Article

Seasonal variations in Greenland Ice Sheet motion: Inland extent and behaviour at higher elevations

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Abstract

We present global positioning system observations that capture the full inland extent of ice motion variations in 2009 along a transect in the west Greenland Ice sheet margin. In situ measurements of air temperature and surface ablation, and satellite monitoring of ice surface albedo and supraglacial lake drainage are used to investigate hydrological controls on ice velocity changes. We find a strong positive correlation between rates of annual ablation and changes in annual ice motion along the transect, with sites nearest the ice sheet margin experiencing greater annual variations in ice motion (15 -18 %) than those above 1000 m elevation (3 - 8 %). Patterns in the timing and rate of meltwater delivery to the ice-bed interface provide key controls on the magnitude of hydrologically-forced velocity variations at each site. In the lower ablation zone, the overall contribution of variations in ice motion to annual flow rates is limited by evolution in the structure of the subglacial drainage system. At sites in the upper ablation zone, a shorter period of summer melting and delayed establishment of a hydraulic connection between the ice sheet surface and its bed limit the timeframe for velocity variations to occur. Our data suggest that land-terminating sections of the Greenland Ice Sheet will experience increased dynamic mass loss in a warmer climate, as the behaviour that we observe in the lower ablation zone propagates further inland. Findings from this study provide a conceptual framework to understand the impact of hydrologically-forced velocity variations on the future mass balance of land-terminating sections of the Greenland Ice Sheet.

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... Although channel initiation and growth occur relatively early in the melt season [depending on ice thickness, surface slope, and viscosity (Schoof, 2010;Cowton et al., 2013)], the timescale of variability in runoff supply (hours) is typically much shorter than that of channel adjustment (days to weeks), hence channels are typically not in steady-state (Pimentel and Flowers, 2010;Schoof, 2010;Bartholomew et al., 2012). The implication is that, regardless of time of year, if runoff supply exceeds the rate at which it can be evacuated, then there will be an increase in water pressure and ice acceleration (Bartholomew et al., 2011a(Bartholomew et al., , 2012. ...
... There is observational and modelling evidence to suggest that, at least in the ablation area of western Greenland, this is indeed the case. Firstly, GPS observations along transects parallel and transverse to ice flow show that the patterns of short-term speedup in response to runoff variations were strikingly similar up to at least ∼75 km from the margin (∼1,500 m a.s.l, ∼1,200 m ice thickness; Bartholomew et al., 2011a) and extend at least 2.8 km transverse to an inferred subglacial channel (Tedstone et al., 2014). Secondly, Hewitt (2011) suggested a theoretical subglacial channel spacing of ∼2-15 km depending on the permeability of the substrate-similar to the spacing of Canadian eskers, which are interpreted as relict subglacial drainage pathways (Storrar et al., 2014). ...
... These observations are broadly consistent with channelinduced depressurisation-lower channel water pressure relative to the surroundings, particularly during periods of steady or declining runoff supply, would result in increased drainage of water from variable pressure axes until channels close after the melt season, allowing re-pressurisation to begin. As discussed in section Spatial Variations in Subglacial Hydrological Structure, observations, and modelling of channel formation in the upper ablation area are sparse and equivocal (Bartholomew et al., 2011a;Chandler et al., 2013;Dow et al., 2014;Koziol and Arnold, 2018). In the upper ablation area, similar mechanisms may operate that do not require channel formation, but which produce similar seasonal ice velocity patterns (Andrews et al., 2014;Hoffman et al., 2016)-these are discussed below. ...
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Coupling between runoff, hydrology, basal motion, and mass loss (“hydrology-dynamics”) is a critical component of the Greenland Ice Sheet system. Despite considerable research effort, the mechanisms by which runoff influences ice dynamics and the net long-term (decadal and longer) dynamical effect of variations in the timing and magnitude of runoff delivery to the bed remain a subject of debate. We synthesise key research into land-terminating ice sheet hydrology-dynamics, in order to reconcile several apparent contradictions that have recently arisen as understanding of the topic has developed. We suggest that meltwater interaction with subglacial channels, cavities, and deforming subglacial sediment modulates ice flow variability. Increasing surface runoff supply to the bed induces cavity expansion and sediment deformation, leading to early-melt season ice flow acceleration. In the ablation area, drainage of water at times of low runoff from high-pressure subglacial environments toward more efficient drainage pathways is thought to result in reductions in water pressure, ice-bed separation and sediment deformation, causing net slow-down on annual to decadal time-scales (ice flow self-regulation), despite increasing surface melt. Further inland, thicker ice, small surface gradients and reduced runoff suppress efficient drainage development, and a small net increase in both summer and winter ice flow is observed. Predicting ice motion across land-terminating sectors of the ice sheet over the twenty-first century is confounded by inadequate understanding of the processes and feedbacks between runoff and subglacial motion. However, if runoff supply increases, we suggest that ice flow in marginal regions will continue to decrease on annual and longer timescales, principally due to (i) increasing drainage system efficiency in marginal areas, (ii) progressive depression of basal water pressure, and (iii) thinning-induced lowering of driving stresses. At higher elevations, we suggest that minor year-on-year ice flow acceleration will continue and extend further into the interior where self-regulation mechanisms cannot operate and if surface-to-bed meltwater connections form. Based on current understanding, we expect that ice flow deceleration due to the seasonal development of efficient drainage beneath the land-terminating margins of the Greenland Ice Sheet will continue to regulate its future mass loss.
... Its hydrological catchment covers >600 km 2 , reaches an elevation of 1500 m, and ranges in width from 10-40 km (Bartholomew et al., 2011a(Bartholomew et al., , 2011b. Meltwater from the catchment is channeled through a single large proglacial river (Leverett River); at peak discharge, typical flows from the river are in the range of 300-400 m 3 s −1 (Bartholomew et al., 2011a(Bartholomew et al., , 2011b. ...
... Its hydrological catchment covers >600 km 2 , reaches an elevation of 1500 m, and ranges in width from 10-40 km (Bartholomew et al., 2011a(Bartholomew et al., , 2011b. Meltwater from the catchment is channeled through a single large proglacial river (Leverett River); at peak discharge, typical flows from the river are in the range of 300-400 m 3 s −1 (Bartholomew et al., 2011a(Bartholomew et al., , 2011b. During an exceptional melting period in 2012, the river reached ∼800 m 3 s −1 . ...
... During both 2011 and 2012, EC was elevated in the early season (60-100 μS cm −1 ) and decreased to 10-20 μS cm −1 with increasing river discharge (Fig. 2;Butler, 2014;Hawkings et al., 2014). These results are consistent with the EC range reported in 2009 (Bartholomew et al., 2011a(Bartholomew et al., , 2011b. Butler (2014) and Hawkings et al. (2014) observed diurnal variations in EC through much of the 2011 and 2012 seasons, which in general were inverse to river discharge. ...
Article
Water flow beneath the Greenland Ice Sheet (GrIS) has been shown to include slow-inefficient (distributed) and fast-efficient (channelized) drainage systems, in response to meltwater delivery to the bed via both moulins and surface lake drainage. This partitioning between channelized and distributed drainage systems is difficult to quantify yet it plays an important role in bulk meltwater chemistry and glacial velocity, and thus subglacial erosion. Radon-222, which is continuously produced via the decay of 226Ra, accumulates in meltwater that has interacted with rock and sediment. Hence, elevated concentrations of 222Rn should be indicative of meltwater that has flowed through a distributed drainage system network. In the spring and summer of 2011 and 2012, we made hourly 222Rn measurements in the proglacial river of a large outlet glacier of the GrIS (Leverett Glacier, SW Greenland). Radon-222 activities were highest in the early melt season (10–15 dpm L−1), decreasing by a factor of 2–5 (3–5 dpm L−1) following the onset of widespread surface melt. Using a 222Rn mass balance model, we estimate that, on average, greater than 90% of the river 222Rn was sourced from distributed system meltwater. The distributed system 222Rn flux varied on diurnal, weekly, and seasonal time scales with highest fluxes generally occurring on the falling limb of the hydrograph and during expansion of the channelized drainage system. Using laboratory based estimates of distributed system 222Rn, the distributed system water flux generally ranged between 1–5% of the total proglacial river discharge for both seasons. This study provides a promising new method for hydrograph separation in glacial watersheds and for estimating the timing and magnitude of distributed system fluxes expelled at ice sheet margins.
... Numerical models and observations of the Greenland Ice Sheet (GIS) link surface meltwater penetration to the bed to both short (hourly, daily) and long (seasonal, decadal) temporal variations in ice velocity (Zwally et al., 2002; Bartholomew et al., 2011; Banwell et al., 2013; Shannon et al., 2013; Mayaud et al., 2014; Tedstone et al., 2015). However, the link between increased basal water inputs and ice sliding is a complex one, largely because viscous heat dissipation (VHD) from water flow beneath ice may melt out efficient drainage tunnels whose presence may decrease, or even reverse, the tendency for ice flow to accelerate with increasing meltwater inputs to the bed (Kamb, 1987; Sundal et al., 2011; Tedstone et al., 2015). ...
... If they do form, they will in general creep closed more quickly than conduits that form under the thinner ice near the margin. Less efficient drainage in the GIS interior, relative to the marginal zone, is one likely cause for the ice accelerations observed there under increased water inputs (e.g., Bartholomew et al., 2011; Shannon et al., 2013; Doyle et al., 2014). Studies examining surface melt, supraglacial routing, subglacial hydrology, and the response of ice sheet outlet glaciers to those various inputs take place predominantly in southwest Greenland, focusing largely on the Russell, Leverett , Paakitsoq, or nearby glaciers (for example, Banwell et al., 2013; Arnold et al., 2014; Andrews et al., 2014; Tedstone et al., 2015). ...
... The interior ice response to increased runoff is less well studied. However, Bartholomew et al. (2011) show that ice does not slow down later in the season as more runoff reaches the bed, and Doyle et al. (2014) show a year-on-year increase in velocity even with increasing runoff. ...
Article
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Basal hydrology of the Greenland Ice Sheet (GIS) influences its dynamics and mass balance through basal lubrication and ice–bed decoupling or efficient water removal and ice–bed coupling. Variations in subglacial water pressure through the seasonal evolution of the subglacial hydrological system help control ice velocity. Near the ice sheet margin, large basal conduits are melted by the viscous heat dissipation (VHD) from surface runoff routed to the bed. These conduits may lead to efficient drainage systems that lower subglacial water pressure, increase basal effective stress, and reduce ice velocity. In this study we quantify the energy available for VHD historically at present and under future climate scenarios. At present, 345 km³ of annual runoff delivers 66 GW to the base of the ice sheet per year. These values are already ∼ 50 % more than the historical 1960–1999 value of 46 GW. By 2100 under IPCC AR5 RCP8.5 (RCP4.5) scenarios, 1278 (524) km³ of runoff may deliver 310 (110) GW to the ice sheet base. Hence, the ice sheet may experience a 5-to-7-fold increase in VHD in the near future which will enhance opening of subglacial conduits near the margin and will warm basal ice in the interior. The other significant basal heat source is geothermal heat flux (GHF), which has an estimated value of 36 GW within the present-day VHD area. With increasing surface meltwater penetration to the bed the basal heat budget in the active basal hydrology zone of the GIS will be increasingly dominated by VHD and relatively less sensitive to GHF, which may result in spatial changes in the ice flow field and in its seasonal variability.
... The rate of water delivery to the bed is of fundamental importance to its effect on basal hydrology (Schoof, 2010). High water fluxes to the bed can channelize the basal system over the course of a melt season (Bartholomew et al., 2011); however, channelization may fail to occur during episodes of extremely high water flux such as rapid lake drainages (Dow et al., 2015) or when the water flux is too low (Bartholomew et al., 2011). Thus, if we are to understand the effect of firn-aquifer water on the subglacial system, we must first know its rate of input to the bed. ...
... The rate of water delivery to the bed is of fundamental importance to its effect on basal hydrology (Schoof, 2010). High water fluxes to the bed can channelize the basal system over the course of a melt season (Bartholomew et al., 2011); however, channelization may fail to occur during episodes of extremely high water flux such as rapid lake drainages (Dow et al., 2015) or when the water flux is too low (Bartholomew et al., 2011). Thus, if we are to understand the effect of firn-aquifer water on the subglacial system, we must first know its rate of input to the bed. ...
... The travel time through the firn aquifer could introduce an offset between the timing of delivery of water to the bed and the timing of the melt season. This would further differentiate this system from western Greenland, where basal water fluxes peak roughly with the melt season (Bartholomew et al., 2011;Smith et al., 2015). Further remote sensing or field observations are required to better constrain any intra-annual variability in water flux. ...
Article
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A firn aquifer in the Helheim Glacier catchment of Southeast Greenland lies directly upstream of a crevasse field. Previous measurements show that a 3.5-km long segment of the aquifer lost a large volume of water (26,000–65,000 m2 in cross section) between spring 2012 and spring 2013, compared to annual meltwater accumulation of 6000–15,000 m2. The water is thought to have entered the crevasses, but whether the water reached the bed or refroze within the ice sheet is unknown. We used a thermo-visco-elastic model for crevasse propagation to calculate the depths and volumes of these water-filled crevasses. We compared our model output to data from the Airborne Topographic Mapper (ATM), which reveals the near-surface geometry of specific crevasses, and WorldView images, which capture the surface expressions of crevasses across our 1.5-km study area. We found a best fit with a shear modulus between 0.2 and 1.5 GPa within our study area. We show that surface meltwater can drive crevasses to the top surface of the firn aquifer (~20 m depth), whereupon it receives water at rates corresponding to the water flux through the aquifer. Our model shows that crevasses receiving firn-aquifer water hydrofracture through to the bed, ~1000 m below, in 10–40 days. Englacial refreezing of firn-aquifer water raises the average local ice temperature by ~4°C over a ten-year period, which enhances deformational ice motion by ~50 m year−1, compared to the observed surface velocity of ~200 m year−1. The effect of the basal water on the sliding velocity remains unknown. Were the firn aquifer not present to concentrate surface meltwater into crevasses, we find that no surface melt would reach the bed; instead, it would refreeze annually in crevasses at depths <500 m. The crevasse field downstream of the firn aquifer likely allows a large fraction of the aquifer water in our study area to reach the bed. Thus, future studies should consider the aquifer and crevasses as part of a common system. This system may uniquely affect ice-sheet dynamics by routing a large volume of water to the bed outside of the typical runoff period.
... Ice velocity variability in Greenland has been observed over a range of timescales, varying from daily (e.g. ; Shepherd et al. (2009); Bartholomew et al. (2012)), seasonal (Joughin et al., , 2012bBartholomew et al., 2010Bartholomew et al., , 2011 to interannual variations Joughin et al., 2008bJoughin et al., , 2012bJoughin et al., , 2014). Our 7 ...
... For example in the south-western region of the ice sheet, the Russel Glacier sector has received a relatively high amount attention due to the propensity of its glaciers to exhibit seasonal speedup and its ease of access for fieldbased research. In-situ GPS observations have shown that seasonal velocity variations are strongly linked to changes in surface melting Bartholomew et al., 2010Bartholomew et al., , 2011Bartholomew et al., , 2012Chandler et al., 2013;Sole et al., 2013;van de Wal et al., 2015). Satellite measurements have provided a large-scale perspective of changes in ice flow Palmer et al., 2011;Sundal et al., 2011;Fitzpatrick et al., 2013) and in the seasonal evolution of supraglacial lakes Howat et al., 2013;Leeson et al., 2013Leeson et al., , 2015. ...
... The area has received a relatively high amount attention due to the propensity of its glaciers to exhibit seasonal speedup. In-situ GPS observations have shown that seasonal velocity variations are strongly linked to changes in surface melting Bartholomew et al., 2010Bartholomew et al., , 2011Bartholomew et al., , 2012Chandler et al., 2013;Sole et al., 2013;van de Wal et al., 2015). Satellite measurements have provided a large-scale perspective of changes in ice flow Palmer et al., 2011;Sundal et al., 2011;Fitzpatrick et al., 2013) and in the extent of supraglacial lakes Howat et al., 2013;Leeson et al., 2013Leeson et al., , 2015. ...
Thesis
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In this thesis, I develop and demonstrate a system for monitoring fluctuations in the speed of Greenland ice sheet outlet glaciers with high temporal frequency from imagery acquired by a range of satellite missions. This work is motivated by an ambition to utilise a new era of operational satellites to better understand how environmental changes are affecting the flow and mass of Greenland’s outlet glaciers. First, I exploited the systematic and frequent acquisition schedule of the Sentinel-1 satellite constellation to track weekly variations in the speed of four fast-flowing, marine-terminating glaciers - Jakobshavn Isbræ, Petermann Glacier, Zachariæ Isstrøm and Nioghalvfjerdsfjorden - between 2015–2017. By combining the Sentinel-1 data with an eight-year time-series derived from TerraSAR-X, I produced a decadal record of variations in glacier flow. On a technical level, I was able to demonstrate the value of Sentinel-1’s 6-day revisit time for glaciology, because it leads to an increase in the degree of correlation between consecutive images and also to improved tracking of movement near to the glacier calving fronts. On a scientific level, I was able to demonstrate that a strong correlation exists between iceberg calving events and glacier speedup, and to show for the first time that Jakobshavn Isbræ has begun to slow down. Next, I assessed the capability of the Sentinel-1 constellation to detect and chart seasonal changes in the speed of five slow-flowing glaciers situated in a 14,000 km2 land-terminating sector of central-west Greenland. These new measurements offer significantly improved spatial and temporal resolution when compared to previous missions, in all seasons. I was able to show that there are marked differences in the degree of seasonal speedup of the five glaciers – with summertime increases in ice flow ranging from 21 to 49 % - reinforcing the need for comprehensive monitoring and the challenges of making regional extrapolations. Thanks to the high temporal frequency afforded by Sentinel-1, I was also able to document for the first time the detailed spatial pattern of speedup persistence, and to show that short- lived peaks of melting match transient spikes in glacier velocity. Finally, I explored the added value and complementarity of the Sentinel-2 multi- spectral instrument (MSI) for tracking ice motion. I was able to combine measurements acquired by Sentinel-1 and Sentinel-2 to detect short-term changes in iceberg drift, iceberg calving, ice motion, and supraglacial lake area at Jakobshavn Isbræ. I also showed that measurements of glacier flow determined from both satellites are in good agreement, and that the spatial coverage they afford is greatest in opposing seasons, illustrating the promise of Sentinel-2 for glaciology.
... Extensive fluvial supraglacial hydrologic networks are observed across the southwest Greenland Ice Sheet (GrIS) ablation zone (Smith andothers, 2015, 2017;Yang and Smith, 2016;Pitcher and Smith, 2019). These networks, consisting of interlinked supraglacial streams, rivers and lakes, drain large volumes of surface meltwater into the ice sheet via moulins and crevasses (Chu, 2014;Flowers, 2018;Pitcher and Smith, 2019), with corresponding impacts on GrIS subglacial water pressures, and ice flow dynamics (Zwally and others, 2002;Bartholomew and others, 2011;Hoffman and others, 2011;Andrews and others, 2014). ...
... This is consistent with previous studies demonstrating reduced ice velocities in response to increased surface melt and development of an efficient subglacial hydrologic system (e.g. Bartholomew and others, 2011;Tedstone and others, 2015). Surface meltwater area proportion peaks on 1 July, after which supraglacial lakes continue expanding (Figs 4a-c), while ice flow decreases at 1200-1400 m (KAN_M station). ...
Article
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Supraglacial lakes and rivers dominate the storage and transport of meltwater on the southwest Greenland Ice Sheet (GrIS) surface. Despite functioning as interconnected hydrologic networks, supraglacial lakes and rivers are commonly studied as independent features, resulting in an incomplete understanding of their collective impact on meltwater storage and routing. We use Landsat 8 satellite imagery to assess the seasonal evolution of supraglacial lakes and rivers on the southwest GrIS during the 2015 melt season. Remotely sensed meltwater areas and volumes are compared with surface runoff simulations from three climate models (MERRA-2, MAR 3.6 and RACMO 2.3), and with in situ observations of proglacial discharge in the Watson River. We find: (1) at elevations >1600 m, 21% of supraglacial lakes and 28% of supraglacial rivers drain into moulins, signifying the presence of high-elevation surface-to-bed meltwater connections even during a colder-than-average melt season; (2) while supraglacial lakes dominate instantaneous surface meltwater storage, supraglacial rivers dominate total surface meltwater area and discharge; (3) the combined surface area of supraglacial lakes and rivers is strongly correlated with modeled surface runoff; and (4) of the three models examined here, MERRA-2 runoff yields the highest overall correlation with observed proglacial discharge in the Watson River.
... Firn aquifers currently occupy areas low in the accumulation zone around much of the Greenland Ice Sheet (Miège et al., 2016), with inland expansion anticipated in future warm climates . If new surface-to-bed connections are made at inland locations, the subsequent evolution of the subglacial hydrologic system may alter ice dynamics (e.g., Bartholomew et al., 2011b;Christoffersen et al., 2018;Clason et al., 2015;Doyle et al., 2014;Poinar et al., 2015). Here, we implement a modeling study whereby a series of idealized scenarios drain surface water to the bed at low-elevation locations and at higher elevations through the downstream end of a firn aquifer. ...
... Observations in western Greenland show clear year-to-year variations in summer ice velocities (Bartholomew et al., 2011a) without substantial variation in annual local downglacier ice displacement (Tedstone et al., 2015), suggesting sparser subglacial channels and less hydrological efficiency there. Inland channel formation may be limited by a small supply of meltwater to the bed (Bartholomew et al., 2011b;Poinar et al., 2015) and generally shallow bed and surface slopes (Meierbachtol et al., 2013;Dow et al., 2015) in western Greenland. ...
Article
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The state of the subglacial hydrologic system, which can modify ice motion, is sensitive to the volume and rate of meltwater reaching it. Bare-ice regions rapidly transport meltwater to the bed via moulins, while in certain accumulation zone regions, meltwater first flows through firn aquifers, which can introduce a substantial delay. We use a subglacial hydrological model forced with idealized meltwater input scenarios to test the effect of this delay on subglacial hydrology. We find that addition of firn-aquifer water to the subglacial system elevates the inland subglacial water pressure while reducing water pressure and enhancing subglacial channelization near the terminus. This effect dampens seasonal variations in subglacial water pressure and may explain regionally anomalous ice velocity patterns observed in Southeast Greenland. As surface melt rates increase and firn aquifers expand inland, it is crucial to understand how inland drainage of meltwater affects the evolution of the subglacial hydrologic system.
... One terrestrial outlet glacier of the GrIS that has been intensively studied throughout the past ten years is Leverett Glacier (Figure 1; Bartholomew et al. 2010Bartholomew et al. , 2011Cowton et al. 2016Cowton et al. , 2013Sole et al. 2013;Tedstone et al. 2014). This exemplar land-terminating margin is the site of a major subglacial drainage portal, and detailed field data have revealed key insights into (1) channelized subglacial drainage ); (2) the evolution of subglacial hydrology during the course of a melt season, and its influence on ice velocity during seasonal (Bartholomew et al. 2010) and interannual Tedstone et al. 2015Tedstone et al. , 2014 timescales; and (3) rates of subglacial erosion and sediment budgets (Cowton et al. 2012). ...
... Leverett Glacier is located in west Greenland, some 15-20 km east from the town of Kangerlussuaq ( Figure 1). The glacier is a distributary of Russell Glacier, and while its tongue is only some 4 km long and 2 km wide, it has a large drainage basin (600 km 2 ) that is thought to extend 80 km inland (Bartholomew et al. 2011). While the terminus itself is small, characterization of its terminus to approximately 15 km inland is important because of the volume of water that routes through the glacier and its meltwater portal (up to 800 m 3 s −1 in summer; Tedstone et al. 2013). ...
Article
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Ice thickness measurements near the margin of the Greenland Ice Sheet (GrIS) are relatively sparse, presenting issues for modelling ice flow dynamics, ice sheet change and subglacial hydrology. We acquired ice thickness data at Leverett Glacier, western Greenland, using a highly portable, low power commercially-available ice-penetrating radar operating at 10-80 MHz. Imaging of the bed to depths of 270 m was possible using this system. Our ice thickness measurements were incorporated into the BedMachine model of bed elevation created using mass conservation methods. The new data significantly modified the modelled ice thickness and routing of subglacial water in both the Leverett and adjacent Russell Glacier. Though changes to the modelled basal topography and subglacial hydrology are consistent with observations, our new data unrealistically reduced the overall size of the Leverett hydrological catchment. Additional ice thickness measurements are therefore required to realistically constrain subglacial topography and subglacial hydrological routing in this area. Our work has significantly enhanced our understanding of basal topography and subglacial hydrology of Leverett Glacier, with implications for glacier dynamics and assessments of water piracy between catchments in the marginal zone of the GrIS.
... Seasonal variations in ice velocity suggest that when integrated over weeks to months, these shorter-term dynamic events associated with lake drainage and diurnal cycles are sufficient to cause faster motion in the summer and slower movement in the winter. It has been suggested recently that it is the variability in magnitude and timing of meltwater input to the subglacial drainage system of the GrIS, rather than the total volume of meltwater input to the system, that has the greatest influence on subglacial water pressures and therefore ice motion [Schoof, 2010;Bartholomew et al., 2011b;Colgan et al., 2011]. Rapid short-term increases in surface water input to a low capacity drainage system early in the summer may cause bigger increases in water pressure and hence uplift and basal motion, than long, sustained higher volumes of surface melt water later in the season. ...
... During this time, subglacial water pressures increase with increased discharge, thus basal shear stress decreases; facilitating increased glacier velocities for as long as the meltwater input exceeds transmissivity [Bartholomew et al., 2010;Colgan et al., 2011a;Hoffman et al., 2011]. However, as surface melt rates increase, discharge into the subglacial system increases and channels enlarge due to wall melt, resulting in the inland development of a channelized system [Bartholomew et al., 2010[Bartholomew et al., , 2011b. While a channelized system exists, the ability of a steady-state water supply to cause further ice sheet acceleration is thought to be suppressed as increased discharge results in decreased subglacial water pressures [Schoof, 2010;Pimentel and Flowers, 2011]. ...
Thesis
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There is increasing recognition that the hydrology of the Greenland Ice Sheet plays an important role in the dynamics and therefore mass balance of the ice sheet. Understanding the hydrology of the ice sheet and being able to predict its future behaviour is therefore a key aspect of glaciological research. To date, the ice sheet’s hydrology has tended to be inferred from the analysis of surface velocity measurements, or modelled in a theoretical, idealised way. This study focuses on the development of a high spatial (100 m) and temporal (1 hour) resolution, physically based, time-dependent hydrological model which is applied to the ~2,300 km2 Paakitsoq region, West Greenland, and is driven, calibrated, and evaluated using measured data. The model consists of three components. First, net runoff is calculated across the ice sheet from a distributed, surface energy- balance melt model coupled to a subsurface model, which calculates changes in temperature, density and water content in the snow, firn and upper-ice layers, and hence refreezing. The model is calibrated by adjusting key parameter values to minimize the error between modelled output and surface height and albedo measurements from the three Greenland Climate Network (GC-Net) stations, JAR 1, JAR 2 and Swiss Camp. Model performance is evaluated in two ways by comparing: i) modelled snow and ice distribution with that derived from Landsat-7 ETM+ satellite imagery using Normalised Difference Snow Index (NDSI) classification and supervised image thresholding; and ii) modelled albedo with that retrieved from the Moderate- resolution Imaging Spectroradiometer (MODIS) sensor MOD10A1 product. Second, a surface routing / lake filling model takes the time-series of calculated net runoff over the ice sheet and calculates flow paths and water velocities over the snow / ice covered surface, routing the water into ‘open’ moulins or into topographic depressions which can fill to form supraglacial lakes. This model component is calibrated against field measurements of a filling lake in the study area made during June 2011. Supraglacial lakes are able to drain by a simulated hydrofracture mechanism if they reach a critical volume. Once water is at the ice / bed interface, discharge and hydraulic head within subglacial drainage pathways are modelled using the third model component. This consists of an adaptation of a component (EXTRAN) of the U.S. Environmental Protection Agency Storm Water Management Model (SWMM), modified to allow for enlargement and closure of ice-walled conduits. The model is used to identify how the subglacial hydrological system evolves in space and time in response to varying surface water inputs due to melt and lake drainage events, driven ultimately by climate data. A key output from the model is the spatially and temporally varying water pressures which are of interest in helping to explain patterns of surface velocity and uplift found by others, and will ultimately be of interest for driving ice dynamics models.
... If forests (including tropical rainforests) in low and middle latitudes play a large role in the adjustment of global change, the effect of ice-albedo on climate is also of concern in high latitudes, especially in areas covered by ice and snow for many years above bare ground in high latitudes zones [14,15]. Generally, GRACE (Gravity Recovery And Climate Experiment) data have been used for assessing ice mass balance in Greenland and most of Antarctica [7,23]. Numerous studies (radar detection of ice layers and ice-core sampling) have confirmed that melting has begun to occur in areas of low elevation in Greenland, such as in melt regions (northeast and northwest, mostly south) [12]. ...
... Additionally, we found that the area demonstrated a high variation frequency across four seasons was located in the south of the study area, and a "boundary line" was easy to be recognized. One possible explanation was that this may be related to local variations in the atmosphere's circulation as well as the local surface elevation [23]. In detail, variations in atmospheric circulation could cause slight changes in meteorological conditions locally; in addition, surface topography (elevation) and global ocean circulation may also contribute to albedo variation. ...
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Land albedo is an essential variable in land surface energy balance and climate change. Within regional land, albedo has been altered in Greenland as ice melts and runoff increases in response to global warming against the period of the pre-industrial revolution. The assessment of spatiotemporal variation in albedo is a prerequisite for accurate prediction of ice sheet loss and future climate change, as well as crucial prior knowledge for improving current climate models. In our study, we employed the satellite data product from the global land surface satellite (GLASS) project to obtain the spatiotemporal variation of albedo from 1981 to 2017 using the non-parameter-based M-K (Mann-Kendall) method. It was found that the albedo generally showed a decreasing trend in the past 37 years (-0.013±0.001 decade-1, p<0.01); in particular, the albedo showed a significant increasing trend in the middle part of the study area but a decreasing trend in the coastal area. The interannual and seasonal variations of albedo showed strong spatial-temporal heterogeneity. Additionally, based on natural and anthropogenic factors, in order to further reveal the potential effects of spatiotemporal variation of albedo on the regional climate, we coupled climate model data with observed data documented by satellite and adopted a conceptual experiment for detections and attributions analysis. Our results showed that both the greenhouse gas forcing and aerosol forcing induced by anthropogenic activities in the past 37 decades were likely to be the main contributors (46.1%) to the decrease of albedo in Greenland. Here, we indicated that overall, Greenland might exhibit a local warming effect based on our study. Albedo–ice melting feedback is strongly associated with local temperature changes in Greenland. Therefore, this study provides a potential pathway to understanding climate change on a regional scale based on the coupled dataset.
... We use RACMO 2.3p2 at 1 km resolution for melt data from the full year 2015 (Noel et al., 2015) to indicate relative melting between different regions of the GIS. All data sets do not necessarily correspond temporally, which is a potential source of error in our analysis since ice velocity, ice surface topography, and bed topography can vary temporally 5 (Sugden, 1978;Hart, 1995;Bartholomew et al., 2011;Sole et al., 2011;Helm et al., 2014). ...
... We find that the time scale for bed topography or basal sliding transfer amplitudes to reach 95% of their steady state values is as much as 60 years for the longest wavelengths of topography in our typical study areas (∼20 km), and is ∼3-20 years for wavelengths that typically exhibit the highest transfer (∼1-10 km). It is probably unlikely that bed topography, ice sheet thickness, or ice sheet surface slope change significantly over 25 these timescales, but ice velocity and basal sliding can vary on day to year timescales, meaning that the steady state assumption is a potential source of error in our analysis (Das et al., 2008;Bartholomew et al., 2011;Sole et al., 2011;Helm et al., 2014;Chandler et al., 2013). ...
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Ice surface topography controls the primary routing of surface meltwater on ablation zones of glaciers and ice sheets. Meltwater routing is important for understanding and predicting ice sheet evolution because surface melt can be both a direct source of ice mass loss and an influence on basal sliding and ice advection. Although controls on ice sheet topography at long wavelengths are well known, smaller scale features relevant for meltwater routing are not well understood. Here we examine the effects of two processes that can influence ice sheet surface topography: bed topography transfer and thermal-fluvial incision by supraglacial streams. We implement 2D bed topography and basal sliding transfer functions in seven study regions of the western Greenland Ice Sheet (GIS) ablation zone to study the influence of basal conditions on ice surface topography. Although bed elevation data quality is spatially variable, we find that ∼ 1–10 km scale ice surface features under variable ice thickness, velocity, and surface slope are well predicted by these transfer functions. We then use flow-routing algorithms to extract supraglacial stream networks from 2–5 m resolution digital elevation models, and compare these with synthetic flow networks calculated on ice surfaces predicted by bed topography transfer. Quantitative comparison of these networks reveals that bed topography can explain ∼ 1–10 km surface meltwater routing patterns without significant contributions from thermal-fluvial erosion by streams. We predict how supraglacial internally drained catchment (IDC) patterns on the GIS would change under time-varying ice flow and/or basal sliding regimes. Basal sliding variations exert a significant influence on IDC spatial distribution, and suggest a potential positive feedback between subglacial hydrologic regime to surface IDC patterning. Increased basal sliding will increase IDC spatial density (by decreasing IDC sizes) and cause more disperse meltwater input to the englacial and subglacial environment. This could result in less efficient subglacial channelization and increased basal sliding that would then further increase IDC density.
... It has now been widely demonstrated that inputs of glacial meltwater to the subglacial environment are directly related to changes in ice dynamics and more particularly rates of basal motion. Studies have shown that terrestrial and marine terminating outlets of the Greenland ice sheet can speed up in response to increased meltwater inputs (Andersen et al., 2010;Bartholomew et al., 2011;Palmer et al., 2011;Sole et al., 2011;Zwally et al., 2002), as well as the drainage of supraglacial lakes (Das et al., 2008). ...
Article
There have been numerous reports that surges of tidewater glaciers in Svalbard were initiated at the terminus and propagated up-glacier, in contrast with downglacier-propagating surges of land-terminating glaciers. Most of these surges were poorly documented, and the cause of this behavior was unknown. We present detailed data on the recent surges of two tidewater glaciers, Aavatsmarkbreen and Wahlenbergbreen, in Svalbard. High-resolution time series of glacier velocities and evolution of crevasse patterns show that both surges propagated up-glacier in abrupt steps. Prior to the surges, both glaciers underwent retreat and steepening, and in the case of Aavatsmarkbreen, we demonstrate that this was accompanied by a large increase in driving stress in the terminal zone. The surges developed in response to two distinct processes. (1) During the late quiescent phase, internal thermodynamic processes and/or retreat from a pinning point caused acceleration of the glacier front, leading to the development of terminal crevasse fields. (2) Crevasses allowed surface meltwater and rainwater to access the bed, causing flow acceleration and development of new crevasses up-glacier. Upward migration of the surge coincided with stepwise expansion of the crevasse field. Geometric changes near the terminus of these glaciers appear to have led to greater strain heating, water production, and storage at the glacier bed. Water routing via crevasses likely plays an important role in the evolution of surges. The distinction between internally triggered surges and externally triggered speedups may not be straightforward. The behavior of these glaciers can be understood in terms of the enthalpy cycle model.
... Rapid lake drainage plays an important role in the GrIS's negative mass balance because the large volumes of lake water can reach the subglacial drainage system, perturbing it from a steady state, lowering subglacial effective pressure, and enhancing basal sliding over hours to days Schoof, 2010;Bartholomew et al., 2011aBartholomew et al., , b, 2012Hoffman et al., 2011;Banwell et al., 2013Banwell et al., , 2016Tedesco et al., 2013;Andrews et al., 2014), particularly if the GrIS is underlain by sediment (Bougamont et al., 2014;Kulessa et al., 2017;Doyle et al., 2018;Hofstede et al., 2018). Rapidlake-drainage events also have two longer-term effects. ...
Article
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Remote sensing is commonly used to monitor supraglacial lakes on the Greenland Ice Sheet (GrIS); however, most satellite records must trade off higher spatial resolution for higher temporal resolution (e.g. MODIS) or vice versa (e.g. Landsat). Here, we overcome this issue by developing and applying a dual-sensor method that can monitor changes to lake areas and volumes at high spatial resolution (10–30 m) with a frequent revisit time (∼3 days). We achieve this by mosaicking imagery from the Landsat 8 Operational Land Imager (OLI) with imagery from the recently launched Sentinel-2 Multispectral Instrument (MSI) for a ∼12 000 km2 area of West Greenland in the 2016 melt season. First, we validate a physically based method for calculating lake depths with Sentinel-2 by comparing measurements against those derived from the available contemporaneous Landsat 8 imagery; we find close correspondence between the two sets of values (R2=0.841; RMSE = 0.555 m). This provides us with the methodological basis for automatically calculating lake areas, depths, and volumes from all available Landsat 8 and Sentinel-2 images. These automatic methods are incorporated into an algorithm for Fully Automated Supraglacial lake Tracking at Enhanced Resolution (FASTER). The FASTER algorithm produces time series showing lake evolution during the 2016 melt season, including automated rapid (≤4 day) lake-drainage identification. With the dual Sentinel-2–Landsat 8 record, we identify 184 rapidly draining lakes, many more than identified with either imagery collection alone (93 with Sentinel-2; 66 with Landsat 8), due to their inferior temporal resolution, or would be possible with MODIS, due to its omission of small lakes
... Thus, channelized systems are associated with slow flowing ice regimes and are often seasonal. Bartholomew et al. (2011) provides evidence that sliding velocities near the margins of the Greenland ice sheet are lower in the late summer than earlier in the summer, likely as an indication of a switch from a distributed to a channelized drainage system. There are two types of channelized drainage systems: Nyechannels that are incised down into the substrate (Walder and Hallet, 1979), and R-channels that are incised up into the ice (Röthlisberger, 1972). ...
Article
Full-text available
We present BrAHMs (BAsal Hydrology Model): a physically based basal hydrology model which represents water flow using Darcian flow in the distributed drainage regime and a fast down-gradient solver in the channelized regime. Switching from distributed to channelized drainage occurs when appropriate flow conditions are met. The model is designed for long-term integrations of continental ice sheets. The Darcian flow is simulated with a robust combination of the Heun and leapfrog–trapezoidal predictor–corrector schemes. These numerical schemes are applied to a set of flux-conserving equations cast over a staggered grid with water thickness at the centres and fluxes defined at the interface. Basal conditions (e.g., till thickness, hydraulic conductivity) are parameterized so the model is adaptable to a variety of ice sheets. Given the intended scales, basal water pressure is limited to ice overburden pressure, and dynamic time stepping is used to ensure that the Courant–Friedrichs–Lewy (CFL) condition is met for numerical stability. The model is validated with a synthetic ice sheet geometry and different bed topographies to test basic water flow properties and mass conservation. Synthetic ice sheet tests show that the model behaves as expected with water flowing down gradient, forming lakes in a potential well or reaching a terminus and exiting the ice sheet. Channel formation occurs periodically over different sections of the ice sheet and, when extensive, displays the arborescent configuration expected of Röthlisberger channels. The model is also shown to be stable under high-frequency oscillatory meltwater inputs.
... GPS station names reflect the distance from the Sermeq Avannarleq terminus and the distance north or south of the flow line, except for FOXX, GULL, and HARE, which include weather stations. These GPS stations fall between 600 and 1,100 m above sea level (asl), similar to previous low-and moderate-elevation GPS stations, boreholes, and dye tracing experiments located at Russell Glacier (e.g., Bartholomew et al., 2011;Chandler et al., 2013;van de Wal et al., 2015;Wright et al., 2016). Therefore, we generally characterize GPS stations below 900 m asl as low elevation (19N1,FOXX,22N4,25N1,and GULL) and stations at elevations between 900 and 1,100 m asl as moderate elevation (28N4, 33N1, HARE, 37N4, 38S3, and 41N1). ...
Article
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The impact of summer surface melt on Greenland Ice Sheet dynamics is modulated by the state of the subglacial hydrologic system. Studies of ice motion indicate that efficiency of the subglacial system increases over the melt season, decreasing the sensitivity of ice motion to surface melt inputs. However, the behavior of the subglacial hydrologic system is complex and some characteristics are still poorly constrained. Here we investigate the coevolution of subglacial hydrology and ice motion in the Pâkitsoq region of western Greenland during the 2011 melt season. We analyze measurements from 11 Global Positioning System stations, from which we derive ice velocity, longitudinal strain rates, and basal uplift, alongside observations of surface ablation and supraglacial lake drainages. We observe ice acceleration after the onset of local surface melting, followed by gradual ice deceleration, consistent with increasing subglacial efficiency. In the study area, supraglacial lake drainages cooccur with a change in regional strain rate patterns and ice deceleration, suggesting that lake drainages contribute to rapid subglacial reorganization. At lower ice surface elevations (below ~900 m above sea level), ice motion is correlated with both total basal uplift and its rate of change, while at higher elevations (~900–1,100 m above sea level), ice motion correlated only with the basal uplift rate. This pattern suggests that continued cavity growth or subglacial sediment dynamics may be important in the apparent increase in subglacial drainage efficiency at higher elevations in the ablation zone. Our results further suggest that transient subglacial behavior is important in the seasonal evolution of ice motion.
... The significant signals of seasonal variation are mainly concentrated in the ablation zone below the equilibrium line identified in McMillan et al. (2016). Thinning in autumn (July-August-September) and thickening in spring (January-February-March) are driven by the seasonal fluctuations in surface melting, snowfall, and ice dynamics (Bartholomew et al., 2011;Slater et al., 2021). Between May and August, surface melting and enhanced ice dynamics when the surface meltwater gains access to the ice-bed interface, lubricating basal motion, lower the surface in the ablation zone. ...
Article
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A long-term time series of ice sheet surface elevation change (SEC) is an essential parameter to assess the impact of climate change. In this study, we used an updated plane-fitting least-squares regression strategy to generate a 30-year surface elevation time series for the Greenland Ice Sheet (GrIS) at monthly temporal resolution and 5×5 km grid spatial resolution using ERS-1 (European Remote Sensing), ERS-2, Envisat, and CryoSat-2 satellite radar altimeter observations obtained between August 1991 and December 2020. The ingenious corrections for intermission bias were applied using an updated plane-fitting least-squares regression strategy. Empirical orthogonal function (EOF) reconstruction was used to supplement the sparse monthly gridded data attributable to poor observations in the early years. Validation using both airborne laser altimeter observations and the European Space Agency GrIS Climate Change Initiative (CCI) product indicated that our merged surface elevation time series is reliable. The accuracy and dispersion of errors of SECs of our results were 19.3 % and 8.9 % higher, respectively, than those of CCI SECs and even 30.9 % and 19.0 % higher, respectively, in periods from 2006–2010 to 2010–2014. Further analysis showed that our merged time series could provide detailed insight into GrIS SEC on multiple temporal (up to 30 years) and spatial scales, thereby providing an opportunity to explore potential associations between ice sheet change and climatic forcing. The merged surface elevation time series data are available at https://doi.org/10.11888/Glacio.tpdc.271658 (Zhang et al., 2021).
... In this system, mass is conserved on some characteristic time scale that is neither too short nor too long. For example, on the seasonal to interannual time step (too short), the ice geometry varies with winter snow accumulation and summer ablation, and velocity fields vary between days, seasons, and years (Bartholomew et al., 2011;Sundal et al., 2011;Sole et al., 2013). On a millennial time step (too long) the ice geometry varies due to climate-driven changes in ice volume (e.g., Alley et al., 2010). ...
Article
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Englacial radar reflectors in the ablation zone of the Greenland Ice Sheet are derived from layering deposited in the accumulation zone over past millennia. The original layer structure is distorted by ice flow toward the margin. In a simplified case, shear and normal strain incurred between the ice divide and terminus should align depositional layers such that they closely approximate particle paths through the ablation zone where horizontal motion dominates. It is unclear, however, if this relationship holds in western Greenland where complex bed topography, three dimensional ice flow, and historical changes to ice sheet mass and geometry since layer deposition may promote a misalignment between present-day layer orientation and the modern ice flow field. We investigate this problem using a suite of analyses that leverage ice sheet models and observational datasets. Our findings suggest that across a study sector of western Greenland, the radiostratigraphy of the ablation zone is closely aligned with englacial particle paths, and is not far departed from a state of balance. The englacial radiostratigraphy thus provides insight into the modern, local, internal flow field, and may serve to further constrain ice sheet models that simulate ice dynamics in this region.
... There is debate, however, over the significance of such drainage-driven speed-up over longer (seasonal-decadal) timescales, since the subsequent evolution of subglacial conduits to hydraulically efficient conditions may increase subglacial effective pressures sufficiently to reduce ice velocities, thus offsetting the impact of the earlier speedups (Palmer and others, 2011;Sundal and others, 2011; Bartholomew and others, 2011a; Banwell andothers, 2013, 2016;Chandler and others, 2013; Andrews and others, 2014; Mayaud and others, 2014; Tedstone andothers, 2014, 2015). Finally, rapid lake drainage can affect ice dynamics through the input of relatively warm surface water to the colder ice beneath, promoting faster ice flow due to the strong dependency of ice-deformation rates on ice temperature others, 2010, 2013), although the significance of this process is debated (Poinar and others, 2017). ...
Article
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The controls on rapid surface lake drainage on the Greenland ice sheet (GrIS) remain uncertain, making it challenging to incorporate lake drainage into models of GrIS hydrology, and so to determine the ice-dynamic impact of meltwater reaching the ice-sheet bed. Here, we first use a lake area and volume tracking algorithm to identify rapidly draining lakes within West Greenland during summer 2014. Second, we derive hydrological, morphological, glaciological and surface-mass-balance data for various factors that may influence rapid lake drainage. Third, these factors are used within Exploratory Data Analysis to examine existing hypotheses for rapid lake drainage. This involves testing for statistical differences between the rapidly and non-rapidly draining lake types, as well as examining associations between lake size and the potential controlling factors. This study shows that the two lake types are statistically indistinguishable for almost all factors investigated, except lake area. Thus, we are unable to recommend an empirically supported, deterministic alternative to the fracture area threshold parameter for modelling rapid lake drainage within existing surface-hydrology models of the GrIS. However, if improved remotely sensed datasets (e.g. ice-velocity maps, climate model outputs) were included in future research, it may be possible to detect the causes of rapid drainage.
... Because surface meltwater runoff entering moulins modifies subglacial effective pressure and ice flow velocity (Andrews et al., 2014;Banwell et al., 2016;Flowers, 2018;Koziol & Arnold, 2018), we speculate that the up-elevation decline in diurnal runoff variability-which correspondingly reduces diurnal variability of moulin inputmight potentially contribute to previous observations of reduced short-term variability in ice velocity at higher elevations previously linked to diurnal cycles in surface melting (Bartholomew et al., 2011;Shepherd et al., 2009). ...
Article
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Supraglacial stream/river catchments drain large volumes of surface meltwater off the southwestern Greenland Ice Sheet surface. Previous studies note a strong seasonal evolution of their drainage density (Dd), a classic measure of drainage efficiency defined as open channel length per unit catchment area, but a direct correlation between Dd and surface meltwater runoff (R) has not been established. We use 27 high‐resolution (∼0.5 m) satellite images to map seasonally evolving Dd for four GrIS supraglacial catchments, with elevations ranging from 1,100 m to 1,700 m. We find a positive linear correlation (r² = 0.70, p < 0.01) between Dd and simulations of runoff production from two climate models (MAR v3.11 and MERRA‐2). Applying this R‐Dd empirical relationship to climate model output enables parameterization of spatial and temporal changes in supraglacial drainage efficiency continuously throughout the melt season, although temporal and spatial skewness of Dd observations likely affects the application of this R‐Dd relationship on crevasse fields and snow/firn surfaces. Incorporating this information into a simple surface routing model finds that high runoff leads to earlier, larger diurnal peaks of runoff transport on the ice surface, owing to increased Dd. This effect progressively declines from low (∼1,100 m) to high (∼1,700 m) elevation, causing a roughly order‐of‐magnitude reduction in diurnal runoff variability at the highest elevations relative to standard climate model output. Combining intermittent satellite Dd mapping with climate model output thus promises to improve characterization of supraglacial drainage efficiency to the benefit of supraglacial meltwater routing and subglacial hydrology models.
... 114,115,116,117,118 The effects of warmer air and ocean temperatures on GrIS can be amplified by ice dynamical feedbacks, such as faster sliding, greater calving, and increased submarine melting. 116,119,120,121 Shallow ocean warming and regional ocean and atmospheric circulation changes also contribute to mass loss. 122,123,124 The underlying mechanisms of the recent discharge speed-up remain unclear; 125,126 however, warmer subsurface ocean and atmospheric temperatures 118, 127, 128 and meltwater penetration to the glacier bed 125,129 very likely contribute. ...
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1. Annual average near-surface air temperatures across Alaska and the Arctic have increased over the last 50 years at a rate more than twice as fast as the global average temperature. (Very high confidence) 2. Rising Alaskan permafrost temperatures are causing permafrost to thaw and become more discontinuous; this process releases additional CO2 and methane, resulting in an amplifying feedback and additional warming (high confidence). The overall magnitude of the permafrost–carbon feedback is uncertain; however, it is clear that these emissions have the potential to complicate the ability to meet policy goals for the reduction of greenhouse gas concentrations. 3. Arctic land and sea ice loss observed in the last three decades continues, in some cases accelerating (very high confidence). It is virtually certain that Alaska glaciers have lost mass over the last 50 years, with each year since 1984 showing an annual average ice mass less than the previous year. Based on gravitational data from satellites, average ice mass loss from Greenland was −269 Gt per year between April 2002 and April 2016, accelerating in recent years (high confidence). Since the early 1980s, annual average Arctic sea ice has decreased in extent between 3.5% and 4.1% per decade, become thinner by between 4.3 and 7.5 feet, and began melting at least 15 more days each year. September sea ice extent has decreased between 10.7% and 15.9% per decade (very high confidence). Arctic-wide ice loss is expected to continue through the 21st century, very likely resulting in nearly sea ice-free late summers by the 2040s (very high confidence). 4. It is virtually certain that human activities have contributed to Arctic surface temperature warming, sea ice loss since 1979, glacier mass loss, and northern hemisphere snow extent decline observed across the Arctic (very high confidence). Human activities have likely contributed to more than half of the observed Arctic surface temperature rise and September sea ice decline since 1979 (high confidence). 5. Atmospheric circulation patterns connect the climates of the Arctic and the contiguous United States. Evidenced by recent record warm temperatures in the Arctic and emerging science, the midlatitude circulation has influenced observed Arctic temperatures and sea ice (high confidence). However, confidence is low regarding whether or by what mechanisms observed Arctic warming may have influenced the midlatitude circulation and weather patterns over the continental United States. The influence of Arctic changes on U.S. weather over the coming decades remains an open question with the potential for significant impact.
... In the context of the Greenland Ice Sheet, periods of inphase change between discharge and solute concentration are best explained by the flushing of a linked cavity or other distributed hydrological system as hydraulic pressure rises. Seasonal changes in ice velocity in this sector of the Greenland ice sheet have been linked to a combination of distributed and channelized subglacial flow (Bartholomew et al., 2011). Dye tracing of the hydrological connections between moulins and glacial outlets has also indicated a mixture of subglacial flow regimes (Chandler et al., 2013). ...
Article
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In order to examine daily cycles in meltwater routing and storage in the Isunnguata Sermia outlet of the Greenland Ice Sheet, variations in outlet stream discharge and in major element hydrochemistry were assessed over a 6-day period in July 2013. Over 4 days, discharge was assessed from hourly photography of the outlet from multiple vantages, including where midstream naled ice provided a natural gauge. pH, electrical conductivity, suspended sediment, and major element and anion chemistry were measured in samples of stream water collected every 3 h. Photography and stream observations reveal that although river width and stage have only slight diurnal variation, there are large diurnal changes in discharge shown by the doubling in width of what we term the active channel, which is characterized by large standing waves and fast flow. The concentration of dissolved solutes follows a sinusoidal diurnal cycle, except for large and variable increases in dissolved solutes during the stream's waning flow. Solute concentrations vary by ∼ 30 % between diurnal minima and maxima. Discharge maxima and minima lag temperature and surface melt by 3–7 h; diurnal solute concentration minima and maxima lag discharge by 3–6 h. This phase shift between discharge and solute concentration suggests that during high flow, water is either encountering more rock material or is stored in longer contact with rock material. We suggest that expansion of a distributed subglacial hydrologic network into seldom accessed regions during high flow could account for these phenomena, and for a spike of partial silicate reaction products during waning flow, which itself suggests a pressure threshold-triggered release of stored water.
... First, they influence surface melt rates, particularly through their effect on lowering albedo (Lüthje et al., 2006;Tedesco and Steiner, 2011;Tedesco et al., 2012). Second, they affect ice-dynamic processes, since their rapid drainage by hydrofracture allows large pulses of surface meltwater to reach the GrIS's bed, which may impact subglacial effective pressures, raising basal water pressures and hence surface ice velocities (Zwally et al., 2002;Alley et al., 2005;Shepherd et al., 2009;Bartholomew et al., 2010Bartholomew et al., , 2011aBartholomew et al., , 2011bBartholomew et al., , 2012Schoof, 2010;Sundal et al., 2011;Hoffman et al., 2011;Colgan et al., 2011a;Cowton et al., 2013;Joughin et al., 2013;Tedstone et al., 2013;Andrews et al., 2014;Bougamont et al., 2014;Dow et al., 2015;Stevens et al., 2015). This process is of particular concern since over 200 rapid SGL drainages are thought to occur annually across the GrIS, affecting~10% of the total SGL population (Selmes et al., 2011). ...
Article
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Supraglacial lakes (SGLs) on the Greenland Ice Sheet (GrIS) influence ice dynamics if they drain rapidly by hydrofracture. MODIS data are often used to investigate SGLs, including calculating SGL area changes through time, but no existing work presents a method that tracks changes to individual (and total) SGL volume in MODIS imagery over a melt season. Here, we develop such a method by first testing three automated approaches to derive SGL areas from MODIS images from the MOD09 level-2 surface-reflectance product, by comparing calculated areas for the Paakitsoq and Store Glacier regions in West Greenland with areas derived from Landsat-8 (LS8) images. Second, we apply a physically-based depth-calculation algorithm to the pixels within the SGL boundaries from the best performing area-derivation method, and compare the resultant depths with those calculated using the same method applied to LS8 imagery. Our results indicate that SGL areas are most accurately generated using dynamic thresholding of MODIS band 1 (red) MOD09 data with a 0.640 threshold value; calculated values from MODIS are closely comparable to those derived from LS8. Third, we incorporate the best performing area- and depth-detection methods into a Fully Automated SGL Tracking (“FAST”) algorithm that tracks individual SGLs between successive MODIS images. Finally, we apply the FAST algorithm to the two study regions, where it identifies 43 (Paakitsoq) and 19 (Store Glacier) rapidly draining SGLs during 2014, representing 21% and 15% of the respective total SGL populations, including some clusters of rapidly draining SGLs. The FAST algorithm improves upon existing automatic SGL tracking methods through its calculation of both SGL areas and volumes over large regions of the GrIS on a fully automatic basis. It therefore has the potential to be used for investigating statistical relationships between SGL areas, volumes and drainage events over the whole of the GrIS, and over multiple seasons, which might provide further insights into the factors that trigger rapid SGL drainage.
... In addition to driving surface mass loss, observations and modelling suggest that higher rates of surface melting may lead to dynamic changes of the ice sheet (Hewitt, 2013;Doyle and others, 2014; Moon and others, 2014;Tedstone and others, 2015;Van De Wal and others, 2015). Correlations between the summer melt season and increased summer ice velocities indicate that surface meltwater entering the subglacial system modulates water pressures, influencing ice velocities through changes in basal drag (Zwally and others, 2002; Bartholomew and others, 2011;Joughin and others, 2013;Fitzpatrick and others, 2013;Moon and others, 2014). Recent modelling studies of the subglacial hydrological system report that the temporal variability of meltwater input (Schoof, 2010;Hewitt, 2013) is an important control on basal drag. ...
Article
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Increased summer ice velocities on the Greenland ice sheet are driven by meltwater input to the subglacial environment. However, spatial patterns of surface input and partitioning of meltwater between different pathways to the base remain poorly understood. To further our understanding of surface drainage, we apply a supraglacial hydrology model to the Paakitsoq region, West Greenland for three contrasting melt seasons. During an average melt season, crevasses drain ~47% of surface runoff, lake hydrofracture drains ~3% during the hydrofracturing events themselves, while the subsequent surface-to-bed connections drain ~21% and moulins outside of lake basins drain ~15%. Lake hydrofracture forms the primary drainage pathway at higher elevations (above ~850 m) while crevasses drain a significant proportion of meltwater at lower elevations. During the two higher intensity melt seasons, model results show an increase (~5 and ~6% of total surface runoff) in the proportion of runoff drained above ~1300 m relative to the melt season of average intensity. The potential for interannual changes in meltwater partitioning could have implications for how the dynamics of the ice sheet respond to ongoing changes in meltwater production.
... Thus, channelized systems are associated with slow flowing ice regimes and are often seasonal. Bartholomew et al. (2011) provides evidence that sliding velocities near the margins of the Greenland Ice Sheet are lower in the late summer than earlier 30 in the summer, likely as an indication of a switch from a distributed to a channelized drainage system. There are two types of channelized drainage systems: Nye Channels that are incised down into the substrate (Walder and Hallet, 1979), and Rchannels that are incised up into the ice (Röthlisberger, 1972). ...
Article
Full-text available
We present BrAHMs (BAsal Hydrology Model): a new physically-based basal hydrology model which represents water flow using Darcian flow in the distributed drainage regime and a fast down-gradient solver in the channelized regime. Switching from distributed to channelized drainage occurs when appropriate flow conditions are met. The model is designed for long-term integrations of continental ice sheets. The Darcian flow is simulated with a robust combination of the Heun and leapfrog-trapezoidal predictor-corrector schemes. These numerical schemes are applied to a set of flux-conserving equations cast over a staggered grid with water thickness at the centres and fluxes defined at the interface. Basal conditions (e.g. till thickness, hydraulic conductivity) are parametrized so the model is adaptable to a variety of ice sheets. Given the intended scales, basal water pressure is limited to ice overburden pressure, and dynamic time-stepping is used to ensure that the CFL condition is met for numerical stability. The model is validated with a synthetic ice sheet geometry and different bed topographies to test basic water flow properties and mass conservation. Synthetic ice sheet tests show that the model behaves as expected with water flowing down-gradient, forming lakes in a potential well or reaching a terminus and exiting the ice sheet. Channel formation occurs periodically over different sections of the ice sheet and, when extensive, display the arborescent configuration expected of Rothlisberger Channels.
... The area has received a relatively high amount attention due to the propensity of its glaciers to exhibit seasonal speedup. In-situ GPS observations have shown that seasonal velocity variations are strongly linked to changes in surface melting [14,19,25,26,[41][42][43]. Satellite measurements have provided a large-scale perspective of changes in ice flow [18,20,21,27] and in the extent of supraglacial lakes [44][45][46][47]. ...
Article
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Land-terminating sectors of the Greenland ice sheet flow faster in summer after surface meltwater reaches the subglacial drainage system. Speedup occurs when the subglacial drainage system becomes saturated, leading to a reduction in the effective pressure which promotes sliding of the overlying ice. Here, we use observations acquired by the Sentinel-1a and b synthetic aperture radar to track changes in the speed of land-terminating glaciers across a 14,000 km2 sector of west-central Greenland on a weekly basis in 2016 and 2017. The fine spatial and temporal sampling of the satellite data allows us to map the speed of summer and winter across the entire sector and to resolve the weekly evolution of ice flow across the downstream portions of five glaciers. Near to the ice sheet margin (at 650 m.a.s.l.), glacier speedup begins around day 130, persisting for around 90 days, and then peaks around day 150. At four of the five glaciers included in our survey the peak speedup is similar in both years, in Russell Glacier there is marked interannual variability of 32% between 2016 and 2017. We present, for the first time, seasonal and altitudinal variation in speedup persistence. Our study demonstrates the value of Sentinel-1’s systematic and frequent acquisition plan for studying seasonal changes in ice sheet flow.
... The latter process of meltwater transport to the bed, also known as hydrofracturing, can rapidly (e.g. in less than 2 h) transport large volumes of water (∼44 million cubic meters) through ice thicknesses of ∼1 km to the base of the ice sheet. The input of surface meltwater at the ice-bedrock interface has been observed to cause local ice uplift and enhanced ice flow (Bartholomew et al., 2011;Joughin et al., 2008). ...
Article
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We analyze Landsat-7 imagery spanning a 13-year period (2000–2012) for the Jakobshavn Ablation Region (JAR) along the west coast of Greenland. In addition, we introduce a new semi-automated technique for the mapping of melt-lakes using FoveaPro image-processing software (plug-in to Adobe Photoshop™), greatly simplifying the process, and resulting in more-precise spatial melt-lake statistics over existing manual methods. We found a total mean melt-lake area of 0.30 ± 0.12 km² (±1σ), with maximum melt-lake area increasing at an average rate of 0.032 km² d⁻¹ across the study periods. Additionally, we note a yearly seasonal increase (∼1.8 m d⁻¹) in the overall mean lake elevation (∼200 m per season) as well as an optimal elevation of the largest-area melt-lakes of ∼1320 ± 20 m (±1σ). We also found an increase in the maximum average melt-lake elevation (MAME) of ∼3.8 m a⁻¹ (∼50 m). Based on data recorded at nearby automated weather stations, the mean seasonal temperature increased ∼1.6°C over the 13-year period at an average rate of 0.125°C a⁻¹. Although temperature is a driver for meltwater production, we conclude that mechanisms related to the surface topography are more likely modulating the spatial pattern and characteristics of melt lakes in the ablation zone.
... On the other hand, increased surface melt may lead to increased seasonal velocities, via basal lubrication and till deformation (e.g. Boulton et al., 2001;Zwally et al., 2002;Schoof, 2010;Bartholomew et al., 2011;Hart et al., 2011;Andrews et al., 2014;Bougamont et al., 2018;Hart et al., 2019a). This process can vary considerably over the melt season, however, depending on several different factors (Bartholomew et al., 2012;Tedstone et al., 2013;Hart et al, 2019b). ...
Article
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Proglacial lakes are becoming ubiquitous at the termini of many glaciers worldwide due to continued climate warming and glacier retreat, and such lakes have important consequences for the dynamics and future stability of these glaciers. In light of this we quantified decadal changes in glacier velocity since 1991 using satellite remote sensing for Breiðamerkurjökull, a large lake‐terminating glacier in Iceland. We investigated its frontal retreat, lake area change and ice surface elevation change, combined with bed topography data, to understand its recent rapid retreat and future stability. We observed highly spatially variable velocity change from 1991 to 2015, with a substantial increase in peak velocity observed at the terminus of the lake‐terminating eastern arm from ~1.00 ±0.36 m d‐1 in 1991 to 3.50 ±0.25 m d‐1 in 2015, with mean velocities remaining elevated from 2008 onwards. This is in stark comparison to the predominately land‐terminating arms which saw no discernible change in their velocity over the same period. We also observed a substantial increase in the area of the main proglacial lake (Jökulsárlón) since 1982 of ~20 km2, equating to an annual growth of 0.55 km2 yr‐1. Over the same period, the eastern arm retreated by ~3.50 km, which is significantly greater than the other arms. Such discrepancies between the different arms is due to the growth and, importantly, depth increase, of Jökulsárlón, as the eastern arm has retreated into its ~300 m deep reverse‐sloping subglacial trough. We suggest that this growth in lake area, forced initially by rising air temperatures, combined with the increase in lake depth, triggered an increase in flow acceleration, leading to further rapid retreat and the initiation of a positive feedback mechanism. These findings may have important implications for how increased melt and calving forced by climate change will affect the future stability of large soft‐bedded, reverse sloped, subaqueous‐terminating glaciers elsewhere.
... The seasonal ice sheet processes at our site align with the glaciological paradigms for basal drainage system and ice flow response to surface melting across the GrIS's widespread ablation zone. Basal water pressures evolve in response to surface melt across several tens of kilometres of the outer ablation zone 16 seasonally accelerates/decelerates across the entire ablation zone 35,36 . Recent decadal trends have been toward intensifying melt, a longer melt season and inland expansion of melt intensity 37 around the ice sheet. ...
Article
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Greenland Ice Sheet mass loss is impacting connected terrestrial and marine hydrologic systems with global consequences. Groundwater is a key component of water cycling in the Arctic, underlying the 1.7e6 km2 ice sheet and forming offshore freshwater reserves. However, despite its vast extent, the response of Greenland’s groundwater to ongoing ice sheet change is unknown. Here we present in-situ observations of deep groundwater conditions under the Greenland Ice Sheet, obtained in a 651-metre-long proglacial bedrock borehole angled under the ice sheet margin. We find that Greenland’s groundwater system responds rapidly and sensitively to relatively minor ice sheet forcing. Hydraulic head clearly varies over multi-annual, seasonal and diurnal timescales, which we interpret as a response to fluid pressure forcing at the ice/bed interface associated with changes in overlying ice loading and ice sheet hydrology. We find a systematic decline in hydraulic head over the eight-year observational period is linked primarily to ice sheet mass loss. Ongoing and future ice thinning will probably reduce groundwater discharge rates, with potential impacts to submarine freshwater discharge, freshwater delivery to fjords and biogeochemical fluxes in the Arctic. Greenland’s groundwater system responds rapidly to ice-sheet change, according to borehole observations from underneath the ice-sheet margin.
... Subglacial channelized (efficient) networks form within 50 km from the ice margin (Bartholomew et al., 2011;Chandler et al., 2013) and leave identifiable traces such as eskers and tunnel valleys. Eskers are the most recognizable landform, representing the sedimentary infill of R-channels (Clark and Walder, 1994). ...
Article
High-resolution LiDAR (Light detection and Ranging) -based digital elevation models (DEM) have greatly improved the mapping of glacial landforms and revealed new ones such as murtoos. Murtoos have extensive diversity in form, relief and size, and they often appear along meltwater routes. However, not all meltwater routes in the recently glaciated terrains include murtoos. We mapped different types of subglacial meltwater routes and the related distribution of murtoos in the Finnish part of Fennoscandian Ice Sheet (FIS). Subglacial meltwater routes represent a previously unknown extension of the subglacial hydrological system that supplements esker networks. Murtoo deposition along the routes is dictated by the marked concentration and routing of subglacial meltwater in a high-pressure environment outside the subglacial tunnel flow and channelized drainage zone. The main environments of murtoo route genesis include the margins of glacial lineation fields, lateral shear margins of ice streams or in between ice-flow sectors or corridors, confluence zones of ice stream onset areas, lee-sides of bedrock protrusions or thresholds, bedrock fracture valleys and potential subglacial lake inputs and outputs. This study adds to the knowledge of the development of subglacial drainage and emphasizes the role of murtoos as the potential missing link between the channelized and distributed subglacial drainage.
... At landterminating margins, while the impacts of variable hydrological forcing on ice flow have been well-studied near the ice margin (van de Wal andothers, 2008, 2015;Bartholomew and others, 2010;Sole and others, 2013;Tedstone and others, 2015), it remains unclear whether meltwater can access the bed, and efficient subglacial channels form, further into the ice-sheet interior where the ice is thicker and rates of surface melting are lower (Nienow and others, 2017). This is particularly important given that as the ELA rises in response to projected increases in surface melt (Hanna and others, 2008), the area of the ice-sheet surface undergoing melt will increase exponentially due to the hypsometry of the ice-sheet surface (Bartholomew and others, 2011;Machguth and others, 2016). Furthermore, some studies have postulated that ice motion will scale positively with surface melting at high elevations (Doyle and others, 2014; Gagliardini and Werder, 2018), and others that the presence of liquid water within the englacial hydrological system may increase ice deformation rates over time scales of years to decades (Phillips andothers, 2010, 2013). ...
Article
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Greenland's future contribution to sea-level rise is strongly dependent on the extent to which dynamic perturbations, originating at the margin, can drive increased ice flow within the ice-sheet interior. However, reported observations of ice dynamical change at distances >~50 km from the margin have a very low spatial and temporal resolution. Consequently, the likely response of the ice-sheet's interior to future oceanic and atmospheric warming is poorly constrained. Through combining GPS and satellite-image-derived ice velocity measurements, we measure multi-decadal (1993–1997 to 2014–2018) velocity change at 45 inland sites, encompassing all regions of the ice sheet. We observe an almost ubiquitous acceleration inland of tidewater glaciers in west Greenland, consistent with acceleration and retreat at glacier termini, suggesting that terminus perturbations have propagated considerable distances (>100 km) inland. In contrast, outside of Kangerlussuaq, we observe no acceleration inland of tidewater glaciers in east Greenland despite terminus retreat and near-terminus acceleration, and suggest propagation may be limited by the influence of basal topography and ice geometry. This pattern of inland dynamical change indicates that Greenland's future contribution to sea-level will be spatially complex and will depend on the capacity for dynamic changes at individual outlet glacier termini to propagate inland.
... 12 of 20 PDDs coincides with records of lower annual velocities. This can be explained by a rapid transition from a hydraulically inefficient, distributed drainage system to an efficient, channelized system (Bartholomew et al., 2011;Dunse et al., 2012;How et al., 2017). Consequently, on an annual scale, frontal ablation in warm years was not balanced by sufficient ice mass influx toward the terminus and Hansbreen significantly retreated ( Figure 7). ...
Article
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The mechanism of glacier recession and its climatic controls are complex processes that differ across the Arctic region. Here, we investigate factors influencing front variations of Hansbreen, a glacier terminated in Hornsund fjord (SW Svalbard). We apply remote sensing data to observe glacier front fluctuations between 1992 and 2015 and compare them to atmospheric and oceanographic data, sea water depth at the terminus and surface velocity. Rate of subglacial eltwater discharge approximated by the seasonal positive degree-day index (PDD) together with sea thermal conditions appear to be the main factors responsible for the fluctuations of the front of Hansbreen, while water depth at the front plays a secondary role. Taking into account ocean and air thermal conditions, the studied period has been divided into warm, cold and moderate years. The glacier retreated considerably throughout a bedrock overdeepening in the very warm period 2012–2014. This recession coincided with a slower ice flow due to intense subglacial runoff and increased submarine melting. The long-term retreat was interrupted by glacier advances in colder years, regardless of water depth at the front. The slower recession rate was the combined effect of decreased subglacial melting and increased glacier movement associated with lower subglacial runoff. Although the seasonal PDD is a good indicator of the front fluctuations, the duration of the retreat and advance periods are strongly correlated with the sea surface temperature. Expected climate warming and an increase of water temperature in the West Spitsbergen Current will stimulate further recession of Hansbreen in future.
... Large volumes of meltwater are routed through supraglacial stream and river networks on the Greenland ice sheet (GrIS) each summer (Smith et al., 2015). In temperate areas of the ice sheet, most of this surface meltwater is injected to the bed via moulins (Catania et al., 2008;Lampkin and VanderBerg, 2014;Smith et al., 2015;Yang and Smith, 2016;Koziol and Arnold, 2018), where it can modulate ice flow (Bartholomew et al., 2011;Palmer et al., 2011;Banwell et al., 2013;Hewitt, 2013;Andrews et al., 2014;de Fleurian et al., 2016). However, the role of the supraglacial system in controlling subglacial hydrology remains poorly studied to date (Flowers, 2018). ...
Article
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Each summer, large volumes of surface meltwater drain off the Greenland ice sheet (GrIS) surface through moulins to the bed, impacting subglacial hydrology and ice flow dynamics. Supraglacial surface routing delays may propagate to englacial and subglacial hydrologic systems, requiring accurate assessment to correctly estimate subglacial effective pressures. We compare hourly supraglacial moulin discharge simulations from three surface meltwater routing models – the synthetic unit hydrograph (SUH), the bare-ice component of surface routing and lake filling (SRLF), and the rescaled width function (RWF) – for four internally drained catchments on the southwestern Greenland ice sheet surface. The routing models are forced identically using surface runoff from the Modèle Atmosphérique Régionale regional climate model (RCM). For each catchment, simulated moulin hydrographs are input to the SHAKTI subglacial hydrologic model to simulate diurnally varying subglacial effective-pressure variations in the vicinity of a single moulin. Overall, all three routing models produce more realistic moulin discharges than simply using RCM runoff outputs without surface routing but produce significant differences in peak moulin discharge and time to peak. In particular, the RWF yields later, smaller peak moulin discharges than the SUH or SRLF due to its representation of slow interfluve flow between supraglacial meltwater channels, and it can readily accommodate the seasonal evolution of supraglacial stream and river networks. Differences among the three routing models are reflected in a series of simple idealized subglacial hydrology simulations that yield different diurnal effective-pressure amplitudes; however, the supraglacial hydrologic system acts as short-term storage for surface meltwater, and the temporal mean effective pressure is relatively consistent across routing models.
... van de Wal et al., 2012), dynamics (e.g. van de Wal et al., 2008;Bartholomew et al., 2011;Palmer et al., 2011;Sole et al., 2013), and supraglacial lakes (Doyle et al., 2013;Fitzpatrick et al., 2014). Recently, two studies have published DEMs of Isunnguata Sermia and Russell Glacier (Jezek et al., 2013;Morlighem et al., 2013), based on the IceBridge dataset (Leuschen and Allen, 2010), also used in this study (see Sect. 2.2). ...
Article
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We present ice thickness and bed topography maps with high spatial resolution (250 to 500 m) of a and-terminating section of the Greenland Ice Sheet derived from combined ground-based and airborne radar surveys. The data have a total area of ~12000 km2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of the Isunnguata Sermia Glacier is over-deepened and reaches an elevation of several hundreds of meters below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The covered area is one of the most studied regions of the Greenland Ice Sheet with studies of mass balance, dynamics, and supraglacial lakes, and our combined dataset can be valuable for detailed studies of ice sheet dynamics and hydrology. The compiled datasets of ground-based and airborne radar surveys are accessible for reviewers (password protected) at doi.pangaea.de/10.1594/pangaea.830314 and will be freely available in the final revised paper.
... However, if hydrofracture can occur within the inland, land-terminating regions of the GrIS (>∼1000 m a.s.l.), substantial uncertainty surrounds its implications for ice-sheet dynamics, notably since field measurements indicate that increased summer velocities are not offset by reduced wintertime ones (Doyle and others, 2014). It is hypothesised that this is because hydraulicallyefficient subglacial drainage systems cannot form within inland regions due to the shallow ice-surface gradients, producing low hydraulic-potential gradients, and the thick overlying ice, producing high creep-closure rates (Bartholomew and others, 2011;Chandler and others, 2013;Meierbachtol and others, 2013;Dow andothers, 2014, 2015;Moon and others, 2014). Instead, inefficient drainage here may persist year-round. ...
Article
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The inland advance of supraglacial lakes (SGLs) towards the interior regions of the Greenland ice sheet (GrIS) may have implications for the water volumes reaching the subglacial drainage system, and could consequently affect long-term ice-sheet dynamics. Here, we investigate changes to the areas, volumes and elevation distributions of over 8000 manually delineated SGLs using 44 Landsat images of a 6200 km ² sector of north-west Greenland over three decades (1985–2016). Our results show that SGLs have advanced to higher maximum (+418 m) and mean (+299 m) elevations, and that there has been a near-doubling of total regional SGL areas and volumes over the study period, accelerating after 2000. These changes were primarily caused by an increased SGL area and volume at high (≥1200 m a.s.l.) elevations, where SGL coverage increased by over 2750% during the study period. Many of the observed changes, particularly the post-2000 accelerations, were driven by changes to regional surface-temperature anomalies. This study demonstrates the past and accelerating response of the GrIS's hydrological system due to climatic warming, indicating an urgent need to understand whether the increasingly inland SGLs will be capable of hydrofracture in the future, thus determining their potential implications for ice-sheet dynamics.
... Given the difficulty of observing the necessary components of tidewater glacier systems, much has been inferred through comparison with land-terminating sectors of the GrIS, which are more accessible. The similarity in seasonal dynamic behavior of type 3 glaciers to that of land-terminating glaciers (e.g., Bartholomew et al., 2010Bartholomew et al., , 2011Sundal et al., 2011) led Vijay et al. (2019) to suggest that the underlying processes controlling dynamics may be the same. Specifically, the development of hydraulically efficient channels during the melt season is thought to reduce water pressure across large areas of the bed (Hoffman et al., 2016;Sole et al., 2013), leading to the extra slow-down. ...
Article
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Surface‐derived meltwater can access the bed of the Greenland ice sheet, causing seasonal velocity variations. The magnitude, timing, and net impact on annual average ice flow of these seasonal perturbations depend on the hydraulic efficiency of the subglacial drainage system. We examine the relationships between drainage system efficiency and ice velocity, at three contrasting tidewater glaciers in southwest Greenland during 2014–2019, using high‐resolution remotely sensed ice velocities, modeled surface melting, subglacial discharge at the terminus, and results from buoyant plume modeling. All glaciers underwent a seasonal speed‐up, which usually coincided with surface melt onset, and subsequent slow‐down, which usually followed inferred subglacial channelization. The amplitude and timing of these speed variations differed between glaciers, with the speed‐up being larger and more prolonged at our fastest study glacier. At all glaciers, however, the seasonal variations in ice flow are consistent with inferred changes in hydraulic efficiency of the subglacial drainage system and qualitatively indicative of a flow regime in which annually averaged ice velocity is relatively insensitive to interannual variations in meltwater supply—so‐called “ice flow self‐regulation.” These findings suggest that subglacial channel formation may exert a strong control on seasonal ice flow variations, even at fast‐flowing tidewater glaciers.
... Because we are looking at warming in the top part of the ice column and because sliding accounts for much of the speed, we do not correct for speed as a function of depth. Furthermore, the wintertime speeds we use, which are ∼10% slower than summertime speeds ( Bartholomew and others, 2011), bias our residence times long by approximately the same margin. Residence times in the ablation zone for all points in our central western Greenland study area are shown in Figure 8a. ...
Article
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Surface meltwater can refreeze within firn layers and crevasses to warm ice through latent-heat transfer on decadal to millennial timescales. Earlier work posited that the consequent softening of the ice might accelerate ice flow, potentially increasing ice-sheet mass loss. Here, we calculate the effect of meltwater refreezing on ice temperature and softness in the Pâkitsoq (near Swiss Camp) and Jakobshavn Isbræ regions of western Greenland using a numeric model and existing borehole measurements. We show that in the Jakobshavn catchment, meltwater percolation within the firn warms the ice at depth by 3–5°C. By contrast, meltwater refreezing in crevasses (cryo-hydrologic warming) at depths of ~300 m warms the ice in Pâkitsoq by up to 10°C, but this causes minimal increase in ice motion (<10 m a ⁻¹ ). Pâkitsoq is representative of western Greenland's land-terminating ice, where the slow movement of ice through a wide ablation zone provides ideal conditions for cryo-hydrologic warming to occur. We find that only ~37% of the western Greenland ice flux, however, travels through such areas. Overall, our findings suggest that cryo-hydrologic warming will likely have only a limited effect on the dynamic evolution of the Greenland ice sheet.
... The inland limit and spatial extent of efficient channel formation is subject to considerable debate 38,61 . Borehole and tracer studies and ice velocity records have been used to infer channels extending 40-80 km inl and 19,25,26,34,[62][63][64] , with high flow velocities of traced waters and the rapid transmission of pulses of meltwater from the ice sheet surface to the margin indicating efficient drainage 38,65 . These observations are supported by modelling studies, whereby efficient channels have been modelled up to 50 km inland, under 900 m thick ice 66,67 . ...
Article
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The subglacial hydrological system critically controls ice motion at the margins of the Greenland Ice Sheet. However, over multi-annual timescales, the net impact of hydro-dynamic coupling on ice motion remains poorly understood. Here, we present annual ice velocities from 1992–2019 across a ~10,600 km² land-terminating area of southwest Greenland. From the early-2000s through to ~2012, we observe a slowdown in ice motion in response to increased surface melt, consistent with previous research. From 2013 to 2019 however, we observe an acceleration in ice motion coincident with atmospheric cooling and a ~15% reduction in mean surface melt production relative to 2003–2012. We find that ice velocity speed-up is greater in marginal areas, and is strongly correlated with ice thickness. We hypothesise that under thinner ice, increases in basal water pressure offset a larger proportion of the ice overburden pressure, leading to reduced effective pressure and thus greater acceleration when compared to thicker ice further inland. Our findings indicate that hydro-dynamic coupling provides the major control on changes in ice motion across the ablation zone of land terminating margins of the Greenland Ice Sheet over multi-annual timescales.
... During this time, there was also considerable research into the role of ice sheet surface meltwater in altering ice dynamics. Much research focused on land-terminating glaciers (Bartholomew et al., 2011;Das et al., 2008;Joughin et al., 2008b;Sole et al., 2013;Zwally et al., 2002) because of the relative ease of making observations in these regions compared to heavily crevassed outlet glaciers. This work revealed the ice velocity response to subglacial surface meltwater input on a range of time scales. ...
Article
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Mass loss from the Greenland ice sheet (GrIS) has increased over the last two decades in response to changes in global climate, motivating the scientific community to question how the GrIS will contribute to sea-level rise on timescales that are relevant to coastal communities. Observations also indicate that the impact of a melting GrIS extends beyond sea-level rise, including changes to ocean properties and circulation, nutrient and sediment cycling, and ecosystem function. Unfortunately, despite the rapid growth of interest in GrIS mass loss and its impacts, we still lack the ability to confidently predict the rate of future mass loss and the full impacts of this mass loss on the globe. Uncertainty in GrIS mass loss projections in part stems from the nonlinear response of the ice sheet to climate forcing, with many processes at play that influence how mass is lost. This is particularly true for outlet glaciers in Greenland that terminate in the ocean because their flow is strongly controlled by multiple processes that alter their boundary conditions at the ice-atmosphere, ice-ocean, and ice-bed interfaces. Many of these processes change on a range of overlapping timescales and are challenging to observe, making them difficult to understand and thus missing in prognostic ice sheet/climate models. For example, recent (beginning in the late 1990s) mass loss via outlet glaciers has been attributed primarily to changing ice-ocean interactions, driven by both oceanic and atmospheric warming, but the exact mechanisms controlling the onset of glacier retreat and the processes that regulate the amount of retreat remain uncertain. Here we review the progress in understanding GrIS outlet glacier sensitivity to climate change, how mass loss has changed over time, and how our understanding has evolved as observational capacity expanded. Although many processes are far better understood than they were even a decade ago, fundamental gaps in our understanding of certain processes remain. These gaps impede our ability to understand past changes in dynamics and to make more accurate mass loss projections under future climate change. As such, there is a pressing need for (1) improved, long-term observations at the ice-ocean and ice-bed boundaries, (2) more observationally constrained numerical ice flow models that are coupled to atmosphere and ocean models, and (3) continued development of a collaborative and interdisciplinary scientific community.
... The study area covers c. This region provides well constrained surface mass balance models with in situ measurements (Van de Wal et al., 2012;, reliable ice surface velocity data Joughin et al., 2010), temperature and discharge data (Bartholomew et al., 2010(Bartholomew et al., , 2011Tedstone et al., 2013;Fitzpatrick et al., 2014). Run-off variability and its effect on the effective pressure have been described by Fleurian et al. (2016) using a double-continuum approach of the sub-glacial hydrology. ...
Thesis
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Glaciers and ice sheet dynamic evolution governs their contribution to sea level change in the context of climate change. Glacier basal sliding modulation by subglacial hydrology through the effective pressure (difference between the ice normal stress applied on the bedrock and the water pressure at this interface) exerts a major control on glaciers flow dynamic. However, modelling the hydrology/sliding coupling at the ice-bedrock interface requires a fine representation of the sub-glacial hydrology network, composed of cavities, lowering the effective pressure and a network of channels, efficiently routing the water thus increasing the effective pressure. In order to better constrain the interaction between subglacial drainage system seasonal evolution and glacial dynamic we study here the Russel Glacier (West Greenland), where an extensive dataset on both ice surface velocity and hydrology is available. We use a subglacial hydrology model to finely characterize the drainage network development. We show a rapid effective pressure lowering as the melt season starts, followed by a progressive increase linked to channels growth. We then couple the hydrology and ice flow models and show that this cause an hydrology network response attenuation to increasing meltwater input. This is supported by a shortening of the periods of effective pressure lowering and rising, thus reducing the seasonal variation amplitude with respect to the non-coupled hydrology model case. Our results imply that an increase of the subglacial water flux, can lead to both a strong seasonal ice flow cceleration and an annual deceleration.
... Hence, the relatively simple glacier dynamics ensures the application and validation of our monitoring approach. Observations such as surface mass balance, changes in supraglacial lakes and ice sheet dynamics, have been conducted by previous studies ( Van de Wal et al. 2008;Bartholomew et al. 2011;Palmer et al. 2011;Sole et al. 2013). Previous studies have been conducted to measure the velocity of ice over entire Greenland, including the Russell glacier area (Joughin et al. 2010;Moon et al. 2012;Rignot and Mouginot 2012;Tedstone et al. 2014). ...
Article
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To fulfil the strong need for monitoring seasonal difference of velocity over the Greenland ice sheet (GrIS), we developed an approach based on the fusion of multiple temporal and multi sensor remote sensing observations. We used spaceborne synthetic aperture radar (SAR) and optical data over the Russell glacier in southwestern Greenland. Firstly, offset tracking and InSAR time series analyses were employed for deriving the glacier's velocity in planimetric and line of sight (LOS) directions. Next, a three-dimensional (3D) decomposition was applied for estimating the 3D velocity vectors of the glacier. Once the reliability of the results was validated, a numerical ice sheet model (ISM) was further applied to derive the modelled basal friction in different seasons. We concluded that the overall data integration using multiple open-accessed satellite image employed in this study demonstrated a decent method to analyze seasonal velocity difference of the Russell glacier. Based on the proposed monitoring strategy, it is of great potential to further investigate other polar and inland glaciers with various remote sensed data.
Thesis
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Glaciers shape high altitude and latitude landscapes in numerous ways. Erosion associated with glacial processes can limit the average height of mountain ranges, while creating the greatest relief on Earth and shaping the highest mountain peaks, but glaciers can also shield pre-existing topography. Glacial erosion processes, though still enigmatic, are central to the evolution of landscapes, particularly since the onset of the Pleistocene. Glacial erosion comprises three fundamental processes: (1) abrasion, (2) quarrying and (3) the direct action of subglacial water flow in sediment transport and bedrock erosion. Glacier sliding and the hydro-mechanical conditions at the ice–bed interface drive erosion processes, and are themselves controlled by the morphology and state of the subglacial drainage system. Although widely acknowledged, the direct and indirect controls of subglacial water flow on glacial erosion have been largely neglected in previous studies. This thesis focuses on exploring these controls using numerical models with an emphasis on sub-annual to annual timescales. This work has three primary objectives: (1) to investigate how the state and morphology of the subglacial drainage system indirectly drive abrasion and quarrying, (2) to devise the first model of direct bedrock erosion by subglacial water flow and (3) to develop a framework for sediment transport in subglacial channels over a rigid bed. The results show that well-known seasonal variations in subglacial hydrology drive patterns of glacial erosion. Abrasion is favoured where the drainage system is the most dynamic, whereas quarrying calculated using a recently published law is hindered; the latter result is at odds with previous theories. Direct erosion by subglacial water flow can explain bedrock channel excavation, but the resulting erosion rates remain negligible compared to expected basin-wide glacial erosion rates. The models predict a bottleneck in sediment transport near the glacier terminus that is inherent to channel dynamics. The resulting sediment accumulation provides a process-based explanation for esker deposition, and could shape proglacial sediment yields. In focusing on spatial and temporal scales commensurate with subglacial processes, this study challenges some of the common assumptions made in glacial erosion studies and provides a starting point for refining models of landscape evolution.
Article
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Although remote sensing is commonly used to monitor supraglacial lakes on the Greenland Ice Sheet, most satellite records must trade-off high spatial resolution for high temporal resolution (e.g. MODIS) or vice versa (e.g. Landsat). Here, we overcome this issue by developing and applying a dual-sensor method that can monitor changes to lake areas and volumes at high spatial resolution (10–30 m) with a frequent revisit time (~ 3 days). We achieve this by mosaicking imagery from the Landsat 8 OLI with imagery from the recently launched Sentinel-2 MSI for a ~ 12 000 km2 area of West Greenland in summer 2016. First, we validate a physically based method for calculating lake depths with Sentinel-2 by comparing measurements against those derived from the available contemporaneous Landsat 8 imagery; we find close correspondence between the two sets of values (R2 = 0.841; RMSE = 0.555 m). This provides us with the methodological basis for automatically calculating lake areas, depths and volumes from all available Landsat 8 and Sentinel-2 images. These automatic methods are incorporated into an algorithm for Fully Automated Supraglacial lake Tracking at Enhanced Resolution (FASTER). The FASTER algorithm produces time series showing lake evolution during the 2016 melt season, including automated rapid (≤ 4 day) lake-drainage identification. With the dual Sentinel-2-Landsat 8 record, we identify 184 rapidly draining lakes, many more than identified with either imagery collection alone (93 with Sentinel-2; 66 with Landsat 8), due to their inferior temporal resolution, or would be possible with MODIS, due to its omission of small lakes
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Melt and supraglacial lakes are precursors to ice shelf collapse and subsequent accelerated ice sheet mass loss. We used data from the Landsat 8 and Sentinel-2 satellites to develop a threshold-based method for detection of lakes found on the Antarctic ice shelves, calculate their depths and thus their volumes. To achieve this, we focus on four key areas: the Amery, Roi Baudouin, Nivlisen, and Riiser-Larsen ice shelves, which are all characterized by extensive surface meltwater features. To validate our products, we compare our results against those obtained by an independent method based on a supervised classification scheme (e.g., Random Forest algorithm). Additional verification is provided by manual inspection of results for nearly 1000 Landsat 8 and Sentinel-2 images. Our dual-sensor approach will enable constructing high-resolution time series of lake volumes. Therefore, to ensure interoperability between the two datasets, we evaluate depths from contemporaneous Landsat 8 and Sentinel-2 image pairs. Our assessments point to a high degree of correspondence, producing an average R2 value of 0.85, no bias, and an average RMSE of 0.2 m. We demonstrate our method’s ability to characterize lake evolution by presenting first evidence of drainage events outside of the Antarctic Peninsula on the Amery Ice shelf. The methods presented here pave the way to upscaling throughout the Landsat 8 and Sentinel-2 observational record across Antarctica to produce a first-ever continental dataset of supraglacial lake volumes. Such a dataset will improve our understanding of the influence of surface hydrology on ice shelf stability, and thus, future projections of Antarctica’s contribution to sea level rise.
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The Department of Geography has been engaged with glaciological research from its early beginnings. The paper concentrates on the period from the appointment of Chalmers Clapperton in 1962 onwards, which coincides with the time when academic staff developed focused areas of research expertise. A brief biography of each of the eleven academic members of staff who worked in the area of glaciology is presented. This is followed by an overview of the recurring research themes and locations which have been revisited within glaciology over the years. It aims to provide an overview and flavour of the Department’s glaciological research.
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tIn a previous study, we demonstrated with a comparative morphometrical analysis the first morpho-metric evidence of a glacial landscape composed of glacial cirques and glacial valleys in the south ofTerra Sabaea at an elevation > 1000 m in two impact craters and one mountain. The purpose of this studyis to use the same method to seek other geomorphologic evidence of glacial landscapes elsewhere inTerra Sabaea. Based on a comparison between current and old glacial landscapes on Earth and Mars, weidentified 81 glacial valleys and possible evidence for a former plateau ice cap dated at 3.6 Ga at thehighest elevation in Terra Sabaea. The identified glacial valleys have the same morphometric propertiesas terrestrial and martian glacial valleys with U-shaped cross-sectional profiles, a V-index >0.2, a lengthto with ratio >1 and a cross-sectional area to drainage area ratio four times higher than the fluvial ones.Moreover, these properties are different from terrestrial and martian fluvial valleys. We did not find wellpreserved glacial cirques in this area, this absence questions the origin of glacial valleys. However, thepresence of an extensive flat plateau, from which the long valleys radiate, could have hosted an ancientplateau ice cap which was the source of these glacial valleys. A comparison with the Cantal and the ShaluliShan in the southeastern Tibetan plateau on Earth reveals morphometrical similarities with our studyarea. In fact, long glacial valleys, originating radially from a plateau at higher elevation are characteris-tics of an ancient plateau ice cap. This analysis allowed us to propose a polythermal regime for martianglacial landscape, namely a cold-based ice cap except at the margin where the regime is warmed-baseddue to the steeper topography. This topography created shear stress which increased the heat at the baseof the ice and created the outlet glacial valleys. Near the plateau, the radial valleys are U-shaped witha V-index >0.2 but downstream, to the low elevation area, these valleys become more V-shaped witha V-index around 0.1. This hypothesis is supported by the presence of an open-basin paleolake makingthe transition between inlet glacial valleys upstream and an outlet V-shaped valley downstream. So themorphometry of the radiating valleys suggests that liquid water played a role in the formation of thislandscape.
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The aim of this study is to present the first evidence of triangular-shaped till landforms and related erosional features indicative of subglacial drainage within the ice stream bed of the Scandinavian ice sheet in Finland. Previously unidentified grouped patterns of Quaternary deposits with triangular landforms can be recognized from LiDAR-based DEMs. The triangular landforms occur as segments within geomorphologically distinguishable routes that are associated with eskers. The morphological and sedimentological characteristics as well as the distribution of the triangular landforms are interpreted to involve the creep of saturated deforming till, flow and pressure fluctuations of subglacial meltwater associated with meltwater erosion. There are no existing models for the formation of this kind of large-scale drainage systems, but we claim that they represent an efficient drainage system for subglacial meltwater transfer under high pressure conditions. Our hypothesis is that the routed, large-scale subglacial drainage systems described herein form a continuum between channelized (eskers) and more widely spread small-scale distributed subglacial drainage. Moreover, the transition from the conduit dominated drainage to triangular-shaped subglacial landforms takes place about 50–60 km from the ice margin. We provide an important contribution towards a more realistic representation of ice sheet hydrological drainage systems that could be used to improve paleoglaciological models and to simulate likely responses of ice sheets to increased meltwater production.
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Continuous GPS measurements on three broad and gently sloping temperate ice-cap outlets in southern and western Vatnajökull, southeast Iceland, and in northern Hofsjökull, central Iceland, are the subject of this thesis. The measurements show events of increased ice velocity and how jökulhlaups (glacial outburst floods) affect glacier motion. Interpretation of these events, with the aid of other available hydrological and glaciological data, such as discharge time series from proglacial rivers and runoff modelling calibrated with mass balance measurements, sheds light on the time-dependent development of the subglacial hydraulic system of the ice cap outlets, and its interaction with ice motion. Motion events unrelated to jökulhlaups are observed: (i) during the early melt season, (ii) contemporaneous with events of increased surface melt or rain, and (iii) during the emptying of supraglacial slush ponds. Events of slower movement than late winter velocities are also observed, prior to early-melt-season motion events and in the wake of motion events during the height of the melt season. We interpret these events, with the aid of runoff modelling on the glacier and estimates of longitudinal stress-gradient coupling lengths, as being induced by hydrological forcing on basal slip. Lack of response in movement to certain runoff pulses and the characteristics of the diurnal variation in measured proglacial discharge indicate the development in the ablation zone of a fast, efficient subglacial hydraulic system early in the summer. The passing of a jökulhlaup and high subglacial groundwater flow do not disturb this development. Three GPS campaigns to measure jökulhlaups have been carried out over known jökulhlaup paths in two outlets from Vatnajökull ice cap, Skaftárjökull and Skeiðarárjökull. Two slowly rising jökulhlaups from Grímsvötn and two rapidly rising jökulhlaups from the western and eastern Skaftá cauldrons were captured in these campaigns, with maximum discharge ranging from 240 to 3300m3 s-1. Glacier surface movements measured in these campaigns are presented along with the corresponding discharge curves. The measurements are interpreted as indicating: (i) initiation of rapidly rising jökulhlaups with a propagating subglacial pressure wave, (ii) decreased glacier basal friction during jökulhlaups, (iii) subglacial accumulation of water in slowly rising jökulhlaups and (iv) lifting of the glacier caused by subglacial water pressure exceeding overburden in both rapidly and slowly rising jökulhlaups. The latter two observations are inconsistent with assumptions typically made in theoretical and numerical modelling of jökulhlaups. Measurements of discharge and water temperature in the Skaftá river, and of the lowering of the ice shelf over the subglacial lake at the western Skaftá cauldron are, furthermore, available for a rapidly rising jökulhlaup in September 2006. Outflow from the lake, flood discharge at the glacier terminus and the transient subglacial volume of floodwater during the jökulhlaup are derived from these data. The 40 km long initial subglacial path of the jökulhlaup was mainly formed by lifting and deformation of the overlying ice, induced by water pressure in excess of the ice overburden pressure. Melting of ice due to the heat of the floodwater from the subglacial lake and frictional heat generated by the dissipation of potential energy in the flow played a smaller role. Therefore this event, like other rapidly rising jökulhlaups, cannot be explained by the jökulhlaup theory of Nye (1976). Instead, our observations indicate that they can be explained by a coupled subglacial-sheet–conduit mechanism where essentially all of the initial flood path is formed as a sheet by the propagation of a subglacial pressure wave. Both viscous and elastic deformation of the glacier as well as turbulent hydraulic fracture at the ice/bedrock interface are important in the dynamics of the subglacial pressure wave at the front of rapidly rising jökulhlaups.
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Ice surface topography controls the routing of surface meltwater generated in the ablation zones of glaciers and ice sheets. Meltwater routing is a direct source of ice mass loss as well as a primary influence on subglacial hydrology and basal sliding of the ice sheet. Although the processes that determine ice sheet topography at the largest scales are known, controls on the topographic features that influence meltwater routing at supraglacial internally drained catchment (IDC) scales (< 10s of km) are less well constrained. Here we examine the effects of two processes on ice sheet surface topography: transfer of bed topography to the surface of flowing ice and thermal–fluvial erosion by supraglacial meltwater streams. We implement 2-D basal transfer functions in seven study regions of the western Greenland Ice Sheet ablation zone using recent data sets for bed elevation, ice surface elevation, and ice surface velocities. We find that ∼ 1–10 km scale ice surface features can be explained well by bed topography transfer in regions with different multiyear-averaged ice flow conditions. We use flow-routing algorithms to extract supraglacial stream networks from 2 to 5 m resolution digital elevation models and compare these with synthetic flow networks calculated on ice surfaces predicted by bed topography transfer. Multiple geomorphological metrics calculated for these networks suggest that bed topography can explain general ∼ 1–10 km supraglacial meltwater routing and that thermal–fluvial erosion thus has a lesser role in shaping ice surface topography on these scales. We then use bed topography transfer functions and flow routing to conduct a parameter study predicting how supraglacial IDC configurations and subglacial hydraulic potential would change under varying multiyear-averaged ice flow and basal sliding regimes. Predicted changes to subglacial hydraulic flow pathways directly caused by changing ice surface topography are subtle, but temporal changes in basal sliding or ice thickness have potentially significant influences on IDC spatial distribution. We suggest that changes to IDC size and number density could affect subglacial hydrology primarily by dispersing the englacial–subglacial input of surface meltwater.
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Glaciated watersheds are regions of intense physical and chemical weathering. In order to gain new insight on subglacial weathering processes, we measured uranium and radium isotopes from a proglacial river draining the Greenland Ice Sheet (GrIS). Time series samples were collected from the spring to mid-summer, a time period during which subglacial drainage pathways are thought to transition from slow-inefficient (distributed) to fast-efficient (channelized) systems. The ²²⁸Ra/²²⁶Ra activity ratio of the dissolved load varied from 5.2 ± 0.9–16.9 ± 3.6, which was significantly higher than the ²²⁸Ra/²²⁶Ra ratio of a suspended sediment load sample of 2.1 ± 0.07 and crustal values of ~1. The high ²²⁸Ra/²²⁶Ra in the dissolved load relative to the source rock material is indicative of mineral surface weathering induced by rapid and continuous flushing of the subglacial drainage network during the course of the melt season and those prior. The ²³⁴U/²³⁸U ratio (δ²³⁴U) varied between 33 and 106‰ with a discharge-weighted mean of 67‰; the seasonal evolution of δ²³⁴U did not correlate to geochemical indicators of subglacial meltwater storage time. An experiment designed to measure changes in δ²³⁴U with increasing meltwater storage times found that δ²³⁴U in the dissolved phase decreased rapidly with increasing storage time. Similarly, samples collected along a transect moving downstream from the ice sheet terminus had decreasing δ²³⁴U values from 63 to 15‰ further indicating that with increased weathering, the δ²³⁴U of meltwater decreases. Coupled with the relatively low δ²³⁴U and high ²²⁸Ra/²²⁶Ra, U appears to be impacted by rapid chemical weathering of subglacial and suspended sediments. The Leverett River discharge weighted U concentration was 0.13 nM; if this system is considered representative of the broader GrIS, then the total dissolved U flux from the GrIS would be on the order of 6.4 × 10⁴ mol/y. Using a similar set of assumptions, the dissolved ²²⁸Ra and ²²⁶Ra flux from the GrIS was ~1.1 × 10¹⁴ dpm/y and ~ 5.5 × 10¹³ dpm/y, respectively. These estimates suggest that the ²²⁶Ra flux to the ocean from the GrIS is globally significant and that the ²²⁸Ra flux in particular is larger than most river inputs.
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The results of systematic movement studies carried out by means of an automatic camera on the Unteraargletscher since 1969 (Flotron, 1973) are discussed together with more recent findings from theodolite measurements made at shorter intervals and over a longer section of the glacier. In addition to the typical spring/early-summer maximum of velocity known from other glaciers, an upward movement of up to 0.6 m has been recorded at the beginning of the melt season. It was followed, after various fluctuations of the vertical velocity, by a similar but slower downward movement which continued at an almost constant rate for about three months. The uplift was not confined to the section covered by the camera but occurred nearly simultaneously in profiles located 1 km below and 2 km above. The times of maximum upward velocity (increases of up to 140 mm/d) coincided approximately with periods of large horizontal velocity and occurred after increases of melt-rate. The following explanations for the variations of vertical velocity are considered: (1) Changes of longitudinal strain-rate. (2) Changes of the sliding velocity in a channel of variable width and with a bed slope deviating from horizontal. (3) Changes of volume due to opening or closing of crevasses. (4) Swelling or contraction of veins at the grain edges. (5) Growth (and closure) of cavities in the interior of the glacier. (6) Changes of large-scale water storage at the bed. Although all of the mechanisms (1)–(5) have some effect on the vertical ice movement, they cannot account for the observed variations of vertical velocity. We therefore conclude that large-scale water storage at the bed is the main cause of the uplift. Apparently the storage system is efficiently connected with the main subglacial drainage channels only during times of very high water pressure in the channels. The findings are of some interest to the concepts of glacier sliding: As mentioned above the maxima of horizontal velocity—and thus of the sliding velocity—have not been measured at the time when the storage had attained a maximum, but at the time of maximum vertical velocity, which we assume to be the time of most rapid growth of cavities at the bed. This behaviour of the sliding velocity agrees with that predicted by a simple finite-element model of the basal ice on a wavy bed with water-filled cavities. In particular, the model shows that the sliding velocity is larger during the process of cavity growth than at the final stage when the cavities have grown to the size which is stable for the applied water pressure.
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Between 3 June 1982 and 8 July 1985, a stake net consisting of up to 32 stakes covering the greater part of Storglaciären was surveyed 70 times, yielding roughly 2000 separate determinations of vertical and horizontal velocity. The time interval between surveys averaged about 1 week during the summer and 2 months during the winter. Horizontal velocities were normally highest during periods of high daily temperature or heavy rain early in the melt season. Comparable or sometimes higher temperatures or rainfalls later in the season usually had less effect, though minor velocity peaks were often present in August and early September. During periods for which bore-hole water-level measurements are available, velocity peaks generally coincided with periods of high basal water pressure, but not all periods of high water pressure resulted in velocity peaks. Despite increasing basal water pressures, velocity decreased gradually during the winter. Vertical velocities also vary seasonally. Beneath the upper part of the ablation area the glacier bed is overdeepened. Vertical velocities here are ˜3 mm/d higher during the summer. Down-glacier from the overdeepening, vertical velocities are ˜1 mm/d lower during the summer. These and other characteristics of the vertical velocity pattern are best explained by appealing to: (1) a decrease in strain-rate with depth, and (2) seasonal variations in this depth-dependence. Five periods of high velocity lasting from 3 to 11d were studied in detail. In an area where the bed is overdeepened, force-balance calculations suggest that basal drag decreased between 16 and 40% during these high-velocity events. This resulted in a decrease in compressive strain-rate at the up-glacier end of the overdeepening, an increase at the down-glacier end, and a slight increase in lateral shear strain-rates. Down-glacier from the overdeepening, basal drag increased during two events owing to an increased push from up-glacier and pull from down-glacier. Lateral shear strain-rates increased sharply here.
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Seasonal variations in ice motion have been observed at several polythermal ice masses across the High Arctic, including the Greenland Ice Sheet. However, such variations in ice motion and their possible driving mechanisms are rarely incorporated in models of the response of High Arctic ice masses to predicted climate warming. Here we use a three-dimensional finite difference flow model, constrained by field data, to investigate seasonal variations in the distribution of basal sliding at polythermal John Evans Glacier, Ellesmere Island, Canada. Our results suggest that speedups observed at the surface during the melt season result directly from changes in rates of basal motion. They also suggest that stress gradient coupling is ineffective at transmitting basal motion anomalies to the upper part of the glacier, in contrast to findings from an earlier flow line study at the same glacier. We suggest that stress gradient coupling is limited through the effect of high drag imposed by a partially frozen bed and friction induced by valley walls and significant topographic pinning points. Our findings imply that stress gradient coupling may play a limited role in transmitting supraglacially forced basal motion anomalies through Arctic valley and outlet glaciers with complex topographic settings and highlight the importance of dynamically incorporating basal motion into models predicting the response of the Arctic's land ice to climate change.
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We report the spatial and temporal pattern of sliding on the 7-km-long Bench Glacier, Alaska. Using five continuously recording GPS antennas following motion of the surface ice, distributed at 1 km spacing along the glacier center line, we documented surface ice motion over 50 days during summer 2002. Surface speeds in two previous winters constrain the motion component associated with ice deformation, allowing isolation of the sliding speed history. We observed two speedup events bracketing 2 weeks of steady slow sliding. The first event was not associated with a meteorological trigger, was more subtle than the second, and propagated up-glacier at a rate of several hundred meters per day. The second event coincided with a warm up-valley wind, which triggered considerable melt of the glacier surface. Sliding speeds in this event reached 0.3 m d-1 and began almost simultaneously at all sites in the ablation area. Both the horizontal and vertical displacement time series can be explained by growth and collapse of cavities in the lee of bumps in the bedrock bed. Cavities grow during rapid sliding and decay by viscous creep. We posit that effective pressure, averaged over some large area of the bed, is inversely proportional to the sliding speed. This effective pressure then controls the collapse rate of cavities, whose dimensions are estimated from a plausible, stepped-bed geometry. This model explains well the horizontal and vertical surface displacement history through the first event and beginning of the second event. The vertical record demands a substantial and abrupt drop in water pressure that departs from the posited sliding-effective pressure relationship. We argue that this pressure drop reflects establishment of efficient subglacial drainage, also manifested in a nearly simultaneous step increase in water discharge in the exit stream. The establishment of an efficient conduit system terminates sliding; its maintenance inhibits further sliding over the remainder of the summer.
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Results of systematic movement studies carried out by means of an automatic camera on Unteraargletscher since 1969 are discussed together with supplementary theodolite measurements made at shorted intervals and over a longer section of the glacier. In addition to the typical spring/early summer maximum of velocity known from other glaciers, an upward movement of up to 0.6m has been recorded at the beginning of the melt season. It was followed, after a few fluctuations of the vertical velocity, by an equal but slower downward movement which continued at an almost constant rate for about three months. Possible explanations of the uplift are discussed, the most satisfactory explanation being water storage at the bed.-from Authors
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[1] Surface melting on the Greenland Ice Sheet is common up to ∼1400 m elevation and, in extreme melt years, even higher. Water produced on the ice sheet surface collects in lakes and drains over the ice sheet surface via supraglacial streams and through the ice sheet via moulins. Water delivered to the base of the ice sheet can cause uplift and enhanced sliding locally. Here we use ice-penetrating radar data to observe the effects of significant basal melting coincident with moulins and calculate how much basal melt occurred. We find that more melting has occurred than can be explained by the release of potential energy from the drainage of surface meltwater during one melt season suggesting that these moulins are persistent for multiple years. We find only a few persistent moulins in our study area that drain the equivalent of multiple lakes per year and likely remain active over several years. Our observations indicate that once established, these persistent moulins might be capable of establishing well-connected meltwater drainage pathways.
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Ice thickness data collected between 1993 and 1999 using a coherent ice-penetrating radar system developed at the University of Kansas have been combined with data collected by the Technical University of Denmark in the 1970s to produce a new ice thickness grid for Greenland. Crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5-km spacing grid using a kriging interpolation procedure. A high-resolution land-ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where, in the past, the relative errors have been largest. The ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of Greenland. The ice thickness grid was compared with the currently available data set. Differences in the center of the ice sheet, where the ice is thickest, were of the order of a few percent. Near the margins, however, large differences, of as much as a factor of 10, were found. The total volume of ice contained in the ice sheet was reestimated and found to have a value of 2.93 x 10(6) km(3). The ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet. Anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow.
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Global Positioning System (GPS) data are now routinely used for many glaciological applications. In some common cases systematic errors are unmodelled at the data processing stage, although they are often presumed insignificant. In this paper I investigate these assumptions for three different scenarios: 1) measurements on a moving glacier; 2) measurements on a floating ice shelf; and 3) precise height determination over large elevation ranges, such as for aircraft positioning in lidar/laser altimeter missions. In each case systematic errors are shown to be present in the coordinate solutions that have a far greater magnitude than the formal error estimates produced by the GPS processing software, under certain conditions. If these coordinate biases go undetected, short and long term measurements of horizontal ice velocity or rates of ice thickness change may be erroneous and the coordinates could not be expected to match rigorously processed data or results from different processing techniques.
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We use ground-based and satellite observations to detect large diurnal and longer-period variations in the flow of the Greenland Ice Sheet (GrIS) during late summer that are strongly coupled with changes in its surface hydrology. The diurnal signals are associated with periodic changes in surface melting, and the longer-period signals are associated with the episodic drainage of supra-glacial lakes. Ice velocity doubles around 2 hours after peak daily melting and returns approximately to wintertime levels around 12 hours afterwards, demonstrating an intimate link between the surface and basal hydrology. During late summer, the ice sheet accelerates by 35% per positive degree-day of melting. The observed link between surface melting and enhanced flow is typical of Alpine glaciers, which may provide an appropriate analogue for the evolution of the GrIS in a warming climate.
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The Greenland ice sheet contains enough water to raise sea levels by 7 m. However, its present mass balance and future contribution to sea level rise is poorly understood. Accelerated mass loss has been observed near the ice sheet margin, partly as a result of faster ice motion. Surface melt waters can reach the base of the ice sheet and enhance basal ice motion. However, the response of ice motion to seasonal variations in meltwater supply is poorly constrained both in space and time. Here we present ice motion data obtained with global positioning system receivers located along a 35 km transect at the western margin of the Greenland ice sheet throughout a summer melt season. Our measurements reveal substantial increases in ice velocity during summer, up to 220% above winter background values. These speed-up events migrate up the glacier over the course of the summer. The relationship between melt and ice motion varies both at each site throughout the melt season and between sites. We suggest that these patterns can be explained by the seasonal evolution of the subglacial drainage system similar to hydraulic forcing mechanisms for ice dynamics that have been observedat smaller glaciers.
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High rates of surface uplift and horizontal velocities were measured during a hydrologically induced spring speed-up event. Spatial patterns of surface uplift are analyzed to estimate components of vertical motion due to flow along an inclined bed and vertical strain. Areas are identified where surface uplift was most likely due in part to the opening or enlargement of subglacial cavities by bed separation. Results suggest a widespread enlargement of subglacial cavities during the event, and survival of residual cavities after the event. The spatial pattern of cavity enlargement closely matches pre-viously identified axes of preferential subglacial drainage. It is suggested that localized cavity opening along axes of preferential drainage may constitute the initial stage in the seasonal development of channelized subglacial drainage. It is concluded that spatial and temporal variations in glacier motion may play an active role in determining the structure and rate of development of subglacial drainage during the summer melt season.
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The authors attribute significantly increased Greenland summer warmth and Greenland Ice Sheet melt and runoff since 1990 to global warming. Southern Greenland coastal and Northern Hemisphere summer temperatures were uncorrelated between the 1960s and early 1990s but were significantly positively correlated thereafter. This relationship appears to have been modulated by the North Atlantic Oscillation, whose summer index was significantly (negatively) correlated with southern Greenland summer temperatures until the early 1990s but not thereafter. Significant warming in southern Greenland since similar to 1990, as also evidenced from Swiss Camp on the west flank of the ice sheet, therefore reflects general Northern Hemisphere and global warming. Summer 2003 was the warmest since at least 1958 in coastal southern Greenland. The second warmest coastal summer 2005 had the most extensive anomalously warm conditions over the ablation zone of the ice sheet, which caused a record melt extent. The year 2006 was the third warmest in coastal southern Greenland and had the third-highest modeled runoff in the last 49 yr from the ice sheet; five of the nine highest runoff years occurred since 2001 inclusive. Significantly rising runoff since 1958 was largely compensated by increased precipitation and snow accumulation. Also, as observed since 1987 in a single composite record at Summit, summer temperatures near the top of the ice sheet have declined slightly but not significantly, suggesting the overall ice sheet is experiencing a dichotomous response to the recent general warming: possible reasons include the ice sheet's high thermal inertia, higher atmospheric cooling, or changes in regional wind, cloud, and/or radiation patterns.
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A hydrologically coupled flowband model of 'higher order' ice dynamics is used to explore perturbations in response to supraglacial water drainage and subglacial flooding. The subglacial drainage system includes interacting 'fast' and 'slow' drainage elements. The fast drainage system is assumed to be composed of ice-walled conduits and the slow system of a macroporous water sheet. Under high subglacial water pressures, flexure of the overlying ice is modelled using elastic beam theory. A regularized Coulomb friction law describes basal boundary conditions that enable hydrologically driven acceleration. We demonstrate the modelled interactions between hydrology and ice dynamics by means of three observationally inspired examples: (i) simulations of meltwater drainage at an Alpine-type glacier produce seasonal and diurnal variability, and exhibit drainage evolution characteristic of the so-called 'spring transition'; (ii) horizontal and vertical diurnal accelerations are modelled in response to summer meltwater input at a Greenland- type outlet glacier; and (iii) short-lived perturbations to basal water pressure and ice-flow speed are modelled in response to the prescribed drainage of a supraglacial lake. Our model supports the suggestion that a channelized drainage system can form beneath the margins of the Greenland ice sheet, and may contribute to reducing the dynamic impact of floods derived from supraglacial lakes. This journal is
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Fluctuations in surface melting are known to affect the speed of glaciers and ice sheets, but their impact on the Greenland ice sheet in a warming climate remains uncertain. Although some studies suggest that greater melting produces greater ice-sheet acceleration, others have identified a long-term decrease in Greenland's flow despite increased melting. Here we use satellite observations of ice motion recorded in a land-terminating sector of southwest Greenland to investigate the manner in which ice flow develops during years of markedly different melting. Although peak rates of ice speed-up are positively correlated with the degree of melting, mean summer flow rates are not, because glacier slowdown occurs, on average, when a critical run-off threshold of about 1.4 centimetres a day is exceeded. In contrast to the first half of summer, when flow is similar in all years, speed-up during the latter half is 62 ± 16 per cent less in warmer years. Consequently, in warmer years, the period of fast ice flow is three times shorter and, overall, summer ice flow is slower. This behaviour is at odds with that expected from basal lubrication alone. Instead, it mirrors that of mountain glaciers, where melt-induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient. A model of ice-sheet flow that captures switching between cavity and channel drainage modes is consistent with the run-off threshold, fast-flow periods, and later-summer speeds we have observed. Simulations of the Greenland ice-sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage; a simple model of basal lubrication alone misses key aspects of the ice sheet's response to climate warming.
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Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 36 (2009): L10501, doi:10.1029/2008GL036765. Water-filled cracks are an effective mechanism to drive hydro-fractures through thick ice sheets. Crack geometry is therefore critical in assessing whether a supraglacial lake contains a sufficient volume of water to keep a crack water-filled until it reaches the bed. In this study, we investigate fracture propagation using a linear elastic fracture mechanics model to calculate the dimensions of water-filled cracks beneath supraglacial lakes. We find that the cross-sectional area of water-filled cracks increases non-linearly with ice sheet thickness. Using these results, we place volumetric constraints on the amount of water necessary to drive cracks through ∼1 km of sub-freezing ice. For ice sheet regions under little tension, lakes larger than 0.25–0.80 km in diameter contain sufficient water to rapidly drive hydro-fractures through 1–1.5 km of subfreezing ice. This represents ∼98% of the meltwater volume held in supraglacial lakes in the central western margin of the Greenland Ice Sheet. Support for this research was provided by NSF and NASA (through ARC-0520077, ARC- 0531345, and ARC-520382) and by the Joint Initiative Awards Fund from the Andrew Mellon Foundation, and the WHOI Ocean and Climate Change Institute and Clark Arctic Research Initiative.
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The surface motion of Haut Glacier d'Arolla, Switzerland, was monitored at a high spatial and temporal resolution. Data are analyzed to calculate surface velocities, surface strain rates and the components of the glacier force budget before, during and after an early melt season speed-up or "spring event". We investigate the extent to which variations in glacier motion can be attributed to hydrologically induced local forcing or to non-local forcing transmitted via horizontal stress gradients. Enhanced glacier motion is dependent on a change in the spatial distribution of areas of high drag across the glacier.
Article
During the snow-melt season of 1982, basal water pressure was recorded in 11 bore holes communicating with the subglacial drainage system. In most of these holes the water levels were at approximately the same depth (around 70 m below surface). The large variations of water pressure, such as diurnal variations, were usually similar at different locations and in phase. In two instances of exceptionally high water pressure, however, systematic phase shifts were observed; a wave of high pressure travelled down-glacier with a velocity of approximately 100 m/h. The glacier-surface velocity was measured at four lines of stakes several times daily. The velocity variations correlated with variations in subglacial water pressure. The functional relationship of water pressure and velocity suggests that fluctuating bed separation was responsible for the velocity variations. The empirical functional relationship is compared to that of sliding over a perfectly lubricated sinusoidal bed. On the basis of the measured velocity-pressure relationship, this model predicts a reasonable value of bed roughness but too high a sliding velocity and unstable sliding at too low a water pressure. The main reason for this disagreement is probably the neglect of friction from debris in the sliding model. The measured water pressure was considerably higher than that predicted by the theory of steady flow through straight cylindrical channels near the glacier bed. Possible reasons are considered. The very large disagreement between measured and predicted pressure suggests that no straight cylindrical channels may have existed.
Article
Water flowing in tubular channels inside a glacier produces frictional heat, which causes melting of the ice walls. However the channels also have a tendency to close under the overburden pressure. Using the equilibrium equation that at every cross-section as much ice is melted as flows in, differential equations are given for steady flow in horizontal, inclined and vertical channels at variable depth and for variable discharge, ice properties and channel roughness. It is shown that the pressure decreases with increasing discharge, which proves that water must flow in main arteries. The same argument is used to show that certain glacier lakes above long flat valley glaciers must form in times of low discharge and empty when the discharge is high, i.e. when the water head in the subglacial drainage system drops below the lake level. Under the conditions of the model an ice mass of uniform thickness does not float, i.e. there is no water layer at the bottom, when the bed is inclined in the down-hill direction, but it can float on a horizontal bed if the exponent n of the law for the ice creep is small. It is further shown that basal streams (bottom conduits) and lateral streams at the hydraulic grade line (gradient conduits) can coexist. Time-dependent flow, local topography, ice motion, and sediment load are not accounted for in the theory, although they may strongly influence the actual course of the water. Computations have been carried out for the Gornergletscher where the bed topography is known and where some data are available on subglacial water pressure.
Article
The temporal and spatial variation in the surface albedo of the Greenland ice sheet during the ablation season of 1991 is investigated. The study focuses on an area east of Søndre Strømfjord measuring 200 km by 200 km and centred at 67°5′ N, 48° 13′W. The analysis is based on satellite radiance measurements carried out by the Advanced Very High Resolution Radiometer (AVHRR). The broad-band albedo is estimated from the albedos in channel 1 (visible) and channel 2 (near-infrared). The results are calibrated with the surface albedo of sea and dry snow. Satellite-derived albedos are compared with GIMEX ground measurements at three stations. There is a high degree of consistency in temporal variation at two of the three stations. Large systematic differences are attributed to albedo variations on sub-pixel scale. In the course of the ablation season four zones appear, each parallel to the ice edge. It is proposed that these are, in order of increasing altitude: (I) clean and dry ice, (II) ice with surface water, (III) superimposed ice, and (IV) snow. An extensive description of these zones is given on the basis of the situation on 25 July 1991. Zones I, III and IV reveal fairly constant albedos (0.46, 0.65 and 0.75 on average), whereas zone II is characterised by an albedo minimum (0.34). Survey of the western margin of the Greenland ice sheet (up to 71° N) shows that the zonation occurs between 66° and 70° N.
Article
Water flowing in tubular channels inside a glacier prod