CR3.4

EDI
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: Rachel CarrECSECS, Angelika Humbert, Adrian Jenkins, Nicolas Jourdain
vPICO presentations
| Tue, 27 Apr, 13:30–17:00 (CEST)

vPICO presentations: Tue, 27 Apr

Chairperson: Inga Monika Koszalka
13:30–13:35
CR3.4 Session 1: Presentations
13:35–13:37
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EGU21-2313
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ECS
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Highlight
Jan-Hendrik Malles, Fabien Maussion, and Ben Marzeion

Mountain glaciers across the world are contributing around one-third to the recent barystatic global mean sea-level rise, and relevant for regional hydrological changes. Although the majority of Earth’s glaciers is land-terminating, roughly one-third of the glaciated area drains into an ocean or a lake. Due to the interrelation of surface and frontal mass budget, marine-terminating glaciers are subject to different dynamics than land-terminating ones, which are only forced by the atmosphere. This means that mass changes of marine-terminating glaciers cannot only be explained by changes in the atmospheric forcing. Thus, if ice-ocean interaction is not explicitly treated in a mass-balance model, calibration using, e.g., geodetic mass balances will lead to an overestimation of these glaciers’ sensitivity to changes in atmospheric temperatures. However, most large-scale glacier models are not yet able to account for this process and frontal ablation remains an elusive feature of glacier dynamics, because direct observations are sparse. We explore this issue by implementing a simple frontal ablation parameterization in the Open Global Glacier Model (OGGM). One of the major changes this entails is the lowering of marine-terminating glaciers’ sensitivities to atmospheric temperatures in the model’s surface mass-balance calibration. We then use this model, forced with an ensemble of atmospheric temperature and precipitation projections from climate models taking part in the Climate Model Intercomparison Project’s sixth phase (CMIP6), to project global glacier mass change until 2100. The main aim of this work is to investigate  the influence of the frontal ablation parameterization on those projections. We find that introducing the parameterization of frontal ablation, but ignoring changes in ocean climate, reduces the spread between different emission scenarios in 2100.

How to cite: Malles, J.-H., Maussion, F., and Marzeion, B.: Exploring the influence of frontal ablation on global glacier mass change projections, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2313, https://doi.org/10.5194/egusphere-egu21-2313, 2021.

13:37–13:39
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EGU21-2274
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ECS
Donald Slater, Doug Benn, Tom Cowton, Jeremy Bassis, and Joe Todd

For tidewater glaciers worldwide, calving is a principal mechanism of mass loss. In turn, undercutting of tidewater glacier termini by submarine melting is understood to be a principal driver of calving. Yet, we currently have no practical and widely-accepted parameterisations that can represent the impact of submarine melting on calving in ice sheet models that are used for sea level projection, reducing confidence in their predictions.

The ‘crevasse-depth calving law’ that broadly relates depth-mean stress to a crevasse depth has been very widely used in models of tidewater glaciers, but this law does not fully account for the impact of submarine melt undercutting on the near-terminus stress field, which may be the key link between tidewater glaciers and the ocean. As such, we here work to incorporate the full impact of melt undercutting into a revised crevasse-depth calving law.

We combine elastic beam theory, linear elastic fracture mechanics and Elmer/Ice simulations to study the propagation of surface and basal crevasses near the front of tidewater glaciers in response to melt undercutting. We work to parameterise these results through a simple revision of the existing crevasse-depth calving law. The revised law explicitly accounts for the impact of melt undercutting on crevasses near the terminus, without increasing the computational demand on ice sheet models that might incorporate such a law, representing an important step towards better projection of ice sheet mass loss driven by the ocean.

How to cite: Slater, D., Benn, D., Cowton, T., Bassis, J., and Todd, J.: Revisiting the crevasse-depth calving law in the presence of melt undercutting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2274, https://doi.org/10.5194/egusphere-egu21-2274, 2021.

13:39–13:41
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EGU21-126
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ECS
Adrien Wehrlé, Martin P Lüthi, Andrea Walter, Guillaume Jouvet, and Andreas Vieli

Glacier calving plays a key role in the recently observed dynamic mass
loss of the Greenland ice sheet. Calving waves, generated by the
sudden detachment of ice from the glacier terminus, can reach tens of
meters of height and have devastating effects upon impact on
surrounding shores. In this study, we describe a new method for the
detection of source location and timing of calving waves, and the
analysis of their magnitude and spreading properties using a
terrestrial radar interferometer (TRI). This method was applied to
11,500 minute-interval TRI acquisitions from Eqip Sermia, Greenland.
More than 2,000 calving waves were detected within seven
days. Quantitative assessment with a Wave Power Index (WPI) showed
spatially distinctive patterns: the sector of the calving front ending
in deep water shows a higher wave activity (+49%) with higher
cumulative WPI (+34%) than the shallow sector. In combination with
a detection of meltwater plume locations, we highlighted a 2.3 times
higher occurrence of visible meltwater plumes in the deep sector than the
shallow one. We found both the cumulated WPI and the number of waves
to increase by more than 80% in the presence of a meltwater plume
in the deep sector while only by 30% in the shallow sector.  We
therefore explain the higher calving activity in the deep sector to be 
strongly related to a combination of higher occurrence of meltwater plumes 
and more efficient calving enhancement linked to better connections 
to deep warm waters.

How to cite: Wehrlé, A., Lüthi, M. P., Walter, A., Jouvet, G., and Vieli, A.: Enhanced calving rates related to meltwater plume occurrence at Eqip Sermia, Greenland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-126, https://doi.org/10.5194/egusphere-egu21-126, 2020.

13:41–13:43
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EGU21-797
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ECS
Felicity Holmes, Eef van Dongen, and Nina Kirchner

Understanding how tidewater glaciers are responding to climatic and oceanographic changes is vital in order to reduce uncertainty in sea level rise estimates. In this project, we are using the 3D calving model in Elmer/Ice to simulate how Kronebreen responds over short time scales to various forcing scenarios. Specifically, a variety of frontal melt scenarios are being implemented to understand how calving and glacier dynamics respond to changing inputs. Both the magnitude and spatial distribution of frontal melt will be varied, with these scenarios being informed by a dataset of glacier proximal water temperatures (spanning Aug 2016 –  Aug 2017) as well as by plume locations as identified from satellite imagery.  The model output will be compared to observational data (frontal position, velocities) collected for the period 2016 – 2017 with the aim of running longer simulations using a ’best fit’ model set up. Details of the experimental set up, as well as some preliminary results, are presented here. 

How to cite: Holmes, F., van Dongen, E., and Kirchner, N.: Modelling calving at Kronebreen, Svalbard using Elmer/Ice, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-797, https://doi.org/10.5194/egusphere-egu21-797, 2021.

13:43–13:45
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EGU21-14678
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ECS
Ben Davison

Ocean-driven retreat of Greenland’s tidewater glaciers remains a large uncertainty in predictions of sea level rise, partly due to limited constraints on glacier-adjacent water properties. Icebergs are likely important modifiers of fjord water properties, yet their effect is poorly understood. Here, we use a 3-D ocean circulation model coupled to a submarine iceberg melt module to investigate the effect of submarine iceberg melting on glacier-adjacent water properties in a range of idealised settings. Icebergs can modify glacier adjacent water properties in three principle ways: (1) substantial cooling and modest freshening in the upper ~50 m of the water column; (2) warming of Polar Water due to iceberg-induced upwelling of warm Atlantic Water, and; (3) the Atlantic Water layer warms on average when vertical temperature gradients through the Atlantic Water layer are steep (due to vertical mixing of warm water at depth), but cools on average when vertical temperature gradients are shallow. When icebergs extend to-or-below sill depth, they can cause cooling throughout the entire water column. All of these effects are more pronounced in fjords with higher iceberg concentrations and deeper iceberg keel depths. These results characterise the important role of icebergs in modifying ice sheet – ocean interaction and highlight the need to improve representations of fjord processes in ice sheet-scale models.

How to cite: Davison, B.: Characterising the effect of submarine iceberg melting on glacier-adjacent water properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14678, https://doi.org/10.5194/egusphere-egu21-14678, 2021.

13:45–13:47
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EGU21-2972
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ECS
Alex Bradley, Paul Holland, and Pierre Dutrieux

In recent years, the ice shelf of Pine Island Glacier has experienced several significant calving events. It is understood that the presence of the ice shelf in conjunction with a subglacial ridge provide a strong topographic barrier to warm Circumpolar Deep Water spilling onto the continental shelf, but it is not known how this barrier will respond to this recent, and possible future, calving events. In this presentation, I shall present results of numerical simulations of ocean circulation under Pine Island Glacier, which indicate a strong sensitivity to such calving events, and discuss the implications of these results for the overall stability of the glacier.

How to cite: Bradley, A., Holland, P., and Dutrieux, P.: Sub-shelf melting consequences of recent and future Pine Island Glacier ice shelf calving events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2972, https://doi.org/10.5194/egusphere-egu21-2972, 2021.

13:47–13:49
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EGU21-9310
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ECS
Mengzhen Qi, Yan Liu, and Xiao Cheng

  Iceberg calving, one of the key processes of Antarctic mass balance, has been regarded as an important variable in fine monitoring the changes of ice shelves. Based on multi-source satellite imagery, all annual calving events larger than 1 km² that occurred from August 2005 to August 2019 were extracted. Also, their area, thickness, mass, and calving recurrence cycle were calculated to derive the annual iceberg calving dataset. This dataset contains the distribution of 14-year annual calving events, along with the attributes of each calving event including calving year, length, area, average thickness, mass, recurrence interval, and calving type, and it can directly reflect the magnitude characteristics and distribution of Antarctic iceberg calving in different years, which fills the gap of fine monitoring dataset of iceberg calving and provides fundamental data for subsequent research on calving mechanism and mass balance of Antarctic ice shelf-ice sheet system.

How to cite: Qi, M., Liu, Y., and Cheng, X.: the Development of an Annual Iceberg Calving Dataset of the Antarctic Ice Shelves , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9310, https://doi.org/10.5194/egusphere-egu21-9310, 2021.

13:49–13:51
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EGU21-6099
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ECS
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Highlight
Maaike Izeboud and Stef Lhermitte

The increasing contribution of the Antarctic Ice Sheet to sea level rise is linked to reductions in ice shelf buttressing, compounded by their thinning, weakening and fracturing. Ice shelf shear zones that are highly crevassed and with open fractures are first signs that these shear zones have structurally weakened. The weakening of shear zones by this damage results in speedup, shearing and further weakening of the ice shelf, hence promoting additional damage development. This damage feedback potentially preconditions these ice shelves for disintegration and enhances grounding line retreat, and is considered key to the retreat of Pine Island Glacier and Thwaites Glacier as well as the collapse of Larsen B. Although damage feedbacks have been identified as key to future ice shelf stability, it is one of the least understood processes in marine ice sheet dynamics. Furthermore, the amount of damage and its changes is yet to be quantified.

Quantifying damage efficiently and accurately is a challenging task due to the highly complex surface of Antarctica, the variations in viewing-illumination geometry, snow or cloud cover and the variable signal-to-noise levels in satellite imagery (e.g. speckle in SAR). As a result, efforts to detect damage from remote sensing are usually limited to regional studies or limited in spatial resolution, thus only identifying large rifts. Or alternatively when used to support models or machine learning techniques, mapping fractures is often done manually, with several shortcomings. Lastly, there has been little to no effort to map the changes of damage state over regional areas.

In this study we construct an Antarctic wide damage and damage change assessment from an automated approach that includes high resolution features, with regional focus on identified weak ice shelves. We apply the radon transform technique to detect damage from both optical (Sentinel-2) and SAR (Sentinel-1) imagery in the past 5 years (2015-2020). The radon transform has been demonstrated to be efficient in detecting along-flow features and also to be used for complex flow patterns with a wide range of crevasse orientations. By using two remote sensing sources, we overcome the stated challenges that relate to the respective individual sources.

In our damage assessment we are able to distinguish shallow surface crevasses from large rifts, and identify mode I (opening; tensile) and mode III (shearing) fractures. With this, we can clearly identify weak ice shelves from our results, such as Pine Island and Thwaites glacier, where the damage area in the shear margins has grown substantially over the years. The changes in mode I and mode III fracture patterns observed on these ice shelves provide additional insights in the development of shear zone. Lastly, we show a good agreement in fracture pattern retrieved from optical and SAR imagery, and the complimentary application of SAR to detect fractures under snow cover.

How to cite: Izeboud, M. and Lhermitte, S.: Damage state and damage change assessment from remote sensing observations at Antarctic ice shelves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6099, https://doi.org/10.5194/egusphere-egu21-6099, 2021.

13:51–13:53
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EGU21-6392
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ECS
Seth Olinger, Brad Lipovsky, Marine Denolle, and Brendan Crowell

Nearly 50% of Antarctic ice discharge into the ocean occurs via iceberg calving (Depoorter et al 2013). Large tabular icebergs calve from ice shelves along large fractures called rifts, but the physics of rifting are poorly understood. How fast does rift propagation occur? Does the timing of rift fracture coincide with episodes of unusual ice motion? We investigate these questions using data from seismometers and GPS sensors deployed on Pine Island Glacier ice shelf (PIG) from January 2012 to December 2013 surrounding the calving of iceberg B31, which exceeded 700 km2 in size and calved in November 2013 along a large rift. Using TerraSAR-X imagery, we identify a large 7km-long rift that must have occurred between May 8 and May 11, 2012. We identify a large-amplitude seismic signal on May 9, 2012, which we attribute to the rifting event. The signal is broadband, containing energy at frequencies higher than 1 Hz and lower than 0.01 Hz, and exhibits pronounced dispersion characterized by high frequencies arriving before low frequencies. We use features of the May 9 “riftquake” to detect thousands of similar events, which we classify using K-shape clustering. We hypothesize that the observed signals are flexural gravity waves generated by a bending moment applied to the ice shelf during fracture. To test this hypothesis, we model the ice shelf as a dynamic beam supported by an inviscid, incompressible ocean. We find that the model reproduces observed riftquake waveforms when forced with a bending moment. We then use a Markov Chain Monte Carlo inversion to model representative events from each cluster of observed events. The inversion reveals that source durations on the order of seconds have the highest likelihood of explaining observed riftquake waveforms, suggesting that rifting occurs on elastic timescales. Finally, we locate the riftquakes and find that a swarm of events originating at the rift tip occurs just after the start of a period of acceleration at PIG, suggesting that the stress concentrations driving rift opening are influenced by changes in ice dynamics.

How to cite: Olinger, S., Lipovsky, B., Denolle, M., and Crowell, B.: Riftquakes: Recording and Modeling Seismic Signals of Rifting at Pine Island Glacier, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6392, https://doi.org/10.5194/egusphere-egu21-6392, 2021.

13:53–13:55
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EGU21-6175
Yuri Konovalov

The propagation of high-frequency elastic-flexural waves through an ice shelf was modeled by a full 3-D elastic models. These models based on the momentum equations that were written as the differential equations (model#1) and as the integro-differential equations (model#2). The integro-differential form implies the vertical integration of the momentum equations from the ice surface to the current coordinate z like, for instance, in the Blatter-Pattyn ice flow model. The sea water flow under the ice shelf is described by the wave equation. The numerical solutions were obtained by a finite-difference method. Numerical experiments were undertaken for a crevasse-ridden ice shelf with different spatial periodicities of the crevasses. In this research the modeled positions of the band gaps in the dispersion spectra dependently on the spatial periodicities of the crevasses is investigated from the point of view of agreement of these positions with the Bragg’s law. The investigation of the dispersion spectra shows that different models reveal different sensitivities of the dispersion spectra (in relation to the appearance of the band gaps in the spectra) dependently on the spatial periodicity of the crevasses and on the crevasses depth.

How to cite: Konovalov, Y.: Modeling of ocean wave propagation across the crevasse-ridden ice shelf: focus on the comparison of two models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6175, https://doi.org/10.5194/egusphere-egu21-6175, 2021.

13:55–13:57
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EGU21-3053
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ECS
Vjeran Visnjevic, Reinhard Drews, Clemens Schannwell, and Inka Koch

Ice shelves buttress ice flow from the continent towards the ocean, and their disintegration results in increased ice discharge.  Ice-shelf evolution and integrity is influenced by surface accumulation, basal melting, and ice dynamics. We find signals of all of these processes imprinted in the ice-shelf stratigraphy that can be mapped using isochrones imaged with radar.

Our aim is to develop an inverse approach to infer ice shelf basal melt rates using radar isochrones as observational constraints. Here, we investigate the influence of basalt melt rates on the shape of isochrones using combined insights from both forward and inverse modeling. We use the 3D full Stokes model Elmer/Ice in our forward simulations, aiming to reproduce isochrone patterns observed in our data. Moreover we develop an inverse approach based on the shallow shelf approximating, aiming to constrain basal melt rates using isochronal radar data and surface velocities. Insights obtained from our simulations can also guide the collection of new radar data (e.g., profile lines along vs. across-flow) in a way that ambiguities in interpreting the ice-shelf stratigraphy can be minimized. Eventually, combining these approaches will enable us to better constrain the magnitude and history of basal melting, which will give valuable input for ocean circulation and sea level rise projections.

How to cite: Visnjevic, V., Drews, R., Schannwell, C., and Koch, I.: Constraining ice shelf basalt melting rates from isochrone data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3053, https://doi.org/10.5194/egusphere-egu21-3053, 2021.

13:57–13:59
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EGU21-4750
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ECS
Erwin Lambert, André Jüling, Paul Holland, and Roderik van de Wal

The contact between ice shelves and relatively warm ocean waters causes basal melt, ice shelf thinning, and ultimately ice sheet mass loss. This basal melt, and its dependence on ocean properties, is poorly understood due to an overall lack of direct observations and a difficulty in explicit simulation of the circulation in sub-shelf cavities. In this study, we compare a number of parameterisations and models of increasing complexity, up to a 2D ‘Layer’ model. Each model is aimed at quantifying basal melt rates as a function of offshore temperature and salinity. We test these models in an idealised setting (ISOMIP+) and in a realistic setting for the Amundsen Sea Embayment. All models show a comparable non-linear sensitivity of ice-shelf average basal melt to ocean warming, indicating a positive feedback between melt and circulation. However, the Layer model is the only one which explicitly resolves the flow direction of the buoyant melt plumes, which is primarily governed by rotation and by the basal topography of the ice shelves. At 500m resolution, this model simulates locally enhanced basal melt near the grounding line, in topographical channels, and near the western boundary. The simulated melt patterns for the Amundsen Sea ice shelves are compared to satellite observations of ice shelf thinning and to 3D numerical simulations of the sub-shelf cavity circulation. As detailed melt rates near the grounding line are essential for the stability of ice sheets, spatially realistic melt rates are crucial for future projections of ice sheet dynamics. We conclude that the Layer model can function as a relatively cheap yet realistic model to downscale 3D ocean simulations of ocean properties to sub-kilometer scale basal melt fields to provide detailed forcing fields to ice sheet models.

How to cite: Lambert, E., Jüling, A., Holland, P., and van de Wal, R.: Simulating detailed spatial patterns of ice shelf basal melt, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4750, https://doi.org/10.5194/egusphere-egu21-4750, 2021.

13:59–14:01
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EGU21-3486
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ECS
Joanna Millstein and Brent Minchew

Glaciers and ice sheets flow as a consequence of ice rheology. At the temperatures and pressures found on Earth, several creep mechanisms allow glacier ice to flow as a non-Newtonian (shear-thinning) viscous fluid. The semi-empirical constitutive relation known as Glen’s Flow Law is often used to describe ice flow and to provide a simple expression for an effective viscosity that decreases with increasing stress and deformation rate. Glen’s Flow Law is a power-law relation between effective strain rate and deviatoric stress, with two parameters defining the rheology of ice: a rate factor, A, and stress exponent, n. The rate factor depends on features such as temperature and grain size, while the stress exponent is primarily representative of the creep mechanism. Neither A nor n are well constrained in natural ice, and the stress exponent is typically assumed to be n = 3 everywhere. Here, we take advantage of recent improvements in remotely sensed observations of surface velocity and ice shelf thickness to infer the values of A and n in Antarctic ice shelves. We focus on areas of ice shelves that flow in a purely extensional regime, where extensional stresses are proportional to observed ice thickness, drag at the base of the ice is negligible, and extensional strain-rates are calculated from the gradients of observed surface velocity fields. In this manner, we use independent observational data to derive spatially dependent constraints on the rate factor A and stress exponent n in Glen's Flow Law. The robust spatial variability provides insights into the creep mechanisms of ice, thereby capturing rheological properties from satellite observations. Our analysis indicates that n ≈ 4 in most fast-flowing areas in an extensional regime, contrary to the prototypical value of n = 3. This finding implies higher non-linearity in ice flow than typically prescribed, influencing calculations of mass flux and the response of ice sheets to perturbations. Additionally, This result suggests that dislocation creep is the dominant creep mechanism in extensional regimes of Antarctic ice shelves, indicative of tertiary creep. This analysis unites theoretical work and synoptic-scale observations of ice flow, providing insights into the rheology and stress-states of ice shelves in Antarctica.

How to cite: Millstein, J. and Minchew, B.: Inferring ice rheology in Antarctic ice shelves from remotely sensed observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3486, https://doi.org/10.5194/egusphere-egu21-3486, 2021.

14:01–14:03
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EGU21-3566
Johannes Feldmann and Anders Levermann

The time scales of the flow and retreat of the outlet glaciers draining Greenland and Antarctica and their potential instabilities are arguably the largest uncertainty in future sea-level projections. The associated stress and velocity fields are highly complex. Here we derive an exact scaling law from first principles that shows that the time scale of outlet-glacier flow is related to the inverse of 1) the fourth power of the width-to-length ratio of its topographic confinement, 2) the third power of the confinement depth and 3) the temperature-dependent ice softness. We show that idealized numerical simulations of marine ice-sheet instabilities (MISI) as found in Antarctica follow this theoretical prediction. In a further step we apply the scaling law to observations of different MISI-prone Antarctic outlets to compare their potential instability time scales. The simple scaling relation incorporates the full complexity of the ice stress field of a fast outlet glacier similar to the predictive power of the thermodynamic equations of an ideal gas. In quantifying the non-linear influence of glacier geometry and temperature on the ice dynamicsscaling law allows to investigate similar ice flow under future global warming.

How to cite: Feldmann, J. and Levermann, A.: A scaling law for similar ice sheet flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3566, https://doi.org/10.5194/egusphere-egu21-3566, 2021.

14:03–14:05
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EGU21-7285
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ECS
Yu Wang, Chen Zhao, Rupert Gladstone, and Ben Galton-Fenzi

The Amery Ice Shelf (AIS), East Antarctica, has a layered structure, due to the presence of both meteoric and marine ice. In this study, the thermal structures of the AIS are evaluated from vertical temperature profiles, and its formation mechanism are demonstrated by numerical simulations. The temperature profiles, derived from borehole thermistor data at four different locations, indicate distinct temperature regimes in the areas with and without basal marine ice. The former shows a near-isothermal layer over 100 m at the bottom and stable internal temperature gradients, while the latter reveals a cold core ice resulting from upstream cold ice advection and large temperature gradients within 90 m at the bottom. The three-dimensional steady-state temperature fields are simulated by Elmer/Ice, a full-stokes ice sheet model, using three different basal mass balance datasets. We found the simulated temperature fields are highly sensitive to the choice of dynamic boundary conditions on both upper and lower surfaces. To better illustrate the formation of the vertical thermal regimes, we construct a one-dimensional temperature column model to simulate the process of ice columns moving on the flowlines with varying boundary conditions. The comparison of simulated and observed temperature profiles suggests that the basal mass balance and meteoric ice advection are both crucial factors determining the thermal structure of the ice shelf. The different basal mass balance datasets are indirectly evaluated as well. The improved understanding of the thermal structure of the AIS will assist with further studies on its thermodynamics and rheology.

How to cite: Wang, Y., Zhao, C., Gladstone, R., and Galton-Fenzi, B.: Thermal structure of the Amery Ice Shelf from borehole observations and simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7285, https://doi.org/10.5194/egusphere-egu21-7285, 2021.

14:05–14:07
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EGU21-935
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ECS
Katharina Streuff and Colm Ó Cofaigh

A new digital database compiling glacial landforms and sediments in the High Arctic was created in order to facilitate and underpin new research on palaeo-ice sheets and tidewater glacier dynamics. The database is in a geographic information system (GIS) format and will be available for web download when published. It documents evidence of previous glacial activity as visible on the contemporary seafloor of fjords and continental shelves around all of Svalbard, Greenland, and Alaska, and north of 66°30’ N in Russia, Norway, and Canada. Extensive literature research was conducted to create the database, compiling a large number of glacial landforms at a range of scales, sediment cores, and radiocarbon dates. Glacial landforms included in the database are cross-shelf troughs, trough-mouth fans, grounding-zone wedges, overridden moraines, glacial lineations, drumlins, crag-and-tails, medial moraines, terminal moraines, debris-flow lobes (including glacier-contact fans), recessional moraines, De Geer moraines, crevasse-fill ridges, eskers and submarine channels. Sediment core locations are attributed with a description of the sampled lithofacies and sediment accumulation rates where available. Radiocarbon dates were included when thought to be relevant for constraining the timing of large-scale palaeo-ice dynamics. Outlines of bathymetric datasets published before December 2020 were also mapped to give an overview of previously investigated research areas. The database will aid researchers in the reconstruction of ice dynamics during and since the Last Glacial Maximum and in the interpretation of High-Arctic glacial landform-sediment assemblages. Moreover, apart from providing a comprehensive bibliography on Arctic glacial geomorphological and sedimentological research, it is intended to serve as a basis for future ice sheet modelling of High-Arctic glacier dynamics.

How to cite: Streuff, K. and Ó Cofaigh, C.: A GIS Database of Submarine Glacial Landforms and Sediments on High-Arctic Continental Shelves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-935, https://doi.org/10.5194/egusphere-egu21-935, 2021.

14:07–14:09
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EGU21-10040
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ECS
Lukas Krieger and Dana Floricioiu

The drainage divides of ice sheets separate the overall glaciated area into multiple sectors and outlet glaciers. These catchments represent essential input data for partitioning glaciological measurements or modelling results to the individual glacier level. They specify the area over which basin specific measurements need to be integrated.

The delineation of drainage basins on ice sheets is challenging due to their gentle slopes accompanied by local terrain disturbances and complex patterns of ice movement. Therefore, in Greenland the basins have been mostly delineated along the major ice divides, which results in large drainage sectors containing multiple outlet glaciers. In [1] we developed a methodology for delineating individual glaciers that was applied to the Northeast Greenland sector and proposed slightly changed separations between 79N and Zachariae basins driven by the ice flow lines. In the present study the method is extended to the entire Greenland Ice Sheet.

We present a fully traceable approach that combines ice sheet wide velocity measurements by Sentinel-1 SAR and the 90 m TanDEM-X global DEM to derive individual glacier drainage basins for the entire Greenland Ice Sheet with a modified watershed algorithm. We delineate a total of 335 individual glacier catchments, a result triggered by the number and location of the selected seed points.

The resulting dataset will be made publicly available online and is extensible by even more granular delineations of individual tributaries upon request. The proposed approach has the potential to produce catchment areas also for the entirety of the Antarctic Ice Sheet.

 

[1] Krieger, L., D. Floricioiu, and N. Neckel (Feb. 1, 2020). “Drainage Basin Delineation for Outlet Glaciers of Northeast Greenland Based on Sentinel-1 Ice Velocities and TanDEM-X Elevations”.  In:Remote  Sensing  of  Environment 237,  p.  111483.

How to cite: Krieger, L. and Floricioiu, D.: Drainage basins and glacier catchments for the Greenland Ice Sheet, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10040, https://doi.org/10.5194/egusphere-egu21-10040, 2021.

14:09–14:11
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EGU21-15097
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Highlight
Johan Nilsson, Martin Jakobsson, Christian Stranne, Matt O'Regan, and Larry Mayer

Here, we report oceanographic observations and multi-beam bathymetry from the previously uncharted Sherard Osborn Fjord in North Greenland, collected in the summer of 2019 by the Swedish icebreaker Oden. Ryder Glacier, which has Greenland's third largest floating ice tongue, discharges into Sherard Osborn Fjord. The observations show that Arctic Atlantic Water interacts with the ice tongue, creating a prominent intermediate layer of glacially-modified water in the fjord. However, a secondary sill in the inner fjord restricts the heat carried by the Atlantic Water towards Ryder Glacier’s floating tongue, thereby reducing basal melt rates. The observations indicate that the inflow of Atlantic Water over the inner sill is limited by hydraulic control, and that shear-driven vertical mixing cools the inflow reaching the ice tongue. The interactions between the flow and the sill geometry suggest a negative feedback, which reduces the sensitivity of the basal melt rate to variations of Atlantic Water temperature. This may help to explain why the extent of Ryders Glacier's ice tongue has remained stable over the past 50 years, whereas the neighbouring Petermann Glacier's ice tongue, with a different sill geometry, has retreated significantly.  

How to cite: Nilsson, J., Jakobsson, M., Stranne, C., O'Regan, M., and Mayer, L.: Ice-ocean interactions on Ryder Glacier in North Greenland , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15097, https://doi.org/10.5194/egusphere-egu21-15097, 2021.

14:11–14:13
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EGU21-1358
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ECS
Henning Åkesson, Mathieu Morlighem, Martin Jakobsson, Johan Nilsson, and Christian Stranne

Ice shelves in Antarctica and Greenland are thinning and breaking up, and marine outlet glaciers are retreating, where the ocean is known to play a key role. This pattern is projected to continue over the next decades to centuries due to ocean warming induced by global carbon emissions. Given that we halt or even reverse the current warming trend and fulfil the Paris agreement, it is unclear whether ice shelves and glaciers can recover from the preceding breakup and retreat. Here, we use the numerical ice sheet model ISSM to assess whether Petermann Glacier in northwest Greenland can recover after future ice shelf breakup and grounding line retreat. Petermann’s ice tongue is one of the few remaining in Greenland, where several major calving events occurred over the last decade.

Our experiments suggest that if Petermann’s ice shelf collapses due to future ocean warming, the ice shelf will not regrow even if that warming is reversed. Neither an ocean warming reversal nor a more positive surface mass balance help the ice shelf to regrow once it has collapsed. Future ocean warming may thus push Petermann into a new dynamic state from where recovery is exceedingly difficult. Finally, we investigate whether reduced calving activity allows for future grounding line readvance and ice shelf recovery. We discuss our findings in light of both potential future recovery and ice shelf collapse and regrowth in the past.

How to cite: Åkesson, H., Morlighem, M., Jakobsson, M., Nilsson, J., and Stranne, C.: Potenial future recovery of Petermann Glacier in northwest Greenland simulated using ISSM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1358, https://doi.org/10.5194/egusphere-egu21-1358, 2021.

14:13–14:15
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EGU21-10758
Adam Stanway, Andrew Wells, Helen Johnson, and Jeff Ridley


Around Greenland, the transport of heat and fresh meltwater between the ocean and Greenland's Ice Sheet is mediated by circulation in several hundred proglacial fjords. These fjords are long and narrow, with circulation controlled by a variety of processes. This circulation, and the resultant heat transported to the ice sheet has global implications. However, the spatial scales of these fjords means that they cannot be directly represented in global scale climate models, as currently achievable horizontal resolutions are too coarse to resolve fjords directly. Therefore, a subgrid-scale parameterization scheme is required, to include the impact of fjord circulation on Greenland's Ice Sheet in these models. The development of such a scheme requires increased theoretical understanding, with the aim of capturing the circulation response simply, over a relevant range of the parameter space.

Current climate models add freshwater runoff from Greenland's Ice Sheet into the ocean model in the surface grid cell, and do not account for the impacts of fjord circulation on melt rates at glacial termini. Therefore, we focus on predicting the depth at which fresh meltwater enters the wider ocean, and the flow structure at the ice face itself, to understand the feedback on ice melt rates. We consider a subglacial discharge driven regime, with a localised source of subglacial discharge into the fjord at the glacial grounding line. We employ a combination of computational modelling using idealised configurations in MITgcm, and theoretical explorations, to capture this circulation as simply as possible. For fjords without sills, we find that the cross-fjord integrated velocity profile at the fjord mouth echoes that at the ice face. Further, we find that a horizontal recirculation cell develops at the ice face, as the fjord responds to horizontal velocities driven by the plume itself, generating flow across the entire ice face. We use scaling laws previously developed for turbulent plumes to provide a simple prediction of the cross-fjord integrated velocity structure at the fjord mouth, predicting the depth level at which meltwater enters the wider ocean. We develop theoretical predictions for the cross-fjord flow at the ice face, as a consequence of the flow directly induced by a buoyant plume and the circulation response in the fjord, allowing prediction of the pattern of melt across the ice face.

How to cite: Stanway, A., Wells, A., Johnson, H., and Ridley, J.: Scalings for subglacial discharge driven circulation in Greenland's proglacial fjords, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10758, https://doi.org/10.5194/egusphere-egu21-10758, 2021.

14:15–14:17
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EGU21-13956
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ECS
Trevor Hillebrand, Matthew Hoffman, Mauro Perego, Stephen Price, Abby Roat, and Ian Howat

Humboldt Glacier drains ~5% of the Greenland Ice Sheet and has retreated and accelerated since the late 1990s. The northern section of the terminus has retreated towards an overdeepening in the glacier bed that extends tens of kilometers towards the ice sheet interior, raising the possibility of a rapid increase in ice discharge and retreat in the near future. Here we investigate the potential 21st century sea-level contribution from Humboldt Glacier with the MPAS-Albany Land Ice (MALI) ice sheet model. First, we optimize the basal friction field using observations of surface velocity and ice surface elevation to obtain an initial condition for the year 2007. Next, we tune parameters for calving, basal friction, and submarine melt to match the observed retreat rates and surface velocity changes. We then simulate glacier evolution to 2100 under a range of climate forcings from the Ice Sheet Model Intercomparison Project for CMIP6 (ISMIP6), using ocean temperatures from the MIROC5 Earth System Model, with surface mass balance and subglacial discharge from MAR3.9/MIROC5. Our simulations predict ~3.5 mm of sea-level rise from the retreat of Humboldt Glacier by 2100 for RCP8.5, and ~1 mm for RCP2.6. The results are insensitive to the choice of calving parameters for grounded ice, but a low stress threshold for calving from floating ice is necessary to initiate retreat. We find that a highly plastic basal friction law is required to reproduce the observed acceleration, but the choice of basal friction law does not have a large effect on the magnitude of sea-level contribution by 2100 because much of the ice is at present close to floatation in the areas that retreat most significantly. Instead, the majority of ice mass loss comes from increasingly negative surface mass balance. Preliminary results from experiments with a subglacial hydrology model suggest that the simple treatment of subglacial discharge used in our 21st century projections (as used in the ISMIP6-Greenland protocol) underestimates spatial variability of melting at the glacier front but gives a reasonable approximation of total melt. When compared to the recent ISMIP6 estimates of 60–140 mm sea-level rise from the entire Greenland Ice Sheet by 2100, our estimate of 3.5 mm from Humboldt Glacier indicates a significant but far from dominant contribution from this single large outlet.

How to cite: Hillebrand, T., Hoffman, M., Perego, M., Price, S., Roat, A., and Howat, I.: Projections of 21st century sea-level contribution from the retreat of Humboldt Glacier, North Greenland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13956, https://doi.org/10.5194/egusphere-egu21-13956, 2021.

14:17–15:00
Break
Chairperson: Nicolas Jourdain
CR3.4 Session 2: Presentations
15:30–15:32
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EGU21-1362
Mahdi Mohammadi-Aragh, Ole Zeising, Angelika Humbert, knut klingbeil, janin schaffer, Ralph Timmermann, and Hans Burchard

 The floating ice tongue of the 79° North Glacier (Nioghalvfjerdsfjorden Glacier) in Northeast Greenland has been found to thin over the past two decades. Recent studies suggest the warming of the ocean as one of the main drivers of destabilizing outlet glaciers of the Greenland ice sheet by enhanced subglacial melting. Using a horizontal two-dimensional numerical plume model, we study the hydrodynamic processes determining basal melt rates beneath the glacial tongue of the 79° North Glacier. We specifically investigate the spatial distribution of submarine melting and assess the importance of ice base morphology in controlling basal melting. For our study, we design a suite of simulations by implementing a synthetic network of basal channels. Additionally, we determine the role of subglacial discharge in driving melting along the glacier base. Our model results lead us to the conclusion that channelised basal topographies at the glacier base are the dominant control on the basal melt rates and its spatial distribution. 

How to cite: Mohammadi-Aragh, M., Zeising, O., Humbert, A., klingbeil, K., schaffer, J., Timmermann, R., and Burchard, H.: Basal melting at the floating tongue of the 79° North Glacier – on the impacts of ice-shelf basal channels, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1362, https://doi.org/10.5194/egusphere-egu21-1362, 2021.

15:32–15:34
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EGU21-10028
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ECS
Julia Christmann, Veit Helm, Shfaqat Abbas Khan, Thomas Kleiner, Ralf Müller, Mathieu Morlighem, Niklas Neckel, Martin Rückamp, Daniel Steinhage, Ole Zeising, and Angelika Humbert

The Greenland ice sheet is the largest contributor to global sea-level rise. Large uncertainties remain in sea level rise projections due to limited insights in the dynamics of outlet glaciers in Greenland. Nioghalvfjerdsbræ (79°NG) is an outlet glacier of the Northeast Greenland Ice Stream (NEGIS), which holds 1.1 m sea-level equivalent of ice. 
While critical progress has been made in ice sheet modelling, the motion of fast-moving ice streams and their interactions with ocean tides remain poorly understood. We combine GPS observations and two-dimensional numerical modelling to show that tides alter lubrication of the glacier as far as 15 km inland. Modelling these systems is highly complex due to the need for an appropriate material model and the interaction of different components of the physical system. We associate a viscoelastic material with subglacial hydrology and get friction parameters by solving an inverse problem. Steep basal topography enhances creep by 14% locally, whereas in the majority of the fast-moving part of NEGIS the ratio of creep to sliding is below 2%. Based on the viscoelastic material model, it is possible to distinguish between elastic and viscous strains that sum up to the total strain. The elastic strain contribution in the considered cross-section is up to 34%, independent of any tidal forcing. Elastic strain contributes significantly to deformation in fast-moving outlet glaciers and appears to coincide with crevasses representing the solid nature of ice. Including sliding and elastic deformation in ice sheet models to represent recent accelerations of outlet glaciers is an important step forward in reducing uncertainties of Greenland’s contribution to future sea-level rise.

How to cite: Christmann, J., Helm, V., Khan, S. A., Kleiner, T., Müller, R., Morlighem, M., Neckel, N., Rückamp, M., Steinhage, D., Zeising, O., and Humbert, A.: Greenland's glacier tidal response and ice sheet motion  , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10028, https://doi.org/10.5194/egusphere-egu21-10028, 2021.

15:34–15:36
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EGU21-15695
Claudia Wekerle, Ralph Timmermann, Qiang Wang, and Rebecca McPherson

The 79° North Glacier (79NG) is the largest of the marine terminating glaciers fed by the  Northeast Greenland Ice Stream (NEGIS), which drains around 15% of the Greenland ice sheet. The 79NG is one of the few Greenland glaciers with a floating ice tongue, and is strongly influenced by warm Atlantic Water originating from Fram Strait and carried towards it through a trough system on the Northeast Greenland continental shelf.

Considering the decrease in thickness of the 79NG and also of the neighboring Zachariae Isstrøm (ZI), we aim to understand the processes that potentially lead to the decay of these glaciers. As a first step we present here an ocean-sea ice simulation which explicitly resolves the cavities of the 79NG and ZI glaciers, applying the Finite-Element Sea ice-Ocean Model (FESOM). We take advantage of the multi-resolution capability of FESOM and locally increase mesh resolution in the vicinity of the 79NG to 700 m. The Northeast Greenland continental shelf is resolved with 3 km, and the Arctic Ocean and Nordic Seas with 4.5 km. The simulation is conducted for the time period 1980 to 2018, using JRA-55 atmospheric reanalysis. Solid and liquid runoff from Greenland is taken from the Bamber et al. 2018 dataset. The flow of warm Atlantic water into the glacier and outflow of meltwater is compared to observational data from measurement campaigns. We further use current and hydrographic data from moorings deployed in Norske Trough to assess the model performance in carrying warm water towards the glacier. This simulation spanning several decades allows us to investigate recent changes in basal melt rates induced by oceanic processes, in particular warm Atlantic Water transport towards the glacier.

How to cite: Wekerle, C., Timmermann, R., Wang, Q., and McPherson, R.: High-resolution ocean/sea ice/ice shelf simulation of the 79° North Glacier and Zachariae Isstrøm, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15695, https://doi.org/10.5194/egusphere-egu21-15695, 2021.

15:36–15:38
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EGU21-2098
Oliver Huhn, Monika Rhein, Klaus Bulsiewicz, Torsten Kanzow, Janin Schaffer, and Jürgen Sültenfuß

The Greenland Ice Sheet faces accelerated melting under warming climate conditions. The involved processes are surface melting, iceberg calving, and submarine melting through the contact of warm water with marine terminating glaciers. The Nioghalvfjerdsfjorden Glacier (79 North Glacier, 79NG) is the largest marine terminating outlet glacier of the Northeast Greenland Ice Stream and has still a floating ice tongue. In the cavity, the heat of inflowing warm and saline Atlantic Water melts the floating ice shelf at its base, and the colder and fresher outflow is exported towards the shelf break and presumably south with the East Greenland Current. However, freshwater from submarine melting is hardly distinguishable from other freshwater sources off the sources by salinity alone. To identify and to quantify the fraction and distribution of submarine melt water on the northeast Greenland shelf, we use helium (He) and neon (Ne) observations, obtained directly at the calving front of the 79NG, in its close and far vicinity on the northeast Greenland shelf, and beyond the shelf break in Fram Strait during a Polarstern expedition in 2016. These lighter and low soluble noble gasses provide a unique tool to identify submarine melt water and to quantify its fractions. We calculate a submarine melt water formation rate of 14.5 ± 2.3 Gt per year, equivalent to a basal melt rate of 8.6 ± 1.4 m per year of the 79NG. Submarine melt water fractions are present on the shelf, but dilute from 1.8% at the 79NG calving front to nonsignificant in Fram Strait. A surplus of Ne on most of the shelf region indicates that up to 10% of the original water mass had been transformed to sea ice.

How to cite: Huhn, O., Rhein, M., Bulsiewicz, K., Kanzow, T., Schaffer, J., and Sültenfuß, J.: Submarine Melt Water from the 79 North Glacier (79NG, Nioghalvfjerdsbræ), northeast Greenland, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2098, https://doi.org/10.5194/egusphere-egu21-2098, 2021.

15:38–15:40
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EGU21-3405
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ECS
Gong Cheng and Mathieu Morlighem

At least half of today’s mass loss of the Greenland ice sheet is due to the retreat of tidewater glaciers. For example, Helheim Glacier, in southwest Greenland, has one of the largest ice discharge records in the Greenland ice sheet during the previous decade. While there is broad agreement that the intrusion of warmer current in the Sermilik Fjord triggers the acceleration and retreat of this marine terminating glacier, other processes such as changes in basal conditions, surface mass balance or calving dynamics may have also played important roles in controlling the retreat of these glaciers. However, our understanding of these processes and their contributions to the retreat and acceleration of the glaciers remains still limited. The individual contributions of these processes have not been quantified, which makes it difficult to determine which of these processes should be included in ice sheet models to correctly capture the present and future retreat and associated mass loss of the ice sheet. Here, we simulate the dynamics of Helheim Glacier from 2007 to 2020 using the Ice-sheet and Sea-level System Model (ISSM) to investigate the model response to changes in external forcings and boundary conditions, such as basal friction, surface mass balance, ice rheology, and ice-ocean interactions at the calving front. The relative importance of each mechanism to the model is quantified within a series of perturbation experiments. We evaluate the ability of the model to match surface speed and terminus position from the observations collected during the simulation period. Preliminary results suggest that Helheim’s dynamics is relatively insensitive to the choice of friction law or the surface mass balance, but that the position of the calving front and changes in basal sliding conditions are critical to explain the high variability of Helheim’s surface speed. This study, as a result, can be used as a guide for model development of similar glaciers.

How to cite: Cheng, G. and Morlighem, M.: Investigating the drivers of Helheim Glacier’s variability from 2007 to 2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3405, https://doi.org/10.5194/egusphere-egu21-3405, 2021.

15:40–15:42
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EGU21-7910
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ECS
Stephen Brough, J. Rachel Carr, Neil Ross, and James Lea

Between 2000 and 2010, glaciers on Greenland’s east coast were shown to have distinct contrasts in patterns and rates of ice front retreat north and south of 69°N latitude. The correspondence of this transition zone with the northern limit of subtropical waters carried by the Irminger Current has led to the hypothesis that variability in coastal heat transport is the dominant mechanisms causing this regional difference (e.g. Seale et al. 2011). However, whether these regional differences exist for recent glacier change is unknown. Here we examine seasonal and interannual variability in Landsat-8 derived ice-front positions with respect to atmospheric and oceanic forcings for 24 east Greenland outlet glaciers between 2013 and 2017.

 

We find that all glaciers exhibit seasonal advance and retreat cycles proportional to glacier width and velocity, though there is a distinct difference between the interannual trends of glacier termini north and south of 69oN throughout our study period. Glaciers above this latitude showed either limited or gradual terminus change over time that was mostly linear on annual timescales. This contrasts with glaciers south of 69°N where step-wise retreat was observed between 2016 and 2017, following a period of relative stability between 2013 and 2016. We find that retreat south of 69°N during 2016 was coincident with periods of anomalously warm atmospheric and subsurface oceanic temperatures, and a marked decline in sea ice/mélange. Warm atmospheric conditions were also experienced north of 69°N, though subsurface oceanic temperature increases and changes in mélange cover were not as marked. Our work supports the hypothesis that differences in the terminus response of glaciers either side of 69°N can be explained by contrasting oceanic forcing regimes above and below this latitude.

 

References: Seale, A., Christoffersen, P., Mugford, R. I. and O’Leary, M. (2011) Ocean forcing of the Greenland Ice Sheet: Calving fronts and patterns of retreat identified by automatic satellite monitoring of eastern outlet glaciers. Journal of Geophysical Research Letters, 116, doi: 10.1029/2010JF001847.

 

How to cite: Brough, S., Carr, J. R., Ross, N., and Lea, J.: Contrasting retreat patterns of east Greenland tidewater glaciers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7910, https://doi.org/10.5194/egusphere-egu21-7910, 2021.

15:42–15:44
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EGU21-10539
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Highlight
Marco Möller, Beatriz Recinos, and Ben Marzeion

The Greenland Ice Sheet is losing mass at increasing rates. Substantial amounts of this mass loss occur by ice discharge. The ice sheet is surrounded by thousands of peripheral glaciers, which are dynamically decoupled from the ice sheet, and which account for ~10 % of the global glacier ice volume outside the two main ice sheets. Rather low-lying along the coasts, these peripheral glaciers are also losing mass at increasing, but disputed, rates. The total absence of knowledge about the role and share of solid ice discharge in this mass loss adds to the controversy. Since the quantification of ice discharge is still pending, a full understanding of ice mass loss processes in this globally important glacier region is substantially hampered.

Here, we present the first estimation of ice discharge from Greenland's peripheral tidewater glaciers. For each of these 760 glaciers, we combine an idealized rectangular flux gate cross sections derived from modelling with the Open Global Glacier Model with surface ice flow velocities derived from the ITS_LIVE and MEaSUREs remote sensing datasets to calculate glacier specific ice discharge on both annual and multi-annual time scales over the period 1985 to 2018. For the few glaciers not covered by either of the employed original datasets or modelling methods we use a regression tree-based extrapolation scheme to estimate the necessary input data for our calculation.

Our findings indicate a significant overall increase of ice discharge over the study period although several individual glaciers show contrasting developments. This increase became especially apparent across the southern parts of Greenland. Our results also show that the total of the ice discharge from Greenland's peripheral tidewater glaciers is dominated by few major contributors and that this dominance is completely time-independent.

How to cite: Möller, M., Recinos, B., and Marzeion, B.: Increase of ice discharge from Greenland's peripheral tidewater glaciers over recent decades, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10539, https://doi.org/10.5194/egusphere-egu21-10539, 2021.

15:44–15:46
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EGU21-2650
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ECS
Jim Jordan, HIlmar Gudmundsson, Adrian Jenkins, Chris Stokes, Stewart Jamiesson, and Bertie Miles
The buttressing strength of Antarctic ice shelves directly effects the amount of ice discharge across the grounding line, with buttressing strength affected by both the thickness and extent of an ice shelf. Recent work has shown that a reduction in ice-shelf buttressing due to ocean induced ice-shelf thinning is responsible for a significant portion of increased Antarctic ice discharge (Gudmundsson et al., 2019, but few studies have attempted to show the effect of variability in ice-shelf extent on ice discharge. This variability arises due to ice-shelf calving following a cycle of long periods of slow, continuous calving interposed with calving of large, discrete sections.  These discrete calving events tend to occur on a comparative timeframe to that of the observational record. As such, when determining observed changes in ice discharge it is crucial that this natural variability is separated from any observed trends.  
 
In this work we use the numerical ice-flow model Úa in combination with observations of ice shelf extent to diagnostically calculate Antarctic ice discharge. These observations primarily date back to the 1970s, though for some ice shelves records exist back to the 1940s. We assemble an Antarctic wide model for two scenarios: 1) with ice shelves at their maximum observed extent and 2) with ice shelves at their minimum observed extent. We then compare these two scenarios to differences in the observed changes in Antarctic ice-discharge to determine how much can be attributed to natural variance .

 

Gudmundsson, G. H., Paolo, F. S., Adusumilli, S., & Fricker, H. A. (2019). Instantaneous Antarctic ice‐ sheet mass loss driven by thinning ice shelves. Geophysical Research Letters, 46, 13903– 13909. 

How to cite: Jordan, J., Gudmundsson, H., Jenkins, A., Stokes, C., Jamiesson, S., and Miles, B.: The effect of Antarctic ice-shelf extent on ice discharge, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2650, https://doi.org/10.5194/egusphere-egu21-2650, 2021.

15:46–15:48
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EGU21-11412
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ECS
Yin Ying Ip, Lukas Krieger, and Dana Floricioiu

The migration of the glacier grounding line, the boundary between grounded ice and floating ice, is an important indicator of tice sheet stability in a warming climate. Ice-shelf thinning induces grounding line retreat, and potentially leads to the collapse of the inland catchment areas in centennial time periods. Therefore, a continuous observation of the grounding line position is of interest for ice sheet modelling also to predict future sea level rise. However, grounding line in nature is not static in position and it is subject to short-term fluctuations which are influenced by changes in ocean tide level and atmospheric pressure. Investigating tidal influence to the grounding line helps separating the tidal signal from the long-term migration because of ice shelf thinning. Also, it helps quantifying ice discharge and ice flow, as well as potential melting underneath the ice, due to intrusion of sea water.

In this study, the correlation between the time series of grounding line, derived from Sentinel-1 double difference interferograms and the ocean tide level computed from CATS2008 tide model and air pressure corrected with NCEP reanalysis data  are investigated. Study regions are chosen at the Filchner-Ronne Ice Shelf, the Amery Ice Shelf and Dronning Maud Land based on the availability of coherent interferograms and the large tidal amplitude at these locations. The result is expected to be presented as qualitative description of changes in the fringe belt pattern in double difference interferograms and statistical analysis of the derived changes in grounding line position, depending on the complexity of the grounding line structure and the topography of the bed rock.

How to cite: Ip, Y. Y., Krieger, L., and Floricioiu, D.: Investigations of DInSAR derived grounding line migration in Antarctica induced by ocean tides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11412, https://doi.org/10.5194/egusphere-egu21-11412, 2021.

15:48–15:50
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EGU21-15197
Reinhard Drews, Christian Wild, Oliver Marsh, Wolfgang Rack, Todd Ehlers, Niklas Neckel, and Veit Helm

Dynamics of polar outlet glaciers vary with ocean tides, providing a natural laboratory to understand basal processes beneath ice streams, ice rheology and ice-shelf buttressing. We apply Terrestrial Radar Interferometry to close the spatiotemporal gap between localized, temporally well-resolved GNSS and area-wide but sparse satellite observations. Three-hour flowfields collected over an eight day period at Priestley Glacier, Antarctica, validate and provide the spatial context for concurrent GNSS measurements. Ice flow is fastest during falling tides and slowest during rising tides. Principal components of the timeseries prove upstream propagation of tidal signatures $>$ 10 km away from the grounding line. Hourly, cm-scale horizontal and vertical flexure patterns occur $>$6 km upstream of the grounding line. Vertical uplift upstream of the grounding line is consistent with ephemeral re-grounding during low-tide impacting grounding-zone stability. On the freely floating ice shelves, we find velocity peaks both during high- and low-tide suggesting that ice-shelf buttressing varies temporally as a function of flexural bending from tidal displacement. Taken together, these observations identify tidal imprints on ice-stream dynamics on new temporal and spatial scales providing constraints for models designed to isolate dominating processes in ice-stream and ice-shelf mechanics.

How to cite: Drews, R., Wild, C., Marsh, O., Rack, W., Ehlers, T., Neckel, N., and Helm, V.: Grounding-zone flow variability of Priestley Glacier, Antarctica, in a diurnal tidal regime, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15197, https://doi.org/10.5194/egusphere-egu21-15197, 2021.

15:50–15:52
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EGU21-16419
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ECS
Cyrille Mosbeux, Laurence Padman, Emilie Klein, Peter Bromirski, Scott Springer, and Helen A Fricker

Ice mass loss from both Antarctic Ice Sheet is increasing, accelerating its contribution to global sea level rise. Interactions between the ice shelves (the floating portions of the ice sheet) and the ocean are key processes in this mass loss. The large Ross Ice Shelf is presently stable but buttresses grounded ice equivalent to about 12 m of global sea level, and geological evidence points to large and sometimes rapid past changes. Recent ocean modeling and observations show that seasonal inflows of warmed upper-ocean water under a thin-ice corridor from Ross Island to Minna Bluff and at the ice front can produce locally high melt rates each summer, suggesting that future increases in summer upper-ocean ocean warming north of the ice front could accelerate ice-shelf flow speeds and mass loss. Recent GPS observations of Ross Ice Shelf velocity have shown seasonal flow variations of several meters per year over a large part of the ice shelf, accelerating in summer and decelerating in winter. A similar seasonal variability has been observed over the floating extension of Byrd glacier (one of the major tributary glaciers of Ross Ice Shelf) by processing Antarctic image pairs in the ITS_LIVE dataset. However, ice-sheet simulations driven by realistic annual cycles of basal melt rates near the ice front produce much smaller seasonal variations than observed, suggesting that other processes could be at play. Here, we investigate a new potential mechanism for a seasonal signal in ice flow: variations of sea surface height (SSH) driven by seasonal changes in thermodynamic and atmospheric forcing of ocean state under the ice shelf. Model annual cycle of SSH under Ross Ice Shelf has an amplitude of up to ~20 cm, with substantial spatial variability. These variations of sea level, similarly to tidal signal but with a longer period, can lead to changes in driving stress over the ice shelf as well as a migration of the grounding line due to hydrostatic adjustment and visco-elastic bending of the ice shelf in the grounding zone. By simulating these SSH variations in an ice-sheet model, we more accurately reproduce the variations observed at GPS stations on Ross Ice Shelf.

How to cite: Mosbeux, C., Padman, L., Klein, E., Bromirski, P., Springer, S., and Fricker, H. A.: Seasonal flow variations of Ross Ice Shelf (Antarctica): from observations to modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16419, https://doi.org/10.5194/egusphere-egu21-16419, 2021.

15:52–15:54
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EGU21-9139
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ECS
Ibeth Celia Rojas Macedo, Wilson Alfredo Suarez Alayza, Edwin Anibal Loarte Cadenas, and Katy Damacia Medina Marcos

This research aims to explain the influence of climatic variables (temperature and precipitation) in King George Island (KGI) glacier shrinkage on the Antarctic Peninsula. It employed Landsat satellite images from 1989 to 2020, climatic data and ONI index from 1980 to 2019.

King George Island glaciers have lost 10% of their coverage in the last 31 years. Greater glacier shrinkage was shown until the first mid-period assessed, while the retreat rate slowed down for the second half of the studied period. Furthermore, of 73 KGI glaciers, 37% were marine- and land-terminating, 42% were land-terminating and 21% were sea-terminating. Nonetheless, the decreases in the ice-coverage of marine-contact glaciers (35% of glacier coverage reduced) were higher than land-terminating glaciers (17% of glacier coverage reduced).

There was a perceivable fluctuation in annual average air temperature for the 1980-2006 period. Nevertheless, from around 2007 to 2015/2016 there was a slight continuous cooling period and precipitation was somewhat above the average. Therefore, these patterns could explain the recent KGI glacier-retreat deceleration.

Unlike the 1982/1983 and 1997/1998 El Niño events, the 2015/2016 El Niño was colder with precipitation reduction from the sustained annual amount (since roughly 2007 to 2015/2016) to values below the average. Moreover, during the 2015/2016 El Niño, KGI glacier coverage reduction was the lowest for the 31 year-long evaluated. However, it was revealed that the glacier's height could increase by accumulation in El Niño years, but glacier mass balance could be more negative due to basal melting. Additionally, land-terminating glaciers have lost more glacier coverage than sea-terminating glaciers throughout this ENSO event.

Hence, climate variability might play a significant role in KGI glacier shrinkage, but calving process, glacier features and so on, further a combination of them should be assessed to reach a better understanding of KGI glacier retreat.

How to cite: Rojas Macedo, I. C., Suarez Alayza, W. A., Loarte Cadenas, E. A., and Medina Marcos, K. D.: Influence of Climate Variability in King George Island Glacier Retreat – Antarctic Peninsula, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9139, https://doi.org/10.5194/egusphere-egu21-9139, 2021.

15:54–15:56
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EGU21-9510
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ECS
Bradley Reed, Mattias Green, Hilmar Gudmundsson, and Adrian Jenkins

The Amundsen Sea sector in West Antarctica is undergoing dramatic changes, with thinning ice shelves and accelerating, retreating glaciers. One of the largest and fastest flowing ice streams in the region is Pine Island Glacier (PIG). In recent decades it has retreated over 30 km, experienced a 75% increase in velocity and thinned by more than 100m. However, these changes have not been constant, there have been alternating periods of acceleration and stabilisation since the start of the observational era in the 1970s. This has been attributed to variable ocean conditions, where interannual and decadal changes in the Circumpolar Deep Water layer have been linked to large-scale climate variability. The initial ungrounding and subsequent retreat of PIG from a submarine ridge is believed to have been caused by extreme changes in ocean conditions linked to El Niño Southern Oscillation (ENSO) events during the 1940s and 1970s. However, the exact role that these events have played over the last century is not fully understood.

In this study the ice flow model Úa is used to assess how the retreat of PIG has been impacted by ENSO events. During these events, variations in thermocline depth affect the amount of heat available for basal melting beneath the ice shelf. To represent these changing ocean conditions a melt rate parameterisation based on a 1D plume model is used, which depends on ice shelf geometry, grounding line depth and ambient ocean properties. Results will show if a gradually warming ocean is enough to initiate grounding line retreat or if brief, large changes in temperature are required. Further investigations will determine whether cooler years contributed to a slow down of the ice stream. This work will help us understand and model the response of other glaciers to extreme changes in ocean conditions caused by ENSO events in a warming future.

How to cite: Reed, B., Green, M., Gudmundsson, H., and Jenkins, A.: Retreat of Pine Island Glacier: The impact of El Nino Southern Oscillation events , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9510, https://doi.org/10.5194/egusphere-egu21-9510, 2021.

15:56–15:58
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EGU21-9437
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ECS
Deborah Verfaillie, Charles Pelletier, Hugues Goosse, Nicolas Jourdain, Vincent Favier, Jonathan Wille, Quentin Dalaiden, and Thierry Fichefet

The climate of polar regions is characterized by large fluctuations and has experienced dramatic changes over the past decades. In the high latitudes of the Southern Hemisphere, the patterns of changes in sea ice and ice sheet mass, in particular, are more complex than for the Northern Hemisphere. Some regions have warmed less than the global average with some sea-ice advance, in particular in the Ross Sea, while other regions such as the Bellingshausen Sea have warmed significantly and displayed sea-ice loss. The Antarctic Ice Sheet has also lost mass in the past decades, with a spectacular thinning and weakening of ice shelves, i.e., the floating extensions of the grounded ice sheet. Despite recent advances in observing and modelling the Antarctic climate, the mechanisms at the origin of those trends are very uncertain because of the limited amount of observations and the large biases of climate models in polar regions, in concert with the large internal variability prevailing in the Antarctic. One of the most important atmospheric modes of climate variability in the Southern Ocean is the Southern Annular Mode (SAM), which represents the position and the strength of the westerly winds. During years with a positive SAM index, lower sea level pressure at high latitudes and higher sea level pressure at low latitudes occur, resulting in a stronger pressure gradient and intensified Westerlies. However, the current knowledge of the impact of these fluctuations of the Westerlies on the Southern Ocean and Antarctic cryosphere is still limited. Some efforts have been devoted over the past few years to the impact of the SAM on the Antarctic sea ice and the surface mass balance of the ice sheet from an atmospheric-specific perspective. Recently, a few studies have focused on the local impact on ice-shelf basal melt in specific regions of Antarctica. However, to our knowledge, there is no such study of the impact of the SAM on ice-shelf basal melt at the pan-Antarctic scale. In this communication, we will address this issue by using simulations performed with the regional ocean and sea-ice model NEMO-LIM3.6 at a spatial resolution of 0.25° forced by the ERA5 reanalysis over the period 1979-2018 CE. The impact of both the annular and the non-annular components of the SAM on ice-shelf basal melt will be assessed through regressions and correlations between the seasonal or annual averages of the SAM index and the ice-shelf basal melt.

How to cite: Verfaillie, D., Pelletier, C., Goosse, H., Jourdain, N., Favier, V., Wille, J., Dalaiden, Q., and Fichefet, T.: Investigating the impact of the Southern Annular Mode on ice-shelf basal melt in Antarctica using a regional ocean model forced by reanalysis data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9437, https://doi.org/10.5194/egusphere-egu21-9437, 2021.

15:58–16:00
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EGU21-2617
Tom Holt and Neil Glasser

Over the latter half of the 20th Century and beginning of the 21st Century, ice shelves around the Antarctic Peninsula have been losing mass at an accelerating rate, attributable to changes in atmospheric and oceanic conditions. Ice shelves have declined in extent and thickness, and some show signs of structural weakening. Here we investigate the glaciological changes to Bach, Stange and George VI ice shelves that fringe the Southwest Antarctic Peninsula. We used satellite imagery from 2009/10 to 2019/20 (Landsat, Sentinel and ASTER) to measure areal changes, calculate flow speeds, and quantify structural changes, focusing on open fracture width and length. We reveal a total net loss of 797.5 km2 from all three ice shelves since 2009/10, though spatial and temporal patterns of ice loss vary at individual ice fronts. Flow speeds have remained largely stable, but notable acceleration was calculated for Bach Ice Shelf, and at the northern and southern extents of George VI Ice Shelf. Open fractures have widened and lengthened over the observation periods. We conclude that Stange Ice Shelf is stable, and not under any immediate threat of enhanced recession. Continued ice-mass loss and consequential speed up of George VI South may cause further fracturing and destabilisation in the coming decades. Of more immediate concern are the glaciological changes noted for Bach Ice Shelf and George VI North; significant areas of passive ice have already, or will be soon removed, that could result in enhanced recession within the next decade.

How to cite: Holt, T. and Glasser, N.: Decadal changes in south west Antarctic Peninsula Ice Shelves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2617, https://doi.org/10.5194/egusphere-egu21-2617, 2021.

16:00–16:02
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EGU21-10418
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Highlight
Andrew Thompson, Mar Flexas, Michael Schodlok, and Kevin Speer

The acceleration of ice-shelf basal melt rates throughout West Antarctica, as well as their potential to destabilize the ice sheets they buttress, is well documented.  Yet, the mechanisms that determine both trends and variability of these melt rates remain uncertain.  Explanations for the intensification of melting have largely focused on local processes in seas surrounding the ice shelves, including variations in wind stress over the continental slope and shelf.  Here, we show that non-local freshwater forcing, propagated between shelf seas by the Antarctic Coastal Current (AACC), can have a significant impact on ice-shelf melt rates.  

We present results from a suite of high-resolution (~3-km) numerical simulations of the ocean circulation in West Antarctica that includes a dynamic sea-ice field, ice-shelf cavities and forcing from ice shelf-ocean interactions.  Motivated by persistent warming at the northern Antarctic Peninsula since the 1950’s, freshwater perturbations are applied to the West Antarctic Peninsula.  This leads to a strengthening of the AACC and a westward propagation of the freshwater signal.  Critically, basal melt rates increase throughout the WAP, Bellingshausen and Amundsen Seas in response to this perturbation.  The freshwater anomalies stratify the ocean surface near the coast, enhancing lateral heat fluxes that lead to greater ice-shelf melt rates.  A suite of sensitivity studies show that changes in meltrates are linearly proportional to the magnitude of the freshwater anomaly, changing by as much as 30% for realistic perturbations, but are relatively insensitive to the distribution of the perturbation across the WAP shelf.  These results indicate that glacial run-off on the Antarctic Peninsula, one of the first signatures of a warming climate in Antarctica, could be a key trigger for increased melt rates in the Amundsen and Bellingshausen Seas.

How to cite: Thompson, A., Flexas, M., Schodlok, M., and Speer, K.: Antarctic Peninsula warming triggers enhanced basal melt rates throughout West Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10418, https://doi.org/10.5194/egusphere-egu21-10418, 2021.

16:02–16:04
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EGU21-15367
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ECS
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Highlight
Bertie Miles, Chris Stokes, Stewart Jamieson, Jim Jordan, Hilmar Gudmundsson, and Adrian Jenkins

It has been widely reported that ice flux from the Antarctic Ice Sheet has increased over the preceding decades. The vast majority of these increases can be attributed to the ongoing destabilization of the Amundsen Sea sector in West Antarctica, with a much more limited change in East Antarctica. However, much less attention has been focussed on the temporal and spatial variations of ice flux in Antarctica over the observational period.

In this study we combine existing velocity products (ITS_LIVE and MEaSUREs) to create 12 timestamped velocity mosaics between 1999 and 2018 to investigate both overall trends in ice flux and the temporal and spatial variability across our observational period. At an ice sheet scale we report a 45 GT yr-1 increase in ice discharge in West Antarctica and no overall change in East Antarctica. However, at an individual catchment scale we observe considerable temporal and spatial variability. For West Antarctica, despite the overall increase in discharge clear periods of deceleration are observed in most individual catchments. In East Antarctica, despite overall consistency, 3-10% variations in ice discharge are observed at several major outlet glaciers (e.g. Denman, Totten, Frost, Cook, Matusevitch, Rennick). These variations can be linked to regional oceanic variability along with highly localised differences in bed topography and ice shelf calving. In some cases, this can result in neighbouring catchments simultaneously undergoing opposing trends. Improving our understanding the processes driving these short-term variations will be important in improving the accuracy of future sea level contributions from Antarctica.

How to cite: Miles, B., Stokes, C., Jamieson, S., Jordan, J., Gudmundsson, H., and Jenkins, A.: Variable Antarctic ice flux linked to ocean forcing, bed topography and ice shelf buttressing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15367, https://doi.org/10.5194/egusphere-egu21-15367, 2021.

16:04–17:00