The speed-up of glaciers following ice shelf collapse can accelerate ice mass loss dramatically. Investigating the deformation of landfast sea ice enables studying its resistive (buttressing) stresses and mechanisms driving ice collapse. Here, we apply offset tracking to Sentinel-1 A/B synthetic aperture radar (SAR) data to obtain a 2014-2022 time-series of horizontal velocity and strain rate fields of landfast ice filling the embayment formerly covered by the Larsen B Ice Shelf, Antarctic Peninsula until 2002. The landfast ice disintegrated in 2022, and we find that it was precipitated by a few large opening rifts. Upstream glaciers did not accelerate after the collapse, which implies little buttressing effect from landfast ice, a conclusion supported by the near-zero correlation between glacier velocity and landfast ice area. Our observations suggest that buttressing stresses are unlikely to be recovered by landfast sea ice over sub-decadal timescales following the collapse of an ice shelf.

The flow and deformation of glacier ice in response to stress is often described using Glen’s Flow Law, a power-law relation that compactly represents ice rheology with a prefactor, A, and stress exponent, n. For natural ice, these parameters (and the parameters subsumed within them) come with large uncertainties that have not been robustly quantified with observations. Modern remote sensing technologies that collect data with finer resolutions and broader coverage provide us with an opportunity to robustly calibrate these rheological parameters for certain environments. Here, we utilize publicly available observations of ice sheet surface velocity and elevation acquired with remote sensing platforms to calibrate the flow law parameters over select Antarctic ice shelves. We build upon recent work that used remote sensing observations to quantify the relationship between ice stress and strain rate in extensional flow to infer an exponent of n = 4.1 +/- 0.4 for Antarctica. Here, we model two-dimensional flow and perform parameter calibration by constructing and training physics-informed neural networks (PINNs) to learn spatially-varying A and uniform n for each ice shelf. We cast the parameter estimation problem as a neural network optimization problem through minimization of a cost function that includes both data reconstruction errors and momentum balance residuals derived from the 2D shallow-shelf approximation. Additionally, we formulate the networks to predict spatially-varying uncertainties for A by using variational inference techniques, which approximate Bayesian inference (traditionally a computationally-intensive procedure) as an additional optimization objective. Finally, we demonstrate the use of time-dependent surface velocities, which are becoming increasingly more available over the ice sheets, to independently constrain the stress exponent n, confirming the appropriateness of n = 4 derived from previous work. Overall, calibration of these parameters with robust uncertainties are critical for placing observational constraints on prognostic ice flow model parameters and to improve our understanding of flow and fracture processes on ice shelves in Antarctica.

Antarctic ice shelves play a key role in regulating the rate of ice flow in tributary ice streams. Temporal variations in the associated ice-shelf buttressing stress are observed to impact ice flow in glaciers and ice streams. Ephemeral grounding induced by tides is an important mechanism for modulating the buttressing stress. Here, we develop an approach to inferring variations in 3-D surface displacements at an ice-shelf-stream system that explicitly accounts for ephemeral grounding. Using a temporally dense nine-month-long synthetic-aperture radar image acquisition campaign collected over Rutford Ice Stream, West Antarctica, by the 4-satellite COSMO-SkyMed constellation, we infer the ephemeral grounding zones and the spatiotemporal variation of the fortnightly ice-flow variability. We find ephemeral grounding zones along the western ice-shelf margin as well as a few prominent ephemeral grounding points in the central trunk and in the vicinity of the grounding zone. Our observations provide evidence for tide-modulated buttressing stress and the temporally asymmetric response of ice-shelf flow to tidal forcing. Long-term oceanic warming and ice-shelf thinning will cause the loss of ephemeral grounding and decrease in ice-shelf buttressing stress.

The Greenland Ice Sheet discharges ice to the ocean through hundreds of outlet glaciers. Recent acceleration of Greenland outlet glaciers has been linked to both oceanic and atmospheric drivers. Here, we leverage temporally dense observations, regional climate model output, and newly developed time series analysis tools to assess the most important forcings causing ice flow variability at one of the largest Greenland outlet glaciers, Helheim Glacier, from 2009 to 2017. We find that ice speed correlates most strongly with catchment-integrated runoff at seasonal to interannual scales, while multi-annual flow variability correlates most strongly with multi-annual terminus variability. The multiple relevant time scales and the influence of subglacial topography on Helheim Glacier’s dynamics highlight different regimes that can inform modeling and forecasting of its future. Notably, our results suggest that the recent terminus history observed at Helheim is a response to, rather than the cause of, upstream changes.

Most of the mass loss from the Antarctic Ice Sheet occurs through glaciers and ice streams, where fast-flow is partially controlled by rapid ice deformation in the margins. Deformation drives thermomechanical and recrystallization processes that influence further deformation, a feedback which may destabilize glaciers. However, few models account for the feedback between deformation and recrystallization. We derive an idealized model for ice temperature and grain-size that partitions deformational work into dissipated heat and changes in strain and surface energy, all of which drive dynamic recrystallization. Under conditions common in glacier shear margins, we show that a large portion of deformational work is stored as elastic energy, with the remainder dissipated as heat. This result revises our current picture of the amount of heat generated in glacier shear margins and suggests that changes in internal strain through dynamic recrystallization of ice likely play an important role in facilitating fast-flowing glacial ice.