Impact of a representation of Antarctic landfast ice on the shelf water properties simulated by NEMO4-SI³

The formation of dense water in the Southern Ocean plays a key role in the global overturning circulation of the ocean, and thus affects the distributions of heat, carbon, oxygen and nutrients across the World Ocean. However, the simulation of dense water properties by climate models remains problematic. These models often generate dense water in incorrect locations and for wrong reasons, primarily through deep convection in the center of the Weddell and Ross Seas. We hypothesize that this inability to simulate the formation and fate of dense water stems partly from the erroneous or absent representation of coastal polynyas and their key drivers, particularly landfast ice. A recent study presented a restoring method that accurately represents Antarctic landfast ice and demonstrated its essential role in shaping coastal polynyas and enhancing sea ice production in the NEMO4-SI3 model. Here, building on this study, we investigate the impact of this landfast ice representation on water mass properties simulated by the model over the Antarctic continental shelf. We perform two simulations: one with the landfast ice scheme activated and one with this scheme turned off. A comparison of the simulation results confirms the expected densification of water masses within polynyas when landfast ice is represented. However, the results also reveal unexpected regions of fresher water beneath landfast ice, which influence the polynya dynamics downstream. On a circumpolar scale, incorporating landfast ice enhances the model's agreement with observations, particularly in terms of bottom salinity, temperature and mixed layer depth. Notably, the mixed layer depth undergoes significant changes, which in turn affect the Southern Ocean's coastal dynamics and lead to enhanced ice shelf melting. Overall, representing landfast ice improves the simulation of dense water formation and shelf ocean dynamics, thereby advancing our understanding of key physical processes in these critical regions.