CR2.3 | Ice shelves and tidewater glaciers - dynamics, interactions, observations, modelling
Ice shelves and tidewater glaciers - dynamics, interactions, observations, modelling
Co-organized by OS1
Convener: Nicolas Jourdain | Co-conveners: Ronja Reese, Peter Washam, Rachel Carr
Orals
| Thu, 18 Apr, 08:30–12:30 (CEST), 14:00–15:40 (CEST)
 
Room L3
Posters on site
| Attendance Fri, 19 Apr, 10:45–12:30 (CEST) | Display Fri, 19 Apr, 08:30–12:30
 
Hall X4
Orals |
Thu, 08:30
Fri, 10:45
Ice shelves and tidewater glaciers are sensitive elements of the climate system. Sandwiched between atmosphere and ocean, they are vulnerable to changes in either. The recent disintegration of ice shelves such as Larsen B and Wilkins on the Antarctic Peninsula, current thinning of the ice shelves in the Amundsen Sea sector of West Antarctica, and the recent accelerations of many of Greenland's tidewater glaciers provide evidence of the rapidity with which those systems can respond. Changes in marine-terminating outlets appear to be intimately linked with acceleration and thinning of the ice sheets inland of the grounding line, with immediate consequences for global sea level. Studies of the dynamics and structure of the ice sheets' marine termini and their interactions with atmosphere and ocean are the key to improving our understanding of their response to climate forcing and of their buttressing role for ice streams. The main themes of this session are the dynamics of ice shelves and tidewater glaciers and their interaction with the ocean, atmosphere and the inland ice, including grounding line dynamics. The session includes studies on related processes such as calving, ice fracture, rifting and mass balance, as well as theoretical descriptions of mechanical and thermodynamic processes. We seek contributions both from numerical modelling of ice shelves and tidewater glaciers, including their oceanic and atmospheric environments, and from observational studies of those systems, including glaciological and oceanographic field measurements, as well as remote sensing and laboratory studies.

Orals: Thu, 18 Apr | Room L3

Chairpersons: Peter Washam, Ronja Reese, Rachel Carr
Ice-ocean interactions in fjords and under ice shelves
08:30–08:40
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EGU24-5666
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ECS
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On-site presentation
Anneke Vries, Lorenz Meire, John Mortensen, Kirstin Schulz, Willem Jan van de Berg, and Michiel van den Broeke

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.

08:40–08:50
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EGU24-17929
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ECS
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On-site presentation
Eleanor Johnstone, Donald Slater, Tom Cowton, Neil Fraser, Mark Inall, and Martim Mas e Braga

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.

08:50–09:00
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EGU24-8352
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ECS
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On-site presentation
Hannah Picton, Peter Nienow, Donald Slater, and Thomas Chudley

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.

09:00–09:10
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EGU24-14126
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ECS
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On-site presentation
Ken Zhao, Tomas Chor, Eric Skyllingstad, and Jonathan Nash

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.

09:10–09:20
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EGU24-789
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ECS
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On-site presentation
Dorothée Vallot, Nicolas Jourdain, and Pierre Mathiot
Ice-shelf basal melting in NEMO, as in most ocean models, is parameterised based on a friction velocity calculated from a drag coefficient that is constant in space and time, usually tuned to approach observed melt. But the drag between the ice and the ocean should depend on the roughness at different scale. This means that roughness evolution in space and time is not taken into account in today's model. In recent decades, some ice shelves, particularly in the Amundsen Sea Embayement (ASE), have experienced an increase of their damage, associated with more surface and basal crevasses so their sub-shelf environment is rougher. There is good chances that this phenomenon is to happen more in the future and in an extended number of ice shelves. Here we present a study using a spatially variable coefficient of drag, which depends on the topography and is applied on the first wet cell height. We use the ice shelf parameterisation of NEMO4.2 on a configuration of ASE at 12th of a degree.

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.

09:20–09:30
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EGU24-3542
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ECS
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On-site presentation
Josephine Anselin, Paul Holland, John Taylor, and Adrian Jenkins

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.

09:30–09:40
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EGU24-10186
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ECS
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On-site presentation
Ann-Sofie Priergaard Zinck, Stef Lhermitte, and Bert Wouters

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.

09:40–09:50
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EGU24-17297
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On-site presentation
Jan De Rydt and Kaitlin Naughten

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.

09:50–10:00
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EGU24-5804
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ECS
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On-site presentation
Franka Jesse, Erwin Lambert, and Roderik van de Wal

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.

10:00–10:10
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EGU24-6749
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ECS
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On-site presentation
Elena Savidge, Tasha Snow, and Matthew R. Siegfried

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.

10:10–10:15
Coffee break
Chairpersons: Rachel Carr, Peter Washam
Iceberg calving
10:45–10:55
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EGU24-20445
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On-site presentation
The breakup of iceberg D-15: Ice shelf-ocean interactions in a changing icescape environment at West Ice Shelf, East Antarctica
(withdrawn)
Catherine C. Walker, Thomas Neumann, Gil Averbuch, and Weifeng Zhang
10:55–11:05
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EGU24-8616
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ECS
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Virtual presentation
Anna Crawford, Jan Åström, Doug Benn, Adrian Luckman, Rupert Gladstone, Thomas Zwinger, Fredrik Robertsén, and Suzanne Bevan

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.

11:05–11:15
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EGU24-3027
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On-site presentation
Yan Liu, Xiao Cheng, Jiping Liu, John Moore, Xichen Li, and Sue Cook

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.

11:15–11:25
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EGU24-6423
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ECS
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On-site presentation
Naomi E. Ochwat, Ted A. Scambos, Robert S. Anderson, Catherine C. Walker, and Bailey L. Fluegel

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.

11:25–11:35
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EGU24-7829
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ECS
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On-site presentation
Nicolas De Pinho Dias, Alban Leroyer, Anne Mangeney, Olivier Castelnau, and Jean-Baptiste Thiebot

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.

11:35–11:45
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EGU24-18558
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ECS
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On-site presentation
Dhruv Maniktala and Oskar Glowacki

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.

11:45–11:55
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EGU24-3423
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On-site presentation
The role of thermal notch erosion in forcing localised calving failure and short-term increases in velocity at a lake-terminating glacier in southeast Iceland 
(withdrawn)
Nathaniel Baurley and Jane Hart
11:55–12:05
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EGU24-797
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ECS
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Virtual presentation
Benjamin Reynolds, Sophie Nowicki, Kristin Poinar, and Sophie Goliber

Many calving laws have been proposed leading to a need to characterize the ability of these laws to predict terminus movement across years. The influence of terminus change on glacier discharge makes this an important source of uncertainty for multi-decadal sea level rise prediction from ice sheet models. Here, we develop a workflow to tune calving laws and then calculate error in predicted terminus positions based on Greenland Ice Sheet Mapping Project (GrIMP) surface velocity data sets compiled from Sentinel, Landsat, TerraSAR-X, TanDEM-X, and COSMO-SkyMed satellites as well as digital elevation models (DEMs) from ASTER mission and ArcticDEM data.  Greenland glaciers with available data are used to test the height above flotation, fraction above flotation, crevasse depth criterion, von Mises criterion, and surface stress maximum calving laws over a multi-year period. Several versions of the crevasse depth law based on stress input are tested providing insight into the law’s dependence on stress calculation. This dependence is important as the crevasse depth law has been recommended by calving law comparison but has been implemented with various stress calculations to work with three-dimensional stress fields. The terminus melt parameterization used in the Ice Sheet Model Intercomparison Project for CMIP6 standard experiments is included as reference to show the degree to which calving laws are needed to accurately model retreat for future model intercomparison efforts. While testing calving laws independent of an ice sheet model will not provide insight into all the challenges of calving implementation for ice-sheet-wide studies, this remote-sensing based workflow can rapidly test calving laws’ terminus prediction errors. With the availability of monthly-averaged velocity data sets and frequent instantaneous DEMs, this method will allow for analysis of calving law success on many regimes of multi-year glacier movement.  

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.

12:05–12:15
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EGU24-3642
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ECS
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On-site presentation
Calving of the Greenland Ice Sheet since 1985
(withdrawn)
Chad Greene, Alex Gardner, Michael Wood, and Joshua Cuzzone
12:15–12:25
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EGU24-18561
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ECS
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On-site presentation
Rabea Sondershaus, Angelika Humbert, and Ralf Müller

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.

12:25–12:30
Lunch break
Chairpersons: Ronja Reese, Rachel Carr, Peter Washam
Ice-sheet and ice-shelf dynamics
14:00–14:10
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EGU24-5087
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ECS
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On-site presentation
Aminat Ambelorun and Alexander Robel

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.

14:10–14:20
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EGU24-3442
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On-site presentation
Douglas Benn, Jan Åström, Iain Wheel, Adrian Luckman, and Faezeh Nick

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.

14:20–14:30
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EGU24-19514
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ECS
|
Virtual presentation
Lielle Stern and Roiy Sayag

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.

14:30–14:40
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EGU24-11490
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ECS
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On-site presentation
Tom Mitcham, G. Hilmar Gudmundsson, and Jonathan L. Bamber

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.

14:40–14:50
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EGU24-6639
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ECS
|
On-site presentation
Hannah Verboncoeur, Matthew Siegfried, J. Paul Winberry, Nicholas Holschuh, Duncan Byrne, Wilson Sauthoff, Tyler Sutterley, and Brooke Medley

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.

14:50–15:00
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EGU24-12071
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ECS
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On-site presentation
Christian Wild, Reinhard Drews, Niklas Neckel, Joohan Lee, Kim Sihyung, Hyangsun Han, Won Sang Lee, Veit Helm, Oliver Marsh, and Wolfgang Rack

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.

15:00–15:10
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EGU24-22433
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On-site presentation
Michał Tympalski, Marek Sompolski, Anna Kopeć, and Wojciech Milczarek

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.

15:10–15:20
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EGU24-12334
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On-site presentation
Alison Banwell, Ian Willis, Laura Stevens, Rebecca Dell, and Douglas MacAyeal

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.

15:20–15:30
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EGU24-13188
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ECS
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On-site presentation
Karla Boxall, Ian Willis, Jan Wuite, Thomas Nagler, Stefan Scheiblauer, and Frazer Christie

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.

15:30–15:40
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EGU24-10590
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ECS
|
On-site presentation
Giacomo Traversa and Biagio Di Mauro

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.

5 min. discussion (3)

Posters on site: Fri, 19 Apr, 10:45–12:30 | Hall X4

Display time: Fri, 19 Apr, 08:30–Fri, 19 Apr, 12:30
Chairpersons: Ronja Reese, Peter Washam, Rachel Carr
X4.1
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EGU24-1569
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ECS
Isabel Nias, Felicity McCormack, Sue Cook, Susheel Adusumilli, Lu An, Daniel Goldberg, Tore Hattermann, Yoshihiro Nakayama, Hélène Seroussi, and Donald Slater

Mass loss from the Antarctic and Greenland Ice Sheets could lead to a rise in global mean sea level of 0.25 m by 2100 and several metres by 2300 if greenhouse gas emissions remain unmitigated. Uncertainties in these estimates are strongly related to ocean-driven ice melt, which can lead to grounding line retreat, thinning and acceleration of the fast-flowing regions of both Antarctica and Greenland. The processes of ocean-driven ice melt on large spatial and temporal scales are imperfectly known, and measurements are sparse, impacting the accuracy of ice sheet and ocean model projection studies. The Joint Commission on Ice-Ocean Interactions (JCIOI) hosted the first community workshop in October 2022 with the aims to: (1) identify critical knowledge gaps surrounding processes that govern ocean-driven melt of ice sheets across a range of spatio-temporal scales; and (2) identify options to address the knowledge gaps through observing, parameterising, and modelling ice-ocean interactions, and their impacts on ice mass loss and ocean dynamics. Community discussions from the workshop highlighted the need for concurrent and sustained measurements of ice, ocean and atmosphere properties at the ice sheet-ocean interface, and making best use of existing observations to improve models, capture observed changes, better understand physical mechanisms and improve future projections. Building on the workshop outputs, we propose to develop a framework for ice-ocean observations that details the essential measurements that need to be collected, and the temporal and spatial scales on which to measure. This framework will require widespread community engagement on key scientific questions, agreement and coordination, including protocols for data collection, processing, and sharing.

How to cite: Nias, I., McCormack, F., Cook, S., Adusumilli, S., An, L., Goldberg, D., Hattermann, T., Nakayama, Y., Seroussi, H., and Slater, D.: A framework for observing and modelling ice-ocean interactions building on a community workshop organised by the Joint Commission on Ice-Ocean Interactions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1569, https://doi.org/10.5194/egusphere-egu24-1569, 2024.

X4.2
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EGU24-9856
Basal melting of Antarctics ice shelves in Amundsen and Bellingshausen seas
(withdrawn)
Lamees Refat Felemban
X4.3
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EGU24-9876
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ECS
Alex Bradley, David Bett, Paul Holland, C. Rosie Williams, and Robert Arthern

Pine Island Glacier is a fast flowing ice stream in West Antarctica. At present, it is rapidly thinning and retreating, and has been since at least the 1970s, when satellite records began. Sediment records indicate that this retreat was initiated in the 1940s, but the influence of climate change on key forcing components only became significant in the 1960s, i.e. the trigger for retreat occurred naturally. However, current ice loss remains responsive to fluctuations in forcing, indicating that Pine Island Glacier is not undergoing a purely unstable retreat after this trigger. This begs the question: to what extent is climate change responsible for the recent retreat of the Pine Island Glacier?

Adopting a recently published framework, we assess this question. One major challenge is the computational expense associated with the large ensemble of simulations required to account for significant uncertainties in ice sheet model parameters; to overcome this, we use a two stage Ensemble Kalman Inversion and Model Emulation approach. Ultimately, this procedure yields posterior distributions of parameters, including the trend in forcing resulting from climate change; essentially, this allows us to address the question: given the observed Pine Island Glacier retreat, how large does the trend in forcing have to have been?

How to cite: Bradley, A., Bett, D., Holland, P., Williams, C. R., and Arthern, R.: Is climate change responsible for recent retreat of the Pine Island Glacier in West Antarctica?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9876, https://doi.org/10.5194/egusphere-egu24-9876, 2024.

X4.4
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EGU24-12347
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ECS
Carolyn Branecky Begeman, Irena Vaňková, Xylar Asay-Davis, Darin Comeau, Alex Hager, Matthew Hoffman, Matthew Maltrud, Courtney Shafer, and Jonathan Wolfe

Subglacial runoff beneath ice shelves is a source of freshwater, and therefore buoyancy, at the grounding line. Being released at depth, it accelerates an ascending plume along the ice-shelf base, enhancing entrainment of ambient waters, and increasing melt rates. By now it is understood that subglacial runoff is a key control on melt rate variability at the majority of Greenland's glaciers. However, its importance in present-day and future Antarctic melt rates is less clear.

To address this point, we use the Energy Exascale Earth System Model (E3SM) and investigate the effects of Antarctic freshwater volume flux addition in both idealized setups and realistic, global, sea-ice ocean coupled configurations. For realistic Antarctic configurations, we use the subglacial hydrology model from the MALI ice-sheet model with both distributed and channelized drainage run at 4-20 km resolution to calculate steady state subglacial discharge across the grounding line under historical ice-sheet conditions.  This meltwater discharge is implemented as a freshwater flux in MPAS-Ocean, the ocean component of E3SM.

How to cite: Branecky Begeman, C., Vaňková, I., Asay-Davis, X., Comeau, D., Hager, A., Hoffman, M., Maltrud, M., Shafer, C., and Wolfe, J.: The effects of including Antarctic subglacial meltwater flux to the ocean in the Energy Exascale Earth System Model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12347, https://doi.org/10.5194/egusphere-egu24-12347, 2024.

X4.5
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EGU24-17177
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ECS
Maxence Menthon, Pepijn Bakker, Aurélien Quiquet, and Didier Roche

The Antarctic ice sheet dynamics is primarily driven by basal melting under the ice shelves. The limitation of computational resources forces the usage of simplified parametrizations in ice-sheet models. Multiple parametrizations have been developed and implemented over the last years (Reese et al. 2018, Lazeroms et al. 2018, Pelle et al. 2019, Jourdain et al. 2020, etc.). The PICO module (Reese et al. 2018) demonstrates to be a good trade-off between complexity and computational resources for paleo ice-sheet reconstructions. Lately, Burgard et al. 2022 suggested that the implementation of a quadratic version of the PICO module could improve it significantly.

Here we test the implementation of the PICO module with a quadratic relationship between the thermal forcing and the melt in the GRISLI ice sheet model. We test a wide range of parameter values to calibrate the module, we compare the quadratic version of the module with the original version, under 2 different resolutions. Eventually, we show the results of simulations on paleo and future applications.

How to cite: Menthon, M., Bakker, P., Quiquet, A., and Roche, D.: Testing a PICO quadratic sub-shelf basal melt module in the GRISLI ice sheet model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17177, https://doi.org/10.5194/egusphere-egu24-17177, 2024.

X4.6
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EGU24-17430
Yuting Dong, Huimin Liu, Angelika Humbert, Ji Zhao, Dana Floricioiu, Lukas Krieger, Michael Wolovick, Thomas Kleiner, and Lea-Sophie Höyns

The Wordie Ice Shelf (WIS) in the Antarctic Peninsula (AP) has continued to retreat since 1966, and it almost completely disintegrated in the late 1990s. Although the main supply glacier of the WIS, the Fleming Glacier (FG), did not respond immediately, increases in the glacier velocity and dynamic thinning have been observed over the past two decades, especially after 2008 when only a small ice shelf remained at the Fleming Glacier front. As FG is now the fastest flowing outlet glaciers in the west Antarctic Peninsula, ice dynamics is the primary cause of mass loss. Basal sliding is the key mechanism for glacier acceleration and as it responds to thinning and changes in basal conditions. Furthermore, changes in ice-ocean interaction, such as changes in buttressing of ice streams and outlet glaciers like Fleming Glacier, are also leading to acceleration.

Here, we use the Shallow Shelf Approximation (SSA) implementation of the Ice-sheet and Sea-level System Model (ISSM) simulating the basal shear stress distribution of FG in the years 2008, 2011, 2014, 2017, 2019 and 2021 using inverse modelling. To better regularize the glaciological inverse problem, we adopt the latest published L-curve analysis to select the optimal regularization level. Considering Fleming Glacier has a relatively small drainage basin, high resolution geometric data is necessary to obtain better constrained information of the basal conditions. We use TanDEM-X DEMs acquired in austral winter of 2011, 2014, 2017, 2019, and 2021 to provide accurate glacier surface elevations. These DEMs were generated from bi-static InSAR data acquired by the TanDEM-X mission and are with the most complete time series and the best quality that can be obtained in this area at present.  We evaluate the existing ice velocity products and performed a spatio-temporal interpolation to obtain the average velocity of the year corresponding to the elevation data. We use the higher Antarctic ice sheet surface mass balance data RACM2.3p2 at 2 km resolution as a boundary condition. Regarding the bedrock topography, one of the main factors restricting the inversion accuracy, we evaluated all the existing subglacial topography data products within our inversions. To more accurately represent friction at the bed, we also tested Budd’s, Weertman’s and Schoof’s sliding laws, with different friction exponents and variable geometric data.

Comparison of simulated basal shear stresses for 2008 and 2021 suggests the migration of the grounding line 8~9 km upstream by 2021 from the 2008 ice front/grounding line positions. This migration is consistent with the change in floating areas deduced from the calculated height above buoyancy. Our results indicate that the reducing basal shear stress may be directly related to the subglacial hydrologic system and lead to rapid increases in basal sliding and ongoing ungrounding. It will further promote the dynamic loss of glaciers when coupled with ocean forcing and retrograde bedrock. 

How to cite: Dong, Y., Liu, H., Humbert, A., Zhao, J., Floricioiu, D., Krieger, L., Wolovick, M., Kleiner, T., and Höyns, L.-S.: The changes of basal conditions on Fleming Glacier, Antarctic Peninsula, between 2008 and 2021, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17430, https://doi.org/10.5194/egusphere-egu24-17430, 2024.

X4.7
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EGU24-9429
Huw Horgan, Natalie Robinson, Craig Stevens, Craig Stewart, Christina Hulbe, Justin Lawrence, Britney Schmidt, and Peter Washam

Melt beneath Antarctica’s large cold-cavity ice shelves remains a major source of uncertainty in ice sheet projections. Beneath these ice shelves melt is typically greatest both at the ice shelf front and at the grounding zone where ice first goes afloat. Grounding zone melt is thought to have a significant influence on ice flow across the grounding line, but can be difficult to estimate using remote sensing methods due to flexure of the overriding ice shelf. Added complexity in the grounding zone is caused by the thin water column, abundant basal crevassing, and the possible addition of subglacial fresh water draining from beneath the ice sheets. Here we present two independent estimates of basal melt from the ocean cavity of Kamb Ice Stream’s grounding zone, Ross Ice Shelf, West Antarctica. The first method uses repeat phase-sensitive radar observations to estimate melt in profiles from approximately 5 km seaward of the grounding line to approximately 3 km upstream of the grounding line. The second method uses an approximately 10-month long time series of oceanographic observations from a site 3.5 km seaward of the grounding line. Both methods are complemented by the high resolution observations provided by the Remotely Operated Vehicle (ROV) Icefin. The spatially distributed estimates show a more than tripling of melt rate within 5 km of the grounding line. The mooring derived melt rates demonstrate a melt-rate dependence on diurnal and spring-neap tidal currents. The average mooring melt rate more closely matches the radar-based estimates when a drag coefficient previously estimated using Icefin observations is used. Lastly we demonstrate an interesting correlation between mooring derived melt rates and ice shelf surface velocities obtained from Global Navigation Satellite System (GNSS) observations.

How to cite: Horgan, H., Robinson, N., Stevens, C., Stewart, C., Hulbe, C., Lawrence, J., Schmidt, B., and Washam, P.: Distributed and time-series estimates of basal melt from Kamb Ice Stream’s grounding zone ocean cavity, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9429, https://doi.org/10.5194/egusphere-egu24-9429, 2024.

X4.8
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EGU24-17109
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ECS
Morag Fotheringham, Noel Gourmelen, Michel Tsamados, and Donald Slater

Arctic glaciers and ice caps are currently major contributors to global sea level rise, with future projections showing a sustained input. The monitoring of these smaller land-ice masses is challenging due to the high temporal and spatial resolution required.

These glaciers and ice caps are losing mass in response to climate forcings, both atmospheric and oceanic. The relative significance of these forcings is currently unknown with most recent catagorisation focusing on separating loss caused by internal dynamics vs surface mass balance changes.

This leaves the specific roles of the atmosphere and the ocean unconstrained; this understanding is key to improving the accuracy of future loss of ice from these smaller land-ice masses and future sea level rise projections.

This study uses CryoSAT-2 swath interferometric radar altimetry to provide high spatial and temporal observations to produce elevation timeseries in order to evaluate the trends of mass loss. It also utilises an ocean thermal model, previously used to study Greenland's outlet glaciers, to gain a better understanding of the relative contributions of atmospheric and ocean forcings to this mass loss.

How to cite: Fotheringham, M., Gourmelen, N., Tsamados, M., and Slater, D.: Ice- Ocean- Atmosphere Interactions in the Arctic: Glaciers and Ice Caps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17109, https://doi.org/10.5194/egusphere-egu24-17109, 2024.

X4.9
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EGU24-10983
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ECS
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Joanna Zanker and Jan De Rydt

The Northeast Greenland Ice Stream (NEGIS) drains approximately 12 % of the Greenland Ice Sheet’s surface area, containing an ice volume of 1.1 m sea-level equivalent. Nioghalvfjerdsbræ (79NG) is one of two main outlet glaciers of NEGIS, extending into a large floating ice tongue, one of few remaining in Greenland. It is currently not well understood how 79NG will respond to the changing atmosphere and warming oceans, with possible implications for the catchment’s surface mass balance (SMB) and ocean-induced ablation. This research aims to assess the importance of feedbacks between ice-sheet geometry, SMB and ocean-driven melt by having a mutually evolving dynamical ice sheet with evolving SMB parameterization and a 3D ocean circulation model utilising the ice-ocean coupled model Úa-MITgcm. The potential feedbacks between changes in ice-sheet surface geometry, ice-tongue cavity geometry and the atmosphere/ocean mass balance are as-of-yet poorly understood, especially in the context of Greenland. Of particular interest for NEGIS is the potential for geometry induced changes in melting of the ice tongue, as found for some Antarctic ice shelves. Development of the Úa ice-flow model will begin with a Greenland-wide setup and experiments based on the ISMIP6 protocol, before focussing on a regional setup of the NEGIS catchment and coupling to a regional configuration of the MITgcm ocean model of the adjacent fjord and continental shelf. The coupled approach of this project aims to improve the representation of the feedbacks between different climate components at a regional scale and draw conclusions about the fidelity of projections of ice sheet-wide mass loss and sea-level rise from ISMIP. 

How to cite: Zanker, J. and De Rydt, J.: Ice-ocean coupled modelling for Nioghalvfjerdsbræ (79NG), Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10983, https://doi.org/10.5194/egusphere-egu24-10983, 2024.

X4.10
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EGU24-17302
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ECS
Lokesh Jain, Donald Slater, and Peter Nienow

Greenland’s marine-terminating glaciers have retreated and accelerated in recent decades, contributing significantly to sea level rise. An increase in ocean temperatures, and in particular the increased submarine melting of calving fronts, is often cited as the dominant driver of this retreat. However, the presence of ice mélange and its associated buttressing force on a glacier terminus also has a substantial impact on glacier advance and retreat. The buttressing force theoretically depends on the mélange thickness, and thickness will be modulated by ocean melt rate, but our understanding of mélange melting remains limited, and it is not yet known how melt rates vary across a range of glacial and environmental conditions.

Here, we perform high-resolution numerical simulations using MITgcm to model the melting of ice mélange. In order to map out the parameter space for mélange melting at Greenland’s marine-terminating glaciers, we vary each of the ocean temperature, ocean stratification, the flux of freshwater emerging from beneath the glacier (subglacial discharge) and the mélange geometry. We study how each factor affects the magnitude and distribution of ocean melt of the ice mélange and seek a parameterisation that would allow us to simply predict mélange melt rate. Furthermore, this work is also a step towards including iceberg melting in larger climate and ice sheet models which is important because of the need to improve the characterisation of freshwater fluxes into fjord systems.

How to cite: Jain, L., Slater, D., and Nienow, P.: Modelling ocean melt of ice mélange at Greenland's marine-terminating glaciers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17302, https://doi.org/10.5194/egusphere-egu24-17302, 2024.

X4.11
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EGU24-10402
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ECS
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Jonathan Wiskandt, Inga Monika Koszalka, and Johan Nilsson

Ocean forcing of basal melt at the Greenland and Antarctic ice sheets remains a major source of uncertainty in climate ice sheet modelling. Previous efforts to represent these effects focused mainly on the properties of the ocean waters reaching the marine terminating glaciers as well as the near-ice boundary layer flows and processes at the ice-ocean interface. We use high resolution, three dimensional modelling to show the influence that rotational effects have on the fjords circulation and the melt rate distribution and compare the total melt to earlier estimates from two dimensional simulations. Furthermore we investigate the influence that the along and across fjord bathymetry of Greenlandic glacial fjords has on the exchange flow of the warm ocean waters towards the ice sheets and the glacially modified water toward the open ocean. We find that the circulation pattern produced by rotational effects has a profound effect on the distribution of the melt rate at the ice base, producing a concentrated outflow and a melt maximum at the eastern side of a fjord that opens to the open ocean in the north even in narrow fjords (width of the order of the local Rossby Radius). The bathymetry in the fjord has a restricting effect on the inflow of warm Atlantic water and hence on the temperature forcing at the ice base. We compare the inflow strengths for different fjord bathymetries to theoretical estimateion using hydraulic theory (Whitehead, 1998).

How to cite: Wiskandt, J., Koszalka, I. M., and Nilsson, J.: Idealized, High Resolution, 3D Modelling of Ice-Sheet Ocean interactions in long and narrow fjords, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10402, https://doi.org/10.5194/egusphere-egu24-10402, 2024.

X4.12
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EGU24-14713
Andreas Vieli, Armin Dachauer, Dominik Gräff, Andrea Walter, Brad Lipovsky, Fabian Walter, and Ethan Welty

About half of the current rapid mass loss of the Greenland ice sheet (GIS) is through dynamic processes driven by calving and frontal ablation. However, related insitu observations in such dynamic environments are challenging and our process understanding is therefore still limited. Within the wider context of the GreenFjord-project on Greenland Fjord ecosystem we introduce here a multi-sensor approach for observing process interactions at high spatial and temporal resolution at the ice-ocean boundary of the major ocean-terminating outlet glacier Eqalorutsit Kangillit Sermiat (EKaS) in South Greenland.

Besides multiple all year-round time lapse cameras, broadband seismometers and tidegauges distributed around the glacier terminus and running since summer 2022, we deployed for the first time in summer 2023 a fibre optic cable at the fjord-bed along the calving front and performed continuous distributed acoustic and temperature sensing measurements (DAS and DTS) during more than two weeks. In parallel, we run a terrestrial radar interferometer (TRI) at 1min repeat intervals that recorded high resolution flow-fields as well as calving events (time, size and location). Our comprehensive observational approach is further complemented by local meteo-station data and more than 20 CTD profiles in the fjord near the calving front. In addition, two ocean bottom seismometers together with a simple CTD mooring have been deployed in summer 2023 and are planned to be recovered in the coming summer.

Besides our observational approach, we present here a broad overview and preliminary analysis of this unique observational dataset. We are not only able to record and cross-validate the same processes or events (e. g. calving and ice flow) from multiple sensors, but also clearly extend our observational ability (e. g. detection sensitivity, calving type and size, fjord circulation, spatial and temporal resolution).  We further get more insights into related subglacial and submarine processes such as fjord temperature variations, plume discharge and internal waves in the fjord. Our results thereby contribute to improve our understanding of ice-ocean interaction at a calving front and helps to develop sustainable observational systems of related processes.

How to cite: Vieli, A., Dachauer, A., Gräff, D., Walter, A., Lipovsky, B., Walter, F., and Welty, E.: Multi-sensor approach of monitoring ice-ocean interaction at high resolution at a major ocean-terminating glacier in South Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14713, https://doi.org/10.5194/egusphere-egu24-14713, 2024.

X4.13
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EGU24-10287
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ECS
Tian Li, Konrad Heidler, Adam Igneczi, Stefan Hofer, Xiao Xiang Zhu, and Jonathan Bamber

Arctic Amplification is making Svalbard one of the most climatically sensitive regions in the world and it has been undergoing accelerated mass loss over the past several decades. A major uncertainty in predicting the future sea-level rise contribution from marine-terminating glaciers is ice dynamics, which can be driven by non-linear calving processes. However, the relationship between calving and ice dynamics is not well understood in Svalbard, in part due to the lack of high-resolution calving front observations. To improve our understanding of the glacier calving dynamics and its relation to dynamic mass loss, here we use a novel fully automated deep learning framework to produce a new calving front dataset of 149 marine-terminating glaciers in Svalbard. This dataset, which includes 124919 glacier calving front positions from 1985 to 2023, has high spatial and temporal resolutions and is derived from multiple optical and SAR satellite images. We then use this new calving front dataset to systematically quantify the calving front change variabilities at different temporal scales, and identify the key climate drivers controlling the calving dynamics. We show that ocean forcing plays a central role in controlling the glacier calving front changes and mass imbalance. Our study highlights the importance of including ice-ocean interaction in projecting future glacier mass loss from Svalbard.  

How to cite: Li, T., Heidler, K., Igneczi, A., Hofer, S., Zhu, X. X., and Bamber, J.: Ocean-induced glacier retreat drives mass loss in Svalbard , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10287, https://doi.org/10.5194/egusphere-egu24-10287, 2024.

X4.14
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EGU24-15935
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ECS
Felicity Holmes, Jamie Barnett, Henning Åkesson, Johan Nilsson, Nina Kirchner, and Martin Jakobsson

The Greenland Ice Sheet is currently the largest single contributor to global sea level rise, with recent decades having been characterised by an acceleration of mass loss. The Northern sector of the Greenland Ice Sheet has been relatively understudied, but is also the sector containing several of the last remaining ice tongues in Greenland. If these floating ice tongues are lost, the associated reduction in buttressing has the potential to lead to large increases in velocities and mass loss. One such glacier is Ryder glacier which, in contrast to the nearby Petermann glacier, has been reasonably stable in recent decades. As such, this glacier was targeted during the Ryder 2019 expedition with Swedish Icebreaker Oden, leading to a wealth of data on its present-day setting and Holocene history. In conjunction with this observational data, the numerical Ice Sheet and Sea Level System Model (ISSM) is used to investigate both the controls on glacier behaviour since 1900 and the likely trajectory of Ryder glacier towards 2100 under different emissions scenarios. The key focus is on understanding under which circumstances Ryder glacier may lose its ice tongue and what the impacts of this are likely to be in terms of glacier dynamics and sea level rise contribution.

How to cite: Holmes, F., Barnett, J., Åkesson, H., Nilsson, J., Kirchner, N., and Jakobsson, M.: Modelling Ryder Glacier in Northern Greenland until 2100 under various emissions scenarios; Under which circumstances is the ice tongue lost?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15935, https://doi.org/10.5194/egusphere-egu24-15935, 2024.

X4.15
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EGU24-15939
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ECS
Jamie Barnett, Felicity Holmes, Henning Åkesson, Johan Nilsson, Nina Kirchner, and Martin Jakobsson

Coupling paleo numerical simulations of the Greenland Ice Sheet with physical geological evidence of past ice sheet extent can greatly improve our understanding of the factors driving ice loss. Geological observations can be used to reconstruct the state of the Greenland Ice Sheet at snap shots in time, thus acting as constraints to test the fidelity of ice sheet models that can tell a continuous story of retreat over the same geologic timescales. Swedish Ice Breaker Oden’s visit to Sherard Osborn Fjord and Ryder Glacier in 2019 collected a plethora of marine-geological data that describes the glacier’s behaviour and retreat during the Holocene. Here we use a 3D thermo-coupled Higher-Order ice flow module incorporated in the Ice-sheet and Sea-level System Model (ISSM) to simulate the dynamics of Ryder Glacier from 12500 ka to present day. By focusing on a specific individual glacier, we can run the model at resolutions <1km near the grounding line to shed light on the marine (calving and submarine melt) and atmospheric factors that potentially drove Ryder’s retreat from its Younger Dryas position. Of particular interest is understanding whether the glacier withdrew from its marine setting during the Holocene Thermal Maximum and what conditions were required for Ryder to regrow its modern-day ice tongue during the neoglacial cooling at the end of the Holocene.

How to cite: Barnett, J., Holmes, F., Åkesson, H., Nilsson, J., Kirchner, N., and Jakobsson, M.: Modelling the evolution of Ryder Glacier, Greenland, through the Holocene to investigate its responses to marine and atmospheric forcings., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15939, https://doi.org/10.5194/egusphere-egu24-15939, 2024.

X4.16
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EGU24-3138
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ECS
Armin Dachauer, Andrea Kneib-Walter, and Andreas Vieli

Frontal ablation at tidewater outlet glaciers is responsible for a major part of mass loss of the Greenland Ice Sheet. This underscores the need to understand the underlying processes, such as calving and ice flow, with regard to global sea level rise. In this study we explore the temporal and spatial variability of calving activity and ice flow at the major tidewater outlet glacier Eqalorutsit Kangilliit Sermiat (also referred to as Qajuuttap Sermia) in South Greenland and thereby try to get insights into the forcing and relationships between these two processes. This requires high-resolution data which we achieve by using a terrestrial radar interferometer. The instrument provides a temporal resolution of 1 minute and a spatial resolution of a few meters and was running continuously for a two-week field period in August 2023. The data shows considerable spatial and temporal variability of both calving activity and ice flow. Parts of the flow variability can be attributed to a diurnal cycle that is forced by surface melt, whereas enhanced calving activity seems to be tightly linked to locations of major subglacial discharge plumes.

How to cite: Dachauer, A., Kneib-Walter, A., and Vieli, A.: Variability of calving and ice flow during a two-week period using terrestrial radar interferometry, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3138, https://doi.org/10.5194/egusphere-egu24-3138, 2024.

X4.17
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EGU24-9102
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ECS
Zhenfu Guan, Yan Liu, Teng Li, and Xiao Cheng

Ice calving around Antarctica has a significant impact on glacier dynamics, sea ice, and marine productivity, which in turn affect global sea level and climate.  However, there is limited documented knowledge of the causes of ice calving triggered by internal ocean processes throughout Antarctica, especially during the austral winter.  A total of 3708 iceberg calving events were observed along the circum-Antarctic coastline over a three-month winter period.  These events included the calving of ice cliffs, ice shelves, and icebergs, spanning seven orders of magnitude in spatial scale.  The results suggest that ice cliff calving is primarily driven by internal glacier stresses and is widespread along the Antarctic coast.  The frequency of calving is primarily controlled by glacier ice velocity.  About 70% of the calving in Antarctica occurs on the Antarctic Peninsula.  Internal waves generated by ice cliff calving cascade to small enough scales to induce shear that leads to near-field (~40 km) calving of floating ice shelves and icebergs in regions of high topographic relief.  This study presents a newly discovered mechanism for ice shelf and iceberg calving driven by oceanic forces.  The mechanism has broad applicability and can serve as a catalyst for calving modeling and the study of oceanic internal waves.

How to cite: Guan, Z., Liu, Y., Li, T., and Cheng, X.: Calving of floating ice shelves and icebergs in Antarctica triggered by internal ocean waves driven by marine ice-cliff, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9102, https://doi.org/10.5194/egusphere-egu24-9102, 2024.

X4.18
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EGU24-9500
Robert Arthern, Jakub Stocek, and Oliver Marsh

Iceberg calving accounts for around half of the ice lost annually from Antarctica, but realistic representation of fracture and calving in large-scale ice sheet models remains a major unsolved problem in glaciology. We present a new phase-field viscoelastic model for fracture that simulates the slow deformation of ice and the distribution and evolution of cracks. Cracks nucleate and propagate in response to the evolving stress field, and are influenced by water pressure below sea level. The model incorporates nonlinear-viscous rheology, linear-elastic rheology, and a phase-field variational formulation, which allows simulation of complex fracture phenomena. We show that this approach is capable of simulating the physical process of calving. Numerical experiments supported by a simplified model suggest that calving rate will scale with the fourth power of ice thickness for a floating ice front that has no variation across flow. The equations make no assumptions about the style of calving, so they would also simulate numerous more realistic settings in Antarctica for which material parameters and three-dimensional effects can be expected to influence the calving rate.

How to cite: Arthern, R., Stocek, J., and Marsh, O.: A viscoelastic phase-field model for iceberg calving, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9500, https://doi.org/10.5194/egusphere-egu24-9500, 2024.

X4.19
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EGU24-11023
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ECS
Emma Carr, Rachel Carr, Chris Stokes, Emily Hill, Hilmar Gudmundsson, and Neil Ross

Many tidewater glaciers in Greenland terminate in near-vertical ice cliffs from which icebergs calve. Marine Ice Cliff Instability (MICI) is the hypothesis that above a subaerial ice cliff height limit, the tensile or shear stresses at the glacier terminus surpass the ice yield strength, causing catastrophic cliff failure and self-sustaining ice frontal retreat as sequentially taller subaerial cliffs are exposed. Previous modelling studies have proposed this threshold subaerial cliff height is at least 100 m, with estimated thresholds including 100 m and 110 m for damaged ice, and up to 540 m when ice is treated as undamaged. However, modern-day observations to test MICI are limited because few marine-terminating outlet glaciers without a buttressing ice shelf are known to terminate in subaerial ice cliffs greater than 100 m high. Here, we expand the observations of subaerial ice cliff heights at ten marine-terminating outlet glaciers in northwest Greenland using 2 m spatial resolution Arctic DEM strips. Our results identify three marine-terminating outlet glaciers that currently terminate in exposed subaerial ice cliffs approaching or exceeding the stability thresholds estimated for MICI. During at least two years between 2016 and 2021, subaerial ice cliffs at Nuussuup Sermia (NuS), Nunnatakassaap Sermia (NkS) and Sermeq North (SqN) exceeded heights of 100 m and 110 m. Despite being above these postulated thresholds thought conducive for cliff failure, SqN underwent relatively limited net retreat (0.25 km), and NuS and NkS exhibited distinct seasonal cycles of terminus advance (up to 0.92 km) from March to June/July each year prior to the disintegration and removal of proglacial ice mélange. Consequently, none of the glaciers identified as potentially susceptible to MICI underwent rapid, unforced retreat. We hypothesise that MICI processes were mitigated by dynamic thinning lowering the ice surface elevation immediately up-glacier of the ice cliff so that progressively taller subaerial cliffs were not exposed after retreat. Further research is required to monitor and model the evolution of subaerial ice cliffs to better understand the potential for unstable retreat in West Antarctica due to MICI.

How to cite: Carr, E., Carr, R., Stokes, C., Hill, E., Gudmundsson, H., and Ross, N.: Spatiotemporal evolution of subaerial ice cliff heights at marine-terminating outlet glaciers in Northwestern Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11023, https://doi.org/10.5194/egusphere-egu24-11023, 2024.

X4.20
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EGU24-11541
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ECS
Emily Hill, G. Hilmar Gudmundsson, and David Chandler

Changes in ocean conditions surrounding the Antarctic ice sheet, and the impact on melt rates beneath buttressing ice shelves, is one of the largest sources of uncertainty in future ice loss projections. If conditions were to suddenly undergo a regime-shift from cold to warm, melt rates could increase drastically and trigger large and potentially irreversible changes in the interior of the ice sheet. Here, we take an ensemble of ocean-circulation model melt rates as input to an ice-sheet model, to quantify ice loss and the potential for irreversible retreat under such warm conditions. We find that the currently cold-cavity basins of the Filchner-Ronne and Ross ice shelves, in contrast to present-day, could become large contributors to future sea level relevant ice loss. In major basins in West Antarctica, we find high-melt rates can trigger instances of irreversible grounding line retreat, which could only be recovered if arguably unattainable melt rate conditions prevailed over timescales of 100s of years.

How to cite: Hill, E., Gudmundsson, G. H., and Chandler, D.: Regime-shifts in ice-shelf melt could trigger irreversible ice loss from the Antarctic Ice Sheet, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11541, https://doi.org/10.5194/egusphere-egu24-11541, 2024.

X4.21
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EGU24-12332
Shfaqat Abbas Khan, Mathieu Morlighem, Youngmin Choi, Shivani Ehrenfeucht, Eric Rignot, Angelika Humbert, and Javed Hassan

The dynamics of The North East Greenland Ice Stream (NEGIS) are influenced by various factors such as ice thickness, topography, basal conditions, and surface meltwater inputs. The presence of basal lubrication significantly affects NEGIS ice flow by reducing friction at the ice-bed interface. Consequently, alterations in subglacial hydrology and the prevalence of meltwater can result in significant variations in ice stream velocity and mass discharge. In this study, we utilize GPS data from six stations along the main trunk to identify the inland propagation of summer speed-ups, peaking between June and August. Complementing the GPS data, we incorporate ice speed information from mosaics based on ESA Sentinel-1 SAR offset tracking, covering the entire NEGIS. These velocity maps, derived from intensity-tracking of ESA Sentinel-1 data with a 12-day repeat and utilizing the operational interferometric post-processing chain IPP for analysis, reveal substantial acceleration in surface speed from June onwards, followed by a deceleration in August. To simulate the observed summer speed-up, we employ the Ice-sheet and Sea-level System Model (ISSM). Our model results indicate that hydrology is the primary driver of the summer speed-up, leading to changes in speed that extend deep into the interior, reaching over 150 km inland. Understanding the dynamics of NEGIS is essential for predicting its future behavior and potential contributions to sea level rise in a warmer climate with increased meltwater.

How to cite: Khan, S. A., Morlighem, M., Choi, Y., Ehrenfeucht, S., Rignot, E., Humbert, A., and Hassan, J.: Summer speedup at Zachariæ Isstrøm, northeast Greenland, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12332, https://doi.org/10.5194/egusphere-egu24-12332, 2024.

X4.22
|
EGU24-12499
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ECS
Luisa Wagner, Martin Rückamp, and Johannes Fürst

Ice shelves on the western Antarctic Peninsula have partially or completely disappeared due to widespread thinning and retreat. The loss of floating ice results in a reduction of the buttressing on the upstream grounded ice body. As a consequence, tributary glaciers are accelerating and retreating further, leading to increased ice discharge and, in turn, an increased contribution to sea-level rise. Improving projections of the rate of sea-level rise from the area demands an in-depth understanding of the current mechanisms at play.

In order to gain this, we aim to quantify and characterise the buttressing effect of the ice shelves. To achieve this, we model hypothetical upper-end scenarios by either an immediate complete collapse of all floating ice or a sustained extreme melting. The main focus here is on the stability of the tributary glaciers and the ability of the ice shelf to rebuild itself.

To run the scenarios, we operate ISSM based on surface and basal topography from BedMachine and MEaSURE velocities. A Shallow-Shelf-Approximation with Budd and Weertman sliding laws, Beckmann and Goosse basal forcing parameterisation and von Mises calving is used. To initialise the retreat scenarios, we determine the basal friction coefficient of the grounded area and the ice shelf rheology using a joint inversion technique with regularisation.

How to cite: Wagner, L., Rückamp, M., and Fürst, J.: Role of buttressing in the dynamic response to Western Antarctic Peninsula ice shelf collapse, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12499, https://doi.org/10.5194/egusphere-egu24-12499, 2024.

X4.23
|
EGU24-12886
Faezeh M. Nick and Doug Benn
The crevasse depth (CD) calving law predicts the position of glacier termini from the penetration of surface and basal crevasses computed from stresses in the ice. When applied to Greenland tidewater glaciers, it has high skill when implemented in a full-Stokes 3D model, although its performance in 2D and 1D models is still subject to debate, especially its ability to induce ice shelf calving without the addition of unrealistic amounts of  water in surface crevasses.  This study re-evaluates the CD law within a 1D flowline model of an ice shelf.
 
We show that the model predicts deep crevasse penetration at locations where drag at the shelf boundaries diminishes,such as the grounding line or embayment mouths. Crevasse depth depends on the rate at which these resistance sources decrease along-flow, influencing the longitudinal stress gradient. While full-depth penetration may occur in thinned shelves (due to extensive basal melt), full-depth calving is generally not predicted for unconfined ice shelves. Observations of Antarctic ice shelves and floating ice tongues well beyond embayments or basal pinning points suggest that additional triggers, like slow rift growth, basal melting, or oceanographic stresses, are essential for calving.
 
The addition of water to surface crevasses can greatly facilitate calving. In some cases, reflecting real-world conditions, such as the hydrofracturing-induced collapse of vulnerable ice shelves. However, the need for water-depth tuning in other situations has raised concerns about the physical fidelity of the model. We propose a modified stochastic CD calving criterion in which the probability of calving ramps from zero for a threshold crevasse depth to one for full-depth penetration. This non-deterministic approach captures the statistical structure of calving events, and allows a range of observed behaviours to emerge, such as long Antarctic ice shelf calving cycles (ice-tongue advance punctuated by rare calving events), and short-term fluctuations of tidewater glaciers (frequent calving retreat back to pinning points). We argue that a probabilistic approach represents an important step towards a universal calving law.  

How to cite: M. Nick, F. and Benn, D.: The Crevasse Depth Calving Law Applied to Ice Shelves: Insights from a 1D Flowline Model , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12886, https://doi.org/10.5194/egusphere-egu24-12886, 2024.

X4.24
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EGU24-11297
Reinhard Drews, Falk Oraschewski, M. Reza Ershadi, Jonathan Hawkins, Christian Wild, Rebecca Schlegel, Inka Koch, Ole Zeising, and Olaf Eisen

Ekström Ice Shelf is a representative ice shelf for the ice-shelf belt of the Dronning Maud Land Coast in East Antarctica. It has cold ocean-cavity with moderate basal melt rates averaging a few meters per year across the ice shelf. In spite of the comparatively small average basal melt rates, we find basal terraces in a ground-penetrating radar dataset revealing near-vertical walls of more than 30 meters height. Such features have also been observed  elsewhere and linked to large localized basal melt rates which is in parts oriented in the horizontal direction. Here we use a ground-penetrating radar dataset with a profile spacing of <100 m which was revisited in an Eulerian sense in two consecutive field seasons 2021 and 2022. This dataset images the 3D extent of basal terracing and shows that these are remarkably stable and can be clearly identified in both seasons. They are  laterally offset  by along-flow advection and possibly also horizontal basal melting oriented perpendicular to the vertical walls. There is very little vertical difference between both datasets which is consistent with the small sub-daily melt rates derived from a continuously measuring ApRES located above a horizontal plateau linking two basal terraces at the ice base. These two 3D time slices are a unique dataset to better understand how such basal terraces initially form, how they are maintained over time and whether or not ocean-induced melting in the horizontal direction (which is typically not picked up by the ApRES data) is relevant on larger spatial scales.

How to cite: Drews, R., Oraschewski, F., Ershadi, M. R., Hawkins, J., Wild, C., Schlegel, R., Koch, I., Zeising, O., and Eisen, O.: Temporal evolution of basal terraces at Ekström Ice Shelf, East Antarctica , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11297, https://doi.org/10.5194/egusphere-egu24-11297, 2024.

X4.25
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EGU24-16868
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ECS
Dakota Pyles, Nora Gourmelon, Vincent Christlein, and Thorsten Seehaus

Frontal ablation is an important component of tidewater glacier mass loss, however, high temporal resolution estimates have remained elusive due to difficulty in reliably capturing terminus position changes with satellite imagery. Recent developments in machine learning-based radar image segmentation to automatically delineate glacier fronts has opened an opportunity to calculate frontal ablation over fine timescales. Through segmentation of Sentinel-1 synthetic aperture radar image sequences, we aim to quantify seasonal and annual frontal ablation across several Arctic regions, using a deep learning-based terminus segmentation algorithm. Svalbard, an Arctic region characterized by variable and complex glacier and fjord geometries, will serve as a methodological test site before expanding the scope to the Canadian Arctic, Greenland periphery, and Russian Arctic, or ~1400-1500 marine-terminating glaciers in the Northern Hemisphere. The derived frontal ablation information is valuable to climate and glacier models, which could benefit from high-resolution reference data, resulting in improved calibrations and parameterizations. Future project efforts will include quantifying total mass budget for all glaciers in the study by integrating frontal changes, ice discharge calculations from ice thickness and surface velocity products, and climatic mass balance data. To identify and evaluate external drivers of glacier change, the frontal ablation and mass balance products will be combined with modeled and observational atmospheric, oceanic, and sea ice data. Through multivariate statistical analyses between these Earth system datasets and mass balance components, we look to provide an improved understanding of dynamic tidewater glacier processes, their spatio-temporal variability, and the influence of glacier geometry on observed changes throughout the Arctic.

How to cite: Pyles, D., Gourmelon, N., Christlein, V., and Seehaus, T.: Large-scale and High-resolution Frontal Ablation Estimates in the Arctic through a Machine Learning Approach, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16868, https://doi.org/10.5194/egusphere-egu24-16868, 2024.

X4.26
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EGU24-18382
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ECS
Donald Slater, Doug Benn, and Till Wagner

The complexity of the processes and the difficulty of collecting observations mean that the treatment of the ice-ocean boundary remains one of the most challenging aspects of running models of the Greenland ice sheet. With geometry, climate forcing, ice properties and feedbacks between these factors all playing a role, tidewater glaciers display a range of calving styles that are hard to capture within the simple parameterisations that are necessary for large-scale ice sheet modeling.

Here we attempt to place some dominant calving styles within a single framework. We study submarine melt undercut-driven calving using linear elastic fracture mechanics within 2D elastic simulations, together with analytical approaches to calving driven by the intersection of basal and surface crevasses and to ice cliff failure. Taken together, these approaches give a prediction of calving style as a function of the calving front ice thickness, ocean depth and submarine melt undercut length, or equivalently as a function of the frontal tension, bending moment and shear. We discuss possible implementations in ice sheet models.

How to cite: Slater, D., Benn, D., and Wagner, T.: An attempt to capture diverse tidewater glacier calving styles within a single framework, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18382, https://doi.org/10.5194/egusphere-egu24-18382, 2024.