side of the atmosphere.