A recent development in the modeling of marine ice sheet response to ocean forcing has been the postulation of a Marine Ice Cliff Instability (MICI) mechanism (Pollard et al., 2015). When implemented in numerical ice sheet models, MICI can allow for dramatic thinning and retreat beyond that already expected from the Marine Ice Sheet Instability. The specific mechanism proposed is brittle failure of tall ice cliffs that remain following hydrofracture-induced ice-shelf collapse. Invoking this mechanism can result in simulations in which the Antarctic contribution to future sea level rise is substantially larger than simulations that do not invoke MICI. However, Antarctic marine ice sheet experiments using the high-resolution BISICLES ice sheet model have not demonstrated this effect, even when explicit MICI physics are implemented in the model.

Initiation of MICI and the rate of mass wastage at the ice front depend on the size of ice cliffs remaining after ice-shelf collapse; few studies have examined the dependence of post-collapse cliff heights on model resolution. We test the resolution dependence of MICI in the BISICLES model. The adaptive mesh refinement capability in BISICLES enables us to assess the potential for MICI onset following collapse of ice shelves with a range of grid resolutions. We find scenarios in which incorporating MICI physics dramatically affects ice sheet evolution in models with coarse resolution, but this impact essentially vanishes as the mesh resolution approaches the local ice thickness. This sensitivity of MICI to model resolution is in line with previous work showing the sensitivity of grounding line dynamics to grid resolution and points to the need to adequately capture the relatively fine-scale stress divergence at the grounding line and the upstream diffusion of pre-shelf-collapse thinning to achieve accurate representations of post-shelf-collapse ice cliffs.

As a result, we find that the cliff-collapse mechanism plays a minor role in ice-sheet response to ice shelf loss in our simulations, even in the presence of hydrofracture-like shelf loss. We examine the reasons behind this discrepancy, considering in particular the roles of model resolution and model physics in the formation of persistent ice cliffs.

Reference:

D. Pollard, R. M. DeConto, R. B. Alley, EPSL, (412), 2015