Greenland's glacial fjords transport heat and freshwater between the shelf and the outlet glaciers of the Greenland Ice Sheet. Therefore they are crucial to understand ice-ocean interaction in the Norhern Hemisphere. Despite increasing attention from the research community, much of the seasonal variability of fjord circulation remains unknown, especially in the non-summer months. This study presents current velocity and water mass data for a full year in Nuup Kangerlua. We provide insights into the dynamics of this South West Greenland fjord, focusing on winter and the upper layer currents. We show that in winter fjord circulation remains active, including a large cross fjord component that has not been observed before. There is a disconnect between the mouth and the inner part of the fjord, causing heat to be stored in the inner fjord. The stored heat could potentially act as reservoir of melt energy for glaciers in winter.

How to cite: Vries, A., Meire, L., Mortensen, J., Schulz, K., van de Berg, W. J., and van den Broeke, M.: Circulation, mixing and heat transport in a Greenland fjord, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5666, https://doi.org/10.5194/egusphere-egu24-5666, 2024.

Glacial fjords form a crucial coupling between the Greenland ice sheet and the surrounding ocean, but observational data is scarce and their complex multi-scale physics can be difficult to model. Thus, glacial fjord processes are often excluded from large-scale ice sheet models that project  sea level contribution and ocean models that are forced by ice sheet freshwater. A key driver of fjord dynamics is the input of ice sheet freshwater, primarily from subglacial discharge rising in a buoyant plume and from iceberg melt. These freshwater sources set up a density gradient between the fjord and shelf, driving fjord circulation and exporting freshwater to the ocean. Observational evidence from a few fjords suggests that fjords can store this freshwater, leading to an export to the shelf that is modified in properties and lagged in time compared to the input of the freshwater to the fjord. Yet little is known about how this freshwater modification varies across Greenland’s diverse fjords, and the relative importance of the sources of freshwater in this process has not been quantified.  

Here, we use a two-layer box model to simulate fjord dynamics in a simple yet realistic way. We isolate the circulation driven by freshwater input from each of subglacial discharge and iceberg melting to assess the relative impact of each process on (i) strength of circulation and (ii) modification and export of freshwater. The model suggests that fjord geometry and the strength of the fjord-shelf exchange are the key controllers of the lag time for freshwater export, with strong fjord-shelf exchange and smaller fjords promoting nearly instant freshwater export, and weak fjord-shelf exchange and large fjords giving long lags in freshwater export. The wider aims of the project are to quantify freshwater export and heat import at glacial fjords on a Greenland-wide scale.

How to cite: Johnstone, E., Slater, D., Cowton, T., Fraser, N., Inall, M., and Mas e Braga, M.: The relative importance of subglacial discharge and iceberg melt forcing in Greenlandic glacial fjord circulation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17929, https://doi.org/10.5194/egusphere-egu24-17929, 2024.

Jakobshavn Isbræ (Sermeq Kujalleq) has been the largest single contributor to mass loss from the Greenland Ice Sheet over recent decades. Previous work has emphasised the dominant role of oceanic forcing on ice dynamics, with the short-lived (2016-2018) advance, deceleration and thickening of Jakobshavn attributed to decreased ocean temperatures within Disko Bay. Here, we use satellite imagery to extend observations of ice dynamics at Jakobshavn Isbræ between 2018 and 2023. We then employ hydrographic measurements, weather station data, and modelled estimates of surface runoff, to explore the role of climatic forcing on ice dynamics over this most recent five-year period. 

Between 2018 and 2022, Jakobshavn Isbræ accelerated significantly, with peak summer terminus velocity increasing by 79%, from 9.4 to 16.8 km/yr. Despite sustained surface lowering, peak solid ice discharge also increased, rising from 39.4 Gt/yr in 2018 to 54.7 Gt/yr in 2021. Whilst the initial onset of re-acceleration occurred in 2019, a dramatic speedup occurred between May and August 2020, with ice velocity increasing from 7.6 to 13.8 km/yr. In contrast to previous years, ice velocity remained high throughout the subsequent winter, thereby facilitating a peak velocity of 16.8 km/yr in July 2021.

Jakobshavn Isbræ exhibited a typical seasonal calving cycle of winter advance and summer retreat throughout 2018 and 2019. However, a clear switch in dynamics was observed in 2020, with the terminus undergoing minimal readvance over the winter months. This shift coincided with a clear reduction in the extent of rigid mélange within Ilulissat Icefjord, in contrast to preceding years. Although sparse, hydrographic measurements indicate that the mean water temperature within Disko Bay was ~ 0.75⁰C higher in 2020, relative to 2019.

We argue that the initial onset of reacceleration and thinning at Jakobshavn Isbræ was driven primarily by atmospheric forcing, with annual runoff in 2019 approximately double that observed in the other years. Furthermore, we emphasise that at glaciers close to floatation, such as Jakobshavn, surface thinning can significantly impact buoyant flexure, and hence rates of calving. However, we also provide evidence of oceanic forcing, postulating that increased water temperatures reduced the formation of rigid mélange in 2020, thereby facilitating sustained calving and elevated ice velocities throughout the winter months. Our study therefore highlights the critical importance of considering both atmospheric and oceanic forcing when investigating and predicting the future behaviour of ice dynamics at marine-terminating outlet glaciers.

How to cite: Picton, H., Nienow, P., Slater, D., and Chudley, T.: A reassessment of the role of atmospheric and oceanic forcing on ice dynamics at Jakobshavn Isbræ, Ilulissat Icefjord, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8352, https://doi.org/10.5194/egusphere-egu24-8352, 2024.

Glacial melt rates at ice-ocean interfaces are critical to understanding ice-ocean interactions in polar regions and are commonly parameterized as a turbulent shear boundary with a time-invariant drag coefficient. This assumes the exchange of heat and freshwater across the mm-scale diffusive thermal and salinity boundary layers varies proportionally with the strength of external momentum. However, this is only appropriate when melt/buoyancy-driven turbulence and the suppression of turbulence by stratification is weak.

Guided by GPU-accelerated Direct Numerical Simulations (10 micron resolution) of the ice-ocean boundary layer for varying geometric and ocean forcing parameters, I will present an updated understanding of the basic principles of ice-ocean boundary layers as a complex interplay between diffusive freshwater/thermal and viscous shear layers nested within different types of turbulent boundary layers. I will present numerical simulation results that seek to merge the different turbulent ice-ocean boundary layer regimes: (1) meltwater-driven buoyancy, (2) meltwater-driven shear, and (3) externally-driven shear from both horizontal and vertical sources of momentum.

This updated understanding allows us to develop more accurate predictions for the turbulently-constrained momentum, thermal, and freshwater boundary layer thicknesses, which is required to predict the ocean-driven melt rate of ice in polar regions.

How to cite: Zhao, K., Chor, T., Skyllingstad, E., and Nash, J.: Improved Parameterizations of Ice-Ocean Boundary Layers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14126, https://doi.org/10.5194/egusphere-egu24-14126, 2024.

How to cite: Vallot, D., Jourdain, N., and Mathiot, P.: Spatio-temporal variable drag for the sub-ice-shelf melt parameterisation in NEMO, ocean model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-789, https://doi.org/10.5194/egusphere-egu24-789, 2024.

The majority of Antarctica’s contribution to sea level rise can be attributed to changes in ocean-driven melting at the base of ice shelves. Turbulent ocean currents and melting are strongest where the ice base is steeply sloped, but few studies have systematically examined this effect. Here we use 3-D, turbulence-permitting large-eddy simulations (LES) of an idealised ice shelf-ocean boundary current to examine how variations in ice base slope influence ocean mixing and ice melting. The range of simulated slope angles is appropriate to the grounding zone of small Antarctic ice shelves and to the flanks of relatively wide ice base channels, with far-field ocean conditions representative of warm-water ice shelf cavities. Within this parameter space, we derive formulations for the friction velocity, thermal forcing, and melt rate in terms of total melt-induced buoyancy input and ice base slope. This theory predicts that melt rate varies like the square root of slope, which is consistent with the LES results and differs from a previously proposed linear trend. With the caveat that further simulations with an expanded range of basal slope angles and ocean conditions would be necessary to evaluate the validity of our conclusions across the full Antarctic ice base slope parameter space, the derived scalings provide a potential framework for incorporating slope-dependence into parameterisations of mixing and melting at the base of ice shelves.

How to cite: Anselin, J., Holland, P., Taylor, J., and Jenkins, A.: Ice base slope effects on the turbulent ice shelf-ocean boundary current, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3542, https://doi.org/10.5194/egusphere-egu24-3542, 2024.

Ice shelves play a pivotal role in stabilizing the Antarctic ice sheet by providing crucial buttressing support. However, their vulnerability to basal melting poses significant concerns for ice sheet and shelf stability. Our study focuses on assessing basal melt rates at a 50 m posting of 12 ice shelves where earlier studies have identified high melt rates. We make use of the Reference Elevation Model of Antarctica (REMA) strips to generate surface elevation- and melt rates using the Basal melt rates Using Rema and Google Earth Engine (BURGEE) methodology.

BURGEE reveals higher melt rates in areas with thinner ice than existing remote sensing basal melt products. This is for instance the case for basal channels on both Dotson, Totten and Pine Island ice shelves. Modelling studies have already shown that remote sensing inferred basal melt rates are underestimated at the thinnest part of basal channels, and that this underestimation scales with resolution coarsening. Since the thinner parts of an ice shelf also represent its weakest part, it is crucial that we capture its melting well to fully grasp the vulnerability of the ice shelf.

Our work, therefore, represents a crucial step in uncovering the vulnerability of Antarctic ice shelves. By exposing detailed melting patterns, particularly in areas like basal channels, we highlight not just extensive melting but also potential weak points, significantly contributing to our understanding of ice shelf stability. These findings bear substantial importance in comprehending the broader implications of ongoing climate changes on Antarctica's ice sheet integrity and, consequently, global sea levels.

How to cite: Zinck, A.-S. P., Lhermitte, S., and Wouters, B.: Exposing Underestimated Channelized Basal Melt Rates in Antarctic Ice Shelves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10186, https://doi.org/10.5194/egusphere-egu24-10186, 2024.

Ice shelves along the Amundsen Sea coastline in West Antarctica are continuing to thin, albeit at a decelerating rate, whilst ice discharge across the grounding lines has been observed to increase by up to 100% since the early 1990s. Here, the ongoing and future evolution of ice-shelf mass balance components (basal melt, grounding line flux, calving flux) is assessed in a high-resolution coupled ice-ocean model that includes the Pine Island, Thwaites, Crosson and Dotson ice shelves. For a range of idealized ocean-forcing scenarios, the combined evolution of ice-shelf geometry and basal melt rates is simulated over a 200-year period. For all ice-shelf cavities, a reconfiguration of the 3D ocean circulation in response to changes in cavity geometry is found to cause significant and sustained changes in basal melt rate, ranging from a 75% decrease up to a 75% increase near the grounding lines, irrespective of the far-field ocean conditions. These poorly explored feedbacks between changes in ice-shelf geometry, ocean circulation and basal melting have a demonstrable impact on the net ice-shelf mass balance, including grounding line discharge, at multidecadal timescales. They should be considered in future projections of Antarctic mass loss, alongside changes in ice-shelf melt due to anthropogenic trends in the ocean temperature and salinity.

How to cite: De Rydt, J. and Naughten, K.: Geometric amplification and suppression of ice-shelf basal melt in West Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17297, https://doi.org/10.5194/egusphere-egu24-17297, 2024.

Observations show that some of the ice shelves surrounding Antarctica are thinning, driven by warming of the underlying ocean. These ice shelves play an important role in moderating the rate of mass loss from the ice sheet by buttressing the ice flow from the grounded parts of the ice sheet. The increased ocean-induced sub-shelf melt is therefore an important process for the stability of the ice sheet and representing it in ice sheet models is essential to study the evolution of the Antarctic Ice Sheet. Here, we present a coupled ice-ocean setup, applied to an idealised ice shelf.

The sub-shelf melting occurs in highly heterogeneous patterns, typically exhibiting higher melt rates near the grounding line. Currently, most ice sheet models rely on parameterisations which derive sub-shelf melt rates from far-field ocean hydrography. Despite their computational advantage and ease in handling grounding line migration, these parameterisations fall short of accurately representing the right details in the melt patterns. To capture more physically consistent melt patterns, we implemented an online coupling between the ice sheet model IMAU-ICE and the sub-shelf melt model LADDIE. The latter resolves the necessary physics governing the melt, including the Coriolis deflection and topographic steering of meltwater, and provides sub-shelf melt fields at sub-kilometre spatial resolution.

We will show the impact of detailed sub-shelf melt fields in an idealised set-up. We compare IMAU-ICE simulations using existing sub-shelf melt parameterisations with simulations in the coupled set-up with IMAU-ICE and LADDIE. Three parameterisations are considered for this comparison: the quadratic scaling with temperature, the box model PICO, and the plume model. All simulations are performed in the idealised MISMIP+ domain. We consider a range of oceanic temperature forcings similar to present-day temperatures in warmer and colder basins surrounding Antarctica. We present and discuss the results, primarily focusing on the evolution of three key indicators for ice sheet stability: grounding line position, ice shelf extent, and grounding zone shape. These results demonstrate the importance of accounting for realistic melt patterns in ice sheet models.

How to cite: Jesse, F., Lambert, E., and van de Wal, R.: Sub-shelf melt patterns… does detail matter?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5804, https://doi.org/10.5194/egusphere-egu24-5804, 2024.

Thermodynamically maintained open ocean areas surrounded by sea ice, or sensible-heat polynyas, are linked to key ice-sheet processes, such as ice-shelf basal melt and ice-shelf fracture, when they occur near ice-shelf fronts. However, the lack of detailed multi-year records of polynya variability pose a barrier to assessing the potential interconnectivity between polynya and frontal dynamics. Here, we present the first multi-decadal record (2000–2022) of polynya area at Pine Island Glacier (PIG) from thermal and optical satellite imagery. We found that although polynya area was highly variable, there were consistencies in the timing of polynya maximal extent, and opening and closing. Furthermore, we found that the largest polynya (269 km²) in our record occurred at PIG’s western margin just 68 days before iceberg B-27 calved, suggesting that polynya size and position may influence rifting dynamics. We suspect that large sensible-heat polynyas have the potential to reduce both ice-shelf buttressing (via reduced landfast ice) and shear margin dynamics (via reduced contact with slower marginal ice), which may lead to structural instability and eventually contribute to calving. Our new dataset provides a pathway to assess coevolving polynya and frontal dynamics, demonstrating the importance of building long-term records of polynya variability across the continent.

How to cite: Savidge, E., Snow, T., and Siegfried, M. R.: Two Decades of Satellite Observations: Sensible-Heat Polynya Variability at Pine Island Glacier, West Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6749, https://doi.org/10.5194/egusphere-egu24-6749, 2024.

Thwaites Glacier, a large outlet glacier of the West Antarctic Ice Sheet, holds over a half meter of sea level rise equivalent. The large potential contribution to sea level is concerning given that the glacier may be vulnerable to self-sustaining processes of rapid retreat due to the retrograde bed slope that characterises much of the glacier’s bed. Such a reverse-sloping bed exists behind the relatively high ridge on which the western calving front (WCF) of the Thwaites Glacier terminus currently rests. Our study focuses on the factors that control the calving dynamics of the WCF and the ability of mélange to influence these dynamics. Employing the 3D Helsinki Discrete Element Model (HiDEM), we find that calving at this location currently occurs as rifts form and widen due to longitudinal tensile stresses associated with ice flow across the grounding line. Calving is restricted in HiDEM simulations that include a constricted mélange field that is confined within the bounds of the model domain. A thicker, constricted mélange field fully suppresses calving. These simulations show the development of robust force chains that transmit resistive forces to the Thwaites WCF. In the future, the ability for mélange to influence the calving dynamics at the WCF will depend on the degree to which it is constrained in the wide Amundsen Sea Embayment, either through binding in land-fast sea ice or jamming behind large, grounded icebergs. As such, sea-ice conditions and iceberg characteristics will need to be considered along with the presence of mélange in investigations of the future retreat of the prominently recognised Thwaites Glacier.

How to cite: Crawford, A., Åström, J., Benn, D., Luckman, A., Gladstone, R., Zwinger, T., Robertsén, F., and Bevan, S.: Calving dynamics and mélange buttressing conditions at the Thwaites Glacier calving face, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8616, https://doi.org/10.5194/egusphere-egu24-8616, 2024.

Since 2015, there has been a significant increase in iceberg calving rates from Antarctic ice shelves. It is crucial to comprehend the climate-related reasons for this enhanced iceberg calving to improve coupled simulations with the ice sheet and predict their future effects on sea-level rise. Based on continuous observations of iceberg calving around Antarctica over 15 years, we demonstrate that sea ice extent is the primary control on iceberg calving rates in Antarctica, regardless of ice shelf size, location, or ocean regime. The recent increase in calving rates coincides precisely with a significant reduction in sea ice area in most sectors around the continent. We propose a calving model, where iceberg calving is dominated by ocean-wave induced flexure and basal shear and enhanced by ice-shelf basal melt. We also find links between iceberg calving rate and El Niño/Southern Oscillation (ENSO), which are particularly strong in East Antarctica. Given that further decreases in sea ice extent and increases in extreme ENSO events are predicted in future, we raise concern that previously stable East Antarctic ice shelves may soon begin to retreat, with potential to trigger significant mass loss from this massive ice sheet.

How to cite: Liu, Y., Cheng, X., Liu, J., Moore, J., Li, X., and Cook, S.: Strong ice-ocean interaction drives and enhances calving of Antarctic ice shelves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3027, https://doi.org/10.5194/egusphere-egu24-3027, 2024.

Hektoria Glacier on the Eastern Antarctic Peninsula underwent a heretofore unseen rate of tidewater-style glacier retreat from 2022 to 2023 after the loss of decade-old fast ice in the Larsen B embayment. The glacier has retreated 25 km between February 2022 and January 2024, of which at least 8-13 km was grounded ice. Remote sensing data in the months following the fast ice break-out reveals an ice flow speed increase of up to 4-fold, and rapid elevation loss up to 20-30 m, representing an 8-fold increase in the glacier thinning rate. Hektoria and Green Glaciers underwent three phases of retreat displaying differing calving styles. During the first two months after the loss of the fast ice in January 2022 the Hektoria-Green ice tongue calved large tabular bergs. In March 2022, an abrupt change in Hektoria’s calving style was observed, changing from large tabular icebergs to buoyantly rotated smaller bergs. Following this transition, Hektoria underwent several short periods of rapid retreat. In December 2022, 2.5 km of grounded ice were lost over 2.5 days. These retreat rates for grounded tidewater ice are greater than any reported in the modern glaciological record. Here we examine the evidence for locating the pre-fast ice break-out grounding zone as well as the drivers that could cause such a rapid retreat. We link these observations to known causes of glacier instability, such as Marine Ice Sheet Instability and Marine Ice Cliff Instability, as well as the classical tidewater glacier retreat cycle.

How to cite: Ochwat, N. E., Scambos, T. A., Anderson, R. S., Walker, C. C., and Fluegel, B. L.: Re-evaluating Rapid Glacier Retreats: Hektoria Glacier’s Unprecedented Tidewater Collapse, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6423, https://doi.org/10.5194/egusphere-egu24-6423, 2024.

One of the major questions in climate science is to improve the accuracy of sea-level rise prediction, for which mass loss of the polar ice caps has a significant contribution. In this work, the focus is on buoyancy-dominated capsize of large icebergs. The capsizes generate specific seismic signals, which in turn can be analysed and used as a unique tool to study the long term evolution of such large icebergs capsize and the glacier response.

To better quantify ice mass loss due to iceberg calving at marine terminating glaciers, coupling iceberg calving simulation and inversion of the seismic waves generated by these events and recorded at teleseismic distances is necessary. To achieve our task, a complex fluid/structure model of the iceberg capsize is required to obtain accurate forces history acting on the glacier terminus. The simulated forces can then be compared to the force inverted from the seismic signal. Therefore, based on our recent work, we implement a Computation Fluid Dynamics (CFD) approach to reach a high fidelity modelling of the iceberg capsize. First work using the experimental data of an iceberg capsize showed the need and ability of CFD computations to precisely reproduce the iceberg kinematics for different cases. We will present more advanced CFD configurations, including the contact between the capsizing iceberg and a rigid glacier front. Computation results are compared and validated against lab scale experiments, where we outline that some 3D effects cannot be neglected. We will also present full scale capsize simulations, in which the mixing of ocean layers occurs. In particular, we will quantify the transport of particles within the ocean to illustrate the potential change of nutriments distribution or of pressure experienced by local fauna due to iceberg calving.

How to cite: De Pinho Dias, N., Leroyer, A., Mangeney, A., Castelnau, O., and Thiebot, J.-B.: High fidelity modelling of iceberg capsize, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7829, https://doi.org/10.5194/egusphere-egu24-7829, 2024.

The marine-terminating glaciers in Svalbard are retreating and losing mass at an alarming rate due to the rapidly warming climate. And glacier calving is one of the most important process contributing to the glacier mass loss. Hence, it is very important to observe the glacier termini to understand calving variability and the influence of local environmental conditions on them.

Here, high frequency time-lapse images have been used to observe the calving front of Hansbreen (a tidewater glacier in the Honrsund fjord, Svalbard) at a 15-minute interval. The time-lapse images have been visually analyzed from April 2016 to October 2016, to observe the calving variability. The calving events are identified and then classified based on several parameters. The environmental parameters like air temperature, sea surface temperature, tidal cycle, water salinity, etc, for the same region have been understood to see if they have any influence spatial and temporal distribution of the observed calving events. [This research has been supported by the National Science Centre, Poland (grant no. 2021/43/D/ST10/00616) and the Ministry of Education and Science, Poland (subsidy for the Institute of Geophysics, Polish Academy of Sciences).]

How to cite: Maniktala, D. and Glowacki, O.: Studying the influence of environmental parameters on calving variability at Hansbreen, in Svalbard, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18558, https://doi.org/10.5194/egusphere-egu24-18558, 2024.

How to cite: Reynolds, B., Nowicki, S., Poinar, K., and Goliber, S.: Annual Terminus Prediction Errors for Greenland Glaciers from Calving Laws and Melt Parameterizations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-797, https://doi.org/10.5194/egusphere-egu24-797, 2024.

Calving is still a poorly understood process, hence a physically based calving law has not yet been found. Large ice sheet models are using simplified parameterisations to describe calving, which are tuned by observational data. Therefore the demands for the development of physically based models for calving are large.

Calving is facilitated by fracture formation and propagation, which description is the objective of fracture mechanics. Based on the fundamental theory for fracture proposed by Griffith a numerical approach has been developed to describe cracks: the so-called phase field method. This method represents the state of a material, whether it is intact or broken, by means of an additional continuous scalar field. The advantage of the phase field method is its simple numerical implementation and the avoidance of explicit representation of crack faces as well as costly remeshing.

This work adjust the phase field method for fracture to simulate fracture in glacier ice. Thereby the ice rheology is considered by using a viscoelastic material description where a nonlinear viscosity, based on Glen’s flow law, is taken into account. Furthermore finite strain theory is used to capture the large deformations occurring in ice shelves and floating glacier tongues.

The developed theoretical framework is utilized to simulate crack initiation and propagation at ice rises. Here the calving front geometry of the 79N Glacier in Greenland is used to validate the proposed model by comparing the simulated crack paths to satellite imagery.

How to cite: Sondershaus, R., Humbert, A., and Müller, R.: Simulating cracks in glacier ice by means of the phase field method, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18561, https://doi.org/10.5194/egusphere-egu24-18561, 2024.

Iceberg calving is one of the dominant sources of ice loss from the Antarctic and Greenland Ice sheets. Iceberg calving is still one of the most poorly understood aspects of ice sheet dynamics due to its variability at a wide range of spatial and temporal scales. Despite this variability, current large-scale ice sheet models assume that calving can be represented as a deterministic flux. Failure to parameterize calving accurately in predictive models could lead to large errors in warming-induced sea-level rise. In this study, we introduce stochastic calving within a one-dimensional depth-integrated tidewater glacier and ice shelf models to determine how changes in the calving style and size distribution of calving events cause changes in glacier state. We apply stochastic variability in the calving rate by drawing the calving rate from two different probability distributions.e also quantify the time scale on which individual calving events need to be resolved within a stochastic calving model to accurately simulate the probabilistic distribution of glacier state. We find that incorporating stochastic calving with a glacier model with or without buttressing ice shelves changes the simulated mean glacier state, due to nonlinearities in glacier terminus dynamics. This has important implications for the intrinsic biases in current ice sheet models, none of which include stochastic processes. Additionally, changes in calving frequency, without changes in total calving flux, lead to substantial changes in the distribution of glacier state. This new approach to modeling calving provides a framework for ongoing work to implement stochastic calving capabilities in large-scale ice sheet models, which should improve our capability to make well-constrained predictions of future ice sheet change.

How to cite: Ambelorun, A. and Robel, A.: The integrated ice sheet response to stochastic iceberg calving, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5087, https://doi.org/10.5194/egusphere-egu24-5087, 2024.

Marine-terminating glaciers and ice shelves are notoriously complex, with a wide range of ice-dynamic and calving processes occuring in response to oceanographic, atmospheric and glaciological influences. Within this complexity, however, we can recognise order on at least two scales. First, marine ice fronts typically form vertical cliffs, reflecting competition between oversteepening (ice flow and melt-undercutting) and failure. Calving magnitude-frequency distributions have power-law form with an exponent of -1.2, characteristic of self-organising criticality (SOC). Such systems have a critical point as an attractor, such that the system converges on the failure threshold.

The second scale is that of the whole ice tongue. Tidewater glaciers and ice shelves typically oscillate around stable positions for multiple years, punctuated by transitions to new quasi-stable positions. Stability is encouraged by pinning points which function as attractors at thresholds between stable and metastable states. Ice tongues may exist in metastable states for variable amounts of time, from days to decades. Factors encouraging rapid relaxation to the threshold include large stress gradients and rapid basal melt, and factors encouraging long relaxation times include low stress gradients, low melt rates, and buttressing from mélange or sea ice. Calving magnitude-frequency distributions have exponential form, reflecting the stochastic nature of calving in the metastable zone.

Both scales of self-organisation emerge spontaneously from physically-based calving models such as the Helsinki Discrete Element Model (HiDEM) and the crevasse-depth (CD) calving law implemented in Elmer/Ice. Purely deterministic models, however, are not optimal for long-term simulations, especially in Antarctic contexts. We present results of preliminary simulations using a stochastic CD calving law, which opens up the possibility of a universal calving model applicable to both the Greenland and Antarctic ice sheets.

How to cite: Benn, D., Åström, J., Wheel, I., Luckman, A., and Nick, F.: Tidewater Glaciers and Ice Shelves as Self-Organising Systems, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3442, https://doi.org/10.5194/egusphere-egu24-3442, 2024.

Ice shelves that spread into the ocean can develop rifts, which can trigger ice-berg calving and enhance ocean-induced melting. Fluid mechanically, this system is analogous to the radial propagation of a non-Newtonian, strain-rate-softening fluid representing ice that displaces a relatively inviscid and denser fluid that represents an ocean. Laboratory experiments showed that rift patterns can emerge in such systems and that the number of rifts declines in time. Such a dynamics was confirmed theoretically, but only for the earlier stage of the flow and for a fluid layer of uniform thickness. We investigate numerically the stability and late-time evolution of radially spreading, axisymmetric fluid layer of non-uniform thickness. We validate the two dimensional finite-element Úa model using similarity solutions of radially spreading layers of Newtonian fluid that were found consistent with laboratory experiments. We then explore the stability of the flow by introducing geometric perturbations to the initial front and tracing their evolution. Our simulations show that the front of Newtonian fluids is stable, though memory of the perturbation spectral form persists.

How to cite: Stern, L. and Sayag, R.: Stability of radially spreading extensional flows and ice shelves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19514, https://doi.org/10.5194/egusphere-egu24-19514, 2024.

Ice shelves can control the flux of ice across the grounding line of the Antarctic Ice Sheet (AIS), and hence the rate of mass loss, through the process of ice-shelf buttressing. Recent, increased mass loss from the AIS, particularly in the Amundsen Sea Embayment and the Antarctic Peninsula, has been attributed to a reduction in buttressing due to ice-shelf thinning, calving or ice-shelf collapse events. To determine how further changes in ice-shelf geometry might affect the contribution of the AIS to global sea levels, it is therefore important to quantify the total amount of buttressing that the ice shelves currently provide and to determine where within the ice shelves that buttressing is generated.

Previous work has sought to characterise the buttressing of Antarctic ice shelves by, for example, calculating the sensitivity of grounding line flux (GLF) to small perturbations in ice-shelf thickness, or defining regions of passive shelf ice. In this work, we calculate the total buttressing capacity of all Antarctic ice shelves for the first time and then explore the spatial distribution of that total buttressing capacity within each ice shelf.

We use the ice-flow model Úa to conduct a series of diagnostic, idealised calving experiments on a present-day, Antarctic-wide model domain, with high spatial resolution over ice shelves and grounding lines. We calculate the total buttressing capacity of each ice shelf as the relative change in GLF in response to the complete removal of the shelf and find that the total buttressing capacity varies by over two orders of magnitude around the ice sheet.

We then conduct a series of idealised calving perturbations, using a range of procedures for generating new calving front locations, and explore the spatial distribution of the total buttressing capacity within each ice shelf. We find that the vast majority of the buttressing is typically generated in ice shelf regions within a few kilometres of the grounding line. Thus, we suggest that a greater area of Antarctica’s ice shelves could be considered passive than previously proposed.

How to cite: Mitcham, T., Gudmundsson, G. H., and Bamber, J. L.: The buttressing capacity of Antarctic ice shelves, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11490, https://doi.org/10.5194/egusphere-egu24-11490, 2024.

The ongoing deceleration of Whillans Ice Stream, West Antarctica, provides an opportunity to investigate the role of grounded ice flux in downstream pinning point evolution on decadal time scales. Here, we construct and analyze a 20-year, multi-mission satellite altimetry record of dynamic ice surface-elevation change (dh/dt) in the grounded region between lower Whillans Ice Stream and Crary Ice Rise, a major Ross Ice Shelf pinning point. We developed a new method for generating multi-mission time series that reduces spatial bias and implemented this method with altimetry data from the Ice, Cloud, and land Elevation Satellite (ICESat; 2003–09), CryoSat-2 (2010–present), and ICESat-2 (2018–present) altimetry missions. We then used the 20-year dh/dt time series to identify persistent patterns of surface elevation change and to evaluate regional mass balance. Our results suggest that changes in ice flux associated with Whillans Ice Stream stagnation drive non-linear mass change responses isolated to the Crary Ice Rise region, producing persistent, spatially heterogeneous thickness changes. The resulting mass redistribution modifies the grounding zone and mass balance of the Crary Ice Rise region, in turn adjusting the buttressing regime of the southern Ross Ice Shelf embayment.

How to cite: Verboncoeur, H., Siegfried, M., Winberry, J. P., Holschuh, N., Byrne, D., Sauthoff, W., Sutterley, T., and Medley, B.: Multi-decadal evolution of Crary Ice Rise region, West Antarctica, amidst modern ice stream deceleration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6639, https://doi.org/10.5194/egusphere-egu24-6639, 2024.

The stability of polar ice sheets is governed by the seaward movement of ice streams which is decelerated by resistance originating from lateral shear zones. We explore the impact of crystal-scale anisotropy on effective ice stiffness, with regional-scale consequences on ice dynamics. Using the flexural response of Priestley Glacier to tidal forcing as an experimental framework, we constrain isotropic and anisotropic elastic models of vertical tidal ice-shelf flexure. We find that a five-fold reduction of local ice stiffness within narrow lateral shear-zone best fits DInSAR measurements from Sentinel-1. Our modeling not only reproduces 31 double-differential interferograms but also resolves them into 56 individual maps of vertical displacement during SAR image acquisition. Validated with GPS measurements, the inclusion of effective shear-zone weakening significantly reduces the root-mean-square-error of predicted and observed vertical displacement by 84%, from 0.182 m to 0.03 m. These results highlight the untapped potential of DInSAR imagery for mapping ice anisotropy along the feature-rich Antarctic grounding zone, an essential parameter for advancing current ice-sheet flow models.

How to cite: Wild, C., Drews, R., Neckel, N., Lee, J., Sihyung, K., Han, H., Lee, W. S., Helm, V., Marsh, O., and Rack, W.: Monitoring Shear-Zone Weakening in East Antarctic Outlet Glaciers through Differential InSAR Measurements, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12071, https://doi.org/10.5194/egusphere-egu24-12071, 2024.

Determining the grounding lines of ice shelf glaciers (the border at which the ice begins to float in the ocean) is obligatory in precise measuring and understanding of ice sheet mass balance and glacier dynamics. Awareness of its migration range (grounding zone) is also crucial when estimating the impact of glacier/ice sheet waters on the ocean water level. Currently, the most precise large-scale method is based on the viscoelastic tidal movement of the ice shelf identified on a 4-pass DInSAR results. In some places, however, the measurements are impossible or significantly difficult due to the decorrelation between scenes. According to our preliminary results, it may be possible to use unwrapped phase interferograms as a new/supportive method for detecting ground lines. Combined with algorithms for automatic delineation, it can become a powerful solution for obtaining results with unprecedented frequency.

The latest results revealed that for many glaciers the grounding zone width is two orders of magnitude larger than expected. This contradicts existing physical models, which are based on zero ice melt and fixed grounding line position. Irregular interactions between ice and seawater might have a strong impact on glacier evolution and projections if implemented in physical models. We employed a long-time series of Sentinel-1 differential radar interferometry from 2017 to 2021 to detect the variability in grounding line position on Orville Coast, the region of the western Ronne Ice Shelf. The research carried out over a long period and with high frequency allowed a more detailed study of changes occurring in the grounding zone. Observation from a broader perspective gave us the opportunity to detect seasonality and a persistent trend. We compared changes in grounding line migration with external factors e.g. ocean tides. This might provide a better understanding of the behavior of the ice sheet and glaciers, which are currently undergoing such rapid changes.

How to cite: Tympalski, M., Sompolski, M., Kopeć, A., and Milczarek, W.: Grounding line migration at Orville Coast, Ronne Ice Shelf, West Antarctica, based on long interferometric Sentinel-1 time series, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22433, https://doi.org/10.5194/egusphere-egu24-22433, 2024.

Hundreds of surface lakes are known to form each summer on north George VI Ice Shelf, Antarctic Peninsula. To investigate surface-meltwater induced ice-shelf flexure and fracture, we obtained Global Navigation Satellite System (GNSS) observations and ground-based timelapse photography over north George VI for three melt seasons from November 2019 to November 2022.

In particular, we used these field observations to characterize the flexure and fracture behaviour of a mature doline (i.e. drained lake basin formed in a prior melt season) on north George VI Ice Shelf. The GNSS displacement timeseries shows a downward vertical displacement of the doline centre with respect to the doline rim of ~60 cm in response to loading from the development of a central meltwater lake. Viscous flexure modelling indicates that this vertical displacement generates flexure tensile surface stresses of ~>75 kPa. The GNSS data also show a tens-of-days episode of rapid-onset, exponentially decaying horizontal displacement, where the horizontal distance from the rim of the doline with respect to its centre increases by ~70 cm. We interpret this event as the initiation and/or widening of a single fracture, possibly aided by stress perturbations associated with meltwater loading in the doline basin. This observation, together with our observations of circular fractures around the doline basin in timelapse imagery, suggests the first such documentation of “ring fracture” formation on an ice shelf, equivalent to the type of fracture proposed to be part of the chain reaction lake drainage process involved in the 2002 breakup of Larsen B Ice Shelf.

How to cite: Banwell, A., Willis, I., Stevens, L., Dell, R., and MacAyeal, D.: Observed and modelled meltwater-induced flexure and fracture at a doline on north George VI Ice Shelf, Antarctica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12334, https://doi.org/10.5194/egusphere-egu24-12334, 2024.

Recent advances in high-temporal-resolution satellite imaging has revealed the occurrence of seasonal ice-flow variability in the Antarctic Peninsula for the first time. This newly documented phenomenon provides motivation for identifying the as-yet-unknown ice, ocean and climate interactions responsible for driving the seasonal signals observed across the Antarctic Peninsula, and raises important questions about the possible presence and drivers of seasonality elsewhere in Antarctica. Knowledge of such mechanisms and the extent of seasonality around Antarctica will be important for refining discharge-based ice-sheet mass balance estimations, and for improving predictions of Antarctica’s future response to climate change.

Here, we identify the likely drivers of the recently observed ice-flow seasonality in the western Antarctic Peninsula by carrying out statistical time series analysis using our published Sentinel-1-derived velocity observations (Boxall et al., 2022; doi:10.5194/tc-16-3907-2022) and an array of environmental variables. Our results reveal that both surface and oceanic forcing are statistically significant controls upon ice-flow seasonality in the western Antarctic Peninsula, although each mechanism elicits a unique lag between forcing and the ice-velocity response.

By upscaling our Sentinel-1-derived velocity observations, we also report upon the nature of ice-flow seasonality along Antarctica’s entire coastal margin for the first time and, through additional time series analysis, assess the glacier- to regional-scale importance of surface and ocean forcing upon circum-Antarctic rates of flow.

How to cite: Boxall, K., Willis, I., Wuite, J., Nagler, T., Scheiblauer, S., and Christie, F.: Circum-Antarctic seasonality in grounded ice flow, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13188, https://doi.org/10.5194/egusphere-egu24-13188, 2024.

The penetration of shortwave radiation at the surface of an ice shelf has the potential to induce internal melting, resulting in the formation of a porous layer close to the surface commonly known as the weathering crust. This dynamic hydrological system is known to host light-absorbing impurities and microbes, forming a highly porous layer at the ice sheet's surface. The presence of the weathering crust significantly impacts the overall volume of generated meltwater by modulating the extent to which shortwave radiation is absorbed or reflected by the ice. Beyond external meteorological forcing, local conditions leading to weathering crust formation can be influenced by biological impurities on ice surfaces. This interplay between surface ice structures, cryoconite holes (CHs) and weathering crust contributes to the spatial and temporal variability of albedo and surface melt. In this study, we analysed uncrewed aerial vehicle (UAV) data and ground-based field spectroscopy data collected during the 2022/23 austral summer in Antarctica. The aim is to map CHs spatial distribution and to evaluate their radiative impact on blue ice fields at the Hells Gate Ice Shelf in Northern Victoria Land (East Antarctica). Furthermore, we documented the formation of the weathering crust and supraglacial ponds at Hells Gate and Nansen Ice Shelves across the summer solstice. By analysing Sentinel-2 satellite data, we were able to determine the spatial variability in surface albedo before and after the formation of the weathering crust. In detail, at the Hells Gate Ice Shelf, we estimated < 1% of area covered by CHs. Over frozen ponds and ice bands the area covered in CHs reached almost 10%. The corresponding spatially integrated-radiative forcing resulted to be about 1 Wm^-2 in average, but locally it reached values of over 200 Wm^-2, thus sustaining liquid water inside the CHs. As for the weathering crust, the delta albedo (Δα) was found to be about +0.10 and +0.40 respectively where weathering crust covered blue and marine ice. On the other hand, the supraglacial pond and stream formation provided an opposite Δα of about -0.30 over blue ice and -0.50 over areas previously characterised by snow cover. However, the fractional area interested by positive Δα resulted to be significantly higher than positive Δα areas over the two ice shelves.

How to cite: Traversa, G. and Di Mauro, B.: Weathering crust and cryoconite holes on the Hells Gate and Nansen Ice Shelves (East Antarctica), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10590, https://doi.org/10.5194/egusphere-egu24-10590, 2024.