A Hybrid Particle-Continuum Method for Simulating Fast Ice via Subgrid Iceberg Interaction

Abstract: A significant fraction (4%-13%) of Antarctic sea ice remains stationary as landfast sea-ice ("fast ice"), typically anchored by grounded icebergs. Current global climate models do not represent fast-ice formation due to iceberg grounding, as iceberg-sea-ice interaction mostly occurs at subgrid scales. We propose a novel subgrid-scale coupling mechanism between Lagrangian iceberg particles and an Eulerian sea-ice continuum model. This hybrid particle-continuum approach integrates feedback from icebergs into the sea-ice momentum equation via a Green's function, a Stokeslet, representing the drag exerted by a point force on the viscous-plastic medium. The coupled system, including the Stokeslet induced drag, is discretized using a finite-element method with piecewise linear basis functions. The approach assumes that individual icebergs have diameters smaller than the grid spacing. The presented finite-element discretization is compatible with existing unstructured-mesh ocean model frameworks such as FESOM and ICON, ensuring practical applicability in Earth system modeling. This work provides and analyzes, for the first time, a stable numerical framework to capture the effects of individual subgrid-scale icebergs on sea-ice dynamics. We derive an a-priori stability estimate bounding a functional of the sea-ice system and show that the momentum equation including the subgrid iceberg-sea-ice drag remains stable. Numerical test cases demonstrate the capability of the approach to capture fast-ice formation due to subgrid iceberg grounding on coarse horizontal grids.