Displays

The evolution of surface and shallow subsurface meltwater bodies across Antarctic ice shelves has important implicationsfor their (in)stability, as demonstrated by the 2002 rapid collapse of the Larsen B Ice Shelf. Ice-shelf break-up may be triggered by stress variations associated with meltwater movement, ponding and drainage, causing ice-shelf flexure and fracture. We have recently begun a four year, jointly-funded US-NSF / UK-NERC project that will provide important geophysical insights into the stability of the George VI Ice Shelf on the Antarctic Peninsula, where hundreds of surface lakes form each summer.

In November 2019, we deployed global positioning systems, pressure transducers, automatic weather stations, and in-ice thermistor strings to record ice-shelf flexure, surface water depths, and surface and subsurface melting, respectively, in and around several surface lakes. Next austral summer (November 2020), we also plan to record fracture seismicity with a passive seismometer deployment, and to conduct ground penetrating radar surveys to detect subsurface water. Instruments, which are all within ~30 km of BAS's Fossil Bluff Station, will remain on the ice shelf until January 2022, resulting in a 27-month observational record in total.

Here, we report results of satellite image analysis of surface and shallow subsurface meltwater bodies, together with preliminary field and modelling results associated with our project. Using NDWI_ice thresholds applied to Landsat 8, Sentinel-2 and WorldView optical imagery, we show how patterns of surface meltwater evolve within and between summer melt seasons. Using Sentinel-1 SAR imagery and a convolutional neural network technique, we detect and track bodies of shallow subsurface water and show how they relate to patterns of surface water. We also report on field reconnaissance surveys made to two dolines (drained lake basins) on the ice shelf, and present a simple model to describe the process of doline formation. Throughout the project, we will combine field and remotely sensed data to extend and validate our existing approach to modelling ice-shelf flexure and stress, and possible ‘Larsen-B style’ ice-shelf instability and break-up at less geographically confined ice shelves.

How to cite: Banwell, A. F., Dell, R., Dunmire, D., MacAyeal, D., Stevens, L. A., and Willis, I. C.: Ice-shelf instability due to surface meltwater systems on the George VI Ice Shelf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6190, https://doi.org/10.5194/egusphere-egu2020-6190, 2020.

We predict the location of perennial firn aquifers (PFAs) in the Antarctic Peninsula using the updated regional atmospheric climate model RACMO2.3p2, that is specifically adapted for use over the polar regions. With RACMO2 output we force two sophisticated firn models, IMAU-FDM and SNOWPACK, with surface mass fluxes and surface energy fluxes, respectively. These firn models explicitly calculate processes in the snowpack, such as densification, meltwater penetration, refreezing, retention and runoff.

In this presentation, we focus on the Antarctic Peninsula (AP), where conditions are favorable for the formation of PFAs: there is both sufficient meltwater production and snowfall to store the meltwater in the firn during winter without refreezing, as the fresh snow insulates the meltwater from the winter cold wave. These conditions are similar to those locations where PFAs were discovered in Greenland and Svalbard.

While slightly different in behavior, both firn models calculate PFAs on Wilkins ice shelf and the northwestern AP mountain range, but also near the grounding lines of unstable or disintegrated ice shelves such as Prince Gustav, Larsen B and Wordie. The PFAs exist in different forms, e.g. long-lasting, shallow, deep or multi-layer, and are sensitive to the magnitude and timing of atmospheric forcing conditions. We carefully explore processes controlling their formation and/or longevity, discuss their implications for ice shelf stability, and their potential to exist elsewhere in Antarctica.

How to cite: van Wessem, J. M., van den Broeke, M., Steger, C., Wever, N., and Ligtenberg, S.: Modelling perennial firn aquifers in the Antarctic Peninsula, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7574, https://doi.org/10.5194/egusphere-egu2020-7574, 2020.

Mass loss from glacier surface melt is buffered by percolation and refreezing in the underlying snowpack, processes of particular importance in the percolation zone of the Greenland ice sheet and increasingly in Antarctica under a warming climate. Retention and refreezing is dependent on a number of micro-scale factors such as snow grain size, density and temperature that are heavily parameterized in models. Melt and snowfall in preceding seasons are also important in determining retention rates in the current season due to initialization of the snowpack .

In the retention model intercomparison project (RetMIP) we use a common atmospheric forcing from the HIRHAM5 regional climate model to drive participating  models, to study the effect of different internal parameterisations. We compare 9 different 1D models and four 2D models with each other and with observations from 4 key field sites. We show that initialisation of snowpack models is important but evolution of retention through time is strongly determined by melt rates.

Models that explicitly account for deep meltwater percolation tend to overestimate percolation depth and consequently firn temperature at the percolation and ice slab sites although they simulate accurately the recharge of the firn aquifer. Models using Darcy’s law and bucket scheme compare favourably to observations at the percolation site but only the Darcy models accurately simulate firn temperature and thus meltwater percolation at the ice slab site. We find that Eulerian models that transfer firn through fixed layers, diffuse over time the gradients in firn temperature and density. No model outperforms all others at our four test sites indicating that all models have potential for development.

A first look at Antarctic firn processes emphasises the importance of long spin-up times to initialise the snowpack and as the ice sheet surface evolves in the future parameterising the specific Antarctic retention process is likely to become more important.

How to cite: Mottram, R., Vandecrux, B., Olesen, M., Boberg, F., Hansen, N., Langen, P., and Fausto, R. and the RetMIP contributors: How does the meltwater flow? Retention and refreezing in firn on ice sheets and ice shelves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15912, https://doi.org/10.5194/egusphere-egu2020-15912, 2020.

Abstract: Very little is known about the subglacial hydrologic system under the Antarctic Ice Sheet due to the difficulty of directly observing the bottom of the ice sheet. Hydrology modeling is a powerful tool to simulate the spatial distribution of crucial hydrologic properties under the ice sheet. Here, we use the state-of-art two-dimensional Glacier Drainage System model (GlaDS) to simulate both distributed sheet flow and continuous channels under Pine Island Glacier (PIG), West Antarctica, one of the largest contributors to sea level rise in Antarctica.

We adopt an unstructured triangular mesh which enables channels to form along element edges. We drive the model with meltwater computed from an inversion and steady temperature simulation of PIG using a Stokes flow ice dynamic model. Our domain comprises the full PIG catchment. We aim to study the pattern and development of water pressure, hydraulic potential, water sheet thickness and discharge, as well as channel area and flux, which together describe the state of the basal system.

Our results for hydraulic potential correctly route water towards the grounding line, while we find near-zero effective pressure underneath the main trunk of PIG, consistent with the low basal drag and low driving stress there. This has implications for the representation of sliding in ice dynamic models: typical assumptions about hydrology connectivity to the ocean will overestimate effective pressure. When run forward in time, efficient channels evolve near the grounding line indicating an efficient drainage system where water fluxes are high in the downstream part of the PIG.

By applying GlaDS to a real marine ice sheet catchment we can better understand how basal hydrology modulates ice dynamics through basal sliding. We plan to compare our model predictions of effective pressure and drainage system with driving stress and inversions of basal drag. This will allow us to see the relationship between basal hydrology and basal sliding under PIG, and provide us better tools to predict the evolution of the region in view of future climate scenarios. Moving forward, we plan to couple the hydrology model with the ice dynamics model to make more accurate projections of sea level rise from PIG.

Key Words: West Antarctica, subglacial hydrology, drainage system, GlaDS, Elmer/Ice, Pine Island Glacier

How to cite: zhang, Y., Moore, J., Wolovick, M., Gladstone, R., Zwinger, T., and guo, X.: Simulating Antarctic subglacial hydrology processes underneath Pine Island Glacier, West Antarctica, using GlaDS model in Elmer/Ice, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8344, https://doi.org/10.5194/egusphere-egu2020-8344, 2020.

In a warming world, increased meltwater will form on Antarctica’s ice shelves. The fate of this meltwater will be critical to future ice-shelf and ice-sheet stability. Two main observations define the current theoretical framework for understanding the influence of surface hydrology on ice-shelf stability. The first is the collapse West Antarctica’s Larsen B Ice Shelf that was triggered by the formation of thousands of surface ponds atop the ice shelf. The second is the observation of a waterfall on the Nansen Ice Shelf, in East Antarctica, that is hypothesized to protect the ice shelf from hydrofracture by removing meltwater from the ice-shelf surface.

We present a third process that couples ice-shelf hydrology to atmospheric and ocean forcing: the development of an ice-shelf estuary on the Petermann Ice Shelf in northwest Greenland.  High-resolution imagery and digital elevation models (DEMs) shows that channelized surface meltwater on the Petermann Ice Shelf in northwest Greenland incises into underlying ice to form an estuary that propagates fractures along the ice shelf. The estuary at the front of the Petermann Ice Shelf is indicated by the convergence of sea ice at the river mouth, the upstream transport of sea ice in the channel as far as 460 m from the calving front, and the persistence of water in the channel following the end of seasonal surface melt. Between 2013 and 2018, the estuarine reach of the river tripled in width and a 1.5 km longitudinal crack propagated along the bottom of the channel. The Petermann Ice Shelf Estuary forms on top of a basal channel, where basal melting has led to ice-shelf thinning, and the creation of the linear surface depression in which the estuary forms.

The Petermann Estuary may be the first of several ice-shelf estuaries to develop in a warming climate. Widespread surface melting on ice shelves in Greenland and Antarctica increases the urgency to determine the influence of surface hydrology on ice-shelf stability. We hypothesize that surface rivers may initially buffer ice shelves from collapse by terminating in waterfalls and preventing the formation of damaging lakes. However, with increased meltwater transport across ice shelves, channels can incise to sea level and establish estuaries. Once an estuary is established, estuarine weakening can lead to fracture propagation and enhanced calving, destabilizing ice-shelves, and increased ice-sheet mass loss.

How to cite: Boghosian, A., Pitcher, L., Smith, L., and Bell, R.: The Petermann Ice Shelf Estuary and its impact on ice-shelf stability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19947, https://doi.org/10.5194/egusphere-egu2020-19947, 2020.

Numerous studies have documented that water at the ice-bed interface can affect ice flow dynamics of both, mountain glaciers and the Greenland ice sheet. Water at the bed is routed through a complex network of conduits that form a subglacial drainage system. The subglacial drainage system evolves over the melt season in response to the changes in the meltwater supply. However, it is challenging to study due to the inaccessibility of the glacier bed. We use an extensive near-bed water pressure data set from an ablation zone of a small, polythermal, mountain glacier in St. Elias Mountains, Yukon.  Pressure sensors, that exhibit common diurnal variations, are considered to be connected to a hydraulically active drainage system. 

We use a simplified two-dimensional continuum version of the subglacial drainage model with an additional assumption that changes in drainage configuration are negligible over a short time period. Spatially varying permeability function is used as a proxy for the subglacial drainage configuration, assuming that the areas of high (low) permeability correspond to the areas that are connected (disconnected) to a hydraulically active system.  In order to study the evolution of the subglacial drainage system over the melt season, we divide the melt season in a series of short time periods. We then use the inverse model to estimate the permeability function for each of these time periods. Continuity is ensured by using, respectively, the final pressure distribution and the estimated permeability function of the previous period, as the initial condition and the a priori estimate for the consequent time period.

How to cite: Racz, G. C., Schoof, C., Rada, C., Koschitzki, R., Haber, E., and Flowers, G.: Estimating the configuration of the subglacial drainage system under a mountain glacier in St. Elias Mountains, Yukon, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12775, https://doi.org/10.5194/egusphere-egu2020-12775, 2020.

Glacier surfaces are active microbial ecosystems which contribute to melt feedback cycles and biogeochemical processes. Despite this recognition, there is a lack of knowledge regarding the transport dynamics and residence time of microbes in this supraglacial habitat. Throughout the ablation season, meltwater is generated across a glacier’s surface and flows through the porous near-surface weathering crust before entering the channelised supraglacial network. Within the weathering crust, solar radiation provides a “photic zone” which, combined with nutrient availability, is conducive for microbial activity. The water flow through this porous near-surface layer provides a transport mechanism for these microbes. However, the nature of controls upon this phenomenon remain unexplored, despite the relevance for cellular export to downstream ecosystems, glacier surface albedo and biogeochemical cycling.

To determine potential controls on microbial transport in the weathering crust, we present 913 measurements of microbial cell abundance in supraglacial meltwaters from 11 glaciers across the northern hemisphere. Each measurement is coupled with weathering crust hydraulic conductivity or stream discharge. These data reveal a mean microbial abundance of 2.2 × 10⁴ cells mL^-1 (with a range of 10³ – 10⁶) in supraglacial meltwaters within the weathering crust and stream channels. Modal microbe size was 1 – 2 μm (56 % of microbes), with 89 % of microbes smaller than 4 μm. No substantiated difference in size distributions between weathering crust and stream meltwaters were observed. No correlation between microbial abundance and near-surface hydraulic conductivity or stream discharge were observed, either across the entire dataset or when considered on a glacier-by-glacier basis. At three glaciers, water temperature and electrical conductivity (a proxy measure for ionic load) were also recorded; but we observe no correlation between these two variables and microbial abundance. Our data suggests weathering crust microbe abundance is consistent across differing glacial environments, and the concentrations entrained in the near-surface equal those seen in supraglacial streams. As such, despite the low transfer rate of meltwater, there appears to be limited evidence for substantial storage or accumulation of biomass in the near-surface weathering crust. Moreover, microbe entrainment does not appear to be driven by primary hydrological controls. Assuming that once liberated within the weathering crust entrained microbes reach channelised supraglacial networks, we estimate a delivery of 1.1 × 10⁹ kg C a^-1 to downstream environments globally (excluding Antarctica) to 2100, using existing discharge forecasts. This study represents a crucial first step in examining microbial abundance within, and transport across glacier surfaces and their potential role in biogeochemical process-feedbacks and the inoculation of downstream environments.

How to cite: Stevens, I., Irvine-Fynn, T., Edwards, A., Porter, P., Cook, J., Holt, T., Moorman, B., Hodson, A., and Mitchell, A.: Microbial abundance and transport in glacial near-surface meltwater , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16998, https://doi.org/10.5194/egusphere-egu2020-16998, 2020.

Supraglacial lakes on the Greenland Ice Sheet influence surface mass balance, the delivery of water from the surface to the bed, and the rate of basal sliding.  Summertime lake drainage behavior has been catalogued thoroughly through the use of optical remote sensing with a variety of satellites.  Radar results from Operation IceBridge demonstrated the presence of liquid water buried in lakes under ice lids but this platform is limited in its capability to examine short-term changes over the winter season.  This study describes the drainage of multiple buried lakes through the winter season using Sentinel-1 C-Band SAR.  Sudden positive anomalous changes in mean backscatter of surface lakes that are sustained over time are used to pick out wintertime (October through May) lake drainages over a four-year study period.  These changes are confirmed using late-Autumn and early-Spring Landsat-8 photoclinometry changes.  Drainages are detected from November through February, pointing to the likelihood of water injection to the bed through the winter season.  Our automated techniques involve quantifying patterns and trends in SAR backscatter and are also being developed to contribute to our understanding of water storage vs. refreezing in lakes and firn on the surface and in the shallow sub-surface regions of the Greenland Ice Sheet throughout the year.

How to cite: Benedek, C. and Willis, I.: Winter draining lakes on the Greenland ice sheet observed by Sentinel-1, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-377, https://doi.org/10.5194/egusphere-egu2020-377, 2020.

Supraglacial lakes (SGLs) enhance surface melting and their development and subsequent drainage can flex and fracture ice shelves, leading to their disintegration. However, the seasonal evolution of SGLs and their potential influence on ice shelf stability in East Antarctica remains poorly understood, despite a number of potentially vulnerable ice shelves. Using optical satellite imagery, climate reanalysis data and surface melt predicted by a regional climate model, we provide the first multi-year analysis (1974-2019) of seasonal SGL evolution on Shackleton Ice Shelf in Queen Mary Land, which is Antarctica’s northernmost remaining ice shelf. We mapped >43,000 lakes on the ice shelf and >5,000 lakes on grounded ice over the 45-year analysis period, some of which developed up to 12 km inland from the grounding line. Lakes clustered around the ice shelf grounding zone are strongly linked to the presence of blue ice and exposed rock, associated with an albedo-lowering melt-enhancing feedback. Lakes either drain supraglacially, refreeze at the end of the melt season, or shrink in-situ. Furthermore, we observe some relatively rapid (≤ 7 days) lake drainage events and infer that some lakes may be draining by hydrofracture. Our observations suggest that enhanced surface meltwater could increase the vulnerability of East Antarctic ice shelves already preconditioned for hydrofracture, namely those experiencing high surface melt rates, firn air depletion, and extensional stress regimes with minimum topographic confinement. Our results could be used to constrain simulations of current melt conditions on the ice shelf and to investigate the impact of increased surface melting on future ice shelf stability.

How to cite: Arthur, J., Stokes, C., Jamieson, S., Carr, R., and Leeson, A.: Evolution of supraglacial lakes on Shackleton Ice Shelf, East Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1427, https://doi.org/10.5194/egusphere-egu2020-1427, 2020.

Surface meltwater is transmitted to the bed of the Greenland Ice Sheet via supraglacial lake drainages, moulins, and crevasses. Of these, comparatively little research has been performed on the melt infiltration occurring in crevasse fields, which are widespread  in fast-flowing, marine-terminating sectors of the ice sheet. Here, we explore the relationships between crevassing, incidence of surface meltwater, and glacier dynamics at a fast-flowing, marine-terminating sector of West Greenland. Data were collected at high spatial resolution from unmanned aerial vehicle (UAV) surveys on Store Glacier, Greenland, in July 2018. Crevasses and surface water were identified using an object-based machine learning classifier, and strain rates and subsequent stress fields were derived from feature-tracked velocities. Contemporaneous observations of  crevasses and surface water on a larger regional scale were made using ArcticDEM and Sentinel-2 data processed in the Google Earth Engine cloud-based geospatial analysis platform, while stress fields are derived from MEaSUREs velocity data. We find that, whilst previous studies have focussed on relationships between crevassing and stress regime through yield criterion such as the Von Mises stress, we can additionally link the observed spatial distribution of surface meltwater over crevasse fields to the mean stress (defined as the arithmetic mean of the maximum and minimum stress). Crevasse fields existing in tensile mean stress regimes were less likely to exhibit ponded meltwater through a melt season, which we interpret as meltwater being able to continuously drain into the englacial system. Conversely, crevasse fields in compressive mean stress regimes were more likely to exhibit ponded meltwater, which we interpret to be as a result of englacial conduit closure. We show that in these compressive regions, water transfer takes place via intermittent drainage events (i.e. hydrofracture), as envisaged in linear elastic fracture mechanics (LEFM) models. Hence, stress regime can inform spatially heterogeneous styles of crevasse drainage across the ablation zone of an ice sheet: a continuous, low-intensity mode in extensional regimes, in contrast to an episodic, high-intensity mode in compressional regimes. These processes may have distinctly different impacts on basal processes, including subglacial drainage efficiency, diurnal variability, and cryo-hydrologic warming.

How to cite: Chudley, T., Christoffersen, P., Doyle, S., Dowling, T., Bougamont, M., Schoonman, C., Law, R., and Hubbard, B.: Controls on crevasse water transmission to the bed of an ice sheet from remotely sensed datasets, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1616, https://doi.org/10.5194/egusphere-egu2020-1616, 2020.

Subglacial hydrology plays an important role in the evolution of ice dynamics. Primarily, it affects basal processes such as basal sliding. Further, subglacial water exiting a calving front incites submarine melt, increasing calving, resulting in a thinning of the interior ice sheet. Knowledge of it is therefore crucial towards the development and improvement of ice sheet models. We implement a model representing the routing of subglacial water below the Greenland ice sheet in either a one, four or eight directional manner. Due to its computational efficiency, the model is suited for coupling with continental scale ice sheet models on very high resolutions (e.g. 150 m).

Routing depends on the hydraulic potential of individual grid cells which is therefore heavily dependent on accurate estimates of the ice thickness as well as the grid utilized. Sensitivity analyses brought to life that the routing exhibits artefacts resulting in significant flow diversions on high resolutions if the gradients are only considered over the distance of a single grid cell, this is overcome by incorporating a smoothing procedure.

With the basal water model in place and input of the basal melt rate from the VUB Greenland Ice Sheet Model (GISM) as well as runoff input from the Modèle Athmospherique Régional (MAR), we calculate the inflow of freshwater to several reference fjords for the last thirty years and investigate its temporal and spatial patterns. Jakobshavn Isbrae experiences by far the most freshwater inflow compared to the other reference fjords. Despite limited runoff in the northeast of Greenland, high basal melt rates and a significant catchment area provide the outlets of the Northeast Greenland Ice Stream (NEGIS) with substantial inflow too.

How to cite: Vanhulle, A., Le Clec’h, S., and Huybrechts, P.: Modelling the basal hydrology under the Greenland Ice Sheet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2226, https://doi.org/10.5194/egusphere-egu2020-2226, 2020.

The Larsen B ice shelf collapsed in 2002 losing an area twice the size of Greater London to the sea (3000 km²), in an event associated with widespread supraglacial lake drainage. Here, we use optical and radar satellite imagery to investigate the evolution of the ice shelf’s lakes in the decades preceding collapse. We find 1) that lakes spread southwards in the preceding decades at a rate commensurate with meltwater saturation of the shelf surface, 2) no trend in lake size, suggesting an active supraglacial drainage network which evacuated excess water off the shelf and 3) lakes mostly re-freeze in winter but the few lakes that do drain are associated with ice break up 2-4 years later. Given the relative scale of lake drainage and shelf break up, however, it is not clear from our data whether lake drainage is more likely a cause, or an effect, of ice shelf collapse.

How to cite: Leeson, A., Foster, E., Rice, A., Gourmelen, N., and van Wessem, M.: Evolution of supraglacial lakes on the Larsen B ice shelf in the decades before it collapsed, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2441, https://doi.org/10.5194/egusphere-egu2020-2441, 2020.

Due to the lack of direct observations, subglacial hydrology is still marginally considered in Antarctic ice sheet modelling studies, albeit that several approaches exist (e.g., LeBrocq, Bueler and Van Pelt). Subglacial hydrology impacts basal friction through a reduction in effective pressure and through changing properties of subglacial sediments, both factors influencing the lubrication at the bottom of the ice sheet. Several approaches exist to represent subglacial hydrology in ice sheet models (Bueler and Brown, 2009, Goeller et al., 2013) and are generally coupled to either a Coulomb or a Weertman friction law. However, the type of subglacial process determines to a large extent the sensitivity of Antarctic mass change (Sun et al, submitted).

In this study we investigate the sensitivity of subglacial dynamics on the behaviour of the Antarctic ice sheet on centennial time scales. For this purpose we employ a subglacial hydrology model for subglacial water routing (Lebrocq et al., 2009) coupled to a thermomechanical ice-sheet model (f.ETISh; Pattyn, 2017). We consider different parametrizations and representations of effective pressure and till water content at the base.  We also consider the combination of different friction laws and hydrological models (sheet flow, till deformation) depending on estimates of the subglacial conditions of the Antarctic ice sheet. Results show that the way of coupling subglacial hydrology influences the sensitivity of the ice-sheet system on centennial time scales. However, the type and power of the friction law (Coulomb versus Weertman)  has the most dominant impact on ice sheet sensitivity.

How to cite: Kazmierczak, E., Zipf, L., and Pattyn, F.: Coupling subglacial hydrology to basal friction in an Antarctic ice sheet model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7540, https://doi.org/10.5194/egusphere-egu2020-7540, 2020.

Widespread surface meltwater systems have been identified across numerous Antarctic ice shelves and have been implicated in their possible instability and eventual breakup. It is crucial to better understand the seasonal and year-to-year development of these surface meltwater systems, which comprise saturated firn (slush) as well as distinct water bodies (lakes and streams). It has been suggested that repeated melting and re-freezing of the surface firn pack over successive years reduces the firn air content, and therefore its porosity, encouraging the formation of surface water bodies over time. Firn air depletion and the formation of surface water bodies may contribute to ice shelf instability, as the ice becomes increasingly susceptible to hydrofracture.

Here, we use Google Earth Engine to investigate the distributions of slush and deeper water bodies across all Antarctic ice shelves known to have surface melt, to quantify how surface meltwater systems evolve both seasonally and over successive summers. To do this, we use supervised classification of Sentinel-2 and Landsat 7/8 imagery to guide the selection of suitable NDWI_ice thresholds for both the detection of slush and deep surface meltwater. Preliminary results for the George VI Ice Shelf between 2000 and 2017 reveal seasonal patterns in the overall extent of surface meltwater, and the overall meltwater extent typically peaks between January and March each year. The 2009-2010 melt season was characterised by significant melt, and over the course of the melt season the proportion of the overall surface meltwater extent that was held within deep water bodies varied between 0 % (November) and 60 % (January). An increase in the proportion of deep water vs. slush typically aligns with warmer air surface temperatures and, therefore periods of more intense melt.

How to cite: Dell, R., Willis, I., Arnold, N., Banwell, A., Pritchard, H., and Halberstadt, A. R.: Temporal variations in the surface hydrology across Antarctic ice shelves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9856, https://doi.org/10.5194/egusphere-egu2020-9856, 2020.

There is growing interest in surface and shallow subsurface water bodies across Antarctic ice shelves as they impact the ice shelf mass balance. Additionally, the filling and draining of lakes has the potential to flex and fracture ice shelves, which may even lead to their catastrophic break up. The study of lakes on ice shelf surfaces typically uses optical satellite imagery to delineate their area and a parameterised physically-based light attenuation theory to calculate their depths. The approach has been developed and validated using various data sets collected on the Greenland Ice Sheet, but so far the approach has not been validated for Antarctic ice shelves. Here we use simultaneous field measurements of lake water depth and surface spectral properties (red, blue, green, panchromatic), to parameterise the light attenuation theory for use during the filling and draining of shallow lakes on the McMurdo Ice Shelf during the 2016/2017 austral summer. We then apply the approach to calculate lake areas, depths and volumes across several water bodies observed in high resolution Worldview imagery, which helps validate the approach to calculating water volumes across a larger part of the ice shelf using Landsat 8 imagery. Results suggest that using parameter values relevant to the Greenland Ice Sheet may bias the calculation of water volumes when applied to Antarctic ice shelves, and we suggest more appropriate values.

How to cite: Willis, I., Banwell, A., Macdonald, G., Willis, M., and MacAyeal, D.: Surface lake depths on an Antarctic ice shelf: comparing in-situ measurements with ground and satellite multispectral methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10677, https://doi.org/10.5194/egusphere-egu2020-10677, 2020.

In tidewater glacier fjords, the amount, the spatial distribution, and the timing of meltwater entering the subglacial hydrological system play a key role in modulating ice flow dynamics, as well as in impacting adjacent marine ecosystems. This study aims to describe how meltwater journeys through the polythermal glaciers of Kongsfjord basin in Svalbard, Norway. Our methodology involves the use of a surface runoff timeseries (2003-2017) from a coupled surface-energy-balance-snow model forced by a regional climate model (HIRLAM). Using a program for flow pathways analysis in DEMs (TopoToolbox), we generate a map of surface meltwater streams and drainage catchment areas. Other supraglacial features such as melt lakes, moulins and crevasses are manually detected from satellite imagery. These serve as basis to create four different setups of water input to a subglacial drainage model (GlaDS): (1) a spatially continuous input that equals the surface runoff, (2) a discrete input where the total surface runoff over the whole Kongsfjord basin is equally distributed between moulins, (3) a discrete input where upstream catchment areas are taken into account to weight the runoff drained into each moulin, and (4) a hybrid configuration of (1) and (3) where in crevassed areas the input equals the surface runoff, while in non-crevasses areas moulins are fed by upstream catchment runoff. The subglacial drainage model, which allows for meltwater to flow through both an inefficient distributed network of linked cavities, and a more efficient channelized system, yields spatiotemporal information on basal water pressure, sheet discharge and channel discharge, as well as on channel location. Results for the four water input setups are compared, and we discuss the relevance of using a more realistic configuration of meltwater recharge when modeling hydrological systems underneath glaciers. Finally, based on our model outputs, we provide seasonal maps of Kongsfjord basin’s subglacial hydrology that show the preferential flow path of basal water and through which glacier outlet meltwater is released into the fjord.  

How to cite: Scholzen, C., Vikhamar Schuler, T., and Gilbert, A.: Supraglacial and subglacial meltwater routing in Kongsfjord basin, Svalbard, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10744, https://doi.org/10.5194/egusphere-egu2020-10744, 2020.

Over the last two decades, several ice shelves in the Antarctic Peninsula region have experienced significant volume loss or even total collapse driven by atmospheric, oceanic and hydrological processes. Of the three main drivers of ice shelf change, the role of liquid water on and within ice shelves is perhaps the least well defined, largely due to the paucity of field measurements. This study aims to characterise firn aquifers found within an ice shelf vulnerable to hydrological processes. To achieve this objective we use observations collected during two field seasons on the Müller Ice Shelf. The Müller Ice Shelf, the northernmost ice shelf on the western edge of the Antarctic Peninsula, presents a unique opportunity to accomplish our goal: both surface melt pools and subsurface refreezing are known to occur there, and the shelf straddles the -9ºC annual mean isotherm currently considered the limit of ice shelf viability. Measurements from the 2018/19 and 2019/20 field seasons include firn core stratigraphy, geophysical measurements and thermistor datasets, which when combined help to characterise the size and structure of water bodies found within the ice shelf. Whilst during the initial field campaign, no liquid water was observed at the surface, during the drilling of three firn cores liquid water was present at all sites at depths within 20 m of the surface. The prevalence of water and the characterization of the aquifers will provide a baseline for future dynamical studies using physically based models.

How to cite: MacDonell, S., Valois, R., Fernandoy, F., Villar, P., Casassa, G., Hammann, A., and Marambio, M.: Exploring firn aquifers on the Muller Ice Shelf, Antarctica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11695, https://doi.org/10.5194/egusphere-egu2020-11695, 2020.

Despite the well-researched implications of SGL development and drainage for changes in mass balance and dynamics on Greenland, little is known about the role of energy absorption by lakes on Antarctica. Supraglacial lakes (SGLs) are prevalent features of Antarctic surface hydrology forming mainly on ice shelves (<100 m a.s.l) and efficiently conveying atmospheric energy to the ice interior (Lenaerts et al., 2017; Bell et al., 2018). SGLs on Antarctic Ice Shelves are significant for mass balance given lower surface albedo and drainage-induced collapse of fringing ice shelves and consequent increased discharge from tributary outlet glaciers (Stokes et al., 2019).

There have been few efforts to quantify the energy exchanges between SGLs, atmosphere and ice to calculate their effects on glacier ablation (Law et al., 2018), although Miles et al. (2016) find that ponds on a debris-covered mountain glacier input large amounts of energy to underlying ice. Therefore, it is proposed that ice-sheet ponds also act as a significant energy exchange surface inputting large amounts of energy to the ice.

This study aims to code a computationally efficient surface energy balance model (SEB) in Google Earth Engine Editor to quantify how much extra energy is absorbed by SGLs at the during 2019 melt season. The most prolific surface melt is associated with the Antarctic Peninsula, but several East Antarctic ice shelves experience upwards of 60 days/yr of melting (Bell et al., 2018). Near-grounding line negative mass balance of the Nivlisen Ice Shelf (70 ^∘S, 12 ^∘E) in central Dronning Maud Land, East Antarctica, is sufficient to form SGLs and will be used to test SEB accuracy.

The one-dimensional numerical energy-balance SGL model GlacierLake, developed by Law et al. (2018), will be implemented in Google Earth Engine to code for surface energy exchanges. GlacierLake is most sensitive to the proportion of shortwave radiation absorbed at the surface which indicates that it is responsive to surface energy fluxes and is useful for the purposes of this study. A variety of methods, including NDWI and Principle Components Analysis, will be evaluated for use to classify lake and slush extents.

Given that it takes 3.4 x 10⁵ J/kg of latent heat to melt ice at 0 °C, the volume of liquid water on the Nivlisen ice shelf implies how much atmospheric energy has been transferred to the ice shelf. The modelled quantification of extra energy absorbed by lakes will be compared to the observed water volume on the Nivlisen Ice Shelf to test model accuracy.

Whilst this study will focus on the Nivlisen Ice Shelf, the SEB model may be applied at pan-Antarctic scales to calculate the ice-sheet wide extra energy absorbed by surface meltwater pooling. A precise quantification of the present impact of energy absorption by lakes on mass balance and dynamics provides a baseline to gauge how meltwater contribution could evolve under atmospheric warming.

How to cite: Lefroy, N. and Arnold, N.: Quantification of the Impact of Supraglacial Lakes and Slush on Surface Energy Balance of Ice Shelves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20847, https://doi.org/10.5194/egusphere-egu2020-20847, 2020.

In Antarctica, basal melting in the ice-sheet’s interior generates subglacial water that is routed via the subglacial hydrological system towards the margins. At the grounding zone, the subglacial meltwater comes into contact with ocean water subject to tides. The mixing of the two water masses may be one reason for velocities variations on tidal timescales, providing a window into processes of basal sliding. With this goal in mind, we instrumented a flowline across the grounding zone of Priestley glacier, Antarctica, with 4 differential GNSS stations co-located with advanced phase sensitive radars (ApRES) and tiltmeters all measuring continuously over several months. Moreover, we installed a Terrestrial Radar Interferometer (TRI) overlooking the glacier from an adjacent rock outcrop. The to our knowledge first-time deployment of the TRI in Antarctica reveals a stunning picture of grounding-zone dynamics providing spatially coherent 1D flowfields every 3 hours over a time period of 10 days. This enables interpretations of velocity changes measured by GNNS in an unprecedented spatial context. We complement our on-site geophysical dataset with airborne ice-penetrating radar as well as spaceborne InSAR data using timeseries from TanDEM-X, Sentinel-1A, and the ERS satellites.

TRI and GNSS stations jointly detect tidal velocity fluctuations (> 50 % around the mean) which decay landwards with increasing distance from the grounding line. Triple differences in satellite interferometry reveal transient bull’s eye patterns far upstream of the grounding line quantifying localized surface lowering together with adjacent surface uplift. We interpret this as a result from abruptly migrating subglacial water pockets cascading over obstacles in the basal topography. The TRI also shows such bull’s eye patterns pulsating in our highly resolved time series. Moreover, all GNSS stations and the TRI detect a short-lived acceleration event (~100 % horizontal speedup over 2 hours) paired with spatially coherent surface uplift (~15 cm). Magnitude and duration of this event suggests operation of hydraulic jacking, a mechanism explaining short-lived speed-ups with pressure variations in a linked-cavity system. However, usually this is pre-conditioned to the existence of significant surface meltwater entering the subglacial hydrological system, which is not the case at our study site. Our joint observations with multiple sensors and instruments therefore  provide unique observations to further develop our understanding of basal sliding, particularly it's dependency on upstream water supply and ocean tides.

How to cite: Drews, R., Wild, C., Neckel, N., Marsh, O., Rack, W., and Ehlers, T. A.: The heartbeat of a glacier: Cascading subglacial water pockets and ocean tides cause hourly to daily ice-flow variations of Priestley Glacier, Antarctica, detected with Terrestrial Radar Interferometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10743, https://doi.org/10.5194/egusphere-egu2020-10743, 2020.

In 2019, the Kangerlussuaq transect has experienced a record surface melt season at some stations, exceeding even the melt seasons of 2010 and 2012. We demonstrate that net radiation has been driving the high surface melt rates especially in the higher parts of the transect.

Since 2003, continuous measurements of the surface energy budget are made in a transect of four automatic weather stations, spanning the ablation area close to the ice edge to the accumulation are of the Greenland Ice Sheet. All available data have been homogenized and corrected, and an unprecedented time series of surface energy budget is presented here, including meltwater production. In this contribution, the melt season of 2019 is put into the longer-term context, and precise atmospheric drivers of the melt are exposed.

Sixteen years of data clearly reveal the inland and upward expansion of the ablation area. The weather station closest to the equilibrium line (S9) shows a clear and distinct reduction in albedo, and a relatively strong increase in surface melt, which has started to exceed accumulation during the period of observation. Photographs of the area around S9 show that the surface has undergone major changes between 2003 and 2019, now featuring many surface hydrological features that were completely absent in 2003.

These changes have important implications for the hydrology of the surface, the near-surface, and the underlying firn. A firn model calculation reveals that the entire firn column has been heating by several degrees Celsius in the percolation zone, due to refreezing of meltwater. Sudden, stepwise warming is seen in extreme melt seasons like 2019.

How to cite: Kuipers Munneke, P., Reijmer, C., Smeets, P., and van den Broeke, M.: The extreme Greenland melt season of 2019 in a 16-year time series of surface energy balance at the Kangerlussuaq transect, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15968, https://doi.org/10.5194/egusphere-egu2020-15968, 2020.

Supraglacial lakes represent a fundamental component of the surface hydrology of the Greenland ice sheet. Understanding the relationship of these lakes with ice sheet surface mass balance, geometry, location, and how this has changed through time also informs how their drainage can impact ice sheet subglacial hydrology and seasonal flow dynamics. However, previous studies of supraglacial lakes have been limited in spatial and/or temporal scale relative to the entire ice sheet.

Here we use the entire MODIS Terra archive within Google Earth Engine to derive maps of supraglacial lake cover every day of every melt season for the last 20 years for the entire Greenland ice sheet. Through generating annual composites of where lakes are observed, we identify that the frequency of lakes has on average increased by 27% from 2000-2019. Lakes are observed to be occurring at higher elevations in all sectors of the ice sheet for 2010-2019 compared to 2000-2009. Output from the regional climate model MAR suggests that in the most recent decade higher numbers of lakes are being formed for a given volume of runoff.

The observation of lakes that can form more easily, further inland and at higher elevations have significant implications for future surface mass balance, and potentially the dynamics of inland regions of the Greenland ice sheet.

How to cite: Lea, J. and Brough, S.: Greenland’s supraglacial lakes increase by a quarter in the last 20 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17968, https://doi.org/10.5194/egusphere-egu2020-17968, 2020.