Surface crevasses are predominately mode I fractures that penetrate tens of metres deep into grounded glaciers and floating ice shelves. However, elevated surrounding temperatures have resulted in the production of surface meltwater, which accumulates in neighbouring crevasses and applies additional tensile stresses to crack walls. This process is known as hydrofracture; and if sufficient, can promote full thickness crevasse propagation, and lead to iceberg calving events. Net ablation of ice sheets has become of great concern, as it has become the largest contributor to sea-level rise. To overcome the limitations of empirical and analytical approaches to crevasse predictions, we here propose a thermo-dynamically consistent phase field damage model to simulate damage growth in both ice sheets and floating ice shelves using the finite element method.

The model incorporates the three elements needed to mechanistically simulate hydrofracture of surface and basal crevasses: (i) a constitutive description of glacier flow, incorporating the non-linear viscous rheology of ice using Glen’s flow law, (ii) a phase field formulation capable of capturing cracking phenomena of arbitrary complexity, such as 3D crevasse interaction, and (iii) a poro-damage mechanics representation to account for the role of meltwater pressure on crevasse growth. To assess the suitability of the method, we simulated the propagation of surface and basal crevasses within grounded glaciers and floating ice shelves and compared the predicted crevasse depths with analytical methods such as linear elastic fracture mechanics and the Nye zero stress
method, with results showing good agreement for idealised conditions.

References
T. Clayton, R. Duddu, M. Siegert, E. Martínez-Pañeda, A stress-based poro-damage phase field
model for hydrofracturing of creeping glaciers and ice shelves, Engineering Fracture Mechanics
272 (2022) 108693

How to cite: Clayton, T., Duddu, R., Siegert, M., and Martinez-Pañeda, E.: Phase field modelling of glacial crevasses subject to meltwater-driven hydro-fracture, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2804, https://doi.org/10.5194/egusphere-egu23-2804, 2023.

Mountain glaciers are a major source of sea-level rise and also represent an important freshwater resource in many mountainous regions. Thus, accurate estimations of their thickness and, therefore, the total ice volume are important both in predicting and mitigating the global and local effects of climate change. However, to date, only 2% of the world’s glaciers outside the ice sheets have any thickness data, due to the logistical difficulties of obtaining such measurements, creating a large and policy-relevant scientific gap.

The recent development of a global-scale ice-velocity dataset, however, provides an ideal opportunity to fill this gap and determine ice thickness across the 98% of glaciers for which no thickness data is available. This can be done by inverting an ice-dynamics model to solve for the ice thickness. For accurate thickness results, this needs to be a full-Stokes model, but such a model is far too computationally cumbersome to apply on a global scale, and simpler, quicker methods usually based on the shallow ice approximation (SIA) are too inaccurate, particularly where sliding dominates glacier motion. The only attempt that has been made to leverage the global velocity dataset to retrieve ice thickness has, though, used the SIA, simply because higher-order approaches are not computationally realistic at this scale. Consequently, most of the widely-used global glacier models have made no concerted attempt to invert for global ice thickness, owing to these limitations.

As an additional related problem, failing to fully assimilate ice-velocity data into an ice-flow model necessarily introduces a shock when initialising prognostic glacier simulations, resulting in model glaciers and predictions that may diverge substantially from their real-world counterparts.

As a solution to these problems, we present results from a deep-learning-driven inversion model that emulates the performance of state-of-the-art full-Stokes models at a thousandth of the computational cost. This model, by solving an optimisation problem, can fully use and assimilate all available input datasets (surface velocity and topography, ice thickness, etc.) as components of its cost function to simultaneously invert for and optimise multiple control parameters (here, we focus on ice thickness). This approach also gives us the possibility of using the same ice-velocity field for inversion and forward modelling, reducing the magnitude of the shock inherent in traditional modelling approaches. With a view to a large-scale application to all the world’s 200,000 glaciers, we present initial thickness-inversion results for the relatively well-documented European Alps to help constrain model parameters and provide a test bed for extension to other glaciated regions, with initial extension to the Caucasus and the Southern Alps.

How to cite: Cook, S., Jouvet, G., Millan, R., and Rabatel, A.: Glacier ice thickness estimation using deep-learning-driven emulation of Stokes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2607, https://doi.org/10.5194/egusphere-egu23-2607, 2023.

Through their role in buttressing upstream ice flow, Antarctic ice shelves play an important part in regulating future sea level change. Reduction in ice-shelf buttressing caused by increased ocean-induced melt along their undersides is now understood to be one of the key drivers of ice loss from the Antarctic Ice Sheet. However, despite the importance of this forcing mechanism, most ice-sheet simulations currently rely on simple melt-parametrisations of this ocean-driven process since a fully coupled ice-ocean modelling framework is prohibitively computationally expensive. Here, we provide an alternative approach that can capture the greatly improved physical description of this process provided by large-scale ocean-circulation models over currently employed melt-parameterisations, but with trivial computational expense.  This new method brings together deep learning and physical modelling to develop a deep neural network framework, MELTNET, that can emulate ocean model predictions of sub-ice shelf melt rates. We train MELTNET on synthetic geometries, using the NEMO ocean model as a ground-truth in lieu of observations to provide melt rates both for training and to evaluate the performance of the trained network. We show that MELTNET can accurately predict melt rates for a wide range of complex synthetic geometries, with a normalized root mean squared error of 0.11m/yr compared to the ocean model. MELTNET calculates melt rates several orders of magnitude faster than the ocean model and outperforms more traditional parameterisations for 96% of geometries tested. Furthermore, we find MELTNET's melt rate estimates show sensitivity to established physical relationships such as changes in thermal forcing and ice shelf slope. This study demonstrates the potential for a deep learning framework to calculate melt rates with almost no computational expense, that could in the future be used in conjunction with an ice sheet model to provide predictions for large-scale ice sheet models.

How to cite: Rosier, S., Bull, C., Woo, W., and Gudmundsson, H.: Predicting ocean-induced ice-shelf melt rates using deep learning, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15264, https://doi.org/10.5194/egusphere-egu23-15264, 2023.

Ice sheet models, calibrated using observational data, provide a means of projecting our current best state of the knowledge of the system state into the future, so as to obtain information about possible future behaviour. However it is important to be able to estimate the uncertainty associated with these projections. The problem of quantifying ice sheet parametric uncertainty is considered, focusing specifically on the problem of quantifying the posterior uncertainty in inferred basal sliding and rheology coefficients. These measures of uncertainty are projected forwards in time to obtain measures of uncertainty in future quantities of interest. Automated code generation and automated differentiation tools are utilised, leading to an extensible approach. The role of the observational error model in defining parametric uncertainty is considered.

How to cite: Maddison, J., Recinos, B., and Goldberg, D.: A framework for quantifying parametric ice sheet model uncertainty, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5414, https://doi.org/10.5194/egusphere-egu23-5414, 2023.

The calving of ice tongues and ice shelves can generate large, tabular icebergs that have climatological implications given their role in dispersing freshwater from the Greenland and Antarctic ice sheets. These ‘ice islands’ also pose potential risk to marine industry. It is therefore critical that influential deterioration mechanisms be accurately represented in simulations of ice island drift and deterioration, both for risk mitigation in offshore industry and for climatological studies that are focused on the Polar Regions. The majority of ice island deterioration is the result of sidewall erosion, and specifically that which results from waterline wave-erosion leading to ram growth and buoyancy-forced fracture. This study therefore focuses on the inclusion of the buoyancy-driven “footloose” calving mechanism (Wagner et al., 2014) in simulations of ice island length and areal change. Using size and lineage information of ice islands tracked in the Canadian Ice Island Drift, Deterioration and Detection (CI2D3) Database, we quantitatively assess the performance of the footloose calving model by simulating the deterioration of 172 ice islands. The mean model error was +15 (+/- 400) m over 20 d and increased to +401 (+1400/-800) m for simulations that ran to 80 d. The performance of the footloose calving model is a substantial improvement when compared to simulations that did not include this calving mechanism. For example, a thermal-melt model had mean errors of -252 and -1403 m at 20 and 80 d of simulation, respectively, and the mean error of a zero-melt model was -281 and -1545 m over the same time periods. We also present a new approach to modelling ice island areal change resulting from footloose calving. This simple, two-parameter approach simulates discrete footloose calving events and adjusts the ice island surface area to maintain a constant aspect ratio. Mean model error remained under 1 km² over 80 d of simulation, showing that the model performs well over numerous months. Using the CI2D3 Database, we were able to conduct the first large-scale assessment of the footloose model’s performance in simulating change to the ice island length dimension. The morphological data included in the database also provided the opportunity to develop an approach for modelling areal deterioration resulting from footloose calving events. The model assessments would benefit from more observations of long-duration ice island tracks, as there were a limited number of ice islands that were tracked in the CI2D3 Database for over 40 d. Future work can look to implement the presented approaches in operational and climatological modelling while the iceberg modelling community also develops an approach to simulate larger-scale ice island fracture.

How to cite: Crawford, A., Crocker, G., Mueller, D., and Smith, J.: Developments to 2D modelling of ice island deterioration using the CI2D3 Database, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6969, https://doi.org/10.5194/egusphere-egu23-6969, 2023.

Antarctic and Greenland ice sheets lose most of their mass by a few corridors of rapidly flowing ice. These ice conveyor belts constitute fast drainage routes whose flow velocities are undoubtedly sensitive to climate perturbations directly impacting sea-level. Observations suggest the ice is rather sliding than flowing, the key being where sliding is accommodated. Commonly, sliding occurs at the ice-bedrock interface, but recent studies favour englacial sliding to explain data from Western Margin of Greenland. 

Our aim is to demonstrate that the mechanisms controlling the spontaneous formation of englacial sliding explains the transition from slow flowing to fast sliding ice over Greenland. We employ a new thermo-mechanical ice flow model to predict thermally activated creep instability leading to the spontaneous rearrangement of ice motion in three dimensions. Accurately resolving these nonlinear interactions on regional to ice sheet scales requires high spatial and temporal resolution which can only be achieved using a supercomputer.

We present a new thermo-mechanical ice flow model, FastIce.jl, that is capable of predicting the evolution of ice sheet at unprecedented scale. The model uses the full-Stokes formulation for the ice flow and the enthalpy method for describing the polythermal ice behaviour. FastIce.jl uses GPU acceleration for solving the flow equations, resulting in close to ideal scaling in distributed computing benchmarks. We compare our simulation results with other full-Stokes models, and present the results of simulating the 120x120 km regions of Greenland ice sheet at 10m resolution. High resolution allowed us to capture the transition from slow to sliding flow regimes without any simplifying assumptions.

How to cite: Utkin, I., Räss, L., and Werder, M.: Large-scale thermo-mechanical modelling of Greenland ice sheet, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16383, https://doi.org/10.5194/egusphere-egu23-16383, 2023.

Jakobshavn Isbræ (JI), on the West coast of Greenland, is one of the fastest flowing outlet glaciers of the Greenland Ice Sheet, draining 7% of the ice sheet area. Since the late 1990s it has dramatically accelerated, thinned and retreated in a series of phases alternating with periods of quiescence. It exhibits strong seasonal variations in flow speed and calving front position. Between 2012 and 2015, JI attained its point of furthest retreat and flow speeds in excess of 17 km/yr. Since 2016 it has modestly thickened, concurrent with deceleration and readvance of the calving front.

The very fast flow and strong annual and interannual variability present significant challenges for ice sheet modellers. We model the evolution of JI between 2009 and 2018 using the BISICLES ice sheet model. The standard modelling technique of assimilating surface velocity observations to infer a power law basal friction coefficient for a snapshot in time fails to account for rapidly changing basal conditions, underestimating the annual variability. We implement a time-series inverse method in which regular velocity observations are assimilated throughout the study period to produce a time-evolving basal friction coefficient. This method is able to reproduce the large annual variations in flow speed much more accurately than the static method.

This reliance on regular observations to drive the model poses a problem for future projections. We compare a range of sliding laws applied with the normal snapshot inverse method. A modern regularized Coulomb friction sliding law is better able to reproduce JI’s annual variations in flow speed due to its ability to modulate the basal friction in response to movement of the grounding line. As a result, it may be a more appropriate choice of sliding law for modelling the future evolution of fast-flowing outlet glaciers.

How to cite: Trevers, M., Payne, T., and Cornford, S.: A comparison of inverse methods and basal sliding laws applied to a hindcast model of Jakobshavn Isbræ from 2009 to 2018., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8265, https://doi.org/10.5194/egusphere-egu23-8265, 2023.

Rapid climate modifications perturb the long-term dynamic equilibrium of many natural systems. Polar and high-altitude regions such as alpine environments represent locations where perturbations such as glacier collapse features, become visible. Glacier collapse features are characterized by a circular depressions on the ice surface, are bounded by low-angle crevasses and are the surface expression of a cavity developing most often over a subglacial channel, commonly occurring at the glacier snout. Understanding the physical processes governing the collapse feature dynamics is essential to assess hazards and processes related to them, such as, rapid glacier length variations, snout collapses and sudden blockage of the subglacial drainage system.

Field observations from an on-going collapse feature developing at the snout of Rhonegletscher (Switzerland) in Summer 2022 suggest mechanical failure of ice lamellas from the underlying cavity roof to drive the collapse. In order to test this hypothesis, namely mechanical failure to drive glacier collapse features, we developed full-Stokes 2D and 3D mechanical models implementing a temperature and pressure dependent visco-elasto-plastic rheology. We use the extensive dataset from Rhonegletscher to constrain the numerical models to predict possible failure patterns as function of increasing cavity size. We use vertical displacement located in the centre of the collapse feature to validate our models. Preliminary results show the formation of tension failure patterns on the ice surface at locations coinciding with the low-angle circular crevasses. The model results will advance our understanding of the physics of collapse features and provide predictive tools to assess future occurrences and their related risks.

How to cite: Räss, L., Ogier, C., Utkin, I., Werder, M., Bauder, A., and Farinotti, D.: Mechanical failure to drive the glacier collapse feature at Rhonegletscher, Switzerland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9021, https://doi.org/10.5194/egusphere-egu23-9021, 2023.

Glacier surges produce iconic valley-scale folds which encode a history of polyphase deformation resulting from switches between quiescent and surging flow. The folding is passive, resulting from disturbances to ice foliation during surging flow, and subsequently altered during quiescent flow. We investigate the kinematic evolution of these kilometre-scale folds, using Elmer/Ice, by modelling folds through multiple surge cycles using idealized synthetic glacier confluence configurations, and identifying how differences in glacier flow regimes imprint themselves onto three-dimensional fold geometry. The surge and quiescent phases are simulated by changing the basal conditions of one of the tributaries, and matching the scale of velocity variations observed in temperate glacier surges. We determine fold geometry using a particle tracking algorithm applied to the modelled velocity fields, where, mimicking a medial moraine, vertically-spaced particles are injected at the flow unit confluence and advected downglacier. Using structural analysis of the model outputs, we present an archetype of kinematic evolution that describes the transition from cylindrical, vertically plunging, gentle folds emplaced during the surge phase, to complex, non-cylindrical, depth-varying folds following multiple cycles of surging and quiescent flow. The initial fold geometry is controlled by longitudinal and lateral shear stress regimes during surging, while fold evolution is governed primarily by lateral shearing after emplacement. We examine the sensitivity of fold geometry to valley geometry, glacier dynamics, and mass balance. Finally, we illustrate the potential of our approach to reconstruct more complex fold geometries as observed in nature, by applying it to a large surge-type glacier in the St. Elias Mountains of Northern Canada.

How to cite: Young, E., Flowers, G., Jiskoot, H., and Gibson, D.: Modelling the kinematic evolution of valley-scale folding in surge-type glaciers using Elmer/Ice, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8982, https://doi.org/10.5194/egusphere-egu23-8982, 2023.

Observations worldwide have shown that glaciers are receding. Thinning snow cover can result in crevasses becoming exposed at the glacier surface for longer time periods. Crevasses can increase surface roughness, change the surface wind flow fields, and impact the air temperatures within and outside the crevasses (Purdie et al., 2022). Therefore, more can be learnt about regarding the impact of crevasses on energy exchange with glacier atmospheric boundary layer. In order to understand and investigate the atmosphere-crevasse energy exchange, we carried out numerical Large Eddy Simulation (LES) experiments using the PALM model system 6.0 with crevasse-resolving grid spacings less than 1 m. The PALM model system 6.0 has been used for atmosphere and marine boundary layer studies to understand complex processes of atmospheric dynamics and energy balance. Our preliminary results show that the air inside a crevasse does not cool as fast as the air outside a crevasse resulting in net warming from the crevasse relative to the glacier surface. These results agree with the field study conducted at Tasman Glacier (Purdie et al., 2022), and confirm that crevasses could lead to heat storage and increased melting. During the daytime, air temperature inside a crevasse could be 1 °C higher than the air above the glacier surface. After sunset, the presence of the crevasse entrains and traps the warm air such that the centre of the crevasse could still be warmer than the glacier surface during the first half of the evening.  During windy evenings, our results show that turbulent heat exchange associated with eddy entrainment leads to exchange of air mass between the crevasse and the glacier surface, which then causes glacier surface warming. Our preliminary simulations only included one crevasse, while future work will include a field of crevasses to investigate the impact of crevasses in a more realistic environment. This study highlights the importance of including crevasses energy balance in glacier modelling, neglecting which would lead to significant bias in snow melt and mass balance estimations.

Reference:

Purdie, H., et al. (2022). "Variability in the vertical temperature profile within crevasses at an alpine glacier." Journal of Glaciology: 1-5.

How to cite: Lin, D., Katurji, M., and Purdie, H.: Impact of crevasses on surface energy balance at an alpine glacier, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10981, https://doi.org/10.5194/egusphere-egu23-10981, 2023.

How to cite: Sergienko, O. and Haseloff, M.: Basal controls of the grounding line dynamics of buttresses marine ice sheets, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2908, https://doi.org/10.5194/egusphere-egu23-2908, 2023.

Ice is a non-Newtonian fluid whose rheology is typically described using Glen's flow law, a power-law relationship between stress and strain rate with a stress exponent, n, generally taken to be 3. Recent observation-based work suggests that a more accurate choice for the Glen’s law exponent in high-strain regions like ice shelves may be n=4, implying that ice viscosity is more sensitive to changes in stress than is generally assumed. The implications of a higher stress exponent for ice sheet models and their projections of ice sheet response to climate forcing are unclear and likely to be complex. Rheological parameters, such as ice viscosity, are fundamental to ice sheet dynamics and influence the evolution of marine ice sheets. 

Here, we present work that explores the rheological parameter space within the idealized MSIMIP+ marine ice sheet configuration using the BISICLES model. We explore the impacts of increasing  the stress exponent from 3 to 4, highlighting the considerable changes to the ice sheet system caused by increasing the stress exponent. Beyond dynamic changes in the ice behavior, changes become necessary to the other flow law parameters generally computed during initialization. For example, it may be that viscosity modifiers typically interpreted as “damage” may instead be indications of mismatches in rheology.  This study underscores the dynamic sensitivity of glacial ice to changes in the rheological parameters and calls attention to the key variables influencing ice sheet evolution. 

How to cite: Martin, D., Kachuck, S., Millstein, J., and Minchew, B.: Examining the sensitivity of ice sheet models to updates in rheology (n=4), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2844, https://doi.org/10.5194/egusphere-egu23-2844, 2023.

Understanding glaciers evolution is of major concern to evaluate their contribution to sea level rise in the context of global warming. Among the various processes involved in glacier dynamics, fractures like the calving of ice at the front of marine terminating glacier and crevasse formation affect the stress state, potentially modifying the glacier’s velocity. Crevasses also impact the melting rate. The fractures alter the roughness of the ground, increasing the amount of absorbed radiation, and open new networks in which the meltwater is likely to penetrate deeper toward the glacier bed.

In this work we propose to model fractures in glacier based on finite strain elastoplasticity, using the Material Point Method (Wolper et al. 2021): we solve the classical governing equations for ice deformation in an Eulerian-Lagrangian framework and we use a strain softening Drucker-Prager constitutive model to simulate plasticity. Thanks to the Lagrangian part of the model, the fractures appear explicitly where high levels of total plastic deformation are reached. The behaviour of a glacier flowing over a step in the bedrock is investigated. The simulations show that crevasse patterns appear with regular spacing between the fractures. We perform a parametric study to determine which parameters affect the length of these patterns and potential dimensionless numbers are outlined.

How to cite: Rousseau, H., Gaume, J., Blatny, L., and Lüthi, M. P.: Modelling discontinuities in ice flow using the Material Point Method and elastoplasticity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11460, https://doi.org/10.5194/egusphere-egu23-11460, 2023.

Commonly, the parts of the glacier bed that are hydraulically connected to the surface experience significant diurnal variations in water pressure, in response to cycles of surface melting. Closely spaced points on the bed often exhibit nearly identical temporal variations in water pressure, suggesting that they are connected not only to the surface but to each other through conduits along the bed. This behaviour is typically observed directly through instrumented boreholes drilled to the glacier bed. A ‘switching event’ occurs when one of a pair of boreholes abruptly changes from being connected, in the sense of exhibiting the same diurnal oscillations as the other borehole, to being disconnected, or vice versa. A switching event is indicative of a connection through a subglacial conduit being closed, or opened, and therefore provides a limited but highly specific window into the evolution of subglacial conduits and permeability.

However, in most subglacial drainage models, conduits are not represented individually but averaged over a small area of the bed to produce a macroporous continuum representation as a ‘water sheet’, quantified by a mean conduit depth h. The most common assumption is that the water sheet consists of linked cavities and that these open due to basal sliding over bed roughness, and close due to viscous creep (e.g. Hewitt, 2011). Within that framework, the simplest mechanism for a switching event is that a connection is established or closed when the sheet thickness h passes through some percolation threshold h_c (Rada and Schoof, 2018).

We want to test whether the observed switching events can be explained by that mechanism, which in turn implies that two conditions must be met: water sheet depth indeed evolves according to a competition between opening due to basal sliding and creep closure, and that a simple threshold in h suffices to capture the geometric complexity involved in creating or closing connections at the bed.

In a large dataset of borehole water-pressure time series, we identify borehole pairs that exhibit strong evidence of switching behaviour. We assume that switching events can be described by the evolution of a water sheet, with connections between boreholes being opened and closed as sheet thickness passes through a threshold value as described above. We use the switching event catalogue we have created to invert for parameters in the sheet evolution model using a binary indicator function for connectedness to compute the model data mismatch in the absence of any other direct measures of sheet thickness.

This procedure allows us to capture the majority of observed switching events with plausible parameter values. The exception is a set of short-lived periods of connectedness characterized by switching events that are clustered in space and time. In a complementary study (Racz et al, 2023 in prep.), we, therefore, investigate if this class of switching events can instead be explained by an alternative mechanism in which the sudden resumption of surface water supply, following a period of snow cover, drives the propagation of a hydrofracture (e.g. Tsai and Rice 2010, 2012).

How to cite: Racz, G. C., Yeo, K. M. E., Thobani, A., Henry, S., Zarrinderakht, M., Rada, C., and Schoof, C.: Physical processes driving 'switching events', EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11076, https://doi.org/10.5194/egusphere-egu23-11076, 2023.

The stability of the grounding lines of Antarctica is a fundamental question in term of sea level rise The strong mass loss of the Antarctic Ice Sheet (AIS) in recent years has raised concerns about the possibility of the ice sheet reaching a tipping point, beyond which it would experience rapid and irreversible loss of mass. Such a tipping point could be triggered by a combination of external forcing factors, including continued warming of the ocean and atmosphere, as well as changes in ice sheet dynamics. As shown by Urruty et al. (in review), the current mass loss and retreat is mainly due to external forcing such as melt induced by the ocean and current grounding lines are not yet engaged in an unstable retreat. But if forcing remains similar or increases, some irreversible and fast mass loss may occur as a result of grounding lines crossing a tipping point.

As part of the TiPACCs project, we are conducting experiments to evaluate the stability of the grounding lines of the AIS in the future. Building on the stability experiment described in Urruty et al. (in review), we are using the same initial state created with Elmer/Ice to perform a new set of experiments. In these experiments, we are applying large-amplitude perturbations to the grounding line of a steady-state AIS by increasing ocean temperature by 1°C, 3°C, and 5°C for periods ranging from 20 to 100 years in order to push the grounding line far from its current position. After the perturbation is removed, we then apply 80 years of constant forcing to see if an unstable position is reached. These experiments will help us better understand the stability of the grounding line at different positions that could be reached in the near future if current observed forcing trends continue.

How to cite: Urruty, B., Gagliardini, O., Gillet-Chaulet, F., Durand, G., and Chekki, M.: Large-amplitude perturbation experiments to assess the unstable behaviour of AIS in the near future, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11337, https://doi.org/10.5194/egusphere-egu23-11337, 2023.

We use numerical modelling to address several questions related to the future evolution of Thwaites Glacier over the next 50 years. The importance of Thwaites Ice Shelf for upstream grounded flow is investigated by quantifying the buttressing stresses along the grounding line. Removing the ice shelf changes the stress regime along the grounding line by less than 20%. This change is small compared to many, if not most, grounding lines of the Antarctic Ice Sheet, and much smaller than corresponding changes for the neighboring Pine Island and Pope, Smith and Kohler glaciers.  Transient ice-flow modelling experiments show that mass loss from Thwaites Glacier over the next 50 years is insignificantly affected by removal of the ice shelf. We then explore the consequences of the proposed marine ice-cliff instability for Thwaites Glacier. For recently proposed calving laws, where the calving rate increases sharply with cliff height, we do not observe an onset of an unstable calving front retreat. Further numerical modelling experiments for future climatic forcing scenarios will be presented, including uncertainty quantification. Interactions between the ice and the ocean are studied using a coupled ice+ocean modelling framework. As shown before in several studies, we find when simulating its future evolution, that Thwaites Glacier can enter unstable periods of self-enhancing retreat. This appears to be a very robust result, and this behavior is found in all model runs, including coupled ice+ocean simulations.

How to cite: Gudmundsson, G. H., De Rydt, J., Rosier, S., Barnes, J., Goldberg, D., and Morlighem, M.: The Future of Thwaites Glacier, West Antarctica., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13025, https://doi.org/10.5194/egusphere-egu23-13025, 2023.

Ice mass loss from Antarctic Ice Sheet is increasing, accelerating its contribution to global sea level rise. In the Amundsen Sea sector, recent observations of rapid ice-shelf thinning and grounding line retreat have been attributed to increased basal melting driven by inflows of warm Circumpolar Deep Water. However, recent studies have shown that basal melting alone might not be sufficient to explain the recent acceleration, retreat and thinning of the outlet glaciers in the sector.

As part of the European Horizon 2020 research project PROTECT ­— that assesses and projects changes in the land-based cryosphere to produce robust projections of SLR — we conduct numerical simulations to determine the role of damage on changes observed over the last two decades in the Amundsen Sea Sector. More particularly, we use a Stokes flow formulation combined with a Continuum Damage Mechanics model of the open-source ice flow model Elmer/Ice to simulate the ice flow evolution. We initialize our ice sheet model with data assimilation methods using 1996 observations of surface velocities as well as a corrected geometry based on the current ice-sheet geometry and the ice thickness rates of change observed over the past 20 years. From this initial state, we run forward simulations over 20 years with and without damage mechanics, and compare the model evolution to observed surface velocities and ice thickness rates of change, as well as observations of grounding line positions. Our results shed light on the importance of damage in the evolution of the region, in particular an acceleration of several hundred meters per year due to the decreasing buttressing effect of the ice shelves triggered by the increasing damage.

How to cite: Mosbeux, C., Jourdain, N., Gagliardini, O., Råback, P., and Gilbert, A.: On the effect of damage on the recent changes in the Amundsen Sea Sector, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9198, https://doi.org/10.5194/egusphere-egu23-9198, 2023.

Ice shelves around the coastal portion of the Antarctic ice sheet are sensitive indicators of climate change. The thinning of ice shelves diminishes buttressing, promotes longitudinal spreading, and increases ice flux across the grounding line, leading to accelerated glacier discharge into the ocean. Thwaites Glacier in the Amundsen Sea Embayment is one of the fastest-changing outlet glaciers in Antarctica. Damaged areas with densely distributed crevasses and open fractures on Thwaites Glacier are key to future ice shelf stability, grounding line retreat and sea level contribution. The damage feedback processes should be taken into consideration when simulating the evolution of Thwaites Glacier using ice sheet models.

Here, we add the continuum damage mechanics approach to the F.ETISh/Kori ice flow model, to simulate the present-day and near future behavior of the ice sheet and ice shelf system, including brittle ice physics. The damage field is described by equating it to the total crevasse depths used in Nick et al. (2010) and Sun et al. (2017). 100 years simulations under present-day climate conditions with and without damage in different scenarios have been conducted, and the change in ice velocity, ice thickness, the grounding line retreat and the sea level contribution of Thwaites Glacier have been analysed. Moreover, the change in ice velocity along four ice flow profiles in the first 20-year simulation has been analysed and the impact of damage on velocity has been assessed by comparing the simulated velocity fields with the observations (e.g., the MEaSUREs and ITS_LIVE ice velocity products).

Results indicate that damage drastically increases the ice velocity over the ice shelves and weakens them as such that grounding line retreat ~18 km after 20 years, accelerates (~1.5 times) compared to the observed increase in flow speed and contributes around an order of magnitude to the sea level rise in 50 years. Change in ice velocity profiles of Thwaites Glacier also show that local damage may overestimate ice velocity, especially in the grounded ice near the grounding line, while it underestimates observed ice flow when local damage is omitted. Through a series of further sensitivity experiments, an analysis on the timing and magnitude of damage has been carried out to gauge the current and near future state of the Thwaites glacier basin.

How to cite: Li, Y., Navarro, J. B., Pattyn, F., and Qiao, G.: Modelling the impacts of ice damage on the response of Thwaites Glacier, West Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11798, https://doi.org/10.5194/egusphere-egu23-11798, 2023.

Projections of sea level rise are subject to large uncertainties in the contribution of the Antarctic Ice Sheet (AIS), as it is unclear how AIS dynamics will evolve over time. Ice sheet models use spin-up techniques to initialize the ice sheet to the present-day state. Previous attempts using the Community Ice Sheet Model (CISM) assumed that the ice sheet is in equilibrium at the end of the spin-up. This assumption limits the contribution of model drift, but does not match observations and might bias future projections.

For this reason, we have incorporated present-day thickness change rates from Smith et al. (2020) in our Antarctic spin-ups. As in previous spin-ups, we tune basal friction coefficients beneath grounded ice, and ocean temperatures beneath floating ice, to match observed present-day thickness. In the new procedure, CISM is also forced to match thickening and thinning rates, with the surface mass balance (SMB) adjusted to allow the AIS to maintain the observed thickness and grounding-line locations. This technique improves the modelled velocities in regions with substantial thinning. When the SMB adjustments are removed, the modelled ice sheet exhibits the observed thickness change rates. We use this initialised state to project AIS evolution without additional forcing (‘committed climate change’). For a range of parameter settings, this causes Thwaites glacier to collapse irreversibly, without further ocean warming. The time of initiation of collapse is sensitive to model parameters, but once initiated the collapse is largely complete within two to three centuries. The sensitivity tests are carried out for a range of parameters related to basal sliding and various ocean warming scenarios.

How to cite: van den Akker, T., Lipscomb, W. H., Leguy, G. R., van de Berg, W. J., and van de Wal, R.: New Antarctic spin-up method results in committed Thwaites glacier collapse, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11300, https://doi.org/10.5194/egusphere-egu23-11300, 2023.

The Amundsen Sea Sector has some of the highest thinning rates of ice shelves in Antarctica, thought to be driven by high, but interannually variable, ocean driven melt rates. This thinning can lead to increased ice flow speeds, eventually leading to sea level rise. To fully represent these processes and other feedbacks, a fully coupled ice/ocean model must be used. Therefore, a fully synchronous mass conservative coupled ice-sheet/ocean model of the Amundsen Sea Sector has been developed. This new coupled model builds upon previous coupling developments and involves coupling of the WAVI ice-sheet model to the 3D ocean model MITgcm, via the Streamice ice-sheet model. Coupled model projections are presented, examining ice grounding line retreat rates and ice mass loss, along with ocean driven melt rate evolution. The sensitivity of these results to ocean forcings is shown, specifically the thickness of the relatively warm Circumpolar Deep Water layer and its variability. In addition, we discuss the impact of the present-day initialisation and tuning of the coupled model.

How to cite: Bett, D., Bradley, A., Williams, R., Holland, P., Arthern, R., and Goldberg, D.: Fully synchronous coupled ice/ocean modelling of future changes in the Amundsen Sea Sector, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13533, https://doi.org/10.5194/egusphere-egu23-13533, 2023.

Vanderford Glacier is one of the fastest retreating glaciers in East Antarctica, with approximately 18.6 km of grounding line retreat since 1996. Together with the Totten Glacier, the Vanderford Glacier is a key outlet glacier of the Aurora Subglacial Basin (ASB), which contains approximately 7 m of global sea level equivalent, of which ~3.5 m is vulnerable to ocean driven melting, and is rapidly losing mass. While the Totten Glacier currently discharges almost twice as much ice as the Vanderford Glacier, sediment records from the Sabrina and Knox Coast Sectors indicate that the Vanderford Glacier has had sedimentation rates over twice that at Totten in the past. Here, we examine the current flow configuration between Vanderford and Totten Glaciers and drivers of it, including interactions between the subglacial topography, hydraulic potential, climate, and ice sheet dynamics. We use the Ice-sheet and Sea-level System Model (ISSM) under experiments of heightened ocean warming concentrated at Vanderford Glacier, and heightened surface mass balance at Totten Glacier, to show that the present-day flow configuration between the Totten and Vanderford Glaciers is tenuous. Rerouting towards Vanderford Glacier could occur under even minor changes in surface elevation at both glaciers. Such rerouting potentially exposes large parts of the underbelly of the ASB to enhanced ocean-driven ice shelf melting in the event of rapid retreat of Vanderford Glacier, with implications for global sea level rise.

How to cite: McCormack, F., Kulessa, B., and Roberts, J.: Controls on the flow configuration of Vanderford Glacier, East Antarctica, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3216, https://doi.org/10.5194/egusphere-egu23-3216, 2023.

The warm Pliocene was a period of comparable atmospheric carbon dioxide concentrations to modern, but with sea levels up to ~20 m higher. High Pliocene sea level implies collapse of the West Antarctic ice sheet, and mass loss from East Antarctica. Modelling studies have sought to reproduce Pliocene deglaciation, and use sea level reconstructions as a constraint on future projections, despite their large uncertainties. We simulated the Pliocene Antarctic ice sheet under warm Pliocene climate with the BISICLES ice sheet model, capturing grounding line and ice stream dynamics down to 4 km resolution. Our perturbed parameter ensemble approach explores uncertainties in basal sliding, surface mass balance processes, bedrock-ice sheet interactions, ice shelf basal melt sensitivity to ocean forcing and choice of climate model. We simulated a mean Antarctic sea level contribution of 1.85 m and a range of -15.90 to 28.27 m, largely driven by uncertainty in the perturbed basal sliding parameter. We applied a joint calibration, combining a Pliocene Antarctic sea level contribution range and a comparison of regional grounding line with reconstructed Pliocene retreat. This reduced the mean to 1.46 m. The calibration reduced the simulated range by a factor of ~4, and was more effective in reducing uncertainty than comparing to sea level reconstructions alone. Further ensembles explored initial condition uncertainty, and the impact of perturbing the control. The Pliocene initial condition was tested for a subset of main ensemble simulations (mean = -2.35 m), increasing the mean contribution by 11.45 m with all simulations passing the joint calibration. We perturbed the control simulation for the same subset of ensemble members. This increased the mean Antarctic contribution by 7.78 m, and by 8.45 m in combination with the two Pliocene data constraints. We demonstrate a modelling framework that captures important interactions between the ice sheet and other components of the Earth system, whilst being efficient for ensemble studies. Moreover, we used two Pliocene data constraints to rule out ensemble members. This Pliocene-calibrated modelling framework can be run under future climate scenarios, to reduce uncertainty in projections of Antarctica’s long-term contribution to sea level under anthropogenic climate change.

How to cite: ONeill, J., Edwards, T., and Gregoire, L.: Pliocene Antarctic ice sheet model ensembles with joint constraints from reconstructed sea level and margin retreat, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11803, https://doi.org/10.5194/egusphere-egu23-11803, 2023.

The geometry of englacial isochrones is a product of the past and present ice velocity field and is useful for our understanding of steady-state ice flow dynamics, flow regime re-organisation, and calibration of models. Ice rises contain various flow regimes (divide flow, flank flow, and grounding zones) on small spatial scales, meaning they are ideal locations to study ice-flow dynamics and stratigraphy to constrain model parameters. We run full Stokes, thermo-mechanically coupled simulations of Derwael Ice Rise in East Antarctica and simulate the three-dimensional stratigraphy of the ice rise and the surrounding ice shelf using the finite element model Elmer/Ice. Over the ice rise, we derive the accumulation rate from internal reflection horizons and use RACMO2.3 surface mass balance data over the surrounding ice shelf. Simulations are run for Glen's flow law exponents of n=3 and n=4 with appropriate values derived for the Arrhenius law.

To calibrate the model, comparisons are made with the BedMachine surface elevation and density-adjusted internal reflection horizons observed in many transects recorded by AWI’s ultra-wide band radar covering the divide, the flanks, and the grounding zones. To understand ice flow dynamics where the velocity field of the ice rise and the ice shelf converge in the compressive and shear zones, we analyse the modelled englacial stress and strain rate fields. Our results allow us to investigate isochronal structures where observed internal reflection horizons are too steep or obscured to be adequately picked up by radar. A comparison between the model and observed fracturing can be used to infer threshold stress and strain rates for fracture initiation. These simulations are a blueprint for the full Stokes, three-dimensional modelling of ice rises and have further relevance in the study of three-dimensional influences on Raymond arch evolution, the constrained coupling of the anisotropy equations, comparisons with ice core data and the automated inference of ice flow parameters from internal reflection horizons.

How to cite: Henry, C., Drews, R., Schannwell, C., Višnjević, V., Koch, I., Spiegel, H., Muhle, L., Eisen, O., Jansen, D., Franke, S., and Bons, P.: Modelling the three-dimensional stratigraphy of an ice rise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12764, https://doi.org/10.5194/egusphere-egu23-12764, 2023.

Ice shelf calving is responsible for roughly half the mass lost by Antarctic Ice Shelves and is of vital importance for determining ice-shelf stability. Despite this, it is currently poorly represented in numerical models and as such has seen limited inclusion in numerical simulations of the future Antarctic Ice Sheet. A first step towards improving this situation is assessing the capabilities and limitations of current numerical models regarding calving. The Calving Model Intercomparison Project (CaMIP) is, therefore, being undertaken to address this issue as part of the EU funded Horizon 2020 project PROTECT.

We present here an overview of the project, as well as preliminary results from the first round of experiments by a range of participating modelling groups from across the cryospheric community.

How to cite: Jordan, J. and Pattyn, F.: Calving Model Intercomparison Project (CaMIP): Overview and preliminary results, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7264, https://doi.org/10.5194/egusphere-egu23-7264, 2023.

Ice-calving plays a major role in the mass balance of the water-ending glaciers. Thus, it is crucial to have a well-adapted calving law for simulations over long periods. Due to its dependence on several physical parameters, this phenomenon is usually poorly parametrized in long-term numerical simulations. A worldwide model intercomparison project on ice damage and calving, CalvingMIP (see https://github.com/JRowanJordan/CalvingMIP/wiki), is proposed as part of the European project, PROTECT. The CalvingMIP project aims to evaluate the uncertainties in modelling the ice and to provide recommendations to improve the calving laws in the ice-sheet models. This intercomparison project consists of five experiments using two topographic profiles: a hill and Thule bathymetry. For the first phase, a steady-state configuration is implemented for a fixed calving position. In the second phase: the front velocity is prescribed, forcing the front to advance and, then, to retreat. Finally, the last experiment aims to test a realistic calving law. We study the experiments of this configuration by using the community finite element code, Elmer/Ice (see http://elmerice.elmerfem.org/). We study the front evolution using a level-set function (φ), defined as a signed distance to the front. Here, we present the results obtained with our model for this intercomparison experiment, discuss the sensitivity to different physical and numerical parameters, and its application to a realistic configuration.

How to cite: Garcia-Molina, C., Gillet-Chaulet, F., Chekki, M., Durand, G., Gagliardini, O., and Jourdain, N.: Elmer/Ice results on the CalvingMIPintercomparison project using a level-set function, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6947, https://doi.org/10.5194/egusphere-egu23-6947, 2023.

Some ice sheets and glaciers experience long quiescent periods interspersed with short periods of rapid ice advance, such as the binge-purge-type cycling hypothesized to be associated with Heinrich Events. Modeling ice stream activation/de-activation, however, is numerically challenging given the relatively abrupt changes at surge onset and the high ice velocities. In spite of this, a number of high-profile modeling papers have explored Heinrich events and ice surges, but generally with very limited consideration of numerical aspects. Here we test the ability of the 3D Glacial Systems Model (GSM) and Parallel Ice Sheet Model (PISM) to simulate binge-purge-type surges and explore the stability of the simulations with respect to relevant numerical and discretization uncertainties. 

We find surge characteristics exhibit a resolution dependency but converge at higher horizontal grid resolutions (order 5 km). In accordance with theoretical and experimental work, our model results suggest that the thermal activation of basal sliding should start below the pressure melting point. A resolution-dependent basal temperature ramp for the thermal activation of basal sliding as well as a subglacial hydrology model can reduce the discrepancies between high and coarse horizontal grid resolutions. Furthermore, incorporating a bed thermal and at least a minimal complexity subglacial hydrology model significantly affects surge characteristics and is, therefore, essential for modeling large-scale ice stream cycling.

How to cite: Hank, K., Tarasov, L., and Mantelli, E.: Towards confidence in numerical modeling of ice stream cycling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-548, https://doi.org/10.5194/egusphere-egu23-548, 2023.

Ice flow models often suffer from numerical instabilities that restricts time-step sizes. For higher-order models this constitutes a severe bottleneck. We present a method for increasing the largest stable time step in full Stokes models, allowing for a significant speed-up of simulations.  This type of stabilisation was originally developed for mantle-convection simulations and is here extended to ice flow problems. The method is mimicking an implicit solver but the computational cost per time step is nearly as low as for an explicit solver. As it only consists of adding a stabilisation term to the gravitational force in the full Stokes equations, it is very easy to implement. We test the method using both Elmer/Ice and FEniCS on artificial glaciers with varying bedrock roughness, slip rate and surface inclination, as well as on a real world case.

How to cite: Löfgren, A., Ahlkrona, J., Zwinger, T., Helanow, C., and Cohen, D.: Increasing the largest stable time-step size in ice flow models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14849, https://doi.org/10.5194/egusphere-egu23-14849, 2023.