Elmer/Ice News

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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Evidence of Seasonal Uplift in the Argentière Glacier

vincent2022Glacier basal motion is responsible for a large part of ice flux in temperate alpine glaciers and outlet glaciers of ice sheets. However, the hydromechanical processes by which basal water controls sliding at the glacier bed are poorly known in large part because observations are very scarce. Consequently, the impact of surface melting and meltwater input on the future of mountain glaciers and outlet glaciers of ice sheets remains unclear. Here, we use a comprehensive data set of in situ measurements performed over 2 years on the Argentière Glacier in the French Alps, complemented by state-of-the-art ice flow and hydrology modeling, to investigate changes in water storage at the ice-bedrock interface. We find strikingly large uplifts ranging between 0.20 and 0.90 m over the winter/spring seasons in the ablation zone. We show that this uplift is mostly related to enhanced bed separation as a result of increased basal water storage. We expect this study to be helpful to the glaciological community studying basal sliding and its modulation by sub-glacial hydrology with a view of improving predictions of the future behavior of mountain glaciers and outlet glaciers of ice sheets.

Read more: Vincent C., A. Gilbert, A. Walpersdorf, F. Gimbert, O. Gagliardini, B. Jourdain, J. P. Roldan Blasco, O. Laarman, L. Piard, D. Six, L. Moreau, D. Cusicanqui and E. Thibert, 2022. Evidence of seasonal uplift in the Argentière Glacier (Mont Blanc area, France). Journal of Geophysical Research: Earth Surface, 127, e2021JF006454. doi:10.1029/2021JF006454

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