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Abstract. Antarctic ice shelves buttress the flow of the ice sheet but are vulnerable to increased basal melting from contact with a warming ocean and increased mass loss from calving due to changing flow patterns. Channels and similar features at the bases of ice shelves have been linked to enhanced basal melting and observed to intersect the grounding zone, where the greatest melt rates are often observed. The ice shelf of Thwaites Glacier is especially vulnerable to basal melt and grounding zone retreat because the glacier has a retrograde bed leading to a deep trough below the grounded ice sheet. We use digital surface models from 2010–2022 to investigate the evolution of its ice-shelf channels, grounding zone position, and the interactions between them. We find that the highest sustained rates of grounding zone retreat (up to 0.7 km yr−1) are associated with high basal melt rates (up to ∼250 m yr−1) and are found where ice-shelf channels intersect the grounding zone, especially atop steep local retrograde slopes where subglacial channel discharge is expected. We find no areas with sustained grounding zone advance, although some secular retreat was distal from ice-shelf channels. Pinpointing other locations with similar risk factors could focus assessments of vulnerability to grounding zone retreat.

 

The Pine Island and Thwaites glaciers are the two largest contributors to sea level rise from Antarctica. Here we examine the influence of basal friction and ice shelf basal melt in determining projected losses. We examine both Weertman and Coulomb friction laws with explicit weakening as the ice thins to flotation, which many friction laws include implicitly via the effective pressure. We find relatively small differences with the choice of friction law (Weertman or Coulomb) but find losses to be highly sensitive to the rate at which the basal traction is reduced as the area upstream of the grounding line thins. Consistent with earlier work on Pine Island Glacier, we find sea level contributions from both glaciers to vary linearly with the melt volume averaged over time and space, with little influence from the spatial or temporal distribution of melt. Based on recent estimates of melt from other studies, our simulations suggest that the combined melt-driven and sea level rise contribution from both glaciers may not exceed 10 cm by 2200, although the uncertainty in model parameters allows for larger increases. We do not include other factors, such as ice shelf breakup, that might increase loss, or factors such as increased accumulation and isostatic uplift that may mitigate loss. 

Supraglacial lakes have been observed to drain within hours of each other, leading to the hypothesis that stress transmission following one drainage may be sufficient to induce hydro‐fracture‐driven drainages of other nearby lakes. However, available observations characterizing drainage‐induced stress perturbations have been insufficient to evaluate this hypothesis. Here, we use ice‐sheet surface‐displacement observations from a dense global positioning system array deployed in the Greenland Ice Sheet ablation zone to investigate elastic stress transmission between three neighboring supraglacial lake basins. We find that drainage of a central lake can place neighboring basins in either tensional or compressional stress relative to their hydro‐fracture scarp orientations, either promoting or inhibiting hydro‐fracture initiation beneath those lakes. For two lakes located within our array that drain close in time, we identify tensional surface stresses caused by ice‐sheet uplift due to basal‐cavity opening as the physical explanation for these lakes' temporally clustered hydro‐fracture‐driven drainages and frequent triggering behavior. However, lake‐drainage‐induced stresses in the up‐flowline direction remain low beyond the margins of the drained lakes. This short stress‐coupling length scale is consistent with idealized lake‐drainage scenarios for a range of lake volumes and ice‐sheet thicknesses. Thus, on elastic timescales, our observations and idealized‐model results support a stress‐transmission hypothesis for inducing hydro‐fracture‐driven drainage of lakes located within the region of basal cavity opening produced by the initial drainage, but refute this hypothesis for distal lakes.

 

Abstract. The recent influx of remote sensing data provides new opportunities for quantifying spatiotemporal variations in glacier surface velocity and elevation fields. Here, we introduce a flexible time series reconstruction and decomposition technique for forming continuous, time-dependent surface velocity and elevation fields from discontinuous data and partitioning these time series into short- and long-term variations. The time series reconstruction consists of a sparsity-regularized least-squares regression for modeling time series as a linear combination of generic basis functions of multiple temporal scales, allowing us to capture complex variations in the data using simple functions. We apply this method to the multitemporal evolution of Sermeq Kujalleq (Jakobshavn Isbræ), Greenland. Using 555 ice velocity maps generated by the Greenland Ice Mapping Project and covering the period 2009–2019, we show that the amplification in seasonal velocity variations in 2012–2016 was coincident with a longer-term speedup initiating in 2012. Similarly, the reduction in post-2017 seasonal velocity variations was coincident with a longer-term slowdown initiating around 2017. To understand how these perturbations propagate through the glacier, we introduce an approach for quantifying the spatially varying and frequency-dependent phase velocities and attenuation length scales of the resulting traveling waves. We hypothesize that these traveling waves are predominantly kinematic waves based on their long periods, coincident changes in surface velocity and elevation, and connection with variations in the terminus position. This ability to quantify wave propagation enables an entirely new framework for studying glacier dynamics using remote sensing data. 

Abstract. We introduce a new software package called “icepack” for modeling the flow of glaciers and ice sheets.The icepack package is built on the finite element modeling library Firedrake, which uses the Unified Form Language (UFL), a domain-specific language embedded into Python for describing weak forms of partial differential equations.The diagnostic models in icepack are formulated through action principles that are specified in UFL.The components of each action functional can be substituted for different forms of the user's choosing, which makes it easy to experiment with the model physics.The action functional itself can be used to define a solver convergence criterion that is independent of the mesh and requires little tuning on the part of the user. Theicepack package includes the 2D shallow ice and shallow stream models.We have also defined a 3D hybrid model based on spectral semi-discretization of the Blatter–Pattyn equations.Finally, icepack includes a Gauss–Newton solver for inverse problems that runs substantially faster than the Broyden–Fletcher–Goldfarb–Shanno (BFGS) method often used in the glaciological literature.The overall design philosophy of icepack is to be as usable as possible for a wide a swath of the glaciological community, including both experts and novices in computational science. 

Abstract. A system of subglacial lakes drained on Thwaites Glacier from 2012–2014. To improve coverage for subsequent drainage events, we extended theelevation and ice-velocity time series on Thwaites Glacier through austral winter 2019. These new observations document a second drainage cycle in2017/18 and identified two new lake systems located in the western tributaries of Thwaites and Haynes glaciers. In situ and satellite velocityobservations show temporary < 3 % speed fluctuations associated with lake drainages. In agreement with previous studies, these observationssuggest that active subglacial hydrology has little influence on thinning and retreat of Thwaites Glacier on decadal to centennial timescales. 

Abstract. Ocean-induced basal melting is directly and indirectly responsible for much of the Amundsen Sea Embayment ice loss in recent decades, but the total magnitude and spatiotemporal evolution of this melt is poorly constrained. To address this problem, we generated a record of high-resolution Digital Elevation Models (DEMs) for Pine Island Glacier (PIG) using commercial sub-meter satellite stereo imagery and integrated additional 2002&ndash;2015 DEM/altimetry data. We implemented a Lagrangian elevation change (Dh/Dt) framework to estimate ice shelf basal melt rates at 32&ndash;256-m resolution. We describe this methodology and consider basal melt rates and elevation change over the PIG shelf and lower catchment from 2008&ndash;2015. We document the evolution of Eulerian elevation change (dh/dt) and upstream propagation of thinning signals following the end of rapid grounding line retreat around 2010. Mean full-shelf basal melt rates for the 2008&ndash;2015 period were ~82&ndash;93&thinsp;Gt/yr, with ~&thinsp;200&ndash;250&thinsp;m/yr basal melt rates within large channels near the grounding line, ~&thinsp;10&ndash;30&thinsp;m/yr over the main shelf, and ~&thinsp;0&ndash;10&thinsp;m/yr over the North and South shelves, with the notable exception of a small area with rates of ~&thinsp;50&ndash;100&thinsp;m/yr near the grounding line of a fast-flowing tributary on the South shelf. The observed basal melt rates show excellent agreement with, and provide context for, in situ basal melt rate observations. We also document the relative melt rates for km-scale basal channels and keels at different locations on the shelf and consider implications for ocean circulation and heat content. These methods and results offer new indirect observations of ice-ocean interaction and constraints on the processes driving sub-shelf melting beneath vulnerable ice shelves in West Antarctica.