Browsing by Subject "parameterization"

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    Improving simulated Antarctic Sea ice thickness through young ice processes
    (2026) Mangatane, Magata; Vichi, Marcello
    Numerical ocean-sea ice models are essential for understanding the rapidly evolving Antarctic sea-ice cover, yet their performance in the Southern Ocean remains limited by sparse observations, incomplete process representation, and understanding, leading to large intermodel differences. In particular, the formation processes of young Antarctic sea ice, characterized by a granular texture, high ice liquid fraction and thermal permeability and low mechanical strength, are largely unexplored in current models, which are primarily tuned for columnar Arctic first-year and multi-year ice. This thesis addresses these knowledge gaps by investigating the impact of young Antarctic ice formation processes in the NEMO ocean-sea ice model. As a foundation, the thesis conducted a comprehensive intercomparison of six widely used satellite-based sea-ice thickness methods against available field observations, with a focus on ice growth and melt temporal scales. The satellite algorithms have been clustered into two groups: lidar-only approaches, which perform better under thin-ice conditions, and combined-sensor approaches, which are more reliable for thick ice. Building on this framework, the thesis implemented and tested a novel parameterization that enhances sea-ice thermal permeability by using bulk sea-ice salinity as a proxy for young ice type. The increased radiative and turbulent heat fluxes permeability led to enhanced ocean heat loss through the ocean-ice interface, increased ice production in coastal polynyas, and subsequent redistribution of ice through dynamic transport, supporting thickening throughout most of the circumpolar ice cover. Ice thickness increased in winter and summer relative to the standard parameterization and improved the agreement with satellite estimates, with some overestimation in autumn. To test the mechanical contribution of young ice, the thesis proposed a modification of the standard Hibler ice-strength parameterization to incorporate the influence of young ice characteristics such as large brine volume alongside thickness and concentration. This parameterizes weak, deformable ice even under complete ice cover, consistent with recently observed Antarctic features. The revised parameterization was tested in offline simulations and showed reduced ice strength in young-ice regimes, enhancing ice drift and deformation and partly increasing new ice production over open ocean due to increased divergence. While the enhanced thermal permeability largely affects early winter growth (March-April) and leads to a more resilient, thicker summer ice, the modified ice-strength parameterization exerts a more modest influence from mid-winter through spring, which highlights the limits of augmenting the level of detail in an ice-strength parameterization that was designed for larger scale processes. Together, these separate developments indicate that improving the physical realism of Antarctic sea-ice models narrows the gap between simulations and observations. By explicitly accounting for the overlooked role of young Antarctic sea ice, this work provides an initial pathway toward improved representation of the seasonal air-sea-ice interactions, which may influence upper ocean mixing, and longer-term climate-relevant processes such as dense shelf water formation, and Antarctic Bottom Water production.