Circumpolar Effects of a Velocity‐Restoring Scheme for Antarctic Fast Ice on Shelf Waters and Ice Shelf Melt Simulated by NEMO4‐SI 3

Abstract

Dense water formation in the Southern Ocean plays a key role in the global ocean overturning circulation, affecting the distribution of heat, carbon, oxygen, and nutrients. Fast ice stays immobile while blocking sea ice flow, favoring the opening of coastal polynyas, where intense air‐sea exchanges drive sea ice production and ocean densification. Antarctic fast ice representation is notoriously poor in Earth system models, and is thus considered a potential cause of model biases in dense water formation. Here we assess the impact of a sea ice velocity‐restoring representation of Antarctic fast ice on Antarctic shelf water properties and ice shelf melt using NEMO4‐ simulations with and without velocity‐restoring. By improving the location and shape of coastal polynyas, fast ice redistributes sea ice production and promotes Dense Shelf Water formation in several key source regions. In contrast, beneath extensive fast ice tongues, the reduced brine rejection associated with lower sea ice production limits mixed layer deepening, leading to fresher surface waters, which are then advected into downstream polynyas. The enhanced upper‐ocean stratification beneath fast ice isolates warm subsurface waters from the surface and favors their intrusion into nearby ice shelf cavities, increasing ice shelf basal melt. As a result, ice shelf basal melt increases overall by 15.8% in the simulation including fast ice. These results support that ESMs studies with an Antarctic focus would gain from a physically‐based representation of Antarctic fast ice.