Displays

Ice shelves in West Antarctica are melting at an increasing rate due to the flow of relatively warm
Circumpolar Deep Water into the ice shelf cavities. The current that brings heat southward along the
eastern side of a trough towards an ice shelf front is found to have a barotropic and a baroclinic
component. Mooring observations in front of Getz Ice Shelf suggest that 90% (roughly 0.6 Sv) of the
volume transport and 65% of the temperature transport is linked to the barotropic component of the
current towards the ice shelf. It is unknown whether and how much of a barotropic current can
penetrate under the ice shelf across the about 300 m deep ice shelf front, where lines of constant water
column thickness discontinue.
We conduct idealized modelling with MITgcm to investigate the dynamics of a barotropic current at the
ice shelf front. Friction and strong vertical velocities at the ice shelf front break the potential vorticity
constraint and allow the flow to partly enter the ice shelf cavity. Only a small fraction of the current
penetrates deep into the cavity, while a strong current flows parallel to the ice shelf front, where basal
melt is largely enhanced. How much of the current enters the cavity and how far it reaches depends on
the ice shelf- and bedrock topography.

How to cite: Steiger, N., Darelius, E., Kimura, S., Patmore, R., and Wåhlin, A.: Dynamics of a barotropic current at an ice shelf front, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18499, https://doi.org/10.5194/egusphere-egu2020-18499, 2020.

The Getz region is a marine-terminating sector of West Antarctica, characterised by a ~650 km long ice shelf that buttresses the inland ice sheet. The majority of the Getz drainage basin is grounded well below sea level, and while the ice shelf has thinned, its calving front has remained relatively stable since the early ’90s. Satellite observations have shown strong thinning of both the ice sheet and ice shelf over the past 25-years, and mass balance studies have shown that the sector is negatively imbalanced (−16.4 ± 4.0 Gt/year). In this study, we use satellite data to measure ice speed in the Getz region, over a 25-year period from 1994 to 2019. We use Synthetic Aperture Radar (SAR) data from historical missions including ERS-1, 2 and ALOS PALSAR, in combination with newer data from the Sentinel-1a & b satellite constellation, to generate annual velocity maps. The Sentinel-1 data extend the historical velocity record and provides a new high temporal resolution record, 6-day averaged solutions, of velocity change since 2017. We used satellite observations in combination with the BISICLES ice sheet model to fill gaps in the observational record, and to measure ice discharge and from the region. We find there are 14 distinct flow units that drain the Getz coastline, with average speeds ranging from 153 ± 7 to 1053 ± 194 m/yr around the grounding line. Our results show that all of these flow units have sped up during the study period, since 1994. At the grounding line, we measure an average speed increase of ~5 m/yr², with some flow units accelerating by over 11 m/yr². We find that the spatial pattern of change in ice speed is consistent with the regions of strongest surface lowering, which on some flow units occurs at rates of up to -2.4 m/yr. Our observations show that ice speedup is greatest where the ice is thickest (>700 m), and grounded most deeply. This long 25-year record of change also shows that on some ice streams, the rate of change in ice speed has not been constant throughout the study period. In some regions where ocean temperature measurements are also available, we find that increases in ice speed coincide with the periodic presence of circumpolar deep water, which may therefore be responsible for driving this change. In summary, this study provides a new record of change in ice speed for a rapidly evolving region of Antarctica. In the future, it will be important to use both ocean models and observations to improve our understanding of how ocean forcing is driving dynamic imbalance in the region. This will improve our understanding of the physical mechanisms driving change in Antarctica, helping us to better constrain the ice sheets future contribution to global sea level rise.

How to cite: Selley, H., Hogg, A., Cornford, S., Shepherd, A., Dutrieux, P., Wuite, J., Kusk, A., Nagler, T., and Gilbert, L.: Increased ice flow in the Getz region of West Antarctica, from 1994 to 2018 , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15996, https://doi.org/10.5194/egusphere-egu2020-15996, 2020.

Basal melting of floating ice shelves is the main process by which  the Antarctic ice sheet is currently losing ice and is responsible for the accelerating Antarctic contribution to sea-level rise. Moreover, basal melting can strongly vary both spatially and temporally. Detailed observations on high  spatio-temporal scales remain however challenging, not to mention accounting for them in ice-sheet models. 

In this study, we combine CryoSat-2 and TanDEM-X elevation changes to capture in unprecedented detail the spatial (and temporal) variability of ice-shelf basal melting in the entire region of Dronning Maud Land, East Antarctica. The high spatial resolution of TanDEM-X elevations provide us with great  details on the spatial variability of the basal mass balance, whereas CryoSat-2 elevations inform us about temporal changes.

We find sub-shelf melt rates that average 1 m/a for the whole of Dronning Maud Land. Those relatively low melt rates conceal however a significant spatial variability on a wide range of scales (from sub-kilometers to ice-shelf wide scales). Spatially integrated, this basal melting represent  an annual basal loss ~140 Gt/a. This revised estimate corresponds to a two-fold increase compared to previous estimates, which could question the relative stability of ice shelves in this region. 

This study highlights different regimes in sub-shelf melting in Dronning Maud Land and  sheds new light on ice-ocean interactions in a region of Antarctica that has long been considered as very stable and which is therfore regularly overlooked.

How to cite: Berger, S., Helm, V., Hattermann, T., Neckel, N., Glaude, Q., Zeising, O., Sun, S., Pattyn, F., and Eisen, O.: Basal melting of Dronning Maud Land ice shelves twice as high as previously estimated , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15477, https://doi.org/10.5194/egusphere-egu2020-15477, 2020.

Understanding the surface hydrology of ice shelves is an essential first step to reliably project future sea level rise from ice sheet melt. The formation of surface meltwater has been linked with the disintegration of many ice shelves in the Antarctic Peninsula over the last several decades. The most notable ice shelf collapse occurred in 2002 when significant meltwater lake coverage was observed on the surface of the Larsen B Ice Shelf before its collapse over a period of just a few weeks. Such collapse can affect ocean circulation and temperature, and cause a loss of habitat. Additionally, it can cause a loss of the buttressing effect that ice shelves can have on their tributary glaciers, thus allowing the glaciers to accelerate, contributing to sea level rise. Despite the importance of surface meltwater production and transport to ice shelf stability, knowledge of these processes is still lacking and as a result of this, projections of future sea level rise still vary over an order of magnitude.

In order to better understand these processes we present a new 3-D model of surface hydrology for Antarctic ice shelves. This model takes the 1-D surface lake formation model of Buzzard et al. (2018) and expands it to three dimensions. It is the first comprehensive model of surface hydrology to be developed for Antarctic ice shelves, enabling us to incorporate key processes such as the lateral transport of surface meltwater. Recent observations suggest that surface hydrology processes on ice shelves are more complex than previously thought, and that processes such as lateral routing of meltwater across ice shelves, ice shelf flexure and surface debris all play a role in the location and influence of meltwater. Our model allows us to account for these as well as additional key physical processes and is calibrated and validated through both remote sensing and field observations.

Here we present results of coupling the 1-D model with a 3-D meltwater routing scheme. This includes calculations of the surface energy balance, meltwater production, percolation and refreezing and lake formation. Through case studies, calibrated and validated against observations, we will demonstrate the varied applications of the model.

This community-driven, open-access model, has been developed with input from observations, and allows us to provide new insights into surface meltwater distribution on Antarctica’s ice shelves. This enables us to answer key questions about their past and future evolution under changing atmospheric conditions and vulnerability to meltwater driven hydrofracture and collapse.

How to cite: Buzzard, S. and Robel, A.: A 3-D Model of Antarctic Ice Shelf Surface Hydrology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-278, https://doi.org/10.5194/egusphere-egu2020-278, 2020.

Iceberg calving is the reason for more than half of mass loss in both Greenland and Antarctica. It also indirectly contributes to sea-level rise; changes in calving rate can shorten the ice shelves, speed up the grounded ice and increase changes in ice sheets. Therefore, having a better understanding of this phenomenon by a mathematical modeling seems essential.
Lacking of a precise representation of calving in ice sheet and glacier models may yield to nonphysical predictions.
We perform a parameter study to identify groups of key parameters. Here we use boundary element method and compare our result to works done by van der Veen (1998) and Nick et al. (2010). 
 A hydraulic crack propagation is assumed to happen vertically from both base and surface of the shelf. The solution for different scenarios is analysed in the form of stability of a dynamical system. An unstable solution results in an iceberg calving which leads us to a general calving law. 

How to cite: Zarrinderakht, M. and Schoof, C.: A Mathematical Modeling for Stability of Ice Shelves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12645, https://doi.org/10.5194/egusphere-egu2020-12645, 2020.

Under future warming scenarios, both ice sheets on Greenland and Antarctica are likely to discharge ice into the ocean at an accelerating rate. In many regions along the coast of the ice sheets, the icebergs are discharged into a bay. If the addition of icebergs through calving is faster than their transport out of the embayment, the icebergs will be frozen into a mélange with surrounding sea ice. In this case, the buttressing effect of the ice mélange can be considerably stronger than any buttressing by mere sea ice would be. This in turn stabilizes the glacier terminus and leads to a reduction in calving rates. Here we propose a simple but robust buttressing model of ice mélange. 

How to cite: Schlemm, T. and Levermann, A.: A simple but robust model for the buttressing of calving glaciers through ice mélange , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8352, https://doi.org/10.5194/egusphere-egu2020-8352, 2020.

Glaciers calving ice into the ocean is predicted to significantly contribute to sea-level rise and will thus influence future climate. Although numerous factors that induce glacier calving have been identified and studied, it is still extremely challenging to develop a unified and continuum computational framework that simulates ice fracture and glacier calving taking into account all important ingredients such as the interaction between ice and water, including buoyancy and melting, on complex and large scale geometry. This prevents scientists to precisely predict calving rates at the outlet of glaciers. Here, we propose to address this issue through numerical simulations of glacier calving based on the Material Point Method and finite strain elastoplasticity. A non-associative Cam-Clay model was developed to simulate the ice while the water is modeled as a nearly in-compressible fluid. First, simplified 2D simulations were performed to analyse the size of calved icebergs which were in good agreement with analytical solutions. The model reproduces not only the vertical glacier fracture observed in real calving events but also iceberg formation and tsunami-wave generation. Finally, 3D simulations of glacier calving were performed, taking into account opened crevasses on the top of the glacier. Although at a preliminary stage, and lacking experimental validation, we show the promise of our approach for modeling glacier calving, and more generally glacier and sea-ice dynamics.

How to cite: Gaume, J., Gao, M., Wolper, J., Luethi, M. P., Vieli, A., Teran, J., and Jiang, C.: A Material Point Method for Glacier Calving, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21958, https://doi.org/10.5194/egusphere-egu2020-21958, 2020.

Retreat of marine outlet glaciers and ice shelves may initiate depletion of inland ice and lead to ice loss that by far exceeds what would be expected from ocean and atmospheric warming alone. Many marine outlet glaciers draining large parts of past and present ice masses have shown non-linear and variable retreat rates, with adjacent glaciers sometimes showing a highly different response to the same large-scale climate forcing. This suggests that individual glacier characteristics play a dominant role in governing retreat.

There is widespread evidence that the dynamic glacier adjustment to an external forcing is highly influenced by fjord topography. However, whether this stabilizes the glacier, or promotes enhanced retreat, depends on the shape of the fjord. So far, no rigorous, systematic assessment of the exact influence of certain geometric features such as overdeepenings or embayments has been undertaken in a model framework that incorporates all relevant processes in a 3D layout.

Here, we analyze a multitude of topographic settings and scenarios using the Ice Sheet System Model (ISSM), which accounts for all relevant physics in a 3D framework. Using artificial fjord geometries, we investigate glacier-topography interaction and quantify the modeled glacier response directly in relation to topographic features.

In light of our modeled topographic influence on glacier retreat, we consider whether we reliably can extrapolate observations from a few well-monitored glaciers to those less studied. Furthermore, we discuss implications for past and future ice sheet mass loss and associated sea-level rise. Finally, a deeper understanding of processes at the glacier front improves confidence in the climate signal derived from the deglacial landscape, as glacier-proximal landforms can more confidently be linked to climate.

How to cite: Frank, T., Åkesson, H., de Fleurian, B., and Nisancioglu, K. H.: Geometric Controls of Fjord Glacier Dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-838, https://doi.org/10.5194/egusphere-egu2020-838, 2020.

The rate of ice flux from the Greenland Ice Sheet to the ocean depends on the ice flow velocity through outlet glaciers. Ice flow velocity, in turn, evolves in response to multiple geographic and environmental forcings at different timescales. For example, velocity may vary daily in response to ocean tides, seasonally in response to surface air temperature, and multi-annually in response to long-term trends in climate. The satellite observations processed as part of the NASA MEaSUREs Greenland Ice Sheet Velocity Map allow us to analyse variations in ice surface velocity at multiple timescales. Here, we decompose short-term and long-term signals in time-dependent velocity fields for Greenland outlet glaciers based on the methods of Riel et al. (2018). Patterns found in short-term signals can constrain basal sliding relations and ice rheology, while the longer-term signals hint at decadal in/stability of outlet glaciers. We present example velocity time series for outlets including Sermeq Kujalleq (Jakobshavn Isbrae) and Helheim Glacier, and we highlight features indicative of dynamic drawdown or advective restabilization. Finally, we comment on the capabilities of a time series analysis software under development for glaciological applications.

How to cite: Ultee, L., Riel, B., and Minchew, B.: Seasonal to multi-annual speedup and slowdown of Greenland outlet glaciers inferred from time-dependent remote sensing observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12679, https://doi.org/10.5194/egusphere-egu2020-12679, 2020.

Whilst marine-terminating glaciers in Greenland are significant contributors to global sea level rise, their thermodynamics are poorly constrained by observations. Conventional discrete thermistor borehole sensing studies go some way to addressing this but lack the spatial resolution to effectively resolve key processes. Here, we detail results from fibre optic distributed temperature sensing equipment installed in a 1040 m hot water drilled borehole 28 km inland of the calving front of Store Glacier, Greenland. Surface ice velocity at the borehole is 550 m a^-1 with convergent ice flow into a bedrock trough. Spatial resolution of 0.25 m, temperature differences of 0.03 °C, and an absolute temperature accuracy of 0.15 °C were achieved. 0.5 °C warm anomalies were observed between 0-30 and 220-45 m depth with a central cold section down to -22 °C . We interpret the former anomaly to be a result of cryo-hydrologic warming, although of lower magnitude than in slow-flowing sectors of the Greenland Ice Sheet. The latter is theorised to be strain heating, supported by deformation observed in the cable at this point. The record also reveals a 75 m thick section of temperate basal ice and the nature of the cold-temperate transition as a sharp temperature drop of 0.45 °C over 1.5 m at the top of the temperate layer, with notable temperature changes in the vicinity of the transition. Warming of 0.06 °C is observed over the basal 6 m of the profile. The cable lasted 6 weeks before failure, demonstrating the feasibility of using  fibre optic sensing to study thermal processes in a glacier environment with high deformation rates.

How to cite: Law, R., Christoffersen, P., Hubbard, B., Doyle, S., Chudley, T., Bougamont, M., and Schoonman, C.: Distributed fibre-optic temperature sensing in a 1 km borehole drilled on a fast-flowing glacier in Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2433, https://doi.org/10.5194/egusphere-egu2020-2433, 2020.

The 79°N Glacier (79NG) in northeast Greenland, one of the last glaciers in Greenland with a floating ice tongue, plays a crucial role for buttressing the North-East Greenland Ice Stream (NEGIS). Remote-sensing studies indicate high basal melt rates (> 50 m/a) near the grounding line but these methods are limited by the hinge zone, where the floating ice is not in hydrostatic equilibrium. As part of the Greenland Ice Sheet Ocean Interaction (GROCE) project, we have performed a dense grid of repeated measurements with a phase-sensitive radio echo sounder (pRES) accompanied with autonomous pRES (ApRES) stations to estimate basal melt rates focusing on the hinge zone of 79NG. For analysing the pRES measurements, we additionally used ice thickness information derived from AWI’s ultra-wideband radar (UWB) revealing steep channels at the base. The estimated basal melt rates downstream the hinge zone are approximately the same as satellite-derived melt rates. In the hinge zone we found by far larger basal melt rates exceeding 100 m/a next to basal channels.

How to cite: Zeising, O., Steinhage, D., Neckel, N., Christmann, J., Helm, V., Dörr, N., and Humbert, A.: In-situ basal melt rate distribution of the floating tongue of 79°N Glacier, Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14759, https://doi.org/10.5194/egusphere-egu2020-14759, 2020.

Up to 30% of the global tidewater mass loss corresponds to frontal ablation through submarine melting and calving. However, the glacier-fjord interactions remain poorly understood and challenging to constrain in the models. We have developed a 2D glacier flowline-plume coupled model that includes subglacial discharge, submarine melting and iceberg calving to simulate Hansbreen-Hansbukta system (SW Svalbard). We run the model for 20 weeks, from April to September of 2010, with weekly information exchange between glacier and plume models. The same set up and constraints of a previous glacier-fjord circulation model are used here, making the results of both simulations comparable. We consider a 200 m-width subglacial discharging channel, which was found to be a good approximation in the previous glacier-fjord model. Submarine melt rates show high sensitivity to the subglacial-discharge and ambient fjord-temperature intraseasonal evolution. Calving rates are highly dependent on both submarine melting and crevasse water depth. Glacier-plume and glacier-fjord coupled models differ in vertically-accumulated submarine melt rates (up to 30 % higher for the glacier-plume model) and show different melt-undercutting front shapes, which have an influence on the net stress fields near the glacier front. The quasi-linear melt-undercutting morphology exhibited by the glacier-plume model promotes higher calving rates than the quasi-parabolic front shape resulting from the glacier-fjord model, although both models predict similar front positions. Given that the glacier-plume model diminishes the computational cost by a factor of >50, we think that it is a good option for projection studies, as long as we apply appropriate constraints to subglacial discharge fluxes and ambient fjord temperatures.

How to cite: De Andrés, E., Otero, J., and Navarro, F.: Glacier-plume or glacier-fjord circulation models? A model intercomparison for Hansbreen-Hansbukta system, Svalbard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9749, https://doi.org/10.5194/egusphere-egu2020-9749, 2020.

Recent literature has highlighted the great importance of subglacial meltwater plumes in a variety of processes including subaqueous ice melting, enhanced fjord-scale circulation, nutrient and heat mixing, foraging ground formation, and the movements of seals that apparently use plumes for returning to the sea surface.

However, direct measurements of plume water properties are scarce due to the difficulty of conducting observations near unstable glacier calving fronts. A few studies have succeeded in obtaining snapshot views of plume structures using bio-logging, remotely operated vessels, or helicopter-borne eXpendable Conductivity Temperature Depth (XCTD) probes, but continuous data time-series remain elusive and technically challenging.

In this study, we overcame these limitations by deploying mooring-based equipment between major calving events from a calving front of Bowdoin Glacier, an ocean-terminating glacier in Northwest Greenland. In July 2017, a first-of-its-kind 10 d dataset of plume dynamics was obtained by attaching instruments to the ice cliff for the logging of conductivity, temperature, and pressure at depths of ~5 m and ~100 m, with a sampling interval of 10 s.

Nonlinear and spectral time-series analysis revealed a chaotic system, an extremely turbulent environment, the presence of coherent structures, tide-modulated signals, and a non-intuitive transition in the dynamics of the plume due to a witnessed glacial lake outburst flood. Our observations should provide an important reference for the glacier-science community, including modellers interested in the evolution of ocean-terminating glaciers, fjord-scale circulation, and glacier fjord ecosystems.

How to cite: Podolskiy, E. A., Kanna, N., and Sugiyama, S.: Dynamics of a subglacial meltwater plume revealed by continuous subsurface monitoring directly on the calving front, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2771, https://doi.org/10.5194/egusphere-egu2020-2771, 2020.

Freshwater export from the Greenland Ice Sheet to the surrounding ocean has increased by 50% since the early 1990s, and may triple over the coming century under high greenhouse gas emissions. This increasing freshwater has the potential to influence both the regional and large-scale ocean, including marine ecosystems. Yet quantification of these impacts remains uncertain in part due to poor characterization of freshwater export, and in particular the transformation of freshwater around the ice sheet margin by ice-ocean processes, such as submarine melting, plumes and fjord circulation. Here, we combine in-situ observations, ocean reanalyses and simple models for ice-ocean processes to estimate the depth and properties of freshwater export around the full Greenland ice sheet from 1991 to present. The results show significant regional variability driven primarily by the depth at which freshwater runoff leaves the ice sheet. Areas with deeply-grounded marine-terminating glaciers are likely to export freshwater to the ocean as a dilute mixture of freshwater and externally-sourced deep water masses, while freshwater from areas with many land-terminating glaciers is exported as a more concentrated mixture of freshwater and near-surface waters. A handful of large glacier-fjord systems dominate ice sheet freshwater export, and the vast majority of freshwater export occurs subsurface. Our results provide an ice sheet-wide first-order characterization of how ice-ocean processes modulate Greenland freshwater export, and are an important step towards accurate representation of Greenland freshwater in large-scale ocean models.

How to cite: Slater, D. and Straneo, F.: Depth and properties of freshwater export from the Greenland Ice Sheet modulated by ice-ocean processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12976, https://doi.org/10.5194/egusphere-egu2020-12976, 2020.

Antarctica’s ice shelves play a key role in stabilizing their related ice sheets. The ice shelves of western Dronning Maud Land – including the Ekström, Atka, Jelbart, Fimbul and Vigrid ice shelves – currently buttress a catchment that comprises an ice volume equivalent to 0.95 meters of sea level. Any future increase in ice shelf mass loss, with basal melting likely being the main cause, will inevitably accelerate ice sheet drainage and contribute to global sea level rise. Since basal melting largely depends on ice-ocean interactions, it is crucial to attain reliable and consistent bathymetry models to estimate water and heat exchange beneath these ice shelves. We have constructed bathymetry models for an area of about 63,000 km² beneath the ice shelves of western Dronning Maud Land by inverting airborne gravity data, tied to radar, seismic, and offshore depth reference points. New high-resolution airborne magnetic data across the ice shelves point to Jurassic intrusions and seaward-dipping reflectors originating from Gondwana breakup; enabling us to consider geological density variations as part of the bathymetry modelling process. Our bathymetric models reveal deep glacial troughs beneath the ice shelves, and sills close to the continental shelf breaks which currently limit the possible entry of Warm Deep Water from the Southern Ocean. The present-day average thermocline depth is comparable to the average depths of saddles along the sills, which present gateways into the sub-ice cavities. This leads us to suggest a high sensitivity for these ice shelves to changes in ocean temperature and especially thermocline depth in the future. Once a significant amount of warm water overtops the sills, the deep troughs will allow for fast access to the grounding line, after which it seems there may be little to stop basal melting from rapidly eroding the ice shelves of western Dronning Maud Land.

How to cite: Eisermann, H., Eagles, G., Ruppel, A., Smith, E. C., and Jokat, W.: A look beneath the ice shelves of western Dronning Maud Land, East Antarctica: subglacial topography linked to ice shelf stability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3261, https://doi.org/10.5194/egusphere-egu2020-3261, 2020.

We propose a grounding line detection algorithm for Amery Ice Shelf (AIS) using Cryosat-2 altimetry and Landsat8 optical data. The Grounding line represents the area where ice sheet separates from Antarctic ice cap and extends into the ocean, which is the key indicator of the inland ice sheet instability and the boundary conditions of numerical model for ice velocity calculation. Many studies focus on grounding line retrieval using altimetry data or remote sensing data either with lower spatial resolution or discontinuous, which makes it difficult for large scale and long-terms analysis. In this abstract, Bayesian MAP (Maximum a posteriori probability criterion) based Cryosat-2 altimetry and Landsat8 optical data fusion algorithm is proposed for grounding line extraction in AIS, Antarctic. For Cryosat-2 data, the along track based slope analysis is used to calculate the Gaussian curvature and mean curvature, where the area with largest slope variance is defined as the grounding points, which will act as the control points in the fusion framework. For the Landsat8 imagery with the spatial resolution of 30m, we first generate the 1km grid using cubic Hermite method. Based on the similarity measurement between texture feature and grounding line area, where the area with largest variance of mean value and standard deviation is defined as the grounding line in Landsat8 data.  For the MAP based fusion grounding line extraction step, the optimal procedure is to find the minimum distance between the Cryosat-2 grounding points and Landsat8 grounding line within a given area, so as to maintain the smoothness and discontinuous where the optical data is missing or the texture feature is not obvious. In the experiment part, the proposed result is compared with MODIS grounding line products, and the results indicate that the mean value is similar with Landsat8 result and standard deviation is lower. Moreover, since the Cryosat-2 data is not obstacle by cloud, it can provide continuous observation for AIS grounding line. Besides, the time series analysis shows that from 2016-2018, the grounding line did not change so much, which means that the AIS is stable with lower expansion rate.   

How to cite: Zhang, Y., Zhu, T., Zhang, S., and Li, F.: Amery Ice Shelf Grounding line detection using Cryosat-2 and Landsat8 data fusion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18566, https://doi.org/10.5194/egusphere-egu2020-18566, 2020.

High salinity shelf water (HSSW) is a water mass that drives melting at the Ronne Ice Shelf, entering the sub ice shelf cavity at the western end of the ice front. To monitor the rate of ice shelf basal melting along the path of assumed HSSW inflow, a phase-sensitive radar (ApRES) was deployed and it sampled autonomously for over two years. Although the site is found to melt on average, the data show evidence of freezing occurring intermittently throughout the observed time period. Here we systematically investigate oceanographic processes that could give rise to these observations. Further, we address the question of whether ApRES can be used to quantify the rate of basal freezing.

How to cite: Vankova, I. and Nicholls, K.: Intermittent freeze-melt pattern detected at the base of the Ronne Ice Shelf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6108, https://doi.org/10.5194/egusphere-egu2020-6108, 2020.

Antarctic mass loss is the largest source of uncertainty in current sea level rise projections. Ice shelf instability plays a key role in this uncertainty as ice shelves are the floating gatekeepers that surround 75% of Antarctica’s coastline and that buttress the contribution of grounded ice to sea level rise. Although basal melting has been identified as one of the key processes for ice shelf instability, the quantitative understanding of this process and how much, how fast it weakens ice shelves is limited as it is determined by fine scale processes (e.g. channelized basal melting) that until recently were difficult to quantify. The recent availability of high-resolution, multi-source satellite imagery (e.g. stereoscopic DEMs from the Reference Elevation Model of Antarctica (REMA) or swath-processing of Cryosat-2), however, offers the opportunity to quantify the role of channelized melting on ice shelf instability across Antarctica.

In this study, we use REMA, Cryosat-2 and IceBridge elevation data to develop high-resolution indicators of basal melt across some major Antarctic ice shelves (Dotson, Pine Island, Larsen C). The methodology consists of processing time series of high-resolution REMA strips in a Lagrangian framework while accounting for tilt and tide corrections.

Comparison of different approaches (e.g. simplified REMA-only approach; combined REMA-Cryosat-2 approach, combined REMA-IceBridge approach) shows that the simplified approach can be applied easily to develop Antarctic wide estimates of basal melting across Antarctica, while the combined REMA-Cryosat-2 shows the highest accuracy. Results of this study, finally, show the potential of using REMA for developing high resolution basal melt products across Antarctica and providing insight in the spatial variability of basal melting due to channelized melting.

How to cite: Lhermitte, S., Nederend, J., and Wouters, B.: Channelized Antarctic ice shelf melting from high-resolution remote sensing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13277, https://doi.org/10.5194/egusphere-egu2020-13277, 2020.

Ice shelves that spread into the ocean can develop rifts, which can trigger ice-berg calving and enhance ocean-induced melting. Fluid mechanically, this system is analogues to the propagation of a non-Newtonian, strain-rate-softening fluid representing ice that displaces a relatively inviscid and denser fluid that represents an ocean. Recent scaled laboratory experiments have shown that when the flow geometry is circular the front of the displacing non-Newtonian fluid, which represents the leading edge of a shelf, can become unstable and evolve finger-like patterns comprised of rifts and tongues (Sayag & Worster, 2019a). As the rifts and tongues evolved, their number declined with time through the closure of some rifts.

In this study we focus on the weakly nonlinear stability of the propagating front. We consider an annular ice shelf having a fixed grounding line and an edge that evolves due to constant mass flux across the grounding line. We investigate the time evolution of the perturbed front to quantify the instability mechanism and the reduction of the number of rifts and tongues over time. The model predictions have better agreement with experimental measurements than previous studies. Our analysis elucidates the formation and evolution of rifts in ice shelves and provides testable predictions.

How to cite: Stern, L. and Sayag, R.: Instabilities in extensional flows and the dynamics of rifts in ice-shelves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22338, https://doi.org/10.5194/egusphere-egu2020-22338, 2020.

The propagation of high-frequency elastic-flexural waves through an ice shelf was modeled by a full 3-D elastic model, which also takes into account sub-ice seawater flow. The sea water flow is described by the wave equation. Numerical experiments were undertaken both for an intact ice shelf free of crevasses, which has idealized rectangular geometry, and for a crevasse-ridden ice shelf. The crevasses were modeled as triangle/rectangular notches into the ice shelf. The obtained dispersion spectra (the dispersion curves describing the wavenumber/periodicity relation) are not continuous. The spectra reveal gaps that provide the transition from n-th mode to (n+1)-th mode. These gaps are observed both for an intact ice shelf free of crevasses and for a crevasse-ridden ice shelf. They are aligned with the minimums in the amplitude spectrum. That is the ice shelf essentially blocks the impact wave at this transition. However, the dispersion spectrum obtained for a crevasse-ridden ice shelf, has a qualitatively difference from that obtained for an intact ice shelf free of crevasses. Moreover, the dispersion spectrum obtained for a crevasse-ridden ice shelf reveals the band gap – the zone there no eigenmodes exist (Freed-Brown and others, 2012). The numerical experiments with the crevasse-ridden ice tongue that is 16 km in longitudinal extent, 0.8km width and 100m thick, were undertaken for a wide range of the periodicities of the incident wave: from 5 s to 250 s. The obtained dispersion spectra reveal two band gaps in this range: the first band gap at about 20 s and the second band gap at about 7 s for 1km spatial periodicity of the crevasses. The width of the band gap significantly increases when the crevasses depth increases too. Respectively, the amplitude spectra reveal significantly increasing area of periodicities/frequencies where the ice shelf blocks the impact wave.

References

Freed-Brown, J., Amundson, J., MacAyeal, D., & Zhang, W. (2012). Blocking a wave: Frequency band gaps in ice shelves with periodic crevasses. Annals of Glaciology, 53(60), 85-89. doi:10.3189/2012AoG60A120

Konovalov, Y.V. (2019). Ice-shelf vibrations modeled by a full 3-D elastic model. Annals of Glaciology, 1-7. doi:10.1017/aog.2019.9

How to cite: Konovalov, Y.: The effect of the blocking of impact ocean waves by the crevasse-ridden ice shelf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9665, https://doi.org/10.5194/egusphere-egu2020-9665, 2020.

Shoreward oceanic heat flux in deep channels on the continental shelf typically far exceeds that required to match observed ice shelf melt rates, suggesting other critical controls.  IN the present study we study the depth-independent (barotropic) and the density-driven (baroclinic) components of the flow of warm ocean water towards an ice shelf. Using observations from the Getz Ice Shelf system as well as geophysical laboratory experiments on a rotating platform, it is shown that the dramatic step shape of the ice front blocks the barotropic component, and that only the baroclinic component, typically much smaller, can enter the sub-ice cavity.  A similar blocking of the barotropic component may occur in other areas with comparable ice-bathymetry configurations, which may explain why changes in the density structure of the water column have been found to be a better indicator of basal melt rate variability than the heat transported onto the continental shelf. Representing the step topography of the ice front accurately in models is thus important for simulating the ocean heat fluxes and induced melt rates.

How to cite: Wåhlin, A., Steiger, N., Darelius, E., Assmann, K., Glessmer, M., Ha, H. K., Herraiz-Borreguero, L., Heuzé, C., Jenkins, A., Kim, T. W., Mazur, A., Sommeria, J., and Viboud, S.: Ice front blocking of ocean heat transport to an Antarctic ice shelf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19856, https://doi.org/10.5194/egusphere-egu2020-19856, 2020.

To study Antarctica’s contribution to ongoing and future sea level rise, a coupled ice sheet – ice shelf – ocean model with an explicit representation of ice shelf cavities has been developed. The coupled model is based on a global implementation of the Finite Element Sea ice Ocean Model (FESOM) with a mesh that is substantially refined in the marginal seas of the Southern Ocean. The Antarctic Ice Sheet is represented by a regional setup of the Parallel Ice Sheet Model PISM, comprising the Filchner-Ronne Ice Shelf (FRIS) and the grounded ice in its catchment area up to the ice divides.  At the base of the FRIS, melt rates and ocean temperatures from FESOM are applied. PISM returns ice thickness and the position of the grounding line. Buildung on infrastructure developed for the Regional Antarctic and Global Ocean (RAnGO) model, we use a pre-computed FESOM mesh that is adopted to the varying cavity geometry in each coupling step, i.e. currently once per model year. Changes in water column thickness are easily accounted for by the terrain-following vertical coordinate system in the ice shelf cavity. The ice sheet model is run on a horizontal grid with 1 km resolution to ensure an appropriate representation of grounding line processes. Enhancement factors for the approximation of the stress balance, as often used in coarse-resolution ice sheet models, become obsolete at such high resolution. Ice stream flow is well captured by polythermal coupling of the ice flow and a Mohr-Coulomb yield stress criterion that accounts for properties of the till material and the effective pressure on the saturated till. We present results from model runs with a 20^th-century climate forcing and projections until the end of the 22^nd century. We will show that cavity hydrography, ice shelf basal melt rates and thickness evolution as well as the feedback on grounded ice  in the coupled model simulations are very sensitive to the atmospheric forcing scenario applied.

How to cite: Timmermann, R. and Albrecht, T.: Coupled ocean—ice shelf—ice sheet projections for the Weddell Sea Basin, Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18743, https://doi.org/10.5194/egusphere-egu2020-18743, 2020.

Sub-ice-shelf melting plays a critical role in the dynamics of the Antarctic Ice  Sheet and also feeds back on the regional climate, transforming ocean properties (e.g., affecting deep-water production and sea-ice formation).  A full understanding of these processes, as well as the ability to project their response to a changing climate, requires Earth System Models (ESMs) that include coupling with ice-sheet processes.  However, biases in regional Antarctic climate can be amplified through sub-ice-shelf melting, and biased melt rates can have significant adverse effects on ice-sheet model initialization and evolution.  In preparation for inclusion of dynamic ice sheets in ESMs, this presentation discusses our recent experience in understanding the causes of biases in ocean properties on the Antarctic continental shelf and their relationship to ice-shelf melting.  Differences in model behavior across configurations and simulations using the Energy Exascale Earth System Model (E3SM) demonstrates a sensitivity of melt rates to climate. We assess the sensitivity of those melt rates to changes in the region’s climate, including freshening on the continental shelf and shoaling of the thermocline. We also show that ice-shelf meltwater feeds back onto the climate, for example, by affecting melting under neighboring ice shelves, sometimes dramatically so.  We demonstrate that significant reductions in melt-rate biases can be achieved through modifications to ocean model mixing parameterizations. This work charts a path forward for configuring ESMs to produce realistic Antarctic melt rates.

How to cite: Asay-Davis, X., Begeman, C., Comeau, D., Hoffman, M., Lin, W., Petersen, M., Price, S., Roberts, A., Veneziani, M., and Wolfe, J.: Exploring and reducing biases in sub-ice-shelf melt rates in an Earth system model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12987, https://doi.org/10.5194/egusphere-egu2020-12987, 2020.

Ice sheet-ocean interaction is important to properly understand phenomena such as ice sheet melting and ocean circulation. While the long term goal of this project is to fully couple the ice and ocean in one single numerical framework, we here start by modelling the ocean. We use the full non-hydrostatic equations in order to accurately model the complex ocean dynamics near the ice sheets. As numerical method, we employ finite element methods due to their capability of representing a complex fjord geometry and locally refining the mesh in the areas which require more careful handling, and its strong mathematical foundation. This will allow to overcome classical problems such as representing a moving ice shelf in a discretized setting. We here present an example of modeled fjord circulation obtained simulating the model with the FEniCS computing platform.

How to cite: Ottolenghi, S. and Ahlkrona, J.: Towards tackling ice-sheet ocean interaction with Finite Element Methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11279, https://doi.org/10.5194/egusphere-egu2020-11279, 2020.

Freshwater flux from the melting of Greenland’s Ice Sheet is thought to account for 25% of the observed rise in global mean sea level between 1992 and 2011, with a significant proportion of this associated with increased freshwater flux from marine terminating glaciers within coastal fjords. It has been suggested that increased ocean temperatures have triggered the retreat of Greenland’s outlet glaciers, with the melting of submarine glacier termini leading to an acceleration of inland regions of the ice sheet. Global climate  models  currently  operate  at  resolutions  too  coarse  to resolve  ice-ocean  interaction  on  the length  scales  typical  of  coastal  fjords. Therefore, a parameterization scheme is required to incorporate the relevant physics into such models.

As a first step towards such a parameterisation scheme, we develop theoretical understanding of the first order controls on heat and freshwater exchanges in Greenland’s proglacial fjords, guided by computational simulations in MITgcm. Fjords are modelled with idealised geometries, considering cases with and without bathymetric sills. The model parameterises melting at the glacier terminus, and non-hydrostatic flow in one or more buoyant plumes that form from fresh subglacial discharge at the glacier grounding line. We systematically explore how the overturning circulation and heat transport through a fjord respond to varying subglacial discharge.

In a subglacial-discharge dominated regime with flat bathymetry, we find that the horizontally integrated vertical flow structure set by buoyant plumes at the ice face remains unmodified along the length of the fjord, and is independent of the fjord width. For cases with either single or multiple subglacial-discharge plumes, we derive scaling laws for the heat and freshwater exchanges using buoyant plume theory, finding that the water in contact with the ice face mirrors that outside the fjord. This picture is complicated in the presence of a bathymetric sill, which can inhibit the transportation of deep coastal waters into the fjord. We conclude by discussing how our scaling laws might be used as a simple parameterisation of proglacial fjord dynamics in regimes where subglacial discharge controls the flow strength. We discuss how these results might be extended to incorporate the competing effects of circulation driven by along-fjord and along-shelf winds.

How to cite: Stanway, A., Wells, A., Johnson, H., and Ridley, J.: Parameterizing heat and freshwater exchanges driven by subglacial discharge in Greenland's proglacial fjords, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9101, https://doi.org/10.5194/egusphere-egu2020-9101, 2020.

The melting of icebergs within Greenland’s iceberg-choked fjords provides a large and distributed source of liquid freshwater throughout the year. However, the impact of this freshwater flux on fjord properties and circulation remains unclear, in part because icebergs have typically been neglected in modelling studies that seek to examine interaction between glacier and fjord processes. Here, we modify a general circulation model to simulate the impact of iceberg submarine melting within Kangerdlugssuaq and Sermilik fjords in east Greenland, home to two of Greenland’s largest glaciers. We find that iceberg submarine melting results in cooling of up to 5°C and freshening of up to 0.6 psu throughout the upper 100-200 metres of both fjords, compared to experiments without icebergs. The resulting freshwater flux, which is of the order of hundreds of cumecs, is capable of driving a weak overturning circulation. This augments the runoff-driven circulation at depth but can oppose the up-fjord flow of warm near-surface waters, resulting in an increase in up-fjord heat flux at depth but a decrease near the surface. By increasing subsurface iceberg melt rates, ocean warming will therefore expedite iceberg deterioration within ice mélange and may further increase ocean thermal forcing of tidewater glacier grounding lines. Our results highlight the significant impact that icebergs have on fjord water properties and circulation in Greenland’s iceberg-choked fjords, demonstrating the importance of including these processes in studies that seek to examine interactions between the ice sheet and the ocean.

How to cite: Davison, B., Cowton, T., Cottier, F., and Sole, A.: Modelling the impact of iceberg melt on glacier-ocean interaction, east Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8663, https://doi.org/10.5194/egusphere-egu2020-8663, 2020.

Future mass loss predictions, and thereby sea level rise predictions, are strongly affected by the representation of iceberg calving in numerical ice sheet models. Despite recent advances, gaps in our understanding of calving mechanisms remain and there exists a lack of data to constrain mechanical properties related to ice fracturing. For instance, observed critical strain rates for crevasse initiation span two orders of magnitude.

Bowdoin Glacier in Northwest Greenland provides a unique opportunity to conduct in-situ measurements near the calving front due to its accessibility via a crevasse-free walkable moraine. In July 2019, two major calving events were surveyed by 10 GPS stations installed along the front in close vicinity to the calving events. Measurements show glacier uplift prior to the first calving event and horizontal compression prior to the second major calving event.

In contrast to previously observed major events, no precursor such as a large surface crack was visible on the field. Our data suggest a change in calving behaviour from surface crevasses due to hydro-fracturing to basal crevasse formation due to buoyancy, which may be favoured by observed thinning (~4 m yr^-1 since 2013).

How to cite: van Dongen, E., Jouvet, G., Lindner, F., Bauder, A., Walter, F., and Sugiyama, S.: GPS measurements during two major calving events at Bowdoin Glacier, Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8110, https://doi.org/10.5194/egusphere-egu2020-8110, 2020.

How to cite: Mouginot, J., Rignot, E., Scheuchl, B., Millan, R., Bjørk, A., Ehrenfeucht, S., and Derkacheva, A.: Ice-shelf and glacier changes in Northern Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14461, https://doi.org/10.5194/egusphere-egu2020-14461, 2020.

Successful prediction of the response of the Greenland Ice Sheet to climate warming requires accurate estimation of future ice loss from tidewater glaciers. Patterns of tidewater glacier retreat and advance have acted as an important proxy for understanding the processes associated with frontal ablation. It has not however been possible to effectively constrain commonality in these observed patterns that can then be directly linked to the influence of specific controls on ice loss. Here, we investigate planform changes in calving front morphology, an aspect of glacier dynamics that has received little prior attention; however, an improved understanding and quantification of the role of morphometric change in influencing glacier dynamics and iceberg calving may provide critical insights into tidewater glacier behaviour. We develop a buffer analysis method to quantify changes in calving front morphology at Narsap Sermia, a large tidewater glacier in southwest Greenland that has experienced substantial recent retreat. Our results reveal no distinct temporal or spatial patterns in the timing or magnitude of morphological change. Furthermore, we found no statistically significant relationships between morphological change and a range of forcing factors including air temperatures, modelled estimates of subglacial discharge and variations in glacier bed geometry. Our results therefore suggest that process driven morphological terminus change is not an effective predictor of terminus retreat and instead support the application of generalised parameterisations of tidewater glacier retreat within ice-dynamic models.

How to cite: Bunce, C., Nienow, P., Gourmelen, N., and Cowton, T.: Investigating calving front morphology as a precursor to dynamic behaviour at a large Greenlandic tidewater glacier, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16531, https://doi.org/10.5194/egusphere-egu2020-16531, 2020.

We observed two outlet glaciers in West- and Nordwest-Greenland with a terrestrial radar interferometer (TRI), pressure sensors and time-lapse cameras over six and two years, respectively. The resulting detailed dataset provides us with insights on the calving process and the changes in front geometry over the last years. Since the two glaciers are characterised by different geometries and velocity fields, the influence of those parameters on the calving process can be investigated. The combination of the three different observation methods enable us to overcome their individual disadvantages. With the time-lapse camera taking pictures of the glacier front every 10 seconds, we detect all calving events of different sizes and styles but cannot quantify the volume. We used the TRI to quantify the volumes of aerial calving events by DEM differentiation. Further, calving waves measured with pressure sensors are used to distinguish between different calving types. We develop a relationship between calving volumes and wave heights and use this as an additional indirect method to estimate calving volumes. We find that the calving style and size as well as the front geometry is mainly controlled by the bed topography and the presence of a subglacial discharge plume. The location of the plume is observed to migrate from year to year, which leads also to changes in the calving pattern. Calving style and pattern as well as glacier velocity fields and geometry changes are additionally compared with environmental conditions such as the air temperature and the presence of ice-mélange in the proglacial fjord. In years with an early spring we find different front characteristics and calving patterns than for years with colder conditions.

How to cite: Walter, A., Lüthi, M. P., Funk, M., and Vieli, A.: Multi-year observations of calving and front characteristics of two marine terminating outlet glaciers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18633, https://doi.org/10.5194/egusphere-egu2020-18633, 2020.

Grounding line/zone dynamics of floating-tongue glaciers is of major importance for changes in their contribution to sea-level rise. For floating-tongue glaciers, thermal forcing of oceanic heat and tidal forcing are the major processes acting in that zone. Here we deal with the response to tidal forcing. The 79°N Glacier, an outlet glacier of the North East Greenland Ice Stream, is the focus of the Greenland Ice Sheet Ocean Interaction project (GROCE) funded by the German Ministry of Education and Research. We present a study of this region considering the deformation of the glacier in response to ocean tidal forcing by means of observations and modeling. GPS measurements realized in 2017-2018 are analyzed for vertical and horizontal displacements of the glacier and its floating tongue. Observations on fully-grounded ice reveal a periodic horizontal displacement in response to ocean tidal forcing in a distance of more than 35 km upstream from the grounding line. In the hinge zone, i.e. the transition between grounded and floating ice, the tidal forcing leads to a measurable vertical bending of the ice and a periodic movement of the grounding line. Understanding the mechanisms of grounding line migration is important to better evaluate the contribution of grounded ice discharge to sea-level rise.

In order to model the measured displacements, a viscoelastic material model is required using the observed vertical displacements at the floating ice tongue as external forcing. Geometries obtained from AWI’s new ultrawideband radar form the basis for finite-element simulations in COMSOL. With the viscoelastic Maxwell material model, the response of the ice to ocean tidal forcing can successfully be modeled. Results obtained with a nonlinear Glen-type viscosity agree very well with the observed bending near the grounding line. The expected phase shift of the horizontal displacements upstream from the grounding line is well reproduced in the model.

How to cite: Christmann, J., Rückamp, M., Zeising, O., Steinhage, D., Neckel, N., Helm, V., Ralf, M., Scheinert, M., Khan, S. A., and Humbert, A.: Viscoelastic modeling results of the 79°N Glacier, Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16736, https://doi.org/10.5194/egusphere-egu2020-16736, 2020.

Jakobshavn Isbrae has dramatically accelerated, thinned and retreated since the late 1990s in several stages of retreat and stagnation. Studies have indicated that the loss of buttressing due to retreat of the calving front following the disintegration of its floating ice tongue was the trigger of acceleration and thinning of the terminus, however uncertainty remains over the mechanisms controlling the timing and magnitude of the retreat.

The maximum retreat of the calving front was reached between 2013 and 2015 following the peaking of ice flow speeds in excess of 18 km yr^-1. Since 2016, ice flow speeds have decelerated from this peak and the terminus has experienced a modest readvance and thickening. We calculated a calving rate for the period 2009 to 2018 which shows that terminus flow speeds and calving are closely related. Until 2009 a transient loosely bonded ice tongue formed but this feature appears not to have formed from 2010 onwards.

We aim to demonstrate that the signal of thinning and retreat can be reproduced by driving the glacier with the calculated calving rate. We used the BISICLES ice sheet model to simulate the evolution of Jakobshavn Isbrae over the past decade, with the calving front driven by the calculated 2009 – 2018 calving rate. The results of these simulations show that the response of the glacier to the applied calving rate is in line with its observed evolution over this period. We also present the results of further experiments designed to examine the mechanisms and controls on the calving retreat.

How to cite: Trevers, M., Payne, T., Cornford, S., and Hogg, A.: Modelling Jakobshavn Isbrae from 2009 to 2018, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21999, https://doi.org/10.5194/egusphere-egu2020-21999, 2020.

Rapid grounding line retreat at marine-terminating glaciers could expose ice cliffs with heights greater than those on observational record. However, the finite strength of ice places a limit on the height of subareal cliffs. It is proposed that marine ice-cliff instability (MICI) will begin once a stable height threshold is exceeded. If a glacier is situated over a retrograde slope, as is the case for Thwaites Glacier and much of the West Antarctic Ice Sheet, MICI can be expected to accelerate as retreat progresses and increasingly tall and unstable ice cliffs are formed. This is consequential for global sea level rise, yet large uncertainties remain in the prediction of MICI retreat rates.

We investigate MICI by pairing the full Stokes continuum model Elmer/Ice and the Helsinki Discrete Element Model (HiDEM). Viscous flow, simulated in Elmer/Ice, is found to be a necessary pre-condition for MICI collapse. Forward advance and bulging lead to ice-front instability and pervasive crevassing in HiDEM. This culminates in full-thickness calving events. We do not observe calving at ice faces prior to viscous deformation. HiDEM simulations that implement viscous flow (HiDEM-ve) also show forward advance and waterline bulging, similar to the Elmer/Ice simulations. However, the importance of granular shear is highlighted by pronounced shear bands and patterns of surface lowering in HiDEM-ve output. These results emphasize the importance and complexity of viscous and brittle process interaction during MICI.

A simulation matrix of grounded termini shows that calving frequency and magnitude increase with the thickness of the calving front. The time required for viscous flow to recreate unstable conditions is influenced by thickness as well as ice temperature and basal friction. Simulations of buoyant termini are seen to calve through basal-crevassing and block-rotation, as opposed to incising surface-crevasses. Lastly, we observe that buttressing mélange can suppress retreat rate if a sufficient resistive force is delivered to the calving front. A physically-based law for MICI retreat rate is derived from our simulation matrix; this calving rate law can be incorporated into large-scale ice sheet models to constrain projections of Antarctic retreat and associated global sea level rise. Our results will also be used to investigate the future retreat of Thwaites Glacier, which is vulnerable to MICI due to a retreating grounding line, fragile floating ice shelf, and precarious positioning above an overdeepening basin.

How to cite: Crawford, A., Todd, J., Benn, D., Åström, J., and Zwinger, T.: Deriving a Physically-Based Calving Rate Law for Marine Ice-Cliff Instability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15790, https://doi.org/10.5194/egusphere-egu2020-15790, 2020.

Tidewater glaciers are complex systems, which present numerous modelling challenges with regards to integrating a multitude of environmental processes spanning different timescales. At the same time, an accurate representation of these systems in models is critical to being able to effectively predict the evolution of the Greenland Ice Sheet and the resulting sea-level rise. In this study, we present results from numerical simulations of Store Glacier in West Greenland that couple ice flow modelled by Elmer/Ice with subglacial hydrology modelled by GlaDS and submarine melting represented with a simple plume model forced by hydrographic observations. The simulations capture the seasonal evolution of the subglacial drainage system and the glacier’s response, and also include the influence of plume-induced ice front melting on calving and buttressing from ice melange present in winter and spring.

Through running the model for a 6-year period from 2012 to 2017, covering both high- and low-melt years, we find inputs of surface meltwater to the subglacial system establishes channelised subglacial drainage with channels >1 m² extending 30-60 km inland depending on the amount of supraglacial runoff evacuated subglacially. The growth of channels is, however, not sufficiently fast to accommodate all inputs of meltwater from the surface, which means that basal water pressures are generally higher in warmer summers compared to cooler summers and lowest in winter months. As a result, the simulated flow of Store Glacier is such that velocities peak in warmer summers, though we suggest that higher surface melt levels may lead to sufficient channelisation for a widespread low-water-pressure system to evolve, which would reduce summer velocities. The results indicate that Greenland’s contribution to sea-level rise is sensitive to the evolution of the subglacial drainage system and especially the ability of channels to grow and accommodate surface meltwater effectively. We also posit that the pattern of plume melting encourages further calving by creating an indented calving front with ‘headlands’ that are laterally unsupported and therefore more vulnerable to collapse. We validate our simulations with a three-week record of iceberg calving events gathered using a terrestrial radar interferometer installed near the calving terminus of Store Glacier.

How to cite: Cook, S., Christoffersen, P., Todd, J., Slater, D., Chauché, N., and Truffer, M.: A 3D full-Stokes model of Store Glacier, Greenland, with coupling of ice flow, subglacial hydrology, submarine melting and calving, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1607, https://doi.org/10.5194/egusphere-egu2020-1607, 2020.

   The Greenland ice sheet is currently contributing to global sea level at an approximate rate of 0.8 mm/yr. Ice mass loss of Greenland is primarily due to both thinning and retreat of outlet glaciers. For enhanced calving events, detail dynamics characteristics of hydrological and kinematic precursors and underlying mechanisms which control the development of ice calving remain poorly understood, especially in the absence of high-resolution remote sensing observations. On July 26 2017, a calving event took place along a pre-existing rift in Petermann glacier, northern Greenland, which removed partly of the glacier tongue and formed a tabular iceberg 5 km long. In this study, we used high-temporal satellite remote sensing data to detect changes in ice-flow speed, melt ponds and ice mélange during May and July. These hydrological and kinematic dynamics derived from Sentinel-1/2 satellite images with sub-weekly acquisition repeat cycles can be utilized as retreat precursors to characterize the detailed calving process. Moreover, the stress field and analytical damage solution were calculated by coupling the remote sensing observations with SSA ice sheet model to explain the dynamics mechanism. Our preliminary results show that the ice speed in dense observation reached to 30 m/d on the eve of the calving, which is roughly 10 times quicker than usual ice velocity. Additionally, there exited obviously abnormal stress distribution in crack region. And the landfast sea ice and ice mélange transformed into open water that the  backscatter coefficient decreased to 28 dB. The extent of melt pond reached the peak about 30 square kilometers coverage in duration month of calving event. It is inferred that this calving event of Petermann glacier may be related to weakening of sea ice and ice mélange lost the buttressing for ice glacier terminate, tributary glacier extrusion, related with meltwater infiltrated crevasses. Therefore, dense remote sensing observations and numerical modeling in ice flow system make it possible for early waring and projecting glacier calving in the future.

Key words: Iceberg Calving Precursors, Petermann Glacier, High Resolution Remote Sensing, SSA modeling

How to cite: Li, D. and Jiang, L.: Hydrological and Kinematic Precursors of Iceberg Calving at Petermann Glacier in Northern Greenland Observed High-temporal Resolution Sentinel-2 Images, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7831, https://doi.org/10.5194/egusphere-egu2020-7831, 2020.