Results of the third Marine Ice Sheet Model Intercomparison Project (MISMIP+)

Abstract

Abstract. We present the result of the third Marine Ice Sheet Intercomparison project, MISMIP+. MISMIP+ is intended to be a test of ice flow models which include fast sliding marine ice streams and floating ice shelves and in particular a treatment of viscous stress that is sufficient for buttressing, where upstream ice flow is restrained by a downstream ice shelf. A set of idealized experiments test the models in circumstances where buttressing contributes to a stable steady state, and where a reduction in that buttressing causes ice stream acceleration, thinning, and grounding line retreat. We find that the most important distinction between models in this particular type of simulation is in the treatment of sliding at the bed, with other distinctions – notably the difference between the simpler and more complete treatments of englacial stress, but also the differences between numerical methods – taking a secondary role.

Improvements in one-dimensional grounding-line parameterizations in an ice-sheet model with lateral variations

“…As a first test of the modifications above, we use the MISMIP+ experiments (Cornford et al, 2020). These simulate glacier flow in a rectangular fjord-like channel, and involve significant two-dimensional curvatures of grounding lines.…”

“…Starting from a close-to-equilibrated control state with the centerline grounding line just downstream of the bedrock depression, prescribed perturbations to sub-ice oceanic melt rates (which are zero in the control) are applied for 100 years, and either maintained or re-set to zero for the next 100 years. In the MISMIP+ Ice1 experiment, the applied oceanic melt rate is a smooth function of ice-shelf draft and ocean depth, and in the Ice2 experiment, it is a large uniform value in the downstream section of the fjord (Cornford et al, 2020). The resulting variations of the grounding line are examined, mainly its position along the centerline of the channel.…”

“…At 5 km and coarser resolutions, the curvilinear features in the fjord are not adequately resolved (with only 8 grid points or less in each channel half-width), and results are physically unreasonable. (Cornford et al, 2020). Also shown is the "control" grounding line after spinup at year 0 (thick black line).…”

“…Note the nearly 3x stretching of the channel width relative to the length. With our standard model, i.e., without any of the modifications above, the grounding-line variations were significantly faster and larger than other higher-order, higher-resolution models in the intercomparison (Cornford et al, 2020). Here, we perform various sensitivity experiments with one or more of the modifications above:…”

“…Two types of experiments are used to assess the above modifications. First, simulations are performed for a fjord-like glacier confined to a relatively narrow channel, as in the MISMIP+ intercomparison (Cornford et al 2020). Because of the confining https://doi.org/10.5194/gmd-2020-131 Preprint.…”