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Basal melting is a major component of Antarctic ice mass loss. It is mainly driven by the circulation of water in ice shelf cavities. One approximation of this circulation is by models of buoyant plumes rising along the ice shelf base. For large-scale ice sheet models, a local melt parameterization of plume models that can be applied at an ice shelf-wide scale is desirable, and such a parameterization has recently been proposed and shown to perform well compared to a spatially resolved plume model when the ice shelf profile is fixed. In this study, we test the local parameterization for a dynamically evolving ice shelf, which allows for feedbacks between basal slope and melt rate. We find that these feedbacks enhance melt for steep basal slopes and reduce melt for small slopes. This decreases the parameterization performance, and under certain conditions the parameterization introduces an unphysical melt discontinuity. This can lead to large differences in the predicted melt rate and ice shelf length compared to the nonlocal plume model. More generally, our findings suggest that i) local parameterizations should be applied with caution in coupled ice–ocean frameworks, and ii) melt parameterizations should be evaluated in coupled settings to avoid unwanted behavior. 

Abstract: C files for PETSc code used in Subglacial hydrology regulates oscillations in marine ice streams. For questions please contact Marianne Haseloff. 

Marine ice stream dynamics are sensitive to conditions at the grounding line and basal shear stress. Variations in subglacial hydrology have been implicated in ice stream speed-up and shutdown. To investigate the interplay between marine ice stream flow and subglacial hydrology, we couple models of a marine ice stream and a subglacial drainage system. The coupled system evolves dynamically due to a positive feedback between ice flow, heat dissipation at the ice stream bed, and basal lubrication. Our results show that, depending on the hydraulic conductivity of the bed, distinct dynamic regimes can be identified. These regimes include steady streaming, hydraulically controlled oscillations, and thermally controlled oscillations. Periodic fast flow can be initiated by activation waves travelling upstream or downstream or quasi-simultaneously everywhere. Different dynamical regimes are characterized by large differences in grounded ice volume, even under modest changes of grounding line positions. These results imply a strong dependence of marine ice sheet dynamics on evolving hydrological conditions at the bed and highlight the importance of a better understanding of subglacial hydrology. 

Proglacial lakes along the retreating margin of the Laurentide ice sheet (LIS) significantly influenced the ice sheet's dynamics. This study investigates the interaction between proglacial lake drainage events and ice sheet evolution during deglaciation. Using a flowline ice sheet model, we demonstrate that abrupt lake drainage caused by the opening of spillways during the retreat of the ice sheet can temporarily reverse ice retreat and trigger rapid grounding line advance despite ongoing climate warming. We also show that ice shelf regrounding on a retrograde lake bed can follow lake drainage and further amplify ice sheet advance. These processes can decouple ice dynamics from climate forcing, offering a non‐climatic mechanism to explain the observed highly irregular ice margin fluctuations of the LIS. Our findings suggest that proglacial lakes might play an important role in modulating ice sheet evolution in warming climates. 

This repository provides open access to the bed profile, time-series data for surface mass balance parameter and lake level, and the COMSOL Multiphysics model files (.mph files, version 6.1) for EXP 1-4, presented in Hu & Haseloff -Geophysical Research Letters: Proglacial lake drainage events drive fast grounding line advance in a warming climate. 

Glacial landforms provide a valuable record from which to study the history and dynamics of past ice sheets. Eskers record paleo subglacial hydrologic and sediment transport conditions because they are composed of sediment deposited by water flowing through subglacial channels. Despite decades of study, there is still debate about their formation mechanisms and little investigation of the differences between eskers formed over soft and hard beds. To address this complexity, we analysed eskers formed over soft beds along the southern margin of the Laurentide Ice Sheet (LIS) in the Lake Superior region. This included developing a new method to calculate the basal effective pressure gradient during esker formation along the subglacial channel using grain size estimates from a 20 m tall esker exposure. The morphometry and distribution of eskers were mapped with GIS to quantify their sinuosity and lateral spacing, and to compare those to the underlying bedrock elevation and sediment thickness. Lateral spacing decreased over time as the ice margin retreated, suggesting that melt rates increased during the LIS deglaciation. Furthermore, the relation between esker distribution and sediment thickness showed that eskers formed preferentially over thinner layers of sediment, irrespective of whether erosion occurred before their formation. The sedimentology of the Cable Esker exhibits a non‐monotonic pattern in channel boundary shear stress ranging from 10 to 300 Pa, alongside a basal effective pressure gradient fluctuating between −9 to −70 Pa m⁻¹. Negative basal effective pressure gradients are consistent with esker formation in channels close to the glacier terminus, which suggests lower water pressure than normally assumed. This, combined with dynamic water level fluctuations within the esker channel, supports the theory of the formation of eskers near the ice margin. 

Investigations of the time-dependent behavior of marine ice sheets and their sensitivity to basal conditions require numerical models because existing theoretical analyses focus only on steady-state configurations primarily with a power-law basal shear stress. Numerical results indicate that the choice of the sliding law strongly affects ice-sheet dynamic behavior. Although observed or simulated grounding-line retreat is typically interpreted as an indication of marine ice sheet instability introduced by Weertman (1974), this (in)stability is a characteristic of the ice sheet's steady states – not time-variant behavior. To bridge the gap between theoretical and numerical results, we develop a framework to investigate grounding line dynamics with generalized basal and lateral stresses (i.e. the functional dependencies are not specified). Motivated by observations of internal variability of the Southern Ocean conditions we explore the grounding-line response to stochastic variability. We find that adding stochastic variability to submarine melt rates that produced stable steady-state configurations leads to intermittently advancing and retreating grounding lines. They can also retreat in an unstoppable manner on time-scales significantly longer than the stochastic correlation time-scales. These results suggest that at any given time of their evolution, the transient behavior of marine ice sheets cannot be described in terms of ‘stable’ or ‘unstable’.