Benefits of a Landfast Ice Representation on Simulated Antarctic Sea Ice and Coastal Polynya Dynamics

The Antarctic coastal marine region is a unique and highly complex environment, of which landfast ice and polynyas are key features, especially in the context of dense water formation. Current large-scale ocean-sea ice models used in climate studies simulate hardly any Antarctic landfast ice, which has presumably negative implications on sea ice and polynya dynamics. Here we develop, implement, and evaluate an empirical circumpolar Antarctic landfast-ice representation for large-scale ocean-sea ice models. This representation is based on the restoring of sea ice velocity to zero where and when landfast ice is observed, according to a recently released circum-Antarctic landfast ice database. Using 2001–2017 hindcast simulations with the NEMO-${\text{SI}}^3$ model, we demonstrate that prescribing landfast ice not only ensures accurate landfast ice coverage, as expected, but also largely improves the simulated landfast ice thickness and polynya dynamics. This includes more realistic polynya coverage, individual polynya shape, frequency, and ice production rates. Additionally, the model low bias in summer ice extent is reduced, as prescribing landfast ice locks thicker ice near the coast, taking longer to melt. Our simulations also give the first estimate of landfast ice volume, representing 10.6% of the pan-Antarctic total, compared to 3.8% of the total Antarctic sea ice extent. We argue that velocity restoring is appropriate for some investigations of the Antarctic landfast ice over the recent past, but not for the remote past or future projections, for which a physical representation of landfast ice drivers, particularly iceberg-sea ice interactions, is necessary.