Elmer/Ice News

Hysteretic evolution of ice rises and ice rumples in response to variations in sea level

LowHysteresisIce rises and ice rumples are locally grounded features found in coastal Antarctica and are surrounded by otherwise freely floating ice shelves. In both cases, local highs in the bathymetry are in contact with the ice shelf from below, thereby regulating the large-scale ice flow, with implications for the upstream continental grounding line position. We investigate ice rises and ice rumples using a three-dimensional full Stokes ice flow model under idealised scenarios. The simulations span end-member basal friction scenarios of almost stagnant and fully sliding ice at the ice–bed interface. We analyse the interaction with the surrounding ice shelf by comparing the deviations between the non-local full Stokes surface velocities and the local shallow ice approximation (SIA). Deviations are generally high at the ice divides and small on the lee sides. On the stoss side, where ice rise and ice shelf have opposing flow directions, deviations can be significant. During sea level increase and decrease experiments, transitions from ice rise to ice rumple occur (and vice versa) and divide migration is more abrupt the higher the basal friction. We identify a hysteretic response of ice rises and ice rumples to changes in sea level, with grounded area being larger in a sea-level-increase scenario than in a sea-level-decrease scenario. This hysteresis shows not only irreversibility following an equal increase and subsequent decrease in sea level but also that the perturbation history is important in determining the current ice rise or ice rumple geometry.

Read more: Henry A. C. J., R. Drews, C. Schannwell and V. Višnjević, 2022. Hysteretic evolution of ice rises and ice rumples in response to variations in sea level, The Cryosphere, 16, 3889–3905, doi:10.5194/tc-16-3889-2022

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Threshold response to melt drives large-scale bed weakening in Greenland

maier2022Ice speeds in Greenland are largely set by basal motion, which is modulated by meltwater delivery to the ice base. Evidence suggests that increasing melt rates enhance the subglacial drainage network’s capacity to evacuate basal water, increasing bed friction and causing the ice to slow. This limits the potential of melt forcing to increase mass loss as temperatures increase. Here we show that melt forcing has a pronounced influence on dynamics, but factors besides melt rates primarily control its impact. Using a method to examine friction variability across the entirety of western Greenland, we show that the main impact of melt forcing is an abrupt north-to-south change in bed strength that cannot be explained by changes in melt production. The southern ablation zone is weakened by 20–40 per cent compared with regions with no melt, whereas in northern Greenland the ablation zone is strengthened. We show that the weakening is consistent with persistent basal water storage and that the threshold is linked to differences in sliding and hydropotential gradients, which exert primary control on the pressures within drainage pathways that dewater the bed. These characteristics are mainly set by whether a margin is land or marine terminating, suggesting that dynamic changes that increase mass loss are likely to occur in northern Greenland as temperatures increase. Our results point to physical representations of these findings that will improve simulated ice-sheet evolution at centennial scales.

Read more: Maier N., F. Gimbert and F. Gillet-Chaulet, 2022. Threshold response to melt drives large-scale bed weakening in Greenland, Nature 607, pages 714–720, doi:10.1038/s41586-022-04927-3

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On the nature of melt-rate oscillations in coupled ice-ocean simulations

An emergent feature in several contributing coupled models to the 1st Marine Ice Sheet–Ocean Model melt rate evolution of timeIntercomparison Project (MISOMIP1) was a time-varying oscillation in basal melt rates. In this study, the authors used Elmer/Ice coupled to ROMSIceShelf via the Framework for Ice-Sheet Ocean Coupling (FISOC) to investigate the origin and implications of this observed feature and, more generally, the impact of coupled modeling strategies on the simulated basal melt in an idealized ice shelf cavity based on the MISOMIP setup. Interestingly, melt oscillations emerged in both, the coupled system and the standalone ocean model using a prescribed change of cavity geometry. There appears a close relation to the discretized ungrounding of the ice sheet, probably strengthened by a combination of positive buoyancy–melt feedback and/or melt–geometry feedback near the grounding line, and the frequent coupling of ice geometry and ocean evolution. There is still debate whether this is purely numerical or a numerical artifact enhanced by model physics. The paper includes a short best-practice guide on the choice of parameters to minimize the impact of this effect.

Read more: Zhao, C., R. Gladstone, B.K. Galton-Fenzi, D. Gwyther, and T. Hattermann, 2022. Evaluation of an emergent feature of sub-shelf melt oscillations from an idealized coupled ice sheet–ocean model using FISOC (v1.1) – ROMSIceShelf (v1.0) – Elmer/Ice (v9.0),. Geosci. Model Dev., 15, 5421–5439, doi:10.5194/gmd-15-5421-2022

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