CR4.3

Ice shelves and tidewater glaciers - dynamics, interactions, observations, modelling

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.

Co-organized by OS1
Convener: Inga Monika Koszalka | Co-conveners: Nicolas Jourdain, Adrian Jenkins, Rachel Carr, Angelika Humbert
Presentations
| Tue, 24 May, 15:10–18:29 (CEST)
 
Room E2, Wed, 25 May, 08:30–11:37 (CEST)
 
Room E2

Presentations: Tue, 24 May | Room E2

Chairpersons: Nicolas Jourdain, Rachel Carr
15:10–15:15
15:15–15:21
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EGU22-3613
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Virtual presentation
Clara Henry, Clemens Schannwell, Vjeran Višnjević, and Reinhard Drews

The Antarctic contribution to sea level projections remains poorly constrained, particularly due to the complex dynamical response of the ice sheet to changes in external forcing in coastal regions. In our study we investigate ice rises and ice rumples, features which form in ice shelves where ice is locally grounded due to elevated bed topography. As a consequence, upstream ice is buttressed, regulating the flow of ice. Ice rises and ice rumples differ from one another in their characteristic flow regimes, with ice rises having a local, radial flow regime and ice rumples having a flow regime predominantly aligned with that of the surrounding ice shelf. Ice rises cause the surrounding ice shelf to flow either side of the feature and thereby cause a greater degree of buttressing.

Using a three-dimensional, isothermal,  full Stokes, idealised model setup (Elmer/Ice), we investigate the response of ice rises and ice rumples to sea level change, mimicking a glacial cycle. During sea level increase, a transition from ice rise to ice rumple occurs, and with a subsequent decrease in sea level, hysteretic behaviour is observed, i.e. the current grounded area, dome position and flow regime are dependent on the past state of the system. The hysteretic behaviour seen in the ice rise-rumple system is reflected in the upstream ice shelf and is likely to have an effect on continental grounding line dynamics. These findings have important implications for the initialisation and transient simulation of ice rises and ice rumples within continental-scale ice sheet models given that the evolution of these features is important for the timing and magnitude of sea level projections.

How to cite: Henry, C., Schannwell, C., Višnjević, V., and Drews, R.: Ice rise and ice rumple dynamics, and the consequences for ice sheet evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3613, https://doi.org/10.5194/egusphere-egu22-3613, 2022.

15:21–15:27
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EGU22-5462
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ECS
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On-site presentation
Benjamin Wallis, Anna Hogg, Benjamin Davison, and Michiel van den Broeke

In Antarctica dynamic ice loss dominates the continent’s contribution to sea level rise and the magnitude of dynamic ice loss depends in part on the ice speed at marine-terminating glacier grounding lines. Long term dynamic ice speed variations in Antarctica have been observed on multi-year timescales, most notably in ice speed increases in the Amundsen Sea sector, Getz basin and Antarctic Peninsula. Glacier and ice sheet speed can also be variable on seasonal timescales, due to surface meltwater-induced variations in basal water pressure and changes in the force balance at the terminus due to terminus advance and retreat. While these seasonal changes are well documented on the Greenland Ice Sheet, observations of seasonal ice speed changes in Antarctica are sparse and poorly resolved.

In this study, we show widespread seasonal ice speed fluctuations near the termini of 106 tidewater outlet glaciers across Western Antarctic Peninsula North of 70° S by exploiting the full Sentinel-1 record from 2014 to 2021. The seasonal speed variations were consistent each year, and are characterised by a summertime speed-up, with speed variability on average 13 ± 6.5% of the annual mean. There is good agreement between our observations of seasonal ice speed changes and time-series of potential forcing mechanisms, including surface water flux, terminus position change and reanalyses of ocean temperature. Our results demonstrate that the glaciers of the Western Antarctic Peninsula are sensitive to forcing in the ice-ocean-atmosphere system on seasonal timescales.

By observing widespread seasonal ice speed variations on the Antarctic Peninsula for the first time, we demonstrate a previously unknown sensitivity of part of the Antarctic Ice Sheet to external forcing over short timescales. This is particularly relevant for mass balance calculations by the input-output method, which typically rely on annual estimates of ice speed that do not capture these seasonal changes. Our dataset covers the Sentinel-1 epoch (2014-present), however the Antarctic Peninsula has undergone the greatest warming of any Southern Hemisphere terrestrial area in the latter twentieth century and atmospheric temperatures are projected to rise further in a 1.5°C warming scenario. Therefore, it is essential to understand the historic prevalence of seasonal speed changes on the Peninsula and to determine the impact of these seasonal variations on annual ice motion, to improve future projections of the Antarctic response to continued warming and its contributions to sea level rise.

How to cite: Wallis, B., Hogg, A., Davison, B., and van den Broeke, M.: Seasonal ice velocity variability of Western Antarctic Peninsula tidewater glaciers from high temporal resolution Sentinel-1 imagery, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5462, https://doi.org/10.5194/egusphere-egu22-5462, 2022.

15:27–15:33
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EGU22-11898
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ECS
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On-site presentation
Bradley Reed, Hilmar Gudmundsson, Mattias Green, and Adrian Jenkins
Pine Island Glacier (PIG), in West Antarctica, has undergone dramatic changes in the last few decades, where flow speeds have increased by 75% and grounding lines have retreated over 30km. These recent changes are part of a long term trend of mass loss, believed to have been initiated following climate anomalies in the 1940s and 1970s. The ice shelf cavity first opened around 1945, shortly after a strong El Niño event, and PIG eventually ungrounded from a submarine ridge in the early 1970s, following another notable warm period. Observational records show that intermittent periods of cooler ocean conditions likely slowed the subsequent retreat but were not enough to reverse its progress. 
 
Here we use the ice-flow model Úa to study the recent transient evolution of PIG over the last several decades with the aim of identifying the drivers of observed changes in geometry and grounding line position. We use a depth-dependent melt rate parameterisation driven by present day melt values to represent warm conditions, while experimenting with various cold parameterisations. We ask what happens when the model is forced with alternating periods of cool and warm conditions when PIG is grounded at the submarine ridge. We hypothesise that warm ocean conditions will force the ice stream off the ridge and cooler conditions will slow but not stop the retreat. This work will improve the understanding of how glaciers respond to short, intense warm intervals particularly as El Niño events become more frequent in a warming future. We present the results from the initial investigations into how PIG responds to ocean forcing using an ice flow model. 

How to cite: Reed, B., Gudmundsson, H., Green, M., and Jenkins, A.: Modelling the reversibility of Pine Island Glacier, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11898, https://doi.org/10.5194/egusphere-egu22-11898, 2022.

15:33–15:39
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EGU22-4904
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On-site presentation
Olga Sergienko

On timescales longer than several months, ice flow is treated as a non-Newtonian fluid, which viscosity depends on the second invariant of the strain-rate tensor and the temperature-dependent ice-stiffness parameter. This power-law dependence is known as Glen's flow law. Although results of laboratory experiments and inferences from in situ observations suggest a range of the power-law exponent n  from 1 to 5, the value of 3 is widely used. In studies focused on ice-shelf dynamics, the traditional approach is to use remote-sensing observations to infer the ice-stiffness parameter by means of inverse methods assuming a constant value of n=3. Focusing on the floating tongue of Pine Island Glacier, the inversions of the ice-stiffness parameter are performed for various constant as well as spatially variable values of n using present-day observations. Using the inferred parameters and basal melting derived from remote-sensing observations, the Pine Island Glacier Ice Shelf flow is simulated for hundred years. Results of simulations indicate that the effects of rheological parameters are of the order of 5%. The difference between results of hundred years simulations with observationally derived  and spatially uniform basal melting are of the order of 40%. These results indicate that on centennial timescales the ice-shelf flow is more sensitive to details of basal melting than to rheological parameters, provided the latter are constrained by observations.

How to cite: Sergienko, O.: The effects of rheological parameters on ice-shelf flow on centennial time scales., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4904, https://doi.org/10.5194/egusphere-egu22-4904, 2022.

15:39–15:45
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EGU22-13009
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Virtual presentation
Ian Hewitt

Marine ice sheets have the greatest potential to contribute to rapid sea-level change because of the potential for rapid transfer of grounded ice (with thickness above flotation) to floating ice shelves or icebergs.  Ice loss occurs when this transfer (the 'grounding-line flux') is larger than the rate at which ice accumulates over the grounded ice sheet.  It is well known that the balance between accumulation and grounding-line flux can result in both stable and unstable ice-sheet states, and that this leads to the potential for 'marine-ice-sheet-instability' (MISI).   Various reduced mathematical models have been used to examine the stability of steady states, accounting for different parameterisations of basal drag, lateral buttressing and ice-shelf melting/calving.  However, real-world ice sheets are (presumably) rarely in a steady state, since the timescales taken for an ice-sheet to reach steady state are typically thousands of years, longer than the timescales on which the forcing changes.  The time-dependent dynamics are therefore important. 

Here, we detail a simple depth- and width- integrated model for a marine ice sheet that yields insight into the time-dependent dynamics that result from changing climate forcing.  The model - which reduces to a relatively simple dynamical system - demonstrates how gradual changes in forcing (surface accumulation, ocean temperature, for example) cause changes in the 'landscape' through which the ice-sheet evolves.  It reproduces some existing results for how the stability of steady states depends on the topography, as well as new results for the pace of groundling advance and retreat.  Investigating fundamental aspects of the time-dependent dynamics in a simplified model like this is important in order to understand the extent to which ice-sheet changes are 'irreversible' (or not). 

How to cite: Hewitt, I.: A semi-analytical model for marine ice sheet dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13009, https://doi.org/10.5194/egusphere-egu22-13009, 2022.

15:45–15:51
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EGU22-6193
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ECS
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On-site presentation
Tom Mitcham, G. Hilmar Gudmundsson, and Jonathan L. Bamber

The future viability of the Larsen C Ice Shelf (LCIS) has been called into question following the collapse of its more northerly, neighbouring ice shelves on the Antarctic Peninsula, and the calving of the A68 iceberg in July 2017. Initially, using the ice-flow model Úa, we conduct time-independent experiments and find that the vast majority of the buttressing capacity of the LCIS is generated in the regions of the ice shelf just downstream of the grounding line. We also find that the Bawden and Gipps Ice Rises provide a negligible proportion of the total buttressing capacity of the ice shelf, as determined by modelled instantaneous changes in grounding line flux (GLF) in response to their removal.

We then conduct time-dependent experiments to examine the transient evolution of the LCIS and its tributary glaciers to changes in ice-shelf buttressing. We present, for the first time, simulations of the transient response of the system to the loss of basal contact at the Bawden and Gipps Ice Rises.  We find that the instantaneous increase in ice-shelf velocities is sustained throughout the 100-year model run, with associated dynamic thinning of the ice shelf on the order of tens of metres during this period. However, we find that the impact on the grounded ice dynamics, GLF and ice volume above flotation (VAF) is limited.

Through idealised calving experiments we show that the instantaneous response in GLF to a reduction in ice-shelf buttressing decays rapidly in the first few years following the calving event. We also find an increasing, but non-linear, relationship between the reduction in ice-shelf buttressing and the loss of VAF after 100 years, largely controlled by the bedrock topography of the tributary glaciers. With our model setup, using the BedMachine Antarctica v2 ice thickness and bedrock topography data, we find that the dynamic mass loss 100 years after the complete collapse of the LCIS is ~0.6 mm SLE.

How to cite: Mitcham, T., Gudmundsson, G. H., and Bamber, J. L.: The response of the Larsen C Ice Shelf to changes in ice-shelf buttressing, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6193, https://doi.org/10.5194/egusphere-egu22-6193, 2022.

15:51–15:57
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EGU22-12475
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ECS
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Virtual presentation
Sarah F. Child, Ted Scambos, Winnie Chu, Ella Stewart, and Karen Alley

For almost three decades, the Dotson and Crosson Ice Shelves have withstood various degrees of velocity increases, basal melt, weakening shear margins, and grounding line retreat. The two ice shelves, located along the Amundsen Sea Embayment, are fed primarily by Kohler, Smith, and Pope Glaciers. The ice shelves were once thought to be separated by a landmass connecting Bear Island to the mainland, but there is no evidence from 75 years of available data supporting this partition. Recent research shows that the changes in dynamics undergone by both ice shelves and their outlet glaciers are due largely to basal melt driven by warm circumpolar deep water (CDW); however, the scope of those changes varies between the two sectors with the Crosson Ice Shelf experiencing the more extreme transformation. Observations from trimetrogon aerial imagery (late-1940s) and Landsat Thematic Mapper data (mid-1980s) reveal the northern edge of the Crosson Ice Shelf terminating at Holt Glacier and its southeastern edge buttressing against Haynes Glacier. Studies show that in the mid-1980s, a decrease in sea ice concentrations led to the migration of the detached Thwaites Ice Tongue which, with fast ice, aided in containing fragments of Thwaites Glaciers—around this time the retreat of Haynes Glacier’s ice tongue also began. We hypothesize it is this decrease in buttressing from ~35 years ago that began a continuous trend (still observed today) in ice shelf thinning, initiation of rifts, and outlet glacier speed increases and grounding line retreats. In contrast, the Dotson Ice Shelf is flanked by Bear Island and Martin Peninsula and has undergone less dramatic alterations in dynamics than the Crosson Ice Shelf. We quantify the impact of buttressing on the two connected ice shelves with the following analyses: extended temporal scale of outlet glacier hypsometry from 1960-the present; detailed 16-year study of grounding line migrations and hydrostatic equilibrium boundaries using CReSIS MCoRDS/2 level one data; estimated shear stresses from multi-year velocities; modeled back stresses acting on both ice shelves. Results of this study will help to improve modeling ocean-ice sheet interactions and better constrain CDW impacts.

How to cite: Child, S. F., Scambos, T., Chu, W., Stewart, E., and Alley, K.: Historical buttressing effects on present-day Dotson and Crosson Ice Shelf dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12475, https://doi.org/10.5194/egusphere-egu22-12475, 2022.

15:57–16:03
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EGU22-10529
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ECS
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On-site presentation
Simon Schöll, Ronja Reese, and Ricarda Winkelmann

The increasing dynamical loss of grounded ice in response to thinning of surrounding ice shelves is the main driver of the current sea level rise contribution of Antarctica. The observed acceleration of the ice streams is caused by reduced buttressing of the ice shelves connecting the grounded ice flow to the warming ocean. Several methods have been used to analyze the back-stress of the ice shelves at individual grounding line locations, however none of those quantify the state of the whole shelf. Here we present shelf-wide definitions of buttressing for major Antarctic ice shelves, based on the stress-balance at the grounding line, that respond consistently to ocean warming. We use the Parallel Ice Sheet Model (PISM) at 8km grid resolution and diagnostic output from Úa with a resolution of 200m at the grounding line. We show an increase in buttressing for more confined ice shelves and a decrease under idealized ocean warming. With the shelf-wide buttressing, the role of buttressing in the (de-)stabilizing capabilities of ice shelves on marine ice streams can be investigated.

How to cite: Schöll, S., Reese, R., and Winkelmann, R.: Characteristic buttressing of Antarctic ice shelves, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10529, https://doi.org/10.5194/egusphere-egu22-10529, 2022.

16:03–16:09
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EGU22-12225
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ECS
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Virtual presentation
Jakub Stocek, Robert Arthern, and Oliver Marsh

Antarctic ice sheets grounded under the sea level can break apart if the ice cliffs at the edge of ice shelves collapse under their own weight. The process is known as the marine ice cliff instability and could lead to a rapid retreat of ice shelves, acceleration of the ice sheets, and subsequent increase in global sea levels.
A classical treatment of fracture by Griffith [3] introduced the energy release rate for brittle elastic materials, the energy required for crack propagation, and created the energetic fracture criterion. Unfortunately such theories are insufficient as they cannot reproduce curvilinear cracks, kinks, crack branching, crack arrest, or crack nucleation. One can overcome issues of the classical Griffith theory with a diffusive crack modelling by variational approaches based on energy minimisation [1, 4].
In this talk we present a thermodynamically consistent phase field viscoelastic fracture models relevant for ice sheet dynamics that allows to incorporate additional rheological properties such as creep. By identifying the relevant free energy and dissipation potential functions of interest one can derive relevant viscoelastic models. In the case of Maxwell rheology with Glen's flow law [2], one arrives at two possible systems, one better suited for short timescales and another for longer timescales.
We present robust adaptive numerical schemes that allow to treat both compressible and incompressible materials with pure Dirichlet or Neumann, as well as mixed boundary conditions. 
Computational experiments demonstrate the robustness of the numerical solvers and importance of inclusion of fracture mechanisms into ice sheet models.

[1] B. Bourdin, G.A. Francfort, J.J. Marigo, Numerical experiments in revisited brittle fracture. Journal of the Mechanics and Physics of Solids, Vol. 48, pp. 797--826, 2000.
[2] J.W. Glen, The creep of polycrystalline ice. Proceedings of the Royal Society London A, Vol. 228, pp. 519--538, 1955.
[3] A.A. Griffith, The phenomena of rupture and flow in solids. Philosophical Transactions of the Royal Society London A, Vol. 221, pp. 163--198, 1921.
[4] C. Miehe, F. Welschinger, and M. Hofacker, Thermodynamically consistent phase‐field models of fracture: Variational principles and multi‐field FE implementations. International journal for numerical methods in engineering, Vol. 83, pp. 1273--1311, 2010.

How to cite: Stocek, J., Arthern, R., and Marsh, O.: Phase field viscoelastic fracture models for ice sheet dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12225, https://doi.org/10.5194/egusphere-egu22-12225, 2022.

16:09–16:15
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EGU22-3246
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ECS
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Virtual presentation
Naomi Ochwat, Ted Scambos, Sarah Child, and Mike Willis

The major tributary glaciers of the former Larsen B Ice Shelf have undergone significant changes in the time leading up to, and following, the collapse of the ice shelf in March 2002. Crane and Hektoria-Green-Evans Glaciers (hereafter, Crane; Hektoria) experienced multiple periods of rapid velocity increases and intervening decreases, and dramatic surface lowering and mass loss. Initial results of early (late 1960s) U.S. Navy Trimetrogon aerial image analysis for elevation indicates large elevation losses in the decades prior to the disintegration event. Following the ice shelf collapse, both glaciers developed significant ice cliff fronts, but with markedly different calving styles and ice front heights at different times after the event. Rapid collapse with indications of arcuate listric faulting began at Hektoria almost immediately after ice shelf loss, while Crane also experienced rapid retreat during this time. Maximum elevation of the cliff fronts in the Hektoria collapsed region ranged between 60 and 100 meters. Peak ice cliff height at Crane was approximately 105 m, occurring in late 2004. These cliff heights correspond with periods of very high flow speed, thinning, and rapid ice front retreat that is characteristic with modeled ice cliff failure events. Here we present our analysis of the characteristics that defined the retreat periods. We assess ice velocity changes from optical satellite imagery, hypsometry, and ice cliff front heights from stereo-image DEMs and altimetry data, and use bed topography and bathymetry data. Ice cliff failure that could lead to Marine Ice Cliff Instability (MICI) has never been observed either in situ or through remote sensing. Using the observed dynamics of Crane and Hektoria, we aim to improve our understanding of the parameters that modeling results show as the drivers of ice cliff failure. In doing so, impacts of ice cliff failure on outlet glacier stability in numerical modeling will be better constrained, which will increase predictive sea level rise accuracy.

How to cite: Ochwat, N., Scambos, T., Child, S., and Willis, M.: Pre-breakup drawdown and ice cliff formation on two Larsen B tributaries, 1968-2008, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3246, https://doi.org/10.5194/egusphere-egu22-3246, 2022.

16:15–16:21
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EGU22-12017
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ECS
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On-site presentation
Trystan Surawy-Stepney, Anna E Hogg, Stephen L Cornford, and Benjamin J Davison

The majority of ice mass loss in West Antarctica is due to the ejection of grounded ice into the sea via ice-dynamic processes. Structural changes that impact the flow speed of marine-terminating glaciers can, therefore, impact their contribution to global sea level rise. Thwaites Glacier is among those for which these considerations are particularly important, due to its potential connection to the stability of the West Antarctic ice sheet, and the structural changes that have been observed at its terminus in recent years. However, the interactions between ice structural properties and flow speed are not well established, partly due to the limited availability of coincident observations.

We present weekly ice velocity measurements, derived using Sentinel-1 radar data, showing the recent onset of episodic dynamic variability in the form of two large-magnitude ~30%-45% acceleration/deceleration events between 2017 and 2021, occurring across the majority of the remnant of Thwaites Glacier's floating ice tongue, before a relaxation to the 2015/16 mean speed. Using deep learning methods, we measured a synchronous decrease in the structural integrity of the ice tongue and its eastern shear margin during the study period, and the upstream propagation of these regions of damaged ice. The pattern of change seen in the concurrent damage and ice velocity observations suggests a link between the two, which we explore in the work. The existence of this link is further supported by ice flow modelling, carried out using the BISICLES ice sheet model, in which the spatial pattern and concentration of observed damage are closely reproduced when forced with the observed speed changes.

Our results add to the growing body of evidence that the extent and degree of damaged ice has a significant distributed effect on ice velocity, and further demonstrate that damage processes must be integrated in ice sheet models in order to make accurate predictions of long-term behaviour and sea level contribution.

How to cite: Surawy-Stepney, T., Hogg, A. E., Cornford, S. L., and Davison, B. J.: Damage and Dynamic Activity on the Thwaites Glacier Ice Tongue: 2015 to 2021, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12017, https://doi.org/10.5194/egusphere-egu22-12017, 2022.

16:21–16:27
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EGU22-8440
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ECS
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On-site presentation
Cristina Gerli, Sebastian Rosier, and Hilmar Gudmundsson

Antarctic Ice shelves are fundamentally important components of the cryosphere and key to predictions of global sea level rise. Thinning and fracturing of ice shelf systems can reduce back-stress forces exerted on grounded glaciers upstream, increasing mass flux across their grounding lines (GL). In recent years it has been suggested that a number of ice shelves around Antarctica have rapidly broken apart as a result of hydrofracturing.  Hydrofracture is the process whereby surface crevasses are filled up with meltwater and the resulting hydrostatic pressure cause outward propagation of the crevasse fracture.

Recent work assessed the impact of ice shelf thickness change and crevasse hydrofracturing on the vulnerability of ice shelves and on ice drainage. Using a deep convolutional neural network, high-resolution crevasses and fractures were mapped throughout Antarctica, revealing that 60 ± 10 % of ice shelves are vulnerable to fracturing, if inundated with water.

Here we use these crevasse maps to evaluate their impact on the flow of upstream glaciers, quantifying the change in flux at the GL. We employ a finite element ice flow model, Ua, which solves the vertically integrated shallow shelf approximation with an unstructured mesh, that allows refined resolution in complex areas, such as at the GL. In the absence of information on crevasse depth, we make the assumption that crevasses propagate through the entire thickness, meaning our results represent the maximum possible effect that these crevasses may have on ice flow. We present results for many of the most important ice shelves in East and West Antarctica.

We find that incorporating crevasses in the ice shelf always increases the mass flux of upstream glaciers across their GLs, however, there is substantial variability in flux change among ice shelves. Small increases in flux due to crevassing (7-15%) were detected for Fimbul, Shackleton, Pine Island, Larsen C, and Brunt Ice Shelves, with a more considerable increase for the Dotson & Crosson Ice shelves (38%). The increase in flux due to crevassing was extremely large for the Totten Ice Shelf (248%). The large differences in sensitivity between ice shelves may be a result of various factors, most notably the proximity of the features identified as crevasses to important pinning points. More work investigating these factors is needed in order to have a more complete understanding of the effects of crevasse hydrofracturing on inland glaciers.

How to cite: Gerli, C., Rosier, S., and Gudmundsson, H.: Impact of ice shelf crevasses on Grounding line flux , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8440, https://doi.org/10.5194/egusphere-egu22-8440, 2022.

16:27–16:33
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EGU22-4369
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On-site presentation
Impacts of recent calving events on upstream flow of Pine Island Glacier and modelling of potential future scenarios
(withdrawn)
Sainan Sun and Hilmar Gudmundsson
16:33–16:39
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EGU22-11427
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On-site presentation
Cruz Garcia-Molina, Fabien Gillet-Chaulet, Mondher Chekki, Gael Durand, and Olivier Gagliardini

Calving is one of the most important processes that induce mass loss in Greenland and Antarctic ice shelves. These major ice discharges modify the calving front position with an impact over the whole stress regime of these glaciers. Because the calving rate depends on several physical parameters, having an empirical parameterization for simulations over long periods is a big challenge. We study the calving front position using the open-source finite element, Elmer/Ice (http://elmerice.elmerfem.org/) code. We use a time constant mesh coupled with a time-evolving signed distance to the front (level-set function Φ) that activates or masks nodes as needed. We study the front position (given as the 0 level set value) evolution by solving

 

with w=c+v, where c is the calving rate and v is the velocity of the ice normal to the front. By using a realistic synthetic configuration, based on the intercomparison models (MISMIP), we validate our level-set method, we study the numerical sensibility, and the impact of different calving laws reported in the literature.

How to cite: Garcia-Molina, C., Gillet-Chaulet, F., Chekki, M., Durand, G., and Gagliardini, O.: Comparison of different calving laws using a level set method, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11427, https://doi.org/10.5194/egusphere-egu22-11427, 2022.

Coffee break
Chairpersons: Rachel Carr, Nicolas Jourdain
17:00–17:05
17:05–17:11
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EGU22-5266
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On-site presentation
Rachel Carr, Emily Hill, and Hilmar Gudmundsson

The Greenland Ice Sheet (GrIS) contributed to 10.6 mm of global sea level rise between 1992 and 2018 (Shepherd et al., 2020), which is forecast to increase to 90±50 mm by 2100, under RCP8.5 forcing (Goelzer and others, 2020). Thus, it is crucial that we accurately forecast near future ice losses from the GrIS and assess the relative contribution of surface mass balance (SMB) and accelerated discharge from outlet glaciers. Uncertainties in forecasts of GrIS mass loss, which stem from model uncertainties, climate modelling projections, ocean forcing and the calving process.

Here, we assess the relative importance of two major sources of uncertainty, namely the choice of sliding law and SMB forecasts. To do this we use the ice flow model Úa to perform a series of model experiments using different formulations of the sliding law, and different projections of future SMB. Úa is vertically integrated, uses the shallow ice stream / shelf approximation and has an adaptive mesh. We conducted this work at three major Greenland outlet glaciers: Kangerdlugssuaq (KG), Humboldt (HU) and Petermann (PG) glaciers. These glaciers were selected as they are major sources of ice loss from the GrIS and have a diverse range of characteristics (e.g. terminus type, speed and catchment geometry), meaning that we can assess the variability in the importance of sliding laws and/or SMB forecasts between different types of glacier.

First, we initialised the models for each study glacier using remotely sensed data from 2014/15. We then performed a series of model inversions using four different sliding laws (Weertman, Budd, Tsai and Cornford laws), to all closely match the observed ice flow velocities. For each sliding law, we then ran a forward-in-time model simulation using the rheology and basal slipperiness fields derived from each inversion and compared the difference in ice loss after 100 years between each sliding law. Our results demonstrated that the impact of using different sliding laws varied between our study glaciers, resulting in limited differences at HU and substantially variation at KG and PG. To test the impact of SMB projections we use SMB projections from the Modèle Atmosphérique Régional (MAR) for ISMIP6, which utilised six CMIP5 and five CMIP6 models (Hofer et al., 2020). We then run forward simulations for 100 years for each study glacier, using each of the SMB forecasts, and using the rheology and basal slipperiness fields from each inversion. Initial results demonstrate that the impact of the difference SMB forecasts is far greater than the impact of the choice of sliding law.

References:

Goelzer, H., et al., 2020. The future sea-level contribution of the Greenland ice sheet: a multi-model ensemble study of ISMIP6. The Cryosphere, 14(9), pp.3071-3096.

Hofer, S. et al., 2020. Greater Greenland Ice Sheet contribution to global sea level rise in CMIP6. Nature communications, 11(1), pp.1-11.

Shepherd, A. et al., 2020. Mass balance of the Greenland Ice Sheet from 1992 to 2018. Nature, 579(7798), 233-239.

How to cite: Carr, R., Hill, E., and Gudmundsson, H.: Impact of sliding laws and surface mass projections on Greenland outlet glacier dynamics at 100-year timescales , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5266, https://doi.org/10.5194/egusphere-egu22-5266, 2022.

17:11–17:17
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EGU22-9920
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ECS
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Virtual presentation
Adrien Wehrlé, Martin P Lüthi, Ana Nap, Guillaume Jouvet, and Fabian Walter

Sermeq Kujalleq in Kangia (Jakobshavn Isbræ), Greenland has been extensively investigated over the past decades due to its recent retreat associated with extremely fast ice stream flow and high solid ice discharge. However, its short-term dynamics still remain poorly understood as they consist in transient states that can only be captured by high spatial and temporal in situ measurements. In the new COEBELI project, we aim at combining high resolution field data sets from seismic arrays, global navigation satellite system (GNSS) receivers, long-range uncrewed aerial vehicles and terrestrial radar interferometers (TRI) to achieve a comprehensive and detailed study of the short-term ice stream dynamics. Here, we present TRI and GNSS retrievals of surface velocity and elevation acquired during the first, exploratory field campaign of the COEBELI project in summer 2021. Seven kilometers away from the calving front, we specifically identified a slowdown of 1.12 m d-1 within a single day in the main trunk of the ice stream. While the absolute slowdown is larger in the main trunk than in the outer area of the shear margin (1.12 m d-1 versus 0.75 m d-1), it corresponds to a larger fraction of the pre-slowdown velocity in the latter zone (-4.48% versus -7.94%). We further discuss the challenges associated with the acquisition, processing and analysis of high-resolution data sets for the study of such complex and dynamic environments.

How to cite: Wehrlé, A., Lüthi, M. P., Nap, A., Jouvet, G., and Walter, F.: Short-term dynamics of Sermeq Kujalleq in Kangia (Jakobshavn Isbræ), Greenland derived from TRI and GNSS measurements, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9920, https://doi.org/10.5194/egusphere-egu22-9920, 2022.

17:17–17:23
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EGU22-10979
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Virtual presentation
Enrico Ciracì, Eric Rignot, Pietro Milillo, and Luigi Dini

Petermann Gletscher drains 4% of the Greenland Ice Sheet that contains an estimated volume of ice equivalent to a 0.5 m global sea-level rise. It terminates in the longest floating ice shelves in the Northern Hemisphere. A significant portion of the glacier’s drainage basin is grounded below sea level on a downsloping bed, hence prone to rapid retreat if the glacier was pushed out of equilibrium by climate warming. Previous studies documented near-zero mass balance and a steady grounding line position during the last three decades. However, more recent observations revealed the transition to a new phase characterized by rapid grounding line retreat and accelerated ice flow after 2016. Increased basal melt due to warming ocean temperatures has been identified as the physical mechanism driving the retreat process. Nonetheless, a comprehensive evaluation of the magnitude and spatial variability of basal melt has not been performed yet. Its contribution to the ice mass loss remains, for this reason, poorly constrained.

In this study, we achieve this goal by employing high-resolution digital elevation models acquired by the German Aerospace Centre (DLR) TanDEM-X mission between 2011 and 2021. We derive basal melt estimates from ice elevation changes computed in a Lagrangian framework. The extended temporal coverage provided by TanDEM-X data allows mapping changes in basal malt over different temporal scales with unprecedented resolution and highlights increased melt rates during the second part of the observation period. The melt rate spatial distribution is consistent with the recent inland migration of the grounding line with peak values above 60 meters per year measured along with the western, central, and eastern sectors of the grounding zone.

How to cite: Ciracì, E., Rignot, E., Milillo, P., and Dini, L.: Evaluating Petermann Gletscher ice-shelf basal melt and ice-stream dynamics from high-resolution TanDEM-X elevation data., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10979, https://doi.org/10.5194/egusphere-egu22-10979, 2022.

17:23–17:29
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EGU22-7822
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ECS
|
On-site presentation
Robert Law, Poul Christoffersen, Emma MacKie, Samuel Cook, Marianne Haseloff, and Olivier Gagliardini

Uncertainty tied to the mechanics of the fast motion of the Greenland Ice Sheet plagues sea-level rise predictions. Much of this uncertainty arises from imperfect representations of physical processes in constitutive relationships for basal slip and internal ice deformation, with continued misalignment between model output and borehole field data. To investigate further, we model two isolated cuboid domains from the fast-moving Sermeq Kujalleq (a.k.a Store Glacier), incorporating temperate ice rheology (softer ice at the melting point) and statistically realistic variogram-generated bed topography. Our results indicate a hitherto unappreciated complexity in ice-sheet basal motion over rough beds. Realistic topographic variability leads to highly variable basal slip rates (from <10 to >70% of surface velocity over ~1km), complex and variable deformation patterns, and a basal temperate ice layer that varies greatly in thickness in agreement with borehole observations (from <10 to >150 m). Velocity variations at the relatively smooth upper boundary of the temperate ice layer are significantly less variable, indicating that the slim basal temperate ice layer is an important control on ice motion. These results suggest that inversion procedures for basal traction over rough beds (including parts of Antarctica) may also be accounting for deformation within a temperate ice layer, which is problematic if the inclusion of a temperate ice layer and rough topography means commonly used basal slip relationships are no longer applicable. 

How to cite: Law, R., Christoffersen, P., MacKie, E., Cook, S., Haseloff, M., and Gagliardini, O.: Complex basal motion of a Greenland Ice Sheet tidewater glacier, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7822, https://doi.org/10.5194/egusphere-egu22-7822, 2022.

17:29–17:35
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EGU22-10208
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ECS
|
On-site presentation
Thomas Gregov, Frank Pattyn, and Maarten Arnst

Marine ice sheets are complex systems with a highly non-linear behavior. There remains a large uncertainty about how various physical processes such as the basal friction and the subglacial hydrology affect the dynamics of the grounding line (GL). One possibility to better understand their mechanical behavior consists in adopting a boundary-layer analysis close to the GL. Specifically, one can derive a so-called flux condition, which is an analytical expression for the amount of ice that flows through this GL per unit time. In turn, this flux condition can provide useful information about the grounding-line dynamics, including the presence of hysteresis (Schoof, 2007b).

Several studies have introduced hybrid friction laws to model friction between the grounded part of the ice sheet and the bedrock (Schoof, 2005, Gagliardini et al., 2007). These friction laws behave as power-law friction laws far from the GL and plastically closer to it. Recent experiments have shown that these models are more realistic than the usual power-law friction (Zoet and Iverson, 2020). In parallel, sophisticated models for the subglacial hydrology have been developed (Bueler and van Pelt, 2015).

In this presentation, we show that the flux conditions previously derived for the Weertman friction law (Schoof, 2007a) and the Coulomb friction law (Tsai et al., 2015) can be extended to a flux condition for the general Budd friction law, with two different simple effective-pressure models for the subglacial hydrology. Using asymptotic developments, we provide a justification for the existence and uniqueness of a solution to the boundary-layer problem. Finally, we generalize our results to hybrid friction laws, based on a parametrization of the flux condition.

How to cite: Gregov, T., Pattyn, F., and Arnst, M.: Extension of marine ice-sheet flux conditions to effective-pressure-dependent and hybrid friction laws, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10208, https://doi.org/10.5194/egusphere-egu22-10208, 2022.

17:35–17:41
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EGU22-1207
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ECS
|
Presentation form not yet defined
Validation of frictions laws from historical simulations of Upernavik Isstrøm
(withdrawn)
Eliot Jager, Fabien Gillet-Chaulet, and Jérémie Mouginot
17:41–17:47
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EGU22-13292
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ECS
|
On-site presentation
Iain Wheel, Anna Crawford, Joe Todd, Doug Benn, Eef Van Dongen, and Tom Cowton

Jakobshavn Isbrae, the largest outlet glacier in Greenland, accounts for over 20% of the mass loss from the Greenland Ice Sheet. The calving of such large, marine-terminating glaciers is an important yet largely unconstrained contributor to global sea level rise. Understanding the influence of changing environmental conditions on calving at influential glaciers is critical for projections of glacier retreat and sea level rise. This understanding remains poor, in part due to the inability of 3D calving models to robustly simulate calving dynamics and large-scale retreat at fast-flowing glaciers such as Jakobshavn Isbrae. It is important to overcome these modelling challenges, as modelling calving in 3D is necessary to understand the role of geometry and internal glacier dynamics on calving and identify the key environmental forcers at individual glaciers.

We present results from a new calving algorithm implemented in the 3D full-Stokes continuum model Elmer/Ice and applied at Jakobshavn Isbrae, West Greenland. Elmer/Ice fully resolves the glacier velocity and stress fields, whilst recent developments in the calving algorithm allow the modelled glacier to advance and retreat limitlessly along the fjord. A positional crevasse depth calving law is implemented within the calving algorithm, which we use to investigate the dominant processes behind large scale calving and retreat at Jakobshavn Isbrae. Furthermore, we investigate the robustness of the crevasse depth calving law to simulate terminus position. Preliminary results suggest the current incarnation of the crevasse depth law underestimates calving and the crevasse depth required to calve needs to be reduced to accurately simulate terminus change at Jakobshavn Isbrae. Additionally, the inclusion of an ice mélange backstress in winter simulations is key to seasonal terminus advance.

How to cite: Wheel, I., Crawford, A., Todd, J., Benn, D., Van Dongen, E., and Cowton, T.: Simulating seasonal dynamics of Jakobshavn Isbrae through advancing the Elmer/Ice calving model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13292, https://doi.org/10.5194/egusphere-egu22-13292, 2022.

17:47–17:53
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EGU22-12145
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ECS
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On-site presentation
Martin Schulthess, Nina Kirchner, and Peter Sigray

Sea level rise concerns millions of people in coastal areas across the globe. One of the largest uncertainties to project future sea level rise is the frontal ablation (accounting for calving and submarine melt) at marine ice margins, around the Greenland and Antarctic Ice Sheet. High rates of frontal ablation have been observed to imply, through loss of the buttressing effect but not limited to it, increased mass loss from marine terminating glaciers and hence, associated sea level rise. This study focuses on calving processes at a freshwater lake in northern Sweden, which represents a simpler environment to study calving processes than the marine one, because impacts of tides, salinity, and circulation (all known to be relevant at marine ice-ocean boundaries) can be neglected. A multi-method approach to quantify and characterize calving events is presented here, exploring and analysing the underwater acoustic soundscape at a calving glacier front, in connection with optical, image-based methods such as timelapse photography, and photogrammetry based on footage acquired by an uncrewed aerial vehicle (UAV). An acoustic detector is developed, tested and applied to a data set acquired during 2020, and results indicate that the acoustic detector can be an important complement in the range of tools used to observe, and quantify, calving. Applied in remote locations, where continuous monitoring is difficult and where optical methods are of limited use, collecting acoustic data and monitoring calving by means of its acoustic signature could render insights previously not available (because of lacking data and methodology).

How to cite: Schulthess, M., Kirchner, N., and Sigray, P.: Multi-method based characterization of calving events at Salajiegna glacier - Northern Sweden, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12145, https://doi.org/10.5194/egusphere-egu22-12145, 2022.

17:53–17:59
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EGU22-7731
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ECS
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On-site presentation
Donald Slater and Doug Benn

The impact of submarine melting on calving is thought to be central in the response of marine-terminating glaciers to climate, yet we currently have no settled parameterisation that can represent this process in ice sheet models. The crevasse-depth calving law has been widely applied with arguable success, but in its present form accounts only for depth-mean stresses. As such, it does not account for the bending stresses induced by undercutting that may be key to the impact of submarine melting on calving.

Here, we combine elastic beam theory with linear elastic fracture mechanics to study the propagation of surface and basal crevasses near the front of tidewater glaciers in response to melt undercutting. We check our results against a numerical approach involving 2D elastic simulations and the displacement correlation method for estimating fracture depth. Our results suggest that bending stresses can play a significant role in modifying crevasse depth, with undercutting promoting the opening of surface crevasses and protruding ‘ice feet’ promoting the opening of basal crevasses. Lastly, we seek a revised crevasse-depth calving law that accounts for these effects.

How to cite: Slater, D. and Benn, D.: A crevasse-depth calving law accounting for submarine melt undercutting, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7731, https://doi.org/10.5194/egusphere-egu22-7731, 2022.

17:59–18:05
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EGU22-2903
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ECS
|
On-site presentation
Markus Reinert, Marvin Lorenz, Knut Klingbeil, and Hans Burchard

Melting of the Greenland ice sheet has a big influence on the climate system. Therefore, it is important to understand how the ice melts. Since direct measurements at the underside of floating ice tongues are sparse, high-resolution models for the interaction of ocean and glacial ice are needed to determine sub-glacial melt rates and to understand melt processes. A common problem is that model resolution is often too low, so that the meltwater plume is only represented by one or two layers, and thus the entrainment of warmer water into the plume is not well captured. However, this heat transport towards the ice is crucial for the sub-glacial melt rate.

In this talk, we show how we solve this problem with the General Estuarine Transport Model (GETM). GETM features adaptive vertical coordinates that zoom automatically to areas of interest, in particular strong density gradients. A high density contrast exists in the entrainment layer between the relatively fresh and cold meltwater of the plume, and the ambient ocean water. By zooming towards this interface, our adaptive vertical coordinates resolve the meltwater plume with several layers, while keeping the total number of model layers at a modest level to ensure a feasible computation time. In addition, the coordinate levels align to the moving isopycnals – they “follow” the plume, which strongly reduces numerical mixing and pressure gradient errors.

We present this for the fjord of the 79°N-Glacier (79NG), which has the largest floating ice tongue in Greenland. In our idealized 2D-setup, we obtain layers as thin as 0.2 m to 1 m in the meltwater plume, for only 100 levels over a water column of several 100 m depth. Thanks to this high resolution of plume and entrainment layer, our model reproduces the overturning circulation in the glacier cavity correctly; in particular, it shows that the salinity stratification of the adjacent ocean determines the level at which the meltwater plume detaches from the ice tongue. Almost all sub-glacial melting occurs before this detachment, i.e., where the plume is directly at the ice–ocean interface. Furthermore, we can confirm that the highest melt rates exist near the grounding line of the glacier. Finally, our simulated melt rates are consistent with observations at 79NG.

Our model, developed in the GROCE (Greenland ice sheet–ocean interaction) project, will form the basis of a realistic 3D-model of the 79NG-fjord in the future.

How to cite: Reinert, M., Lorenz, M., Klingbeil, K., and Burchard, H.: Understanding the melting of Greenland's largest glacial ice tongue with high-resolution modelling and adaptive coordinates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2903, https://doi.org/10.5194/egusphere-egu22-2903, 2022.

18:05–18:11
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EGU22-11758
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ECS
|
On-site presentation
Jonathan Wiskandt, Inga Koszalka, and Johan Nilsson

Using a two dimensional, high resolution, non-hydrostatic regional model, this study explores the melt induced circulation under the floating ice tongue of Ryder Glacier (RG) and the influence of different aspects of the simulation, like ambient water temperature and ice base geometry, on the circulation.

RG is located at the southern tip of Sherard Osborn Fjord (SOF) in the north of Greenland, which was for the first time surveyed oceanographically in 2019. Low grounding-line water temperatures, complex ice-tongue and sill geometries, and permanent sea-ice cover outside the fjord, potentially make the ice-ocean interactions in SOF rather different from those in the more well-studied nearby Petermann Fjord. 

The control simulation uses 2019 hydrographic observations as initial conditions. A set of model experiments is conducted to analyze the dependency of the plume behavior on the slope of the ice base and the temperature forcing from the in-flowing Atlantic water. 

 The simulated circulation and melt rates are qualitatively similar to previous modelling studies of North Greenlandic fjords. Based on observed ice-thickness transects along RG, two idealized ice tongue profiles are examined: one steeper and one shallower. The simulations with shallower slopes have a greater net basal melt and a stronger overturning fjord circulation, even though the melt plume initially is faster on the steeper slope. The results further suggest a direct relationship between the thermal forcing and the melt rate and resulting overturning time scale.

Additionally we discuss the possible numerical and physical implications of these results for future model experiments targeting the influence of basal melt on fjord circulations.

How to cite: Wiskandt, J., Koszalka, I., and Nilsson, J.: Idealized high resolution modelling of plume dynamics and basal melting at Ryder Glacier, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11758, https://doi.org/10.5194/egusphere-egu22-11758, 2022.

18:11–18:17
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EGU22-11865
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ECS
|
Virtual presentation
Stefano Ottolenghi, Jonathan Wiskandt, and Josefin Ahlkrona

Modeling interactions between ice sheets and ocean has proved of significant importance in order to properly understand larger-scale phenomena such as ice sheet melting and ocean circulation. We introduce the Finite Elements Framework for Fjord-Iceshelf Interaction (FEFFII), a new simulation framework for fjord dynamics. Open source and Python-based, it employs the full non-hydrostatic Navier-Stokes equations to account for the ocean evolution, while ice shelf behavior is accounted by the 3-equations parametrization. Even though some its features are still under experimentation, FEFFII is already capable of simulating realistic scenarios and handling relatively complex geometries, as well as moving boundaries. The model has been tested against several benchmarks from literature.

How to cite: Ottolenghi, S., Wiskandt, J., and Ahlkrona, J.: A new Finite Elements Framework for Fjord-Iceshelf Interaction (FEFFII), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11865, https://doi.org/10.5194/egusphere-egu22-11865, 2022.

18:17–18:23
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EGU22-1158
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On-site presentation
Hans Burchard, Karsten Bolding, Adrian Jenkins, Martin Losch, Markus Reinert, and Lars Umlauf

Basal melting of marine-terminating glaciers, through its impact on the forces that control the flow of the glaciers, is one of the major factors determining sea level rise in a world of global warming. Detailed quantitative understanding of dynamic and thermodynamic processes in melt-water plumes underneath the ice-ocean interface is essential for calculating the subglacial melt rate. The aim of this study is therefore to develop a numerical model of high spatial and process resolution to consistently reproduce the transports of heat and salt from the ambient water across the plume into the glacial ice. Based on boundary layer relations for momentum and tracers, stationary analytical solutions for the vertical structure of subglacial non-rotational plumes are derived, including entrainment at the plume base. These solutions are used to develop and test convergent numerical formulations for the momentum and tracer fluxes across the ice-ocean interface. After implementation of these formulations into a water-column model coupled to a second-moment turbulence closure model, simulations of a transient rotational subglacial plume are performed. The simulated entrainment rate of ambient water entering the plume at its base is compared to existing entrainment parameterizations based on bulk properties of the plume. A sensitivity study with variations of interfacial slope, interfacial roughness and ambient water temperature reveals substantial performance differences between these bulk formulations. An existing entrainment parameterization based on the Froude number and the Ekman number proves to have the highest predictive skill. Recalibration to subglacial plumes using a variable drag coefficient further improves its performance.

How to cite: Burchard, H., Bolding, K., Jenkins, A., Losch, M., Reinert, M., and Umlauf, L.: The vertical structure and entrainment of subglacial melt water plumes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1158, https://doi.org/10.5194/egusphere-egu22-1158, 2022.

18:23–18:29
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EGU22-13179
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ECS
|
Virtual presentation
Jamie Barnett, Felicity Holmes, and Nina Kirchner

Mass loss at the Greenland Ice Sheet is influenced by atmospheric processes controlling its surface mass balance, and by submarine melt and calving where glaciers terminate in fjords. There, an ice mélange – a composite matrix of calved ice bergs and sea ice – may provide a buttressing force on a glacier terminus, and control terminus dynamics as a function of mélange dynamics and strength. Kangerlussuaq Glacier is a major outlet of the Greenland Ice Sheet, for which recent major retreat events in 2004/2005 and 2016-2018 coincided with the absence of an ice mélange in Kangerlussuaq Fjord. To better understand the response of Kangerlussuaq Glacier to climatic and oceanic drivers, a 2D flowline model is employed. Results indicate that an ice mélange buttressing force exerts a major control on calving frequency and rapid retreat: when an ice mélange forms in Kangerlussuaq Fjord, it provides stabilizing forces and conditions favorable for winter terminus re-advance. When it fails to form during consecutive years, modeled retreat of Kangerlussuaq Glacier occurs into the large overdeepenings in Kangerlussuaq Fjord, and to terminus positions more than 30 km farther inland, necessitating to anticipate excessive mass loss from Kangerlussuaq Glacier by the year 2065.

How to cite: Barnett, J., Holmes, F., and Kirchner, N.: Modelled dynamic retreat of Kangerlussuaq Glacier, southeast Greenland, strongly influenced by the consecutive absence of an ice mélange in Kangerlussuaq Fjord, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13179, https://doi.org/10.5194/egusphere-egu22-13179, 2022.

Presentations: Wed, 25 May | Room E2

Chairpersons: Nicolas Jourdain, Rachel Carr
08:30–08:35
08:35–08:41
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EGU22-13408
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ECS
|
Virtual presentation
Margaret Lindeman, Fiamma Straneo, Hanumant Singh, Claudia Cenedese, David Sutherland, Kristin Schild, and Dan Duncan

As much as half of the freshwater flux from the Greenland Ice Sheet enters the ocean through calving of icebergs into glacial fjords. Remote sensing studies have shown that substantial iceberg melt occurs within fjords, and models indicate that the resulting heat and freshwater fluxes affect fjord circulation and the properties of waters reaching the glacier terminus. Observations are needed to evaluate whether these models accurately represent the distribution of iceberg melt.

Repeat oceanographic surveys around a large iceberg in Sermilik Fjord show anomalously cold, fresh layers consistent with the expected properties of submarine ice melt. We interpret these features as intrusions of iceberg melt and characterize their properties and vertical distribution. We find that iceberg melt drives significant upwelling, with the vertical scale set by the ambient stratification, as predicted by theory and numerical simulations. Our results agree with recent studies suggesting that the typical melt parameterization likely underestimates melt rates in this setting.

How to cite: Lindeman, M., Straneo, F., Singh, H., Cenedese, C., Sutherland, D., Schild, K., and Duncan, D.: Iceberg meltwater intrusions observed in Sermilik Fjord, Southeast Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13408, https://doi.org/10.5194/egusphere-egu22-13408, 2022.

08:41–08:47
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EGU22-8336
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ECS
|
On-site presentation
Karita Kajanto and Kerim Nisancioglu

The interface between ice and ocean in Greenlandic fjords is the main source of uncertainty in the sea level contribution estimates from the Greenland ice sheet in the coming century. So far, research has shown tight coupling between the glacier and the water column in the fjord, but several main processes remain unclear. The role of icebergs in narrow fjords is poorly understood, and until recently research has focused mostly on the buttressing effect iceberg melange can have on the calving front. However, icebergs provide a substantial fresh water flux in the fjord that can exceed subglacial discharge annually. Iceberg melt is distributed at depth and produced throughout the year, and contributes to the stratification of the fjord, impacting the glacier terminus.

We model the high-silled Ilulissat Icefjord in Western Greenland with the MITgcm ocean model, using IceBerg package to study the effect different iceberg distributions have on this fjord. We compare our results to available XCTD profiles from the fjord. Our results demonstrate that including icebergs is essential to correctly understand the stratification of the fjord. We show that larger icebergs with drafts close to, or deeper than sill depth cool the fjord basin at depth. More specifically, we show that — while the inflowing water looses heat as it passes icebergs — a significant part of this iceberg-induced cooling at depth is due to entrainment of iceberg-cooled intermediate waters into the basin. Furthermore, we demonstrate that icebergs affect glacier melt rate by modifying the melt rate distribution along the glacier face both in shape and magnitude.

How to cite: Kajanto, K. and Nisancioglu, K.: Icebergs slow glacier retreat in a Greenland fjord, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8336, https://doi.org/10.5194/egusphere-egu22-8336, 2022.

08:47–08:53
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EGU22-9198
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ECS
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On-site presentation
Connor Shiggins, James Lea, William Harcourt, Siddharth Shankar, Stephen Brough, and Dominik Fahrner

Icebergs are a key component of the ice-ocean interface, and provide the opportunity to gain insight into calving processes, and freshwater budgets in fjords and oceans amongst others. Iceberg area and volume distributions have been characterised for a handful of sites across the Greenland Ice Sheet, though a greater spatial and temporal range of data are required to understand how iceberg dynamics vary between different glaciers. Here we present iceberg area and volume distributions from 141 ArcticDEM scenes from 2010-2017 for 19 marine-terminating glaciers in Greenland, with 588,856 icebergs automatically detected.

The data show emerging evidence for more positive power law slope values (i.e. glaciers with larger icebergs) at glaciers with mean terminus depths exceeding 230 meters. However, the range of these values are generally consistent once a depth of 230 metres is exceeded. Glaciers with shallower depths can generate similar iceberg distributions, though typically these provide more negative exponents (i.e. are dominated by smaller icebergs).

Our results allow a characteristic range of iceberg size distributions to be defined for glaciers with mean terminus depths greater than 230 metres, which is likely controlled by a change in dominant calving processes at/near these depths. While shallower glaciers can in some cases provide similar distributions, most observations show distributions dominated by smaller icebergs. Together these suggest that mean terminus depth exerts a fundamental control on calving processes and the resulting iceberg size distributions.

Having the capability to constrain expected iceberg distributions from these data will be useful for understanding controls on calving processes, how fjord freshwater fluxes may evolve, and characterising how the size of icebergs that are exported from fjords will change as Greenland’s marine-terminating glaciers continue to retreat.

How to cite: Shiggins, C., Lea, J., Harcourt, W., Shankar, S., Brough, S., and Fahrner, D.: Observing iceberg size distributions and implications for calving processes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9198, https://doi.org/10.5194/egusphere-egu22-9198, 2022.

08:53–08:59
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EGU22-7437
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Presentation form not yet defined
Recent evolution of the greenlandic ice shelves 
(withdrawn)
Jeremie Mouginot, Romain Millan, Anders Bjørk, Nicolas Jourdain, and Pierre Mathiot
08:59–09:05
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EGU22-3951
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ECS
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On-site presentation
Benjamin Davison, Anna Hogg, Noel Gourmelen, Julia Andreasen, Richard Rigby, Jan Wuite, and Thomas Nagler

Ice shelves play a crucial role in controlling rates of ice discharge across Antarctica’s grounding lines. Mass loss from ice shelves, predominately due to basal melting and calving, can reduce the buttressing force provided by ice shelves, leading to increased grounded ice discharge. Despite the importance of ice shelves, existing estimates of calving and freshwater fluxes from ice shelves have utilised disparate datasets valid for inconsistent time periods or have relied on simplifying assumptions, resulting in a limited account of the health of many ice shelves and little indication of processes driving ice shelf mass imbalance.

Here, we quantify calving and basal melt fluxes at annual temporal resolution during 2010 to 2019. Our annual measurements account for annual variations in ice velocity and basal melt rate for 183 ice shelves, and annual variations calving front position for 34 major ice shelves (accounting for ~90% of the ice shelf area). On average during the study period, a calving flux of 1283±109 Gt yr-1 is roughly equal to a melt flux of 1247±149 Gt yr-1. Inter-annual variations in the fluxes of both basal meltwater and calving mean that the melt contribution to ice shelf mass loss varies between 35% and 62%, with the lowest contributions in years with large calving events. These large (>100 Gt) calving events are rare (8 events during 2010-2019), yet account for 35% of the total ice shelf calving flux, highlighting the importance of large calving events for ice shelf mass balance over short time scales. Eighty percent of ice shelves, including many in East Antarctica, are melting at or faster than their balance rates, indicating that ocean-driven erosion of ice shelf grounding lines is widespread around Antarctica. Furthermore, we find a significant and strong positive correlation (R=0.68) between basal melt flux and grounding line discharge, implying that ocean-driven melt may pace grounded ice loss from Antarctica.

How to cite: Davison, B., Hogg, A., Gourmelen, N., Andreasen, J., Rigby, R., Wuite, J., and Nagler, T.: Annual estimates of basal melting and calving from Antarctic ice shelves during 2010-2019, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3951, https://doi.org/10.5194/egusphere-egu22-3951, 2022.

09:05–09:11
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EGU22-8546
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ECS
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On-site presentation
Chad Greene, Alex Gardner, Nicole-Jeanne Schlegel, and Alexander Fraser

Ice shelves tend to grow through a steady influx of glacial ice and retreat in discrete calving events that occur on subannual to multidecadal timescales. The impacts of ice shelf calving and retreat are far-reaching, but the evolution of Antarctica’s coastline has not been well characterized, owing to the difficulty of delineating ice fronts in limited satellite data. To create an annual coastline dataset that spans the past quarter century, we combine data from multiple satellite sensors, and we use the known physics of ice flow to constrain ice front positions and fill gaps in the data record. We find that since 1997, Antarctica’s coastlines have retreated by 37,000 km2, led by major calving events from the Ross and Ronne ice shelves in the early 2000s, and sustained by countless loss events from smaller ice shelves ever since. Calving losses total nearly 6000 Gt, which is roughly equivalent to the total mass that has been lost to ice shelf thinning over the same period. Using an ice sheet model, we examine the impacts of observed coastal changes on the buttressing strength of Antarctica’s ice shelves.  

How to cite: Greene, C., Gardner, A., Schlegel, N.-J., and Fraser, A.: Coastal retreat doubles previous estimates of Antarctic ice shelf loss, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8546, https://doi.org/10.5194/egusphere-egu22-8546, 2022.

09:11–09:17
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EGU22-3402
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ECS
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Virtual presentation
Celia A. Baumhoer, Andreas J. Dietz, Konrad Heidler, and Claudia Kuenzer

Antarctica`s coastline is constantly changing by moving ice shelf margins and glacier tongues. This can influence the discharge of the Antarctic Ice Sheet if ice shelf areas with buttressing forces are involved. By now, glacier and ice shelf front changes are not tracked continuously due to time-consuming manual work. Hence, dynamics of the calving front position are often simplified by using the steady-state-calving assumption for modelling. To provide modelers with frequent and continuous time series of calving front change, we introduce the ice shelf front monitoring service “IceLines”. IceLines monitors major Antarctic ice shelf fronts based on Sentinel-1 radar imagery. The data set is automatically updated on a monthly basis and can be accessed via the EOC GeoService (geoservice.dlr.de) hosted by DLR. IceLines automatically downloads and pre-processes Sentinel-1 data for 36 selected shelves and glaciers, extracts the calving front based on a deep neural network and optimizes the result by post-processing. The processing chain of IceLines presents unprecedented dense time series of calving front change during the era of Sentinel-1 (2014-today). Whereas many previous challenges for automatic calving front detection were tackled (e.g. various glacier morphologies, backscatter changes, different polarizations), some limitations exist for ice shelves with excessive surface melt during summer or dry snow facies close to the front. We will present the current implementation, the derived calving front time series and validation results of IceLines. Discussions with the modelling community are welcome to further improve the IceLines data set for ice sheet and ice shelf modelling applications.

How to cite: Baumhoer, C. A., Dietz, A. J., Heidler, K., and Kuenzer, C.: IceLines – A new service to monitor Antarctic ice shelf front dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3402, https://doi.org/10.5194/egusphere-egu22-3402, 2022.

09:17–09:23
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EGU22-2997
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ECS
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On-site presentation
Nick Homer, Julien Dowdeswell, and Frazer Christie

Contemporary glaciological research is increasingly focussed on the long-term stability Antarctic Ice Sheet under different climate change scenarios, where changes to atmospheric and oceanic processes are forecast. The floating ice shelves which extend from the ice sheet are of particular research interest because they exert considerable control over the flow of inland ice and respond relatively rapidly to external forcing mechanisms.

In this study, new ice-shelf extent mapping is undertaken by delineating the calving front of the eastern Weddell Sea Sector of the East Antarctic Ice Sheet, where four of Antarctica’s ten largest ice shelves are located. Calving fronts and other lengths of coastline were mapped using an adapted edge-extraction coastline delineation method, entirely within a GIS computing environment, from a suite of remotely-sensed satellite optical (Landsat-series) and synthetic aperture radar (Sentinel-1) imagery. Combined with pre-existing coastline products, a timeseries of ice-shelf areal extent is presented and discussed in the context of known and theorised ice-ocean-atmosphere interactions occurring in the region. In contrast to what is occurring in other regions of the Antarctic Ice Sheet, ice shelves are found to have been synchronously advancing since the 1960s, with only the occasional detachment of large, tabular icebergs causing ice-shelf retreat on sub-decadal timescales. Most recently, total ice-shelf area along the eastern Weddell Sea coastline from Filchner to Fimbul ice shelves, inclusive, has been increasing by c. 550 km2 yr-1 between 2009 and 2019.

Examination of climate reanalysis and sea-ice observations suggests that increasing southward surface wind-speed anomalies along the eastern Weddell Sea coastline are facilitating increased sea-ice concentrations at the margins of the ice shelves and it is argued that this may be increasing the ice-shelves’ structural integrity, limiting iceberg calving activity. Ulimately, however, the ice shelves in this region are still primarily governed by bed-geometry and internal ice dynamical properties. Although this evidence is indicative of a region of the ice sheet in relative mass balance, the future continuation of identified surface air warming trend will increase the likelihood of increased iceberg calving, or indeed ice-shelf retreat or collapse, aping that perviously observed in the Antarctic Peninsula. Further research is, however, needed to assess what effect warming might have on the large-scale atmospheric processes governing changes to the surface winds and related sea-ice concentration anomalies, so that better predictions as to the future evolution of these ice shelves and their inland feeder ice streams may be made.

How to cite: Homer, N., Dowdeswell, J., and Christie, F.: Satellite Remote Sensing Investigations into Changing Ice-shelf Extents in the eastern Weddell Sea Sector of Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2997, https://doi.org/10.5194/egusphere-egu22-2997, 2022.

09:23–09:29
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EGU22-1486
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ECS
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Presentation form not yet defined
Observed inter-annual melt rate variability beneath the Filchner-Ronne Ice Shelf
(withdrawn)
Irena Vankova and Keith Nicholls
09:29–09:35
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EGU22-6965
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ECS
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Presentation form not yet defined
Guillian Van Achter, Thierry Fichefet, and Hugues Goosse

The Totten Glacier in East Antarctica is of major climate interest because of the large fluctuation of its grounding line and of its potential vulnerability to climate change. The Totten ice shelf melt rate is predicted to increase under future climate conditions, but this increase may differ on whether the landfast ice is represented in the model or not. Using a series of high-resolution, regional NEMO-LIM-based experiments, including an explicit treatment of ocean – ice shelf interactions and a landfast ice representation, we simulate the ocean – ice interactions in the Totten Glacier area for both historical (1995-2014) and future (end of the 21 st following RCP 4.5) periods. We show major changes between historical and projection runs as increased ice shelf melt rate, loss in sea ice production or intensified ocean circulation. Moreover, the representation of landfast ice dampens the ice shelf melt rate increase. The Totten ice shelf melt rate is increased between the two periods by either +41% when landfast ice is taken into account, or by 58% when it is not taken into account. This highlights the importance of including a landfast ice representation in our ocean models in order to predict realistic ice shelf melt rate increase in East Antarctica.

How to cite: Van Achter, G., Fichefet, T., and Goosse, H.: Importance of landfast ice for ice shelves melt rate projection under future climate conditions in the Totten area, East Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6965, https://doi.org/10.5194/egusphere-egu22-6965, 2022.

09:35–09:41
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EGU22-647
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On-site presentation
Stefan Jendersie, Christine Dow, Stephan Paul, and David Gwyther

Terra Nova Bay in the western Ross Sea of Antactica has received increasing attention recently by international oceanographic and sea ice observation campaigns.  In Terra Nova Bay strong katabatic events create one of the most intense sea ice producing polynyas in Antarctica.  The associated deep convection drives the formation of HSSW, the precursor of AABW. It also facilitates the oceanic heat exchange with the adjacent ocean cavity beneath the Nansen Ice Shelf (NIS).  Terra Nova Bay presents us with the unique opportunity of studying many of the primary interactive processes of atmosphere, ocean, ice shelves and sea ice, in a relatively confined region. 
In this talk we will show results of a high resolution, coupled ocean-ice shelf modeling study that synthesizes and contextualizes available data sets from various recent observation campaigns. Our results include the first tidal model of Terra Nova Bay and the NIS cavity, the seasonal heat budget of the cavity and the formation of meso-scale eddies inside the polynya. We have also investigated the oceanographic role of erosion features at the base of the NIS, associated ice shelf melt rates and the impact of fresh water outflow in preconditioning the onset of winter polynya activity as well as the large scale circulation in Terra Nova Bay. 

How to cite: Jendersie, S., Dow, C., Paul, S., and Gwyther, D.: A cold  cavity? Results of a high resolution ice-shelf ocean coupled model of Terra Nova Bay and the ocean cavity beneath the Nansen Ice Shelf., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-647, https://doi.org/10.5194/egusphere-egu22-647, 2022.

09:41–09:47
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EGU22-9275
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ECS
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Presentation form not yet defined
Joanna Zanker, Emma Young, Ivan Haigh, and Paul Brickle

South Georgia is a mountainous and heavily glaciated sub-Antarctic island in the Southern Ocean, lying in the path of the Antarctic Circumpolar Current. Cumberland Bay is the largest fjord on the island, split into two arms, Cumberland East and West Bay, with a large marine-terminating glacier at the head of each arm. Water circulation in such fjords, and associated transport and exchange of heat, directly governs the stability of glaciers at the ice-ocean interface and the subsequent glacier dynamics. Over the past century there has been a markedly different behaviour in the retreat rate of Nordenskjöld glacier in East Bay, compared with that of Neumayer glacier in West Bay. Fjord circulation patterns are complex with influencing factors including winds, meltwater runoff, bathymetry and coastal current systems. Precise understanding of the variability in ocean circulation and exchange in Cumberland Bay cannot be understood from limited observational data alone. Here, we use observations together with a new high-resolution numerical model built using the NEMO4 framework to determine the dominant physical drivers of variability. Nordenskjöld and Neumayer glaciers are represented as a vertical wall with a theoretical annual cycle of freshwater discharge injected at the depth of neutral buoyancy. The model is used to investigate how variability in the circulation regime couples with the associated heat transport within the two fjord arms, and to elucidate the role of such variability on glacier dynamics and rate of retreat. The sensitivity of the system to sill depth, fjord geometry and wind direction will be demonstrated through a series of model experiments, gaining a stronger understanding of the key drivers of the different retreat rates of these glaciers. 

How to cite: Zanker, J., Young, E., Haigh, I., and Brickle, P.: Impacts of variability in fjord circulation on glacier dynamics in Cumberland Bay, South Georgia  , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9275, https://doi.org/10.5194/egusphere-egu22-9275, 2022.

Coffee break
Chairpersons: Rachel Carr, Nicolas Jourdain
10:20–10:25
10:25–10:31
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EGU22-6500
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ECS
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On-site presentation
William Scott, Stephan Kramer, Benjamin Yeager, Paul Holland, Keith W. Nicholls, Martin Siegert, and Matthew Piggott

Accurate modelling of basal melting beneath ice shelves is key to reducing the uncertainty in forecasts of ice-shelf stability and, thus, the Antarctic contribution to sea level rise. However, the lack of flexibility inherent to traditional ocean models can pose problems.

Obtaining accurate melt estimates requires capturing the turbulent exchange of momentum, heat and salt at the ice-ocean interface, which may be modulated by the competing effects of stratification and basal slope. There are still significant uncertainties surrounding the trade-off between the simplicity of the melt parameterisation and the processes that need to be resolved by the numerical ocean model near the boundary.

Real ice-shelf cavity geometries are complicated. Bathymetric valleys are common and provide pathways for warm circumpolar deep water. The ice base is marked by channels, crevasses and terraces. These features will affect the boundary flow, with an added complication that melting plays a role in their formation. It is very difficult to model such flow regimes using a traditional ocean model not only because of the resolution constraints imposed by inflexible grids, but also due to the inbuilt assumptions of large aspect ratio processes and domains that may be violated when flow occurs past these features.

Ice flow models are very sensitive to how they are forced by melting at the grounding line, where the ice starts to float. The grounding line is precisely the region where ocean models are most questionable due to insufficient resolution imposed by limitations on the grid. Subglacial outflow into the cavity will likely break the inherent physical assumptions of hydrostatic, non-negligible vertical accelerations in large aspect ratio domains.

To model these effects requires the use of an ocean model that contains a flexible, unstructured mesh, is applicable at a range of length scales and, crucially, is still valid when the vertical-to-horizontal grid aspect ratio approaches order one. We are developing such a model for simulating flow under ice shelf cavities using the Firedrake finite element framework, primarily because it enables adjoint sensitivities to be calculated automatically. We present our 3d simulations of ISOMIP+ experiments alongside simulations using the MITgcm ocean model, a commonly used z-layer (constant vertical resolution) model. We have found that the ability to vary the mesh resolution flexibly in the horizontal and vertical, even in a relatively simple ISOMIP+ domain (i.e., no channels or crevasses) is very useful to investigate how melt rate depends on grid resolution, which ultimately must be the first aim of any study using a numerical model.

How to cite: Scott, W., Kramer, S., Yeager, B., Holland, P., Nicholls, K. W., Siegert, M., and Piggott, M.: First steps for a 3d flexible, unstructured finite element ocean model for flow under ice shelf cavities: an ISOMIP+ case study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6500, https://doi.org/10.5194/egusphere-egu22-6500, 2022.

10:31–10:37
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EGU22-2764
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ECS
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On-site presentation
Vjeran Visnjevic, Reinhard Drews, Clemens Schannwell, Inka Koch, Steven Franke, and Daniela Jansen

Ice shelves surrounding the Antarctic perimeter buttress ice flow from the continent towards the ocean, and their disintegration leads to an increase in ice discharge and sea level rise. The evolution and integrity of ice shelves is governed by surface accumulation, basal melting, and ice dynamics. We find history of these processes imprinted in the ice-shelf stratigraphy, which is mapped using isochrones imaged with radar. As an observational archive, the radar obtained stratigraphy combined with ice flow modeling has high potential to assist model calibration and reduce uncertainties in projections for the ice-sheet evolution. In this study we use a simplistic and observationally driven ice-dynamic forward model to predict the ice-shelf stratigraphy. We validate this approach with the full Stokes ice-flow model Elmer/Ice, and present a test-case for the Roi Baudouin Ice Shelf (East Antarctica) - where our model predictions agree well with radar obtained observations. The presented method enables us to investigate whether ice shelves are in steady-state, as well as to map spatial variations of how much of the ice-shelf volume is determined by its local surface mass balance. In the case of Roi Baudouin, we find the ice-shelf volume in the western part to be dominated by ice inflowing from the ice sheet, while the eastern part of the ice shelf is dominated by ice locally accumulated on the shelf. Such analysis serves as a metric for the susceptibility of ice shelves to climate change. We further apply our approach to other ice shelves in Antarctica.

How to cite: Visnjevic, V., Drews, R., Schannwell, C., Koch, I., Franke, S., and Jansen, D.: Towards interpretation of the radio-stratigraphy of Antarctic ice shelves from modeling and observations: A case study for the Roi Baudouin Ice Shelf, East Antarctica  , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2764, https://doi.org/10.5194/egusphere-egu22-2764, 2022.

10:37–10:43
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EGU22-6747
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ECS
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On-site presentation
Jamin S. Greenbaum, Christine Dow, Tyler Pelle, Mathieu Morlighem, Helen Fricker, Susheel Adusumilli, Adrian Jenkins, Anja Rutishauser, Donald Blankenship, Richard Coleman, Benjamin Galton-Fenzi, Won Sang Lee, Jason Roberts, and Seung-Tae Yoon

Accurate prediction of sea level rise requires detailed understanding of processes contributing to ice sheet mass loss. Antarctica’s ice shelves are thinning, resulting in enhanced flow of grounded ice due to weakened ice shelf buttressing. Glaciers feeding ice shelves with the highest melt rates are also experiencing some of the most rapid grounding zone retreat. However, these ice shelf melt rates reach values that cannot be explained by ocean forcing alone and are not reproduced in ocean models. We present subglacial hydrology model outputs for four major Antarctic glaciers (Pine Island, Thwaites, Totten and Denman), which flow through the deepest and most extensive Antarctic marine subglacial basins and feed rapidly thinning ice shelves. We show that the areas of high ice shelf melting rates and grounding line retreat coincide closely with areas of high subglacial discharge. We posit that the subglacial discharge provides the missing component driving the high melt rates, and identify positive feedbacks between ice dynamics, steepening of ice shelf basal slope, and subglacial outflow. If surface temperatures increase as expected in Antarctica over the coming decades, surface meltwater could flow to the ice sheet base, as observed in Greenland. The surface meltwater hydrological cycle could therefore contribute to seasonal variations in subglacial meltwater and ice shelf basal melt, leading to accelerated grounding line retreat into Antarctica’s deepest subglacial basins. Invoking these feedbacks could reconcile sea level records and ice sheet model simulations that remain overly stable in warmer periods.

How to cite: Greenbaum, J. S., Dow, C., Pelle, T., Morlighem, M., Fricker, H., Adusumilli, S., Jenkins, A., Rutishauser, A., Blankenship, D., Coleman, R., Galton-Fenzi, B., Lee, W. S., Roberts, J., and Yoon, S.-T.: Antarctic grounding line retreat enhanced by subglacial freshwater discharge, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6747, https://doi.org/10.5194/egusphere-egu22-6747, 2022.

10:43–10:49
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EGU22-2364
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ECS
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Presentation form not yet defined
Tijn Berends, Lennert Stap, and Roderik van de Wal

The loss of ice in Antarctica is dominated by the melting of floating ice shelves due to warming oceans. However, the relation between changing ocean temperatures and rates of sub-shelf melt is poorly constrained. Ice-sheet models currently employ a range of different approaches to this problem, varying in complexity from simple parameterizations based on linear temperature-melt relations to fully coupled ocean models. While several studies have compared two or more parameterisations, such efforts are complicated by the complex geometry of the Antarctic ice-sheet, as well as the uncertainty in (future) patterns of ocean circulation and atmospheric forcing.

The MISMIP/ISOMIP/MISOMIP family of experiments (Asay-Davis et al., 2016) provides a framework for intercomparing basal melt parameterisations in an idealized geometry, reducing the many difficulties of applying them in a realistic setting. Here, we present results of the MISOMIP1 experiment with the ice-sheet model IMAU-ICE. We show that the differences in simulated ice-sheet retreat caused by the use of different basal melt models are much larger than those arising from other model uncertainties such as the formulation of basal sliding, stress balance approximations, and model resolution. This suggests that basal melt is likely the largest source of uncertainty in future projections of Antarctic ice-sheet retreat.

How to cite: Berends, T., Stap, L., and van de Wal, R.: Uncertainties in marine ice-sheet retreat are dominated by basal melt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2364, https://doi.org/10.5194/egusphere-egu22-2364, 2022.

10:49–10:55
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EGU22-12173
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ECS
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Virtual presentation
Erwin Lambert, Andre Jueling, Roderik van de Wal, and Paul Holland

A major source of uncertainty in future sea-level projections is the ocean-driven basal melt of Antarctic ice shelves. Remote sensing estimates of basal melt shows kilometer-scale features, and ice sheet models require kilometer-scale resolution to realistically resolve ice shelf stability and grounding line migration. Yet 3D numerical ocean models are computationally too heavy to produce melt rates at this resolution. To bridge this resolution gap, we here present the 2D numerical model LADDIE which allows for computationally efficient downscaling of basal melt rates, based on coarse 3D ocean model output. As a test case, we apply the model to downscale basal melt rates of the Crosson-Dotson ice shelf in the fast-melting Amundsen Sea region. Due to the model’s computational efficiency, parameters can be tuned extensively. We have tuned the model to a range of parameters, namely average basal melt, melt in the Kohler West grounding zone, melt along the Dotson Ice Shelf Channel, and the overturning circulation of the cavity waters. These tuning targets are taken from a range of remote sensing products and in situ ocean observations. We show that the model can be tuned to agree with all observations, providing confidence that the model contains the essential physical processes governing basal melt. We propose that (sub-)kilometer resolution basal melt rates can be used to improve the realism of ice sheet models and their simulations of contemporary and future mass loss. Here we show that LADDIE can provide these boundary conditions in a computationally efficient way.

How to cite: Lambert, E., Jueling, A., van de Wal, R., and Holland, P.: LADDIE: a one-Layer Antarctic model for Dynamical Downscaling of Ice-ocean Exchanges, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12173, https://doi.org/10.5194/egusphere-egu22-12173, 2022.

10:55–11:01
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EGU22-10439
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ECS
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On-site presentation
Tyler Pelle, Adrian Jenkins, Christine Dow, and Jamin Greenbaum

Ice shelf basal melting is the primary mechanism by which the Antarctic Ice Sheet loses mass. While ocean forcing is the principal driver of basal melting, recent evidence suggests that localized melt maxima are located along deep grounding lines where large quantities of subglacial water are being discharged into sub-ice shelf cavities. As any change in the configuration of the grounding line can drastically influence the stress regime of the entire upstream grounded glacier, it is crucial we resolve this subglacial discharge-driven melting in a basal melt rate parameterization that can be used in standalone ice sheet models. Here, we extend the application of a 1D ocean and subglacial discharge driven melt parameterization into a 2D ice sheet model and apply it in forward simulations of Denman Glacier, East Antarctica.  Using subglacial hydrology model outputs to constrain the discharge inputs, we find that this parameterization resolves both local maxima and the large-scale spatial distribution of melt beneath the ice shelves buttressing Denman, Totten, Thwaites, and Pine Island glaciers. In the forward simulations of Denman Glacier, the melt contribution from subglacial discharge is required to reproduce contemporary patterns of grounding line retreat and rates volume loss. Under realistic 21st century ocean and subglacial forcing scenarios, Denman and Scott glaciers undergo largescale retreat and Denman Glacier retreats upstream to a ~10 km prograde section of bed topography upon which the grounding line stabilizes. However, under enhanced forcing, it is possible that Denman’s grounding line can overcome this topographic high and retreat inland into the deepest submarine trench on Earth beyond 2100

How to cite: Pelle, T., Jenkins, A., Dow, C., and Greenbaum, J.: A new ice shelf melt model that accounts for freshwater discharge and application to Denman Glacier, East Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10439, https://doi.org/10.5194/egusphere-egu22-10439, 2022.

11:01–11:07
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EGU22-138
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ECS
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On-site presentation
Salar Karam, Elin Darelius, Keith Nicholls, and Anna Wåhlin

Basal melting of ice shelves in the Amundsen Sea – caused by inflows of relatively warm and salty ocean water – has caused widespread thinning and acceleration of their tributary glaciers. In this study, we present novel time series from 2016 with sub-weekly resolution of direct measurements of basal melt rate from four sites on the western Getz Ice Shelf, including one site close to a grounding line. We examine spatial differences between the sites and complement these time series with mooring records from outside the cavity to investigate driving mechanisms of the basal melt rate from sub-seasonal down to tidal time scales. Far from the grounding line, melt rates display strong variability at fortnightly frequencies, caused by spring-neap tidal cycles increasing turbulence and subsequently mixing up heat towards the ice base. No variability at fortnightly frequencies is visible close to the grounding line, implying that well-mixed conditions there reduce the effect of the spring-neap tidal cycle. On longer time scales, the melt rate appears to show sensitivity to the depth of the thermocline, which previous studies have linked to wind forcing at the shelf break. As glaciers in West Antarctica are rapidly thinning, contributing significantly to sea level rise, it is becoming increasingly urgent to understand driving mechanisms of the basal melt rate.

How to cite: Karam, S., Darelius, E., Nicholls, K., and Wåhlin, A.: Spatial and Temporal Variability of Basal Melt Rate beneath Getz Ice Shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-138, https://doi.org/10.5194/egusphere-egu22-138, 2022.

11:07–11:13
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EGU22-6634
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ECS
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Virtual presentation
Johannes Feldmann, Ronja Reese, Ricarda Winkelmann, and Anders Levermann

Basal ice-shelf melting is the key driver of Antarctica’s increasing sea-level contribution. In diminishing the buttressing force of the ice shelves that fringe the ice sheet the melting increases the solid-ice discharge into the ocean. Here we contrast the influence of basal melting in two different ice-shelf regions on the time-dependent response of an idealized, inherently buttressed ice-sheet-shelf system. Carrying out three-dimensional numerical simulations, the basal-melt perturbations are applied close to the grounding line in the ice-shelf’s 1) ice-stream region, where the ice shelf is fed by the fastest ice masses that stream through the upstream bed trough and 2) shear margins, where the ice flow is slower. The results show that melting below one or both of the shear margins can cause a decadal to centennial increase in ice discharge that is more than twice as large compared to a similar perturbation in the ice-stream region. We attribute this to the fact that melt-induced ice-shelf thinning in the central grounding-line region is attenuated very effectively by the fast flow of the central ice stream. In contrast, the much slower ice dynamics in the lateral shear margins of the ice shelf facilitate sustained ice-shelf thinning and thereby foster buttressing reduction. Regardless of the melt location, a higher melt concentration toward the grounding line generally goes along with a stronger response. Our results highlight the vulnerability of outlet glaciers to basal melting in stagnant, buttressing-relevant ice-shelf regions, a mechanism that may gain importance under future global warming.

How to cite: Feldmann, J., Reese, R., Winkelmann, R., and Levermann, A.: Shear-margin melting causes stronger transient ice discharge than ice-stream melting according to idealized simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6634, https://doi.org/10.5194/egusphere-egu22-6634, 2022.

11:13–11:19
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EGU22-12729
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ECS
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On-site presentation
Dorothée Vallot, Nicolas Jourdain, Anna Crawford, Jan Åström, Doug Benn, and Pierre Mathiot

Below ice shelves, complex interactions between the ice and the ocean are at stake that have large implications for future sea level rise. Basal melting from the ocean is recognised to have large impacts on the stability. Many studies focus on theses interactions in coupled models at different spatio-temporal scales. However, most of them consider the basal topography of the shelf as smooth ignoring its irregular state due to basal crevassing or channel-like features. We propose to investigate the impact of these features on basal melt and ice shelf stability by using a discrete particule model and an ocean model applied at the ice shelf of Thwaites glacier.

How to cite: Vallot, D., Jourdain, N., Crawford, A., Åström, J., Benn, D., and Mathiot, P.: Modelling the ocean-ice interactions at the broken state of ice shelves, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12729, https://doi.org/10.5194/egusphere-egu22-12729, 2022.

11:19–11:25
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EGU22-6203
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ECS
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Virtual presentation
Ole Zeising, Julia Christmann, Hugh F. J. Corr, Veit Helm, Lea-Sophie Höyns, Coen Hofstede, Ralf Müller, Niklas Neckel, Keith W. Nicholls, Timm Schultz, Daniel Steinhage, Michael Wolovick, and Angelika Humbert

Basal melt channels of ice shelves influence ice-ocean interaction and thus the current and future dynamics of ice sheets and ice shelves. Understanding their evolution is necessary to assess their influence on ice shelves’ stability. In this study, we investigate the evolution of a basal channel, up to 330 m high, located in the southern Filchner Ice Shelf where the ice thickness is between 1150 and 1400 m. Observations with a phase-sensitive Radio Echo Sounder (pRES) reveal decreasing melt rates within the channel, from 1.8 m/a to freezing with increasing distance from the grounding line of Support Force Glacier. At a distance of 20 km from the grounding line, melt rates within the channel fall below those of the ambient ice and the height of the channel starts to decrease. Calculating the evolution of this channel over 250 years, under present-day melt rates, reveals a mismatch when compared with its present geometry: the melt rates would have needed to have been twice as high as those of the present day to form today's channel geometry. In contrast, the present-day melt rates result in a closure of the channel. These results were confirmed by simulations with a viscoelastic model: while the present-day melt rates led to a closure of the channel, higher melt rates reproduced the current channel geometry. The type of melt channel in this study diminishes with distance from the grounding line and is therefore not a destabilizing factor for ice shelves.

How to cite: Zeising, O., Christmann, J., Corr, H. F. J., Helm, V., Höyns, L.-S., Hofstede, C., Müller, R., Neckel, N., Nicholls, K. W., Schultz, T., Steinhage, D., Wolovick, M., and Humbert, A.: The evolution of a basal melt channel on the southern Filchner Ice Shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6203, https://doi.org/10.5194/egusphere-egu22-6203, 2022.

11:25–11:31
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EGU22-7254
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ECS
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On-site presentation
Ann-Sofie Priergaard Zinck, Bert Wouters, and Stef Lhermitte

Like most of the ice shelves in the Antarctic Amundsen Sea Embayment, intrusion of warm circumpolar deep water onto the continental shelf causes basal thinning to the Dotson Ice Shelf (DIS). Studies on other ice shelves have shown how Digital Elevation Models (DEM) of high spatial resolution can reveal basal melt patterns that are crucial in understanding the dynamics of basal melting and the underlying ocean circulation. In this study we aim to achieve high spatial and temporal resolution basal melt rates of the DIS to try to uncover new basal melt patterns which products of coarser resolution do not capture. This will be done by using the high spatial resolution Reference Elevation Model of Antarctica (REMA) and a method based on the Google Earth Engine (GEE). This allows for fast co-registration and subsequent thinning and basal melt rate analysis of the 2-m resolution REMA strips from 2010-2017. Ice shelf thinning is calculated both in a Eulerian and Lagrangian framework, the latter providing information to the basal melt rate analysis. In agreement with other studies of the DIS a melt channel is found on the western side of the ice shelf. Furthermore, our study indicates a second smaller channel, which has not been revealed by existing altimetry studies.  This suggests that high-resolution basal melt rate products could be of great importance. Furthermore, it enlightens the difficulties in coupling ocean and ice models, since such models often run on a coarser grid and therefore, they will not capture the small-scale variabilities.

How to cite: Zinck, A.-S. P., Wouters, B., and Lhermitte, S.: Uncovering basal melt channels on the Dotson Ice Shelf, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7254, https://doi.org/10.5194/egusphere-egu22-7254, 2022.

11:31–11:37
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EGU22-5859
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ECS
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Virtual presentation
Beatriz Recinos, Daniel Goldberg, James Maddison, and Joe Todd

Fenics_ice is a finite element model framework written in Python that quantifies the initialization uncertainty for time-dependent ice sheet models. Here, we apply for the first time this framework to real ice streams in the Amundsen basin: Smith, Pope and Kohler Glaciers. We quantify the degree to which observational uncertainty translates to parametric uncertainty (posterior uncertainty of inversions for basal drag and ice stiffness fields) and to uncertainty in projected quantities of interest (QoIs) such as sea level contribution. The framework implements the Shallow Shelf Approximation (SSA), and implements a control methods approach to invert for the basal drag and ice stiffness fields. Beginning with a cost function optimization which can allow for either gridded or point-cloud velocities, we generate a low-rank approximation to the posterior covariance of the parameters through the use of the cost function Hessian. In our work, the Hessian is calculated through algorithmic differentiation (AD) using the “complete” Hessian rather than the Gauss–Newton approximation. We then project the covariance on a linearization of the time-dependent ice sheet model (again using AD to generate the linearization) to estimate the growth of QoI uncertainty over time. We then show the model framework and capabilities when applied to these ice streams and our future plans to scale our framework into a larger domain.

How to cite: Recinos, B., Goldberg, D., Maddison, J., and Todd, J.: Fenics_ice framework applied to three West Antarctic ice streams: Smith, Pope and Kohler Glaciers., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5859, https://doi.org/10.5194/egusphere-egu22-5859, 2022.