Article

Winter motion mediates dynamic response of the Greenland Ice Sheet to warmer summers

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Abstract

We present ice velocities from a land-terminating transect extending >115km into the western Greenland Ice Sheet during three contrasting melt years (2009-2011) to determine whether enhanced melting accelerates dynamic mass loss. We find no significant correlation between surface melt and annual ice flow. There is however a positive correlation between melt and summer ice displacement, but a negative correlation with winter displacement. This response is consistent with hydro-dynamic coupling; enhanced summer ice flow results from longer periods of increasing surface melting and greater duration ice surface to bed connections, while reduced winter motion is explicable by drainage of high basal water pressure regions by larger more extensive subglacial channels. Despite mean interannual surface melt variability of up to 70%, mean annual ice velocities changed by <7.5%. Increased summer melting thereby preconditions the ice-bed interface for reduced winter motion resulting in limited dynamic sensitivity to interannual variations in surface melting.

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... T he melting of snow and ice creates networks of meltwater streams and ponds on glacier and ice-sheet surfaces [1][2][3] . Surface meltwater is known to influence the dynamics of many glaciers [4][5][6][7] and portions of the Greenland Ice Sheet 8,9 . ...
... Surface meltwater is known to influence the dynamics of many glaciers [4][5][6][7] and portions of the Greenland Ice Sheet 8,9 . At these locations, drainage of surface water to the ice-bed interface impacts basal water pressure, leading to variations in ice motion on sub-daily to decadal timescales 1,4,[7][8][9][10][11][12][13] . This mechanism couples ice flow to atmospheric processes over a range of timescales down to the sub-diurnal level, making affected ice masses respond rapidly to changes in atmospheric circulation patterns induced by climate change 9 . ...
... We suggest that once at the ice base, this water causes a rapid acceleration of ice by promoting enhanced basal sliding through a spike in basal water pressure 4,9,22 . Efficient evacuation of both surface-derived meltwater and water stored at the ice base would then lead to a subsequent reduction in water pressure and ice motion 1,13,22 . This mechanism is known to occur in alpine glaciers 5 , some polythermal glaciers 6,23 , and across some marginal regions of the Greenland Ice Sheet 1,9 , but this is the first time that surface meltwater-induced speed-up has been observed in Antarctica. ...
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Atmospheric warming is increasing surface melting across the Antarctic Peninsula, with unknown impacts upon glacier dynamics at the ice-bed interface. Using high-resolution satellite-derived ice velocity data, optical satellite imagery and regional climate modelling, we show that drainage of surface meltwater to the bed of outlet glaciers on the Antarctic Peninsula occurs and triggers rapid ice flow accelerations (up to 100% greater than the annual mean). This provides a mechanism for this sector of the Antarctic Ice Sheet to respond rapidly to atmospheric warming. We infer that delivery of water to the bed transiently increases basal water pressure, enhancing basal motion, but efficient evacuation subsequently reduces water pressure causing ice deceleration. Currently, melt events are sporadic, so efficient subglacial drainage cannot be maintained, resulting in multiple short-lived (< 6 day) ice flow perturbations. Future increases in meltwater could induce a shift in glacier dynamic regime, characterised by seasonal-scale ice flow variations.
... At land-terminating glacier margins, continual subglacial water flow during the summer months causes the formation of hydraulically efficient subglacial channels. These enable the rapid evacuation of meltwater, decreasing basal water pressure, and ultimately cause the overlying ice to decelerate in late summer to speeds slower than those prior to the melt season (an "extra slow-down") (e.g., Sole et al., 2013). In addition, this late-summer extra slow-down scales with meltwater supply such that annually averaged ice velocity is insensitive to interannual variations in meltwater supply-the so-called "ice flow self-regulation" van de Wal et al., 2015). ...
... 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. It is not clear, however, to what extent subglacial channels can form beneath tidewater glaciers. ...
... This behavior resembles that of land-terminating glaciers and occurs when the drainage system is continually challenged by rapidly increasing meltwater inputs, causing frequent spikes in water pressure (Bartholomew et al., 2012;Harper et al., 2007;Schoof, 2010), cavity expansion Iken, 1981;Kamb, 1987), and/or sediment deformation (Iverson et al., 1999). Our observations of seasonal meltwater-induced speed-ups were relatively small (16-40%) compared to land-terminating glaciers (180-400%; Sole et al., 2013;van de Wal et al., 2008van de Wal et al., , 2015, though the maximum speed-ups we observe are likely reduced by smoothing over the 6-to 12-day image baseline. Similarly, modest seasonal meltwater-induced speed-ups (typically less than 15%) have been observed at several other Greenlandic tidewater glaciers (Ahlstrøm et al., 2013;Andersen et al., 2010;Bevan et and may be subdued relative to land-terminating glaciers because of the already low basal resistance at tidewater glaciers (Shapero et al., 2016). ...
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.
... Interactions between channels and cavities are often observed indirectly from evaluating glacier flow-velocity variations in response to meltwater supply variability. High and sustained water supply over long timescales (e.g. during the peak melt season) has been observed to trigger glacier deceleration ( Bartholomew et al., 2010;Sole et al., 2013;Tedstone et al., 2013Tedstone et al., , 55 2015). This behavior is related to the fact that channels-development increases the drainage system capacity and therefore reduces the average basal water pressure (Fountain and Walder, 1998). ...
... Interactions between channels and cavities are often observed indirectly from evaluating glacier flow-velocity variations in response to meltwater supply variability. High and sustained water supply over long timescales (e.g. during the peak melt season) has been observed to trigger glacier deceleration ( Bartholomew et al., 2010;Sole et al., 2013;Tedstone et al., 2013Tedstone et al., , 55 2015). This behavior is related to the fact that channels-development increases the drainage system capacity and therefore reduces the average basal water pressure (Fountain and Walder, 1998). ...
... This behavior is related to the fact that channels-development increases the drainage system capacity and therefore reduces the average basal water pressure (Fountain and Walder, 1998). On the contrary, during short term water supply increase (e.g. at the early melt season or at diurnal scales), glacier velocity changes have been observed to occur concomitantly with water supply changes ( Parizek and Alley, 2004;Palmer et al., 2011;Sole et al., 2013;Doyle et al., 2014;Vincent and Moreau, 2016). This behavior is mostly related to the pressurization of the cavity-system, causing average basal water pressure rise and 60 subsequent basal sliding speeds increase (e.g. ...
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Abstract. Water flowing below glaciers exerts a major control on glacier basal sliding speeds. However, our knowledge on the physics of subglacial hydrology and its link with sliding is limited by lacking observations. Here we use a two-year long dataset made of on-ice measured seismic and in-situ measured glacier basal sliding speed records on the Glacier d’Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels hydraulic radius and hydraulic pressure gradient. At seasonal timescales we observe, for the first time, that hydraulic radius and hydraulic pressure gradient present a four-fold increase from spring to summer, followed by a comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels behaving at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behavior that is consistent with channels being out-of-equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which demonstrate that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speeds, supporting that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel-system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speeds) through an efficient hydraulic connection between the two systems. Using the two regimes herein observed in channels seasonal-dynamics as constraints for subglacial hydrology/ice dynamics models may allow to strengthen our knowledge on the physics of subglacial processes.
... 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. ...
... 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]. ...
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.
... It has been reported that glaciers can display substantial seasonal variations in surface velocity. Vijay et al. [22] and Sole et al. [23] suggested that these changes occur because the seasonality in the production of meltwater affects how much water is retained in the subglacial system. Moon et al. [24] observed three distinct seasonal velocity patterns in the Greenland Ice Sheet: one pattern exhibits a relatively high correlation with glacier front retreat, while the other two patterns seem to be controlled by meltwater. ...
... Kenner et al. [27] suggested that increased water supply from precipitation might have an impact on glacier deformation and on the seasonality of surface velocity. These changes occur because the seasonal variation of meltwater affects how much water is retained in the subglacial system [23,27]. Hence, it remains an open question why and how the water pressure affects the glacier surface velocity. ...
... To conclude, compared to the long-term average, meteorological conditions during the years covered by this study may have resulted in a trend in seasonal snowfall (see Figure 10a,b), as indicated by the weather station data. The high summer temperature could have generated large glacier melting and triggered sliding processes [23][24][25][26]. ...
Article
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Monitoring glacier flow is vital to understand the response of mountain glaciers to environmental forcing in the context of global climate change. Seasonal and interannual variability of surface velocity in the temperate glaciers of the Parlung Zangbo Basin (PZB) has attracted significant attention. Detailed patterns in glacier surface velocity and its seasonal variability in the PZB are still uncertain, however. We utilized Landsat-8 (L8) OLI data to investigate in detail the variability of glacier velocity in the PZB by applying the normalized image cross-correlation method. On the basis of satellite images acquired from 2013 to 2020, we present a map of time-averaged glacier surface velocity and examined four typical glaciers (Yanong, Parlung No.4, Xueyougu, and Azha) in the PZB. Next, we explored the driving factors of surface velocity and of its variability. The results show that the glacier centerline velocity increased slightly in 2017–2020. The analysis of meteorological data at two weather stations on the outskirts of the glacier area provided some indications of increased precipitation during winter-spring. Such increase likely had an impact on ice mass accumulation in the up-stream portion of the glacier. The accumulated ice mass could have caused seasonal velocity changes in response to mass imbalance during 2017–2020. Besides, there was a clear winter-spring speedup of 40% in the upper glacier region, while a summer speedup occurred at the glacier tongue. The seasonal and interannual velocity variability was captured by the transverse velocity profiles in the four selected glaciers. The observed spatial pattern and seasonal variability in glacier surface velocity suggests that the winter-spring snow might be a driver of glacier flow in the central and upper portions of glaciers. Furthermore, the variations in glacier surface velocity are likely related to topographic setting and basal slip caused by the percolation of rainfall. The findings on glacier velocity suggest that the transfer of winter-spring accumulated ice triggered by mass conservation seems to be the main driver of changes in glacier velocity. The reasons that influence the seasonal surface velocity change need further investigation.
... Under reduced surface melt forcing, we would expect the up-glacier extent of efficient subglacial channels to decrease, allowing regions of the distributed drainage system that were, in previous years, drained by efficient channels to re-pressurise through the gradual recharge of meltwater via basal melting. Numerous GPS data show that this process occurs on a seasonal timescale, whereby following the deceleration of ice motion to a minimum in the late melt season, measured ice velocities show a gradual increase over the following winter 41,47,48 ; this process has not however been observed to-date on a multi-annual timescale. This study therefore extends the West Greenland ice velocity time series, both spatially and temporally, in order to investigate how ice motion has responded to recent reductions in surface melt forcing, with the ultimate aim of improving our understanding of the mechanisms driving ice sheet motion. ...
... Moreover, the response times of ice velocity to increases and decreases in surface melt forcing appear to differ -surface melt displays a long-term increasing trend from the early-mid 1990s before ice velocities begin to decrease in ~2003, whereas ice velocities stabilise and begin accelerating almost instantly in response to the large reduction in surface melt forcing from 2013 onwards. To investigate the impact of year-to-year variability in surface melt production on year-to-year velocity, we calculate a linear regression through detrended velocity and melt production anomaly time series (Fig. S13), which gives an R 2 of 0.08 (p = 0.11), indicating that there is no significant relationship between annual ice velocity and annual runoff, consistent with earlier work 41,43,48 . ...
... In our study, we are unable to determine whether a positive change in ice velocity has occurred at elevations above 1300 m (a.s.l.) due to increasing noise in our dataset further inland. However, GPS observations reveal reduced velocities to at least 80 km inland (at ~1500 m a.s.l) in the years of record surface melt in 201048 and 2012 37 when compared to 2009. Whilst multi-annual ice velocity slowdowns in southwest Greenland since the early-mid 2000s are observed here and in a number of other studies 40-44 , differences between studies exist regarding the magnitude of slowdown, and the proposed mechanism(s) driving this dynamic change. ...
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.
... While hydrofracture is recognised as an important contributor to the GrIS's mass loss, there is thought to be a discrepancy between its impact for different areas of the GrIS. Within ice-marginal, land-terminating regions (≤∼1000 m a.s.l.), any mid-summer accelerations in ice velocity are typically offset by reduced late-summer ( Bartholomew and others, 2010;Hoffman and others, 2011;Sundal and others, 2011;van de Wal and others, 2015) or winter velocities (Sole and others, 2013). This is due to the evolution of the subglacial drainage system to higher hydraulic-efficiency channels, which can evacuate meltwater quickly, and in which there is a direct relation between effective pressure and increased discharge (Schoof, 2010;Chandler and others, 2013;Cowton and others, 2013;Andrews and others, 2014;Mayaud and others, 2014). ...
... This is due to the evolution of the subglacial drainage system to higher hydraulic-efficiency channels, which can evacuate meltwater quickly, and in which there is a direct relation between effective pressure and increased discharge (Schoof, 2010;Chandler and others, 2013;Cowton and others, 2013;Andrews and others, 2014;Mayaud and others, 2014). This therefore tends to produce either a net decrease (van de Wal andothers, 2008, 2015;Tedstone and others, 2015) or no net change in annual ice velocity within ice-marginal regions (Sole and others, 2013). 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). ...
... While there may be small increases to SGL area and volume by enhanced SGL-bottom ablation (Lüthje and others, 2006;Tedesco and others, 2012), this shows that the bedrock topography and local sliding rates are a key control on SGL size (Thomsen and others, 1988;Echelmeyer and others, 1991;Gudmundsson, 2003;Lampkin and VanderBerg, 2011;Johansson and others, 2013). Consequently, statistically significant increases to the areas and volumes of these SGLs were impossible, so the increasing quantities of melt at low elevations during the later parts of the study period were presumably accommodated by the earlier drainage of SGLs, and the resultant opening of surface-to-bed connections, permitting the direct delivery of any future meltwater to the basal system for the remainder of the season (Colgan and others, 2011;Palmer and others, 2011;Banwell andothers, 2013, 2016;Sole and others, 2013;Tedstone and others, 2014;Koziol and others, 2017). Meanwhile, in the earlier part of the study period, the basins at higher (≥600 m a.s.l.) elevations were likely to have been either empty or only partially filled, meaning that they were able to accommodate increasingly larger SGLs over the study period, shown by the increases to the mean and maximum sizes of SGLs at these elevations. ...
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.
... The overall evolution over the complete year of the surface speed and the water pressure obtained from the inversions corresponds well with the behaviour expected from previous observations and the current understanding of the interaction 690 between the subglacial hydrological system and the ice flow (Nienow et al., 2017;Davison et al., 2019). For instance, the synchronous increase in basal water pressure and in surface velocity demonstrated here at the beginning of the melt period fits well with the in-situ observations (Bartholomew et al., 2010;Sole et al., 2013;Van De Wal et al., 2015). In our inferred fields the maximum water pressure and velocity are reached while surface runoff still continues to increase, which also corresponds to the local (Bartholomew et al., 2010;Sole et al., 2013) and larger-scale (Sundal et al., 2011;Fitzpatrick et al., 2013) observations 695 in this region, suggesting that a more efficient drainage system is limiting the increase in water pressure at the glacier bed. ...
... For instance, the synchronous increase in basal water pressure and in surface velocity demonstrated here at the beginning of the melt period fits well with the in-situ observations (Bartholomew et al., 2010;Sole et al., 2013;Van De Wal et al., 2015). In our inferred fields the maximum water pressure and velocity are reached while surface runoff still continues to increase, which also corresponds to the local (Bartholomew et al., 2010;Sole et al., 2013) and larger-scale (Sundal et al., 2011;Fitzpatrick et al., 2013) observations 695 in this region, suggesting that a more efficient drainage system is limiting the increase in water pressure at the glacier bed. The drop in water pressure after the end of the melt season ( Fig.11-b) corresponds to the presence of a more efficient drainage system established during summer that allows for easy evacuation of the remaining water inputs, chiefly the production of water at the bed as surface melting ceases. ...
Preprint
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Increasing surface melting on the Greenland ice sheet requires better constraints on seasonally evolving basal water pressure and sliding speed. Here we assess the potential of using inverse methods on a dense time series of surface speeds to recover the seasonal evolution of the basal conditions in a well-documented region in southwest Greenland. Using data compiled from multiple satellite missions, we document seasonally evolving surface velocities with a temporal resolution of two weeks. We then apply the inverse control method using Elmer/Ice to infer the basal sliding and friction corresponding to each of the 24 surface-velocity data sets. Near the margin where the uncertainty in the velocity and bed topography are small, we obtain clear seasonal variations that can be mostly interpreted in terms of a effective-pressure based hard-bed friction law. We find for valley bottoms or "troughs" in the bed topography, the changes in basal conditions directly respond to local water pressure variations, while the link is more complex for subglacial "ridges" which are often non-locally forced. At the catchment scale, in-phase variations of the water pressure, surface velocities, surface-runoff variations are found.Our results show that time-series inversions of observed surface velocities can be used to understand the evolution of basal conditions over different timescales and could therefore serve as an intermediate validation for subglacial hydrology models to achieve better coupling with ice-flow models.
... Both observational (e.g., Andrews et al., 2014) and modelling (e.g., Schoof, 2010) studies have established a close association between the speed changes and evolving subglacial hydrologic conditions forced by surface melt. Yet, despite little to no surface melt in the winter months, this period is when the vast majority of the overall ice displacement occurs (Sole et al., 2013) due to continuous motion (albeit relatively slow) over about 35 two thirds of the year. ...
... We see no way to link our 360 observations of winter acceleration to a steady increase in driving stress, because the speedup is widespread and irrespective of ice thickness gradients. Earlier works have appealed to pressurization of the basal drainage system over the course of winter as a means for decreasing basal traction, manifested conceptually through either isolation of cavities as the drainage system shuts down (e.g., Fitzpatrick et al., 2013), or widespread re-pressurization of the bed in response to closure of low pressure channels (e.g., Sole et al., 2013). Our observations suggest pressurization occurs over a few weeks in autumn (Figs. 3, 5), but then no monotonic increase 365 in the basal water pressure over the winter period (Fig. 5), a finding in agreement with observations from elsewhere in Greenland . ...
Preprint
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Basal sliding in the ablation zone of the Greenland Ice Sheet is closely associated with water from surface melt introduced to the bed in summer, yet melting of basal ice also generates subglacial water year-round. Assessments of basal melt rely on modelling with results strongly dependent upon assumptions with poor observational constraint. Here we use surface and borehole measurements to investigate the generation and fate of basal meltwater in the ablation zone of Isunnguata Sermia basin, Western Greenland. The observational data are used to constrain estimates of the heat and water balances, providing insights into subglacial hydrology during the winter months when surface melt is minimal or non-existent. Despite relatively slow ice flow speeds during winter, the basal meltwater generation from sliding friction remains many fold greater than that due to geothermal heat flux. A steady acceleration of ice flow over the winter period at our borehole sites can cause the rate of basal water generation to increase by up to 20 %. Borehole measurements show high but steady basal water pressure, rather than monotonically increasing pressure. Ice and groundwater sinks for water do not likely have sufficient capacity to accommodate the meltwater generated in winter. Analysis of basal cavity dynamics suggests that cavity opening associated with flow acceleration likely accommodates only a portion of the basal meltwater, implying a residual is routed to the terminus through a poorly connected drainage system. A forcing from cavity expansion at high pressure may explain observations of winter acceleration in Western Greenland.
... Interactions between channels and cavities are often inferred from evaluating glacier flow-velocity variations in response to meltwater supply variability. High and sustained water supply over monthly timescales (e.g. during the peak melt season) has been linked to glacier deceleration (Bartholomew et al., 2010;Sole et al., 2013;Tedstone et al., 2013Tedstone et al., , 2015. This behavior is related to the fact that channels-development increases the drainage system capacity and is, therefore, expected to reduce the average basal water pressure (Fountain, 1994). ...
... This behavior is related to the fact that channels-development increases the drainage system capacity and is, therefore, expected to reduce the average basal water pressure (Fountain, 1994). On the contrary, during short term water supply increase (e.g. at the early melt season or at diurnal scales), glacier velocity changes have been observed to occur concomitantly with water supply changes (Palmer et al., 2011;Sole et al., 2013;Vincent and Moreau, 2016). This behavior is mostly related to the pressurization of the cavity-system, causing average basal water pressure rise and subsequent basal sliding speed increase (e.g. ...
Thesis
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The way in which water flows in the subglacial environment exerts a major control on ice-bed mechanical coupling, which strongly defines glacier sliding speeds. Today our understanding on the physics of the subglacial hydrology network is limited because of the scarcity of field measurements that yield a partial representation of the heterogeneous subglacial environment. The aim of my PhD work is to use passive seismology to help overcome common observational difficulties and quantify the evolution of the subglacial hydrology network pressure conditions and its configuration. Recent works show that subglacial turbulent water flow generates seismic noise that can be related to the associated hydrodynamics properties. These analyses were conducted over a limited period of time making it unclear whether such approach is appropriate to investigate seasonal and diurnal timescales, I.e. when subglacial water flow influences the most glacier dynamics. In addition, previous studies did not consider spatial changes in the heterogeneous drainage system, and until now, almost no study has located seismic noise sources spatially scattered and temporally varying. In this PhD work I address those seismological-challenges in order to resolve the subglacial hydrology dynamics in time and space.We acquired a 2-year long continuous dataset of subglacial-water-flow-induced seismic power as well as in-situ measured glacier basal sliding speed and subglacial water discharge from the Glacier d'Argentière (French Alps). I show that a careful investigation of the seismic power within [3-7] Hz can characterize the subglacial water flow hydrodynamics from seasonal to hourly timescales and across a wide range of water discharge (from 0.25 to 10 m3/sec). Combining such observations with adequate physical frameworks, I then inverted the associated hydraulic pressure gradient and hydraulic radii. I observed that the seasonal dynamics of subglacial channels is characterized by two distinct regimes. At low discharge, channels behave at equilibrium and accommodate variations in discharge mainly through changes in hydraulic radius. At a high discharge rate and with pronounced diurnal water-supply variability, channels behave out of equilibrium and undergo strong changes in the hydraulic pressure gradient, which may help sustain high water pressure in cavities and favor high glacier sliding speed over the summer.We then conducted a one-month long dense seismic-array experiment supplemented by glacier ice-thickness and surface velocity measurements. Using this unique dataset, I developed a novel methodology to overcome the challenge of locating seismic noise sources spatially scattered and temporally varying. Doing so, I successfully retrieve the first two-dimensional map of the subglacial drainage system as well as its day-to-day evolution. Using this map, I characterize when and where the subglacial drainage system is distributed through connected cavities, which favour rapid glacier flow versus localized through a channelized system that prevents rapid glacier flow. In addition, I also use high frequency seismic ground motion amplitude to study glacier features such as crevasses, thickness or ice anisotropy in a complementary way to what is traditionally done with seismic phase analysis.The first outcome of this cross-boundary PhD work is that one can analyse passive seismic measurements to retrieve the temporal evolution of subglacial channels pressure and geometry conditions over a complete melt-season. The second is that dense seismic array measurements can be used to resolve the subglacial drainage system spatial configuration and observe the switch from distributed to localized subglacial water flow. Such advances open the way for studying similar subglacial process on different sites and in particular in Greenland and Antarctica. This also concerns numerous sub-surface environment that host similar process such as volcanoes, karst, and landslides.
... Tedstone et al., 2014) and numerical modelling (e.g. Werder et al., 2013), suggest that large and / or rapid meltwater inputs can cause spikes in conduit water pressure (Cowton et al., 2013). ...
... While this may be partially attributed to the resistant bed of the shield (Clayton et al., 1985;Kamb, 1987;Stokes and Clark, 2003a, b), we also suggest that efficient evacuation of meltwater through the dense channelised network that developed in this region during the final stages of deglaciation, as the climate warmed ( Storrar et al., 2014b), would have inhibited the development of fast flow and 750 potentially contributed to the shut-down of existing ice streams ( Lelandais et al., 2018). This is consistent with modern observations that link decadal-scale ice-flow decelerations with more pervasive and efficient drainage channelisation driven by increased surface meltwater inputs to the bed ( Sole et al., 2013;Tedstone et al., 2014; van de Wal et al., 2015;Davison et al., 2019). We therefore hypothesise that this large-755 scale inverse relationship between drainage channelisation and ice streaming will exist in other palaeo-ice sheet settings. ...
Preprint
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Abstract. We identify and map traces of subglacial meltwater drainage around the former Keewatin Ice Divide, Canada from ArcticDEM data. Meltwater tracks, tunnel valleys and esker splays exhibit several key similarities, including width, spacing, their association with eskers and transitions to and from different types, which together suggest they form part of an integrated drainage signature. We collectively term these features &apos;meltwater corridors&apos; and propose a new model for their formation, based on observations from contemporary ice masses, of pressure fluctuations surrounding a central conduit. We suggest that eskers record the imprint of a central conduit and meltwater corridors the interaction with the surrounding distributed drainage system. The widespread aerial coverage of meltwater corridors (5–36 % of the bed) provides constraints on the extent of basal uncoupling induced by basal water pressure fluctuations and variations in spatial distribution and evolution of the subglacial drainage system, which will modulate the ice dynamic response.
... Hubbard et al., 1995). However, the impact of meltwater-driven ice acceleration has an uncertain impact on net annual ice displacement, particularly in the upper ablation area of the GrIS (Sundal et al., 2011;Sole et al., 2013;van de Wal et al., 2015;Doyle et al., 2014). ...
... The consistently short transit times and low dispersion of all traces from #3 onward show that the entire flow path between Moulin L41A and the proglacial river remained efficient, consistent with gas and dye tracing of several other moulins in previous melt seasons (Chandler et al., 2013) and supported by GPS observations of surface velocity (e.g. Sole et al., 2013). Efficient drainage development within 2-3 weeks of surface-bed connection is currently underestimated in hydrological models, including those that do eventually develop efficient drainage under similar conditions (e.g. ...
Article
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Intensive study of the Greenland Ice Sheet's (GrIS) subglacial drainage has been motivated by its importance for ice dynamics and for nutrient/sediment export to coastal ecosystems. This has revealed consistent seasonal development of efficient subglacial drainage in the lower ablation area. While some hydrological models show qualitative agreement with field data, conflicting evidence (both field- and model-based) maintains uncertainty in the extent and rate of efficient drainage development under thick (∼1 km) ice. Here, we present the first simultaneous time series of directly-observed subglacial drainage evolution, supraglacial hydrology and ice dynamics over 11 weeks in a large GrIS catchment. We demonstrate development of a fast/efficient subglacial drainage system extending from the margin to beneath ice >900 m thick, which then persisted with little response to highly variable moulin inputs including extreme melt events and extended periods (2 weeks) of low melt input. This efficient system evolved within ∼3 weeks at a moulin initiated when a fracture intersected a supraglacial river (rather than hydrofracture and lake drainage). Ice flow response to surface melt inputs at this site follows a pattern commonly observed in the lower GrIS ablation area, and by assuming a strong relationship between ice dynamics and subglacial hydrology, we infer that efficient subglacial drainage evolution is widespread under 900 m-thick ice in west Greenland. This time series of tracer transit characteristics through a developing and then persistent efficient drainage system provides a unique data set with which to validate and constrain existing numerical drainage system models, extending their capability for simulating drainage system evolution under current and future conditions.
... The changes in the subglacial amount of water, with all ensuing consequences in the basal environment, are considered to be the key factor behind the summer speedup of greenlandic land-terminating glaciers (Davison et al., 2019;Nienow et al., 2017). Indeed, speedups were observed with in-situ (Bartholomew et al., 2012;Sole et al., 2013;Tedstone et al., 2013;Zwally et al., 2002) and remote (Fitzpatrick et al., 2013;Joughin et al., 2008a;Lemos et al., 2018b;Sundal et al., 2011) methods across the majority of the land-terminating sectors. All altitudes at which melting occurs are affected, even far upstream from the equilibrium line altitude, where the melting rate is moderate . ...
... All altitudes at which melting occurs are affected, even far upstream from the equilibrium line altitude, where the melting rate is moderate . The link between the surface runoff and water pressure changes was proven by in-situ measurements of water pressure in boreholes Van De Wal et al., 2015;Wright et al., 2016) and melt condition observations (Bartholomew et al., 2012;Sole et al., 2013;Van De Wal et al., 2015). When considering marineterminating glaciers, this mechanism is also assumed to be an important driver of the annual velocity fluctuations on many glaciers Lemos et al., 2018a;Luckman and Murray, 2005;Moon et al., 2015;Vijay et al., 2019), but the relation is more complex to establish as other drivers are at play as explained previously. ...
Thesis
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The recent changes of outlet glaciers flow speed have vast control of the undergoing mass loss of the Greenland ice sheet. The processes driving the flow variability on different time scales, as well as the associated consequences and feedbacks, are not yet entirely understood. This is partly because the lack of frequent, precise, and large-scale observations limits the development of the numerical models. It is particularly difficult to resolve seasonal speed fluctuations, yet it is crucial to better constrain the physical processes controlling the ice flow.This thesis aims to address (i) the difficulties that exist in establishing robust seasonal time-series of Greenland glacier surface velocities from satellite observations, and (ii) the use of these time-series in numerical models for better understanding of the flow drivers.Satellites are able to cover large areas in a relatively short time and uniform way. Continuous time-series with seasonal temporal resolution have only started to be used recently, due to the limited number of image acquisitions made previously. Nevertheless, the time-series of ice surface velocity derived from individual sensors remain temporally incomplete and relatively noisy. Taking together three suitable satellites (Landsat-8, Sentinel-2, and Sentinel-1) across three case study sites in Greenland (Russell sector, Upernavik Isstrøm and Petermann Gletscher), we demonstrate that it is possible to obtain continuous year-around time-series only by combining results from multiple satellites. It is also shown here that by applying post-processing based on the data redundancy to such multi-sensor datasets, we are able to achieve persistent tracking of ice surface motion with a temporal resolution of about 2 weeks and mean accuracy of about 10 m/yr. With such parameters, we can resolve the seasonal variability of greenlandic glaciers where previous studies had limited success.Elaboration of reliable numerical models which would correctly represent the ice flow processes requires suitable observations for the calibration and validation. In the land-terminating sector around Russell Gletscher, we explore the ability of an existing numerical modelling method to use advantageously the obtained high-frequency satellite-derived maps of surface velocity to infer seasonal variations in subglacial conditions. It is widely recognized that they exert a major control on the flow variability, however, despite recent theoretical and modelling developments, constraining the processes in situ remains a key question in Glaciology. By applying the inverse control method implemented in Elmer/Ice on biweekly velocity maps, we estimate the year-around evolution of glacier basal sliding speed, basal traction, and subglacial water pressure with an unprecedented spatial and temporal resolution. Our analysis shows that such results can be successfully used to reveal the functioning of the subglacial environment over different timescales and its influence on glacier speed. These results also could serve as an intermediate validation for more complex ice-flow/subglacial-hydrology coupled models.
... Both observational (e.g., Andrews et al., 2014) and modeling (e.g., Schoof, 2010) studies have established a close association between the speed changes and evolving subglacial hydrologic conditions forced by surface melt. Despite little to no surface melt in the winter months, this period is when the vast majority of the overall ice displacement occurs (Sole et al., 2013) due to continuous motion (albeit relatively slow) over about two-thirds of the year. A steady increase in speed over the course of the winter has also been reported for certain locations of the western Greenland Ice Sheet with satellite (e.g., Fitzpatrick et al., 2013;Joughin et al., 2008Joughin et al., , 2010Moon et al., 2014) and ground-based (e.g., Stevens et al., 2016;van de Wal et al., 2015) observations. ...
... We see no way to link our observations of winter acceleration to a steady increase in driving stress because the speed-up is widespread and irrespective of ice thickness gradients. Earlier works have appealed to pressurization of the basal drainage system over the course of winter as a means for decreasing basal traction, manifested conceptually through either isolation of cavities as the drainage system shuts down (e.g., Fitzpatrick et al., 2013) or widespread re-pressurization of the bed in response to closure of low-pressure channels (e.g., Sole et al., 2013). Our observations suggest that pressurization occurs over a few weeks in autumn (Figs. 3 and 5), but then with no monotonic increase in the basal water pressure over the winter period (Fig. 5), a finding in agreement with observations from elsewhere in Greenland (Ryser et al., 2014a). ...
Article
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Basal sliding in the ablation zone of the Greenland Ice Sheet is closely associated with water from surface melt introduced to the bed in summer, yet melting of basal ice also generates subglacial water year-round. Assessments of basal melt rely on modeling with results strongly dependent upon assumptions with poor observational constraints. Here we use surface and borehole measurements to investigate the generation and fate of basal meltwater in the ablation zone of Isunnguata Sermia basin, western Greenland. The observational data are used to constrain estimates of the heat and water balances, providing insights into subglacial hydrology during the winter months when surface melt is minimal or nonexistent. Despite relatively slow ice flow speeds during winter, the basal meltwater generation from sliding friction remains manyfold greater than that due to geothermal heat flux. A steady acceleration of ice flow over the winter period at our borehole sites can cause the rate of basal water generation to increase by up to 20 %. Borehole measurements show high but steady basal water pressure rather than monotonically increasing pressure. Ice and groundwater sinks for water do not likely have sufficient capacity to accommodate the meltwater generated in winter. Analysis of basal cavity dynamics suggests that cavity opening associated with flow acceleration likely accommodates only a portion of the basal meltwater, implying that a residual is routed to the terminus through a poorly connected drainage system. A forcing from cavity expansion at high pressure may explain observations of winter acceleration in western Greenland.
... It is certainly understood that many mountain glaciers speed up and slow down throughout the year (Burgess et al., 2013;Armstrong et al., 2017;Yasuda and Furuya, 2015;Kraaijenbrink et al., 2016) and that some of Greenland's glaciers respond to seasonal cycles of subglacial hydrology or calving dynamics (Joughin et al., 2008;Howat et al., 2010;Bartholomew et al., 2010;Sole et al., 2013;Moon et al., 2015;King et al., 2018). Seasonal variability has even been reported in a few studies of Antarctic glaciers (Nakamura et al., 2010;Zhou et al., 2014;Greene et al., 2018), but to date, no global-scale mapping of seasonal dynamics of the world's ice has been completed, due in part to the technical challenge of working with optical data in polar regions, where the surface is not touched by sunlight for monthslong periods each winter. ...
Article
Full-text available
Fully understanding how glaciers respond to environmental change will require new methods to help us identify the onset of ice acceleration events and observe how dynamic signals propagate within glaciers. In particular, observations of ice dynamics on seasonal timescales may offer insights into how a glacier interacts with various forcing mechanisms throughout the year. The task of generating continuous ice velocity time series that resolve seasonal variability is made difficult by a spotty satellite record that contains no optical observations during dark, polar winters. Furthermore, velocities obtained by feature tracking are marked by high noise when image pairs are separated by short time intervals and contain no direct insights into variability that occurs between images separated by long time intervals. In this paper, we describe a method of analyzing optical- or radar-derived feature-tracked velocities to characterize the magnitude and timing of seasonal ice dynamic variability. Our method is agnostic to data gaps and is able to recover decadal average winter velocities regardless of the availability of direct observations during winter. Using characteristic image acquisition times and error distributions from Antarctic image pairs in the ITS_LIVE dataset, we generate synthetic ice velocity time series, then apply our method to recover imposed magnitudes of seasonal variability within ±1.4 m yr−1. We then validate the techniques by comparing our results to GPS data collected on Russell Glacier in Greenland. The methods presented here may be applied to better understand how ice dynamic signals propagate on seasonal timescales and what mechanisms control the flow of the world’s ice.
... Answering these questions is crucial to predicting the behavior of glaciers and ice sheets in the future. Several studies in Greenland suggest that climate warming and enhanced surface melting could increase the sliding and enhance mass loss (Parizek & Alley, 2004;Zwally et al., 2002) while other studies suggest that an increase of melting could generate a more efficient subglacial drainage system draining large discharges in discrete channels, which could lead to a limited, or even reduced, effect on seasonally averaged sliding and the long-term dynamic response to a warming climate (Kamb, 1987;Pimentel & Flowers, 2010;Schoof, 2010;Sole et al., 2013;Sundal et al., 2011;Tedstone et al., 2015;Truffer et al., 2005;van de Wal et al., 2008). Gimbert Gilbert, Florent Gimbert, Olivier Gagliardini Numerous studies highlight the paucity of field observations to investigate the relationships between the basal sliding and water storage or subglacial water pressure. ...
... of the bed, and their gradual drainage over time is hypothesised to reduce regional basal water pressure, thereby increasing ice-bed contact and reducing ice velocity (e.g. Sole et al., 2013;Andrews et al., 2014;Bougamont et al., 2014;Tedstone et al., 2015;Hoffman et al., 2016). ...
Article
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We identify and map visible traces of subglacial meltwater drainage around the former Keewatin Ice Divide, Canada, from high-resolution Arctic Digital Elevation Model (ArcticDEM) data. We find similarities in the characteristics and spatial locations of landforms traditionally treated separately (i.e. meltwater channels, meltwater tracks and eskers) and propose that creating an integrated map of meltwa-ter routes captures a more holistic picture of the large-scale drainage in this area. We propose the grouping of meltwater channels and meltwater tracks under the term meltwater corridor and suggest that these features in the order of 10s-100s m wide, commonly surrounding eskers and transitioning along flow between different types, represent the interaction between a central conduit (the esker) and surrounding hydraulically connected distributed drainage system (the meltwater corridor). Our proposed model is based on contemporary observations and modelling which suggest that connections between conduits and the surrounding distributed drainage system within the ablation zone occur as a result of overpressurisation of the conduit. The widespread aerial coverage of meltwater corridors (5 %-36 % of the bed) provides constraints on the extent of basal uncoupling induced by basal water pressure fluctuations. Geomorphic work resulting from repeated connection to the surrounding hydraulically connected distributed drainage system suggests that basal sediment can be widely accessed and evacuated by meltwater.
... The current poor understanding of the interannual evolution of glacier sliding (e.g. Van de Wal et al., 2008;Sole et al., 2013) precludes us from applying a temporally evolving basal friction. The model employs a crevasse-depth calving criterion based on crevasses opening by longitudinal acceleration towards the calving front. ...
Article
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Marine outlet glaciers on Greenland are retreating, yet it is unclear if the recent fast retreat will persist, and how atmosphere and ocean warming will impact future retreat. We show how a marine outlet glacier in Hardangerfjorden retreated rapidly in response to the abrupt warming following the Younger Dryas cold period (approximately 11,600 years before present). This almost 1000 m deep fjord, with several sills at 300–500 m depth, hosted a 175 km long outlet glacier at the western rim of the Scandinavian Ice Sheet. We use a dynamic ice-flow model constrained by well-dated terminal and lateral moraines to simulate the reconstructed 500-year retreat of Hardangerfjorden glacier. The model includes an idealized oceanic and atmospheric forcing based on reconstructions, but excludes the surface mass balance-elevation feedback. Our simulations show a highly episodic retreat driven by surface melt and warming fjord waters, paced by the fjord bathymetry. Warming air and ocean temperatures by 4–5 °C during the period of retreat result in a 125-km retreat of Hardangerfjorden glacier in 500 years. Retreat rates throughout the deglaciation vary by an order of magnitude from 50 to 2500 m a−1, generally close to 200 m a−1, punctuated by brief events of swift retreat exceeding 500 m a−1, each event lasting a few decades. We show that the fastest retreat rates occur in regions of the bed with the largest retrograde slopes; ice shelf length and fjord water depth is less important. Our results have implications for modern glacial fjord settings similar to Hardangerfjorden, where high retreat rates have been observed. Our findings imply that increasing air temperatures and warming subsurface waters in Greenland fjords will continue to drive extensive retreat of marine outlet glaciers. However, the recent high retreat rates are not expected to be sustained for longer than a few decades due to constraints by the fjord bathymetry.
... It is certainly understood that many mountain glaciers speed up and slow down throughout the year (Burgess et al., 2013;Armstrong et al., 2017;Yasuda and Furuya, 2015;Kraaijenbrink et al., 2016), and that some of Greenland's glaciers respond to seasonal cycles of subglacial hydrology or calving dynamics (Joughin et al., 2008;Howat et al., 2010;Bartholomew et al., 2010;Sole et al., 2013;Moon et al., 2015;King et al., 2018). Seasonal variability has even been reported in a few studies 35 of Antarctic glaciers (Nakamura et al., 2010;Zhou et al., 2014;Greene et al., 2018); but to date, no global-scale mapping of seasonal dynamics of the world's ice has been completed, due in part to the logistical challenge of working with optical data in polar regions, where the surface is not touched by sunlight for months-long periods each winter. ...
Preprint
Full-text available
Fully understanding how glaciers respond to environmental change will require new methods to help us identify the onset of ice acceleration events and observe how dynamic signals propagate within glaciers. In particular, observations of ice dynamics on seasonal timescales may offer insights into how a glacier interacts with various forcing mechanisms throughout the year. The task of generating continuous ice velocity time series that resolve seasonal variability is made difficult by the finite integration time over which ice velocities are measured from optical and repeat SAR imagery, and by a spotty satellite record that contains no optical observations throughout dark, polar winters. In this paper, we describe a method of analyzing feature-tracked velocities to characterize the magnitude and timing of seasonal ice dynamic variability. Our method is agnostic to data gaps and is able to recover climatological average winter velocities regardless of the availability of direct observations during winter. Using characteristic image acquisition times and error distributions from Antarctic image pairs in the ITS_LIVE dataset, we generate synthetic ice velocity time series, then apply our method to recover imposed magnitudes of seasonal variability within ±1.4 m yr−1. We then validate the techniques by comparing our results to GPS data collected on Russell Glacier in Greenland. The methods presented here may be applied to better understand how ice dynamic signals propagate on seasonal timescales, and what mechanisms control the flow of the world’s ice.
... of the bed, and their gradual drainage over time is hypothesised to reduce regional basal water pressure, thereby increasing ice-bed contact and reducing ice velocity (e.g. Sole et al., 2013;Andrews et al., 2014;Bougamont et al., 2014;Tedstone et al., 2015;Hoffman et al., 2016). ...
Article
We identify and map traces of subglacial meltwater drainage around the former Keewatin Ice Divide, Canada from ArcticDEM data. Meltwater tracks, tunnel valleys and esker splays exhibit several key similarities, including width, spacing, their association with eskers and transitions to and from different types, which together suggest they form part of an integrated drainage signature. We collectively term these features 'meltwater corridors' and propose a new model for their formation, based on observations from contemporary ice masses, of pressure fluctuations surrounding a central conduit. We suggest that eskers record the imprint of a central conduit and meltwater corridors the interaction with the surrounding distributed drainage system. The widespread aerial coverage of meltwater corridors (5–36 % of the bed) provides constraints on the extent of basal uncoupling induced by basal water pressure fluctuations and variations in spatial distribution and evolution of the subglacial drainage system, which will modulate the ice dynamic response.
... Carbon and nutrients are released when glaciers melt and exert an important, yet poorly quantified influence on downstream ecosystems. Glacial carbon and nutrient export is mainly derived from geochemical interactions and microbial activity in glacial environments that are connected by a seasonally evolving hydrological system (Bartholomew et al., 2010(Bartholomew et al., , 2011Sole et al., 2013;Chandler et al., 2013;Cowton et al., 2013;Chu, 2014). ...
Article
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In recent decades, the land-ocean aquatic continuum, commonly defined as the interface, or transition zone, between terrestrial ecosystems and the open ocean, has undergone dramatic changes. On-going work has stressed the importance of treating Aquatic Critical Zones (ACZs) as a sensitive system needing intensive investigation. Here, we discuss fjords as an ACZ in the context of sedimentological, geochemical, and climatic impacts. These diverse physical features of fjords are key in controlling the sources, transport, and burial of organic matter in the modern era and over the Holocene. High sediment accumulation rates in fjord sediments allow for high-resolution records of past climate and environmental change where multiple proxies can be applied to fjord sediments that focus on either marine or terrestrial-derived components. Humans through land-use change and climatic stressors are having an impact on the larger carbon stores in fjords. Sediment delivery whether from accelerating erosion (e.g. mining, deforestation, road building, agriculture) or from sequestration of fluvial sediment behind dams has been seriously altered in the Anthropocene. Climate change affecting rainfall and river discharge into fjords will impact the thickness and extent of the low-salinity layer in the upper reaches of the fjord, slowing the rate of the overturning circulation and deep-water renewal – thereby impacting bottom water oxygen concentrations.
... If there are systematic changes in moulin storage volumes with distance from the margin, it would have important implications for understanding GrIS ice velocity response as future warming pushes melt (Bevis et al., 2019;Doyle et al., 2015), and likely moulin drainage ( that are further from the ice margin. For example, while both models (de Fleurian et al., 2016;Schoof, 2010;Shannon et al., 2013) and field data (Chandler et al., 2013;Sole et al., 2013;Sundal et al., 2011;Tedstone et al., 2015;van de Wal et al., 2015) suggest that increases in meltwater have minimal net effect on ice motion within the lower ablation zone of the GrIS, a decrease in pressure variability with distance from the margin, driven by systematic increases in moulin volume, could reduce the capability of channelization, or dewatering of distributed flow systems, to moderate meltwater impacts to ice motion further inland. ...
Article
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Plain Language Summary Each summer season on the Greenland Ice Sheet, meltwater forms stream networks on the ice surface that deliver water to moulins, which are holes in the ice that carry the water to the base of the ice sheet. When water backs up into moulins, and the water pressure beneath the ice increases, glacier sliding accelerates, leading to more rapid loss of ice into the ocean. We directly explored two moulins that contained immense storage volumes that are much larger than previously assumed to exist. Our observations of water levels inside moulins, and a model of water flow through the ice, indicate that storage of water within these large moulins during daily meltwater pulses has a big impact on how much the water pressure beneath the ice changes. Our work suggests that moulin sizes influence the interactions between summer melt and sliding of the Greenland Ice Sheet. Consequently, we need a more complete understanding of how moulin volumes vary in order to better predict how future increases in melt will impact the rate of ice loss from Greenland and to constrain its future contribution to sea level rise.
... It is not uncommon that sliding and substrate deformation dominates the total movement of the ice. For instance, Hooke et al. (1997) estimated that the sliding speed at Storglaciären (valley glacier in NW Sweden) accounts for over 85% of the total (surface) velocity, with a similar value given for a land terminating part of the Greenland Ice Sheet (Sole et al., 2013). In general these processes, summed up by the sliding velocity (to be solved for), become a part of the boundary condition necessary to close and solve the partial differential equations. ...
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The basal sliding of glaciers and ice sheets can constitute a large part of the total observed ice velocity, in particular in dynamically active areas. It is therefore important to accurately represent this process in numerical models. The condition that the sliding velocity should be tangential to the bed is realized by imposing an impenetrability condition at the base. We study the, in glaciological literature used, numerical implementations of the impenetrability condition for non-linear Stokes flow with Navier's slip on the boundary. Using the finite element method, we enforce impenetrability by: a local rotation of the coordinate system (strong method), a Lagrange multiplier method enforcing zero average flow across each facet (weak method) and an approximative method that uses the pressure variable as a Lagrange multiplier for both incompressibility and impenetrability. An analysis of the latter shows that it relaxes the incompressibility constraint, but enforces impenetrability approximately if the pressure is close to the normal component of the stress at the bed. Comparing the methods numerically using a method of manufactured solutions unexpectedly leads to similar convergence results. However, we find that, for more realistic cases, in areas of high sliding or varying topography the velocity field simulated by the approximative method differs from that of the other methods by $\sim 1\%$ (two-dimensional flow) and $> 5\%$ when compared to the strong method (three-dimensional flow). In this study the strong method, which is the most commonly used in numerical ice sheet models, emerges as the preferred method due to its stable properties (compared to the weak method in three dimensions) and ability to well enforce the impenetrability condition.
... 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.
... One area of particular concern is the subglacial hydrology of these tidewater glaciers. Whilst there have been many studies focusing on the subglacial hydrology of landterminating portions of the GrIS and its complex effect on the flow of the overlying ice (Chandler et al., 2013;de Fleurian et al., 2016;Christoffersen et al., 2018;Gagliardini and Werder, 2018;Meierbachtol et al., 2013;Sole et al., 2013;Tedstone et al., 2013Tedstone et al., , 2015van de Wal et al., 2015), the hydrology of tidewater glaciers has received much less attention (e.g. Schild et al., 2016;Sole et al., 2011;Vallot et al., 2017), owing to the greater difficulty of gathering observations in the fast-flowing marine-terminating environment. ...
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We investigate the subglacial hydrology of Store Glacier in West Greenland, using the open-source, full-Stokes model Elmer/Ice in a novel 3D application that includes a distributed water sheet, as well as discrete channelised drainage, and a 1D model to simulate submarine plumes at the calving front. At first, we produce a baseline winter scenario with no surface meltwater. We then investigate the hydrological system during summer, focussing specifically on 2012 and 2017, which provide examples of high and low surface-meltwater inputs, respectively. We show that the common assumption of zero winter freshwater flux is invalid, and we find channels over 1 m2 in area occurring up to 5 km inland in winter. We also find that the production of water from friction and geothermal heat is sufficiently high to drive year-round plume activity, with ice-front melting averaging 0.15 m d−1. When the model is forced with seasonally averaged surface melt from summer, we show a hydrological system with significant distributed sheet activity extending 65 and 45 km inland in 2012 and 2017, respectively; while channels with a cross-sectional area higher than 1 m2 form as far as 55 and 30 km inland. Using daily values for the surface melt as forcing, we find only a weak relationship between the input of surface meltwater and the intensity of plume melting at the calving front, whereas there is a strong correlation between surface-meltwater peaks and basal water pressures. The former shows that storage of water on multiple timescales within the subglacial drainage system plays an important role in modulating subglacial discharge. The latter shows that high melt inputs can drive high basal water pressures even when the channelised network grows larger. This has implications for the future velocity and mass loss of Store Glacier, and the consequent sea-level rise, in a warming world.
... Deviation from a Weertman-type scaling is also expected where melt water throughput rates are sufficient for channels to form near input locations and thus channelized drainage exerts primary control on the overall effective pressure field (Röthlisberger, 1972;Schoof, 2010). Observations on mountain glaciers (Lambrecht et al., 2014;Mair et al., 2003;Nienow et al., 1998;Scherler & Strecker, 2012) and from land-terminating regions of Greenland (Bartholomew et al., 2010;Chandler et al., 2013;Sole et al., 2013;Tedstone et al., 2013;Zwally et al., 2002) suggest that channelized drainage may modulate effective pressure during late summer. However, this only occurs during a small fraction of the year corresponding to a few months when melt rates are high and channels have had sufficient time to fully develop. ...
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Plain Language Summary Basal sliding is an important component of glacier motion. However, our knowledge of the physics that controls basal sliding is incomplete. This causes large uncertainties in the contribution to sea‐level rise predicted for ice sheets over the coming century. Here, we test our understanding of basal sliding against particularly unique observations, made via a rotating bicycle wheel that has been continuously measuring glacier basal motion over three decades within excavated tunnels under the Argentière Glacier in the French Alps. Due to stress changes from significant glacier thinning over the multi‐decadal period we are able to establish an observationally derived sliding law and compare it with expectations from theory. We report many observational features that are in striking agreement with theoretical predictions from glacier sliding over bedrock beds. However, we also observe an undocumented behavior of stress stabilization during the melting period at a specific stress state known as the Iken's limit. This behavior causes long term sliding velocities to follow a simple power law scaling with bed shear stress. This finding has the potential of strongly simplifying and reducing uncertainty on predicting glaciers response to climate change.
... Where cavities are hydraulically well connected to channels they drain into the efficient drainage system, which tends to lower the overall basal water pressure (13). In addition, recent observations (13) suggested that hydraulically isolated areas of the bed with very low permeability (14) regulate glacier basal traction during winter (15) and over multiannual timescales (16). The spatial persistence of high water pressure at the glacier bed (17) thus depends on the subglacial drainage system configuration and the hydraulic connectivity across the cavities and from the cavities to the channels. ...
Article
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Subglacial water flow strongly modulates glacier basal motion, which itself strongly influences the contributions of glaciers and ice sheets to sea level rise. However, our understanding of when and where subglacial water flow enhances or impedes glacier flow is limited due to the paucity of direct observations of subglacial drainage characteristics. Here, we demonstrate that dense seismic array observations combined with an innovative systematic seismic source location technique allows the retrieval of a two-dimensional map of a subglacial drainage system, as well as its day-to-day temporal evolution. We observe with unprecedented detail when and where subglacial water flows through a cavity-like system that enhances glacier flow versus when and where water mainly flows through a channel-like system that impedes glacier flow. Most importantly, we are able to identify regions of high hydraulic connectivity within and across the cavity and channel systems, which have been identified as having a major impact on the long-term glacier response to climate warming. Applying a similar seismic monitoring strategy in other glacier settings, including for ice sheets, may help to diagnose the susceptibility of their dynamics to increased meltwater input due to climate warming.
... Since the advent of satellite records in the 1970s, supraglacial lakes have formed in greater numbers, at higher elevations, and at larger sizes in response to warmer summers (16)(17)(18). However, the net effect of an increasing meltwater supply on the dynamics of the Greenland Ice Sheet is the subject of ongoing debate (7,8,(19)(20)(21). ...
Article
Supraglacial lake drainage events influence Greenland Ice Sheet dynamics on hourly to interannual timescales. However, direct observations are rare, and, to date, no in situ studies exist from fast-flowing sectors of the ice sheet. Here, we present observations of a rapid lake drainage event at Store Glacier, west Greenland, in 2018. The drainage event transported 4.8 × 10 ⁶ m ³ of meltwater to the glacier bed in ∼5 h, reducing the lake to a third of its original volume. During drainage, the local ice surface rose by 0.55 m, and surface velocity increased from 2.0 m⋅d ⁻¹ to 5.3 m⋅d ⁻¹ . Dynamic responses were greatest ∼4 km downstream from the lake, which we interpret as an area of transient water storage constrained by basal topography. Drainage initiated, without any precursory trigger, when the lake expanded and reactivated a preexisting fracture that had been responsible for a drainage event 1 y earlier. Since formation, this fracture had advected ∼500 m from the lake’s deepest point, meaning the lake did not fully drain. Partial drainage events have previously been assumed to occur slowly via lake overtopping, with a comparatively small dynamic influence. In contrast, our findings show that partial drainage events can be caused by hydrofracture, producing new hydrological connections that continue to concentrate the supply of surface meltwater to the bed of the ice sheet throughout the melt season. Our findings therefore indicate that the quantity and resultant dynamic influence of rapid lake drainages are likely being underestimated.
... Direct measurements of water pressure along a subglacial flow path showed that large influxes of meltwater from lake drainages can drain isolated cavities and slow sliding speeds without increasing the drainage system's efficiency. Building upon previous studies (Andrews et al., 2014;Hoffman et al., 2016), our results suggest that inland from the GrIS's margin, the efficient drainage system's ability to readily adjust its hydraulic capacity in response to meltwater inputs may have been overemphasized in the literature (e.g., Sole et al., 2013). Therefore, we caution against attributing ice deceleration to increased channelization without direct hydrologic measurements. ...
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Plain Language Summary Meltwater produced on the surface of the Greenland Ice Sheet reaches the bed by flowing into crevasses or moulins, vertical holes that connect to the ice sheet's base. Early in the summer, meltwater that reaches the bed increases water pressures within the drainage system underneath the ice sheet, increasing sliding speeds. However, later in the summer, ice sliding speeds often slowdown despite continued meltwater inputs. While these slowdowns have been attributed to the growth of subglacial conduits, recent observations suggest the drainage of hydraulically isolated cavities—pockets of water formed by ice sliding over bedrock bumps—may instead be responsible. Here, we measure surface ice motion and water pressures within moulins located several kilometers away from rapidly draining supraglacial lakes. We show the passing of a floodwave underneath the ice sheet slowed sliding to wintertime speeds without enlarging subglacial channels connected to our instrumented moulin. Instead, our results indicate the drainage of isolated cavities may be responsible for slowdowns that occur during the melt season. Accordingly, our results, similar to others, suggest increased channelization of the subglacial drainage system appears unlikely to buffer GrIS ice velocity against future meltwater inputs.
... drainage leads to increased basal water pressure and accelerated ice flow, while fast, channelized drainage encourages decelerated ice flow due to capture of porewater (or water within basal cavities) and subsequent reduction of water pressure in the neighboring region (Hoffman et al., 2016;Sole et al., 2013;Tedstone et al., 2015). However, this simple association is challenged (e.g., Gulley, Grabiec, et al., 2012;Gulley, Walthard, et al., 2012) by nonlinear and spatially heterogeneous ice-flow responses (e.g., Cowton et al., 2013;Minchew et al., 2016;Siegfried et al., 2016). ...
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Realistic characterization of subglacial hydrology necessitates knowledge of the range in form, scale, and spatiotemporal evolution of drainage networks. A relict subglacial meltwater corridor on the deglaciated Antarctic continental shelf encompasses 80 convergent and divergent channels, many of which are hundreds of meters wide and several of which lack a definable headwater source. Without significant surface‐melt contributions to the bed like similarly described landforms in the Northern Hemisphere, channelized drainage capacity varies non‐systematically by three orders of magnitude downstream. This signifies apparent additions and losses of basal water to the bed‐channelized system that relates to bed topography. Larger magnitude grounding‐line retreat events occurred while the channel system was active than once channelized drainage had ceased. Overall, this corridor demonstrates that meltwater drainage styles co‐exist in time and space in response to bed topography, with prolonged impacts on grounding‐line behavior.
... As the season progress the transition to the channelized system occurs and triggers a lowering of both the water pressure and the basal sliding. The Greenland Ice Sheet (GrIS) has been the place for numerous studies suggesting that increased surface meltwater in a warming climate can enhance the dynamic mass loss of grounded ice (e.g., Zwally et al., 2002;Parizek and Alley, 2004;Palmer et al., 2011;Sole et al., 2013;Doyle et al., 2014). As this trend 1. Introduction is generally agreed, the amplitude of this feedback is still debated and other studies support a limited response of the ice sheet (Joughin et al., 2008;Tedstone et al., 2013) and even an inverse effect, i.e. an annual increase of surface melt could induces a decrease of the mean annual surface velocity (Tedstone et al., 2015). ...
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.
... While daily maximum discharge varies with catchment size and day of the season, discharge decreases when melt energy drops off at night (Marston, 1983;Mernild et al., 2006;McGrath et al., 2011;Yang et al., 2018). Diurnal variability and timing of meltwater delivery to the subglacial drainage system have been shown to influence ice sheet velocities in several studies (Bartholomew et al., 2012;Sole et al., 2013;Andrews et al., 2014;Smith et al., 2021), with up to 65 % increase in ice velocity in the lower-ablation area ). Shortterm speedups occur in the lower-ablation regions of southwest Greenland (Shepherd et al., 2009), with an increase in ice velocities by up to 300 %-400 % (compared to pre-melt speeds) that lasts for a few days to a week in response to the variations in surface runoff supply . ...
Article
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Greenland ice sheet surface runoff is drained through supraglacial stream networks. This evacuation influences surface mass balance as well as ice dynamics. However, in situ observations of meltwater discharge through these stream networks are rare. In this study, we present 46 discrete discharge measurements and continuous water level measurements for 62 d spanning the majority of of the melt season (13 June to 13 August) in 2016 for a 0.6 km2 supraglacial stream catchment in southwest Greenland. The result is an unprecedentedly long record of supraglacial discharge that captures both diurnal variability and changes over the melt season. A comparison of surface energy fluxes to stream discharge reveals shortwave radiation as the primary driver of melting. However, during high-melt episodes, the contribution of shortwave radiation to melt energy is reduced by ∼40 % (from 1.13 to 0.73 proportion). Instead, the relative contribution of longwave radiation, sensible heat fluxes, and latent heat fluxes to overall melt increases by ∼24 %, 6 %, and 10 % (proportion increased from −0.32 to −0.08, 0.28 to 0.34, and −0.04 to 0.06) respectively. Our data also identify that the timing of daily maximum discharge during clear-sky days shifts from 16:00 local time (i.e., 2 h 45 min after solar noon) in late June to 14:00 in late July and then rapidly returns to 16:00 in early August. The change in the timing of daily maximum discharge could be attributed to the expansion and contraction of the stream network, caused by skin temperatures that likely fell below freezing at night. The abrupt shift, in early August, in the timing of daily maximum discharge coincides with a drop in air temperature, a drop in the amount of water temporarily stored in weathering crust, and a decreasing covariance between stream velocity and discharge. Further work is needed to investigate if these results can be transferable to larger catchments and uncover if rapid shifts in the timing of peak discharge are widespread across Greenland supraglacial streams and thus have an impact on meltwater delivery to the subglacial system and ice dynamics.
... 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.
... Answering these questions is crucial to predicting the behavior of glaciers and ice sheets in the future. Several studies in Greenland suggest that climate warming and enhanced surface melting could increase the sliding and enhance mass loss (Parizek & Alley, 2004;Zwally et al., 2002) while other studies suggest that an increase of melting could generate a more efficient subglacial drainage system draining large discharges in discrete channels, which could lead to a limited, or even reduced, effect on seasonally averaged sliding and the long-term dynamic response to a warming climate (Kamb, 1987;Pimentel & Flowers, 2010;Schoof, 2010;Sole et al., 2013;Sundal et al., 2011;Tedstone et al., 2015;Truffer et al., 2005;van de Wal et al., 2008). ...
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The hydromechanical processes by which basal water controls sliding at the glacier bed are poorly known, despite glacier basal motion being responsible for a large part of ice flux in temperate alpine glaciers. Previous studies suggest that sliding strongly relates to the quantity of water being stored at the ice‐bedrock interface. However, this water storage is difficult to quantify accurately on the basis of surface‐motion observations, given that uplift can also be affected by changes in vertical‐strain rates and sliding velocity change. Here, we use a comprehensive data set of in situ measurements performed over 2 years on the Argentière Glacier in the French Alps to investigate the relationships between horizontal and vertical velocities, basal sliding, subglacial runoff and bed separation. We observe strikingly large uplifts varying spatially between 0.20 and 0.90 m over the winter/spring seasons between January and June and with a consistent spatial pattern from 1 year to another. We show, based on observations and three dimensional ice‐flow modeling, that these large uplifts cannot be explained solely by changes in strain rates or in sliding up an inclined bed. Our results reveal that more than 80% of the observed uplift is related to enhanced bed separation through cavitation, allowing us to estimate the volume occupied by water‐filled subglacial cavities. Our interpretation of uplift being mainly caused by increased cavitation is also consistent with an associated increase in the observed surface horizontal velocity. These findings provide important observational constraints for testing subglacial hydrological models.
... Using the discharge-weighted mean DOC concentration (0.114 mg L −1 ) and the total annual discharge of 1.45 km 3 (Hatton et al., 2019), the total flux of DOC from LG over the 2015 melt season is 165,000 ± 25,000 kg C year −1 . Because interannual variability in discharge is high, we used the relationship between GrIS runoff (Bamber et al., 2018) and LG Q (Cowton et al., 2012;Hawkings et al., 2014;Sole et al., 2013) to calculate total LG Q for years when no discharge record is available (Hawkings et al., 2015); the 10-year average flux of DOC from LG between 2007 and 2016 is 180,000 ± 50,000 kg C year −1 , about 3 times lower than previously estimated for LG . However, scaling to GrIS runoff, total average annual DOC flux between 2007 and 2016 (Bamber et al., 2018) was 0.065 ± 0.010 Tg C year −1 , similar to previous estimates made for runoff from the GrIS (Bhatia et al., 2013;Hood et al., 2015). ...
Article
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The Greenland Ice Sheet is losing mass at a remarkable rate as a result of climatic warming. This mass loss coincides with the export of dissolved organic matter (DOM) in glacial meltwaters. However, little is known about how the source and composition of exported DOM changes over the melt season, which is key for understanding its fate in downstream ecosystems. Over the 2015 ablation season, we sampled the outflow of Leverett Glacier, a large land‐terminating glacier of the Greenland Ice Sheet. Dissolved organic carbon (DOC) concentrations and DOM fluorescence were analyzed to assess the evolution of DOM sources over the course of the melt season. DOC concentrations and red‐shifted fluorescence were highly associated (R² > 0.95) and suggest terrestrial inputs from overridden soils dominated DOM early season inputs before progressive dilution with increasing discharge. During the outburst period, supraglacial drainage events disrupted the subglacial drainage system and introduced dominant protein‐like fluorescence signatures not observed in basal flow. These results suggest that subglacial hydrology and changing water sources influence exported DOC concentration and DOM composition, and these sources were differentiated using fluorescence characteristics. Red‐shifted fluorescence components were robust proxies for DOC concentration. Finally, the majority of DOM flux, which occurs during the outburst and postoutburst periods, was characterized by protein‐like fluorescence from supraglacial and potentially subglacial microbial sources. As protein‐like fluorescence is linked to the bioavailability of DOM, the observed changes likely reflect seasonal variations in the impact of glacial inputs on secondary production in downstream ecosystems due to shifting hydrologic regimes.
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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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Ice speeds in Greenland are largely set by basal motion1, which is modulated by meltwater delivery to the ice base2–4. Evidence suggests that increasing melt rates enhance the subglacial drainage network’s capacity to evacuate basal water, increasing bed friction and causing the ice to slow5–10. This limits the potential of melt forcing to increase mass loss as temperatures increase11. Here we show that melt forcing has a pronounced influence on dynamics, but factors besides melt rates primarily control its impact. Using a method to examine friction variability across the entirety of western Greenland, we show that the main impact of melt forcing is an abrupt north-to-south change in bed strength that cannot be explained by changes in melt production. The southern ablation zone is weakened by 20–40 per cent compared with regions with no melt, whereas in northern Greenland the ablation zone is strengthened. We show that the weakening is consistent with persistent basal water storage and that the threshold is linked to differences in sliding and hydropotential gradients, which exert primary control on the pressures within drainage pathways that dewater the bed. These characteristics are mainly set by whether a margin is land or marine terminating, suggesting that dynamic changes that increase mass loss are likely to occur in northern Greenland as temperatures increase. Our results point to physical representations of these findings that will improve simulated ice-sheet evolution at centennial scales. An analysis of basal-friction variability across western Greenland shows melt forcing influences bed strength in opposite ways in northern and southern Greenland, establishing melt has an important role in ice-sheet evolution that is mainly dictated by whether a region is land or marine terminating.
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Water flowing below glaciers exerts a major control on glacier basal sliding. However, our knowledge of the physics of subglacial hydrology and its link with sliding is limited because of lacking observations. Here we use a 2-year-long dataset made of on-ice-measured seismic and in situ-measured glacier basal sliding speed on Glacier d'Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels' hydraulic radius and hydraulic pressure gradient. At seasonal timescales we find that hydraulic radius and hydraulic pressure gradient respectively exhibit a 2- and 6-fold increase from spring to summer, followed by comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels' behaviour at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behaviour that is consistent with channels behaving out of equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which prove that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speed, which supports that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speed) through an efficient hydraulic connection between the two systems. The present findings provide an essential basis for testing the physics represented in subglacial hydrology and glacier sliding models.
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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.
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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.
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Surface speeds in Greenland's ablation zone undergo substantial variability on an annual basis which are presumed to mainly be driven by changes in sliding. Yet, meltwater-forced changes in ice–bed coupling can also produce variable deformation motion, which impacts the magnitude of sliding changes inferred from surface measurements and provides important context to flow dynamics. We examine spatiotemporal changes in deformation, sliding and surface velocities over a 2-year period using GPS and a dense network of inclinometers installed in borehole grid drilled in western Greenland's ablation zone. We find time variations in deformation motion track sliding changes through the summer and entire measurement period. A distinct spatial deformation and sliding pattern is also observed within the borehole grid which remains similar during winter and summer flow. We suggest that positively covarying sliding and deformation across seasonal timescales is characteristic of passive areas that are coupled to regions undergoing transient forcing, and the spatial patterns are consistent with variations in the local bed topography. The covarying deformation and sliding result in a 1.5–17% overestimate of sliding changes during summer compared to that inferred from surface velocity changes alone. This suggests that summer sliding increases are likely overestimated in many locations across Greenland.
Article
We present the first fully coupled 3D full-Stokes model of a tidewater glacier, incorporating ice flow, subglacial hydrology, plume-induced frontal melting and calving. We apply the model to Store Glacier ( Sermeq Kujalleq ) in west Greenland to simulate a year of high melt (2012) and one of low melt (2017). In terms of modelled hydrology, we find perennial channels extending 5 km inland from the terminus and up to 41 and 29 km inland in summer 2012 and 2017, respectively. We also report a hydrodynamic feedback that suppresses channel growth under thicker ice inland and allows water to be stored in the distributed system. At the terminus, we find hydrodynamic feedbacks exert a major control on calving through their impact on velocity. We show that 2012 marked a year in which Store Glacier developed a fully channelised drainage system, unlike 2017, where it remained only partially developed. This contrast in modelled behaviour indicates that tidewater glaciers can experience a strong hydrological, as well as oceanic, control, which is consistent with observations showing glaciers switching between types of behaviour. The fully coupled nature of the model allows us to demonstrate the likely lack of any hydrological or ice-dynamic memory at Store Glacier.
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Meltwater produced at the surface of glaciers and ice sheets has important implications for basal sliding rates and therefore ice flow velocities. In order to determine the role of supraglacial water in ice dynamics and predict future changes, we first need to understand and be able to accurately predict moulin input rates. To this end, we present the Subaerial Drainage System (SaDS) model. SaDS is a dynamic model that couples supraglacial runoff in the bare‐ice ablation zone in a distributed sheet with flow in discrete channels. Flow in the distributed sheet drives melt through potential energy dissipation, allowing a channel network to form naturally with no prior assumptions about channel locations. We apply the model to a synthetic ice sheet margin and carry out a suite of sensitivity tests. Modeled moulin inputs show expected behaviors including large diurnal variability, multi‐hour lags following peak surface melt, and demonstrate complex and diverse seasonal dynamics. The sensitivity tests illustrate the range of possible model behaviors and constrain the parameter values for which the model predicts physically realistic moulin inputs. We also apply the model to a ∼20 × 27 km² catchment on the southwestern Greenland Ice Sheet using RACMO melt forcing and previously mapped moulin locations. Modeled supraglacial lake and stream locations match those mapped from Landsat 8 images, and moulin inputs show varied daily and seasonal dynamics. These results demonstrate that the model is a promising tool to provide moulin inputs for subglacial and ice dynamic studies.
Article
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Subglacial hydrology modulates basal motion but remains poorly constrained, particularly for soft-bedded Greenlandic outlet glaciers. Here, we report detailed measurements of the response of subglacial water pressure to the connection and drainage of adjacent water-filled boreholes drilled through kilometre-thick ice on Sermeq Kujalleq (Store Glacier). These measurements provide evidence for gap opening at the ice-sediment interface, Darcian flow through the sediment layer, and the forcing of water pressure in hydraulically-isolated cavities by stress transfer. We observed a small pressure drop followed by a large pressure rise in response to the connection of an adjacent borehole, consistent with the propagation of a flexural wave within the ice and underlying deformable sediment. We interpret the delayed pressure rise as evidence of no pre-existing conduit and the progressive decrease in hydraulic transmissivity as the closure of a narrow (< 1.5 mm) gap opened at the ice-sediment interface, and a reversion to Darcian flow through the sediment layer with a hydraulic conductivity of ≤ 10 ⁻⁶ m s ⁻¹ . We suggest that gap opening at the ice-sediment interface deserves further attention as it will occur naturally in response to the rapid pressurisation of water at the bed.
Article
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Due to increasing surface melting on the Greenland ice sheet, better constraints on seasonally evolving basal water pressure and sliding speed are required by models. Here we assess the potential of using inverse methods on a dense time series of surface speeds to recover the seasonal evolution of the basal conditions in a well-documented region in southwest Greenland. Using data compiled from multiple satellite missions, we document seasonally evolving surface velocities with a temporal resolution of 2 weeks between 2015 and 2019. We then apply the inverse control method using the ice flow model Elmer/Ice to infer the basal sliding and friction corresponding to each of the 24 surface velocity data sets. Near the margin where the uncertainty in the velocity and bed topography are small, we obtain clear seasonal variations that can be mostly interpreted in terms of an effective-pressure-based hard-bed friction law. We find for valley bottoms or “troughs” in the bed topography that the changes in modelled basal conditions directly respond to local modelled water pressure variations, while the link is more complex for subglacial “ridges” which are often non-locally forced. At the catchment scale, in-phase variations in the water pressure, surface velocities, and surface runoff variations are found. Our results show that time series inversions of observed surface velocities can be used to understand the evolution of basal conditions over different timescales and could therefore serve as an intermediate validation for subglacial hydrology models to achieve better coupling with ice flow models.
Article
Record highs of meltwater production at the surface of the Greenland ice sheet have been recorded with a high recurrence over the last decades. Those melt seasons with longer durations, larger intensities, or with both increased length and melt intensity have a direct impact on the surface mass balance of the ice sheet and on its contribution to sea level rise. Moreover, the surface melt also affects the ice dynamics through the meltwater lubrication feedback. It is still not clear how the meltwater lubrication feedback impacts the long-term ice velocities on the Greenland ice sheet. Here we take a modeling approach with simplified ice sheet geometry and climate forcings to investigate in more detail the impacts of the changing characteristics of the melt season on ice dynamics. We model the ice dynamics through the coupling of the Double Continuum (DoCo) subglacial hydrology model with a shallow shelf approximation for the ice dynamics in the Ice-sheet and Sea-level System Model (ISSM). The climate forcing is generated from the ERA5 dataset to allow the length and intensity of the melt season to be varied in a comparable range of values. Our simulations present different behaviors between the lower and higher part of the glacier, but overall, a longer melt season will yield a faster glacier for a given runoff value. However, an increase in the intensity of the melt season, even under increasing runoff, tends to reduce glacier velocities. Those results emphasize the complexity of the meltwater lubrication feedback and urge us to use subglacial drainage models with both inefficient and efficient drainage components to give an accurate assessment of its impact on the overall dynamics of the Greenland ice sheet.
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Glacier motion responds dynamically to changing meltwater inputs, but the multi-decadal response of basal sliding to climate remains poorly constrained due to its sensitivity across multiple timescales. Observational records of glacier motion provide critical benchmarks to decode processes influencing glacier dynamics, but multi-decadal records that precede satellite observation and modern warming are rare. Here we present a record of motion in the ablation zone of Saskatchewan Glacier that spans seven decades. We combine in situ and remote-sensing observations to inform a first-order glacier flow model used to estimate the relative contributions of sliding and internal deformation on dynamics. We find a significant increase in basal sliding rates between melt-seasons in the 1950s and those in the 1990s and 2010s and explore three process-based explanations for this anomalous behavior: (i) the glacier surface steepened over seven decades, maintaining flow-driving stresses despite sustained thinning; (ii) the formation of a proglacial lake after 1955 may support elevated basal water pressures; and (iii) subglacial topography may cause dynamic responses specific to Saskatchewan Glacier. Although further constraints are necessary to ascertain which processes are of greatest importance for Saskatchewan Glacier's dynamic evolution, this record provides a benchmark for studies of multi-decadal glacier dynamics.
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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.
Article
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Late-summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of Haut Glacier d’Arolla, Switzerland. Interpolated surfaces of minimum diurnal water pressure and diurnal water-pressure variation suggest the presence of a subglacial channel within a more widespread, distributed drainage system. The channel flows along the centre of a variable pressure axis (VPA), some tens of metres wide, that is characterized by low minimum diurnal water pressures (frequently atmospheric) and high diurnal water-pressure variations. These characteristics are transitional over a lateral distance of c. 70 m to higher and more stable subglacial water pressures in the adjacent distributed system. Water-pressure variations recorded in boreholes located close to the centre of the VPA reflect the delivery of surface-derived meltwater to the glacier bed and result in a diurnally reversing, transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight. Subglacial observations suggest that such flow occurs through a vertically confined sediment layer. Borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension. Analysis of the propagation velocity and amplitude attenuation cf the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel, falling from c. 10−4 m s−1 at the channel boundary to c. 10−7 m s−1 70 m away. These apparent hydraulic conductivities are consistent with Darcian flow through clean sand and typical glacial till, respectively. We suggest that fine material is systematically flushed from basal sediments located adjacent to large, melt-season drainage channels beneath warm-based glaciers. This process may have important implications for patterns of glacier erosion, hydro-chemistry and dynamics.
Article
Full-text available
Late-summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of Haut Glacier d’Arolla, Switzerland. Interpolated surfaces of minimum diurnal water pressure and diurnal water-pressure variation suggest the presence of a subglacial channel within a more widespread, distributed drainage system. The channel flows along the centre of a variable pressure axis (VPA), some tens of metres wide, that is characterized by low minimum diurnal water pressures (frequently atmospheric) and high diurnal water-pressure variations. These characteristics are transitional over a lateral distance of c. 70 m to higher and more stable subglacial water pressures in the adjacent distributed system. Water-pressure variations recorded in boreholes located close to the centre of the VPA reflect the delivery of surface-derived meltwater to the glacier bed and result in a diurnally reversing, transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight. Subglacial observations suggest that such flow occurs through a vertically confined sediment layer. Borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension. Analysis of the propagation velocity and amplitude attenuation cf the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel, falling from c. 10 ⁻⁴ m s ⁻¹ at the channel boundary to c. 10 ⁻⁷ m s ⁻¹ 70 m away. These apparent hydraulic conductivities are consistent with Darcian flow through clean sand and typical glacial till, respectively. We suggest that fine material is systematically flushed from basal sediments located adjacent to large, melt-season drainage channels beneath warm-based glaciers. This process may have important implications for patterns of glacier erosion, hydro-chemistry and dynamics.
Article
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High resolution measurements of ice motion along a ˜120 km transect in a land-terminating section of the GrIS reveal short-term velocity variations (<1 day), which are forced by rapid variations in meltwater input to the subglacial drainage system from the ice sheet surface. The seasonal changes in ice velocity at low elevations (<1000 m) are dominated by events lasting from 1 day to 1 week, although daily cycles are largely absent at higher elevations, reflecting different patterns of meltwater input. Using a simple model of subglacial conduit behavior we show that the seasonal record of ice velocity can be understood in terms of a time-varying water input to a channelized subglacial drainage system. Our investigation substantiates arguments that variability in theduration and rate, rather than absolute volume, of meltwater delivery to the subglacial drainage system are important controls on seasonal patterns of subglacial water pressure, and therefore ice velocity. We suggest that interpretations of hydro-dynamic behavior in land-terminating sections of the GrIS margin which rely on steady state drainage theories are unsuitable for making predictions about the effect of increased summer ablation on future rates of ice motion.
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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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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.
Article
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Basal motion of glaciers is responsible for short-term variations in glacier velocity. At the calving fronts of marine-terminating outlet glaciers, accelerated basal motion has led to increased ice discharge and thus is tightly connected to sea level rise. Subglacial water passes through dynamic conduits that are fed by distributed linked cavities at the bed, and plays a critical role in setting basal motion. However, neither measured subglacial water pressure nor the volume of water in storage can fully explain basal motion. Here, we use global positioning system observations to document basal motion during highly variable inputs of water from diurnal and seasonal melt, and from an outburst flood at Kennicott Glacier, Alaska. We find that glacier velocity increases when englacial and subglacial water storage is increasing. We suggest that whenever water inputs exceed the ability of the existing conduits to transmit water, the conduits pressurize and drive water back into the areally extensive linked cavity system. This in turn promotes basal motion. Sustained high melt rates do not imply continued rapid basal motion, however, because the subglacial conduit system evolves to greater efficiency. Large pulses of water to the bed can overwhelm the subglacial hydrologic network and incite basal motion, potentially explaining recent accelerations of the Greenland Ice Sheet, where rapid drainage of large surficial melt ponds delivers water through cold ice.
Article
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Ice flow at a location in the equilibrium zone of the west-central Greenland Ice Sheet accelerates above the midwinter average rate during periods of summer melting. The near coincidence of the ice acceleration with the duration of surface melting, followed by deceleration after the melting ceases, indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice-bedrock interface. Interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting, with larger increases accompanying higher amounts of summer melting. The indicated coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming.
Article
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Continuous Global Positioning System observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the Greenland Ice Sheet. Within days, ice velocity reacts to increased meltwater production and increases by a factor of 4. Such a response is much stronger and much faster than previously reported. Over a longer period of 17 years, annual ice velocities have decreased slightly, which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input, which results in a more or less constant ice flux over the years. The positive-feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades.
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
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
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
There is an important distinction between subglacial drainage mechanisms that involve water distributed over the vast majority of the bed and those that involve localized flow in channels. In order to predict variations in water pressure at the bed, and therefore ice sliding velocity, it may be necessary to model both types of drainage, or to use models which can transition between distributed and channelized drainage modes. Distributed drainage can be modelled as flow through a deformable porous medium with variable permeability. On a large enough scale, a channel network can also be described as an effective porous medium, with a (different) varying permeability. A dual porosity model can be used to describe the coupled drainage through both systems. This work derives and examines such a model, and in particular discusses the importance of the spacing between adjacent channels and how this should be determined. More widely spaced channels can capture a larger quantity of water and therefore grow to a larger size, covering a greater extent of the bed and generally predicting lower water pressure than more finely spaced channels. The channel spacing is, however, limited by a `compaction' length of the interconnecting distributed system - that is, the length scale from which the reduced-pressure channel is able to draw in water.
Article
Models are proposed for channelized and distributed flow of meltwater at the base of an ice sheet. The volumes of both channel and distributed systems evolve according to a competition between processes that open drainage space (e.g. sliding over bedrock, melting of the ice) and processes that close it (e.g. viscous creep of the ice due to a positive effective pressure). Channels are generally predicted to have lower water pressure and therefore capture water from the surrounding regions of distributed flow. There is a natural length scale associated with the distributed system that determines the width of the bed from which water can be drawn into a channel. It is suggested that this determines the spacing between major channels and that this may be reflected in the spacing of eskers. A more permeable distributed system results in more widely spaced, and therefore larger, channels. Calculations of the flow into the head of a channel reveal that there is a critical discharge necessary for it to form, and provide a criterion for where channels can exist.
Article
We use interferometric synthetic aperture radar observations recorded in a land-terminating sector of western Greenland to characterise the ice sheet surface hydrology and to quantify spatial variations in the seasonality of ice sheet flow. Our data reveal a non-uniform pattern of late-summer ice speedup that, in places, extends over 100km inland. We show that the degree of late-summer speedup is positively correlated with modelled runoff within the 10 glacier catchments of our survey, and that the pattern of late-summer speedup follows that of water routed at the ice sheet surface. In late-summer, ice within the largest catchment flows on average 48% faster than during winter, whereas changes in smaller catchments are less pronounced. Our observations show that the routing of seasonal runoff at the ice sheet surface plays an important role in shaping the magnitude and extent of seasonal ice sheet speedup.
Article
Ice velocities observed in 2005/06 at three GPS stations along the Sermeq Avannarleq flowline, West Greenland, are used to characterize an observed annual velocity cycle. We attempt to reproduce this annual ice velocity cycle using a 1-D ice-flow model with longitudinal stresses coupled to a 1-D hydrology model that governs an empirical basal sliding rule. Seasonal basal sliding velocity is parameterized as a perturbation of prescribed winter sliding velocity that is proportional to the rate of change of glacier water storage. The coupled model reproduces the broad features of the annual basal sliding cycle observed along this flowline, namely a summer speed-up event followed by a fall slowdown event. We also evaluate the hypothesis that the observed annual velocity cycle is due to the annual calving cycle at the terminus. We demonstrate that the ice acceleration due to a catastrophic calving event takes an order of magnitude longer to reach CU/ETH ('Swiss') Camp (46 km upstream of the terminus) than is observed. The seasonal acceleration observed at Swiss Camp is therefore unlikely to be the result of velocity perturbations propagated upstream via longitudinal coupling. Instead we interpret this velocity cycle to reflect the local history of glacier water balance.
Article
We measure hydrological parameters in meltwater draining from an outlet glacier in west Greenland to investigate seasonal changes in the structure and behaviour of the hydrological system of a large catchment in the Greenland ice sheet (GrIS). Our data reveal seasonal upglacier expansion and increase in hydraulic efficiency of the subglacial drainage system, across a catchment >600 km2, to distances >50 km from the ice-sheet margin. This expansion occurs episodically in response to the drainage of surface meltwaters into a hitherto inefficient subglacial drainage system as new input locations become active progressively further upglacier; this system is similar to Alpine glaciers. These observations provide the first synopsis of seasonal hydrological behaviour in the ablation zone of the GrIS.
Article
We apply a novel one-dimensional glacier hydrology model that calculates hydraulic head to the tidewater-terminating Sermeq Avannarleq flowline of the Greenland ice sheet. Within a plausible parameter space, the model achieves a quasi-steady-state annual cycle in which hydraulic head oscillates close to flotation throughout the ablation zone. Flotation is briefly achieved during the summer melt season along a ∼17 km stretch of the ∼50 km of flowline within the ablation zone. Beneath the majority of the flowline, subglacial conduit storage 'closes' (i.e. obtains minimum radius) during the winter and 'opens' (i.e. obtains maximum radius) during the summer. Along certain stretches of the flowline, the model predicts that subglacial conduit storage remains open throughout the year. A calculated mean glacier water residence time of ∼2.2 years implies that significant amounts of water are stored in the glacier throughout the year. We interpret this residence time as being indicative of the timescale over which the glacier hydrologic system is capable of adjusting to external surface meltwater forcings. Based on in situ ice velocity observations, we suggest that the summer speed-up event generally corresponds to conditions of increasing hydraulic head during inefficient subglacial drainage. Conversely, the slowdown during fall generally corresponds to conditions of decreasing hydraulic head during efficient subglacial drainage.
Article
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.
Article
Based on observations of the 1982-1983 surge of Variegated Glacier, Alaska, a model of the surge mechanism is developed in terms of a transition from the normal tunnel configuration of the basal water conduit system to a linked cavity configuration that tends to restrict the flow of water, resulting in increased basal water pressures that cause rapid basal sliding. -from Author
Article
The Greenland ice sheet is likely to make a faster contribution to sea-level rise in a warming world than previously believed, based on numerical modelling using a parameterization of recent results showing surface-meltwater lubrication of ice flow. Zwally et al. (Science 297(557) (2002) 218) documented correlation between increased ice velocity and increased surface melt (as parameterized by positive degree days). They argued that surface water is piped directly to the bed with little delay, causing increased basal-water pressures and basal-sliding velocities, an effect not included in recent Greenland ice-sheet models known to the authors.Using the Pennsylvania State University/University of Chicago thermomechanical flowline model, numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the Zwally et al. data under three different global warming scenarios (2×CO2, 4×CO2, and 8×CO2). Comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates, additional thinning and retreat of the margin, and a reduction in ice volume leading to an increased contribution to global sea-level rise.
Article
Increased ice velocities in Greenland are contributing significantly to eustatic sea level rise. Faster ice flow has been associated with ice-ocean interactions in water-terminating outlet glaciers and with increased surface meltwater supply to the ice-sheet bed inland. Observed correlations between surface melt and ice acceleration have raised the possibility of a positive feedback in which surface melting and accelerated dynamic thinning reinforce one another, suggesting that overall warming could lead to accelerated mass loss. Here I show that it is not simply mean surface melt but an increase in water input variability that drives faster ice flow. Glacier sliding responds to melt indirectly through changes in basal water pressure, with observations showing that water under glaciers drains through channels at low pressure or through interconnected cavities at high pressure. Using a model that captures the dynamic switching between channel and cavity drainage modes, I show that channelization and glacier deceleration rather than acceleration occur above a critical rate of water flow. Higher rates of steady water supply can therefore suppress rather than enhance dynamic thinning, indicating that the melt/dynamic thinning feedback is not universally operational. Short-term increases in water input are, however, accommodated by the drainage system through temporary spikes in water pressure. It is these spikes that lead to ice acceleration, which is therefore driven by strong diurnal melt cycles and an increase in rain and surface lake drainage events rather than an increase in mean melt supply.
Article
Habschr. ETH Zürich, 1972 (KA).
Article
It has been widely hypothesized that a warmer climate in Greenland would increase the volume of lubricating surface meltwater reaching the ice-bedrock interface, accelerating ice flow and increasing mass loss. We have assembled a data set that provides a synoptic-scale view, spanning ice-sheet to outlet-glacier flow, with which to evaluate this hypothesis. On the ice sheet, these data reveal summer speedups (50 to 100%) consistent with, but somewhat larger than, earlier observations. The relative speedup of outlet glaciers, however, is far smaller (<15%). Furthermore, the dominant seasonal influence on Jakobshavn Isbrae's flow is the calving front's annual advance and retreat. With other effects producing outlet-glacier speedups an order of magnitude larger, seasonal melt's influence on ice flow is likely confined to those regions dominated by ice-sheet flow.
IceBridge MCoRDS L2 Ice Thickness
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