Quoted from: https://e3sm.org/model/e3sm-model-description/v1-description/v1-ocean-sea-ice-land-ice/ 

Since a new ocean model mesh was introduced with MPAS-O, Phase I of E3SM required the development of MPAS-SI, an entirely new sea ice component (Turner et al., 2018). This is because the E3SM coupler requires the ocean and sea ice components to use the same mesh, and the existing sea ice model was incompatible with that mesh. Similar to the approach used in many atmosphere models, we have separated the sea ice dynamical core solver from the vertical, column physics package. Integration of MPAS-SI into the coupled system had several coupling instabilities (with inertial oscillations and sea ice embedding). However, we overcame these challenges and completed stable coupled simulations.