CR3.4

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.

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, 2021.

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.

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.

  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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Over the latter half of the 20^th Century and beginning of the 21^st 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.

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.

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.