High-fidelity modeling of marine ice-cliff instability: Modes of collapse, mélange breaks and a calving-rate law

Abstract:

Thwaites Glacier is susceptible to rapid retreat via marine ice-cliff instability (MICI), a dramatic collapse process that could initiate if ice shelf loss and grounding line retreat expose subaerial cliffs that exceed a stability threshold dictated by the strength of ice. Though such towering ice cliffs are outside our range of observations, it is necessary to adequately represent the processes in Antarctic Ice Sheet simulations and projections of global sea level rise. Using a suite of high-resolution 3D glacier models, we found that MICI will transpire through different modes that can be described by the interactions of viscous deformation, shear-band formation, and brittle failure. Strong, intact ice with low shear stress calves via brittle failure due to tensile strain imposed on a glacier surface following characteristic viscous deformation. This process was simulated in a one-way coupling workflow of the full-Stokes continuum model, Elmer/Ice, and the particle-based Helsinki Discrete Element Model (HiDEM). The workflow allowed us to explicitly determine the duration of a calving cycle, from which we derived a simple and conservative MICI calving rate law that considers the influence of ice temperature, basal friction, and buttressing from proglacial mélange. The latter inhibited MICI collapse if sufficient back force (4.2e⁶ to 5.7e⁷ N m^-1) was exerted. The brittle visco-elastic implementation of HiDEM simulated collapse dominated by shear-band formation, which became more prominent for damaged ice associated with greater ice thicknesses and decreasing bed friction. Our results demonstrate that the MICI collapse will materialise through a variety of modes and serve as a foundation for further MICI-specific calving law development. This work will inform investigations into the vulnerability of Thwaites Glacier, which is precariously situated over a retrograde bed and has an exposed calving face approaching MICI-susceptible cliff heights.