Extreme melt season traction variations recorded on the western Greenland Ice Sheet

2020 ◽  
Author(s):  
Nathan Maier ◽  
Neil Humphrey ◽  
Joel Harper ◽  
Toby Meierbachtol
Keyword(s):  
Conceptual Model ◽  
Dynamic Models ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Late Summer ◽  
Force Balance ◽  
Surface Velocity ◽  
Ice Flow ◽  
Multi Scale ◽  
Basal Ice

<p>Basal traction is fundamental to the dynamics of glaciers and ice sheets. On the Greenland Ice Sheet meltwater delivery to the bed and evolving drainage efficiency and connectivity modulate traction producing a characteristic seasonal velocity response. While numerical modelling and basal pressure observations have linked these velocity variations to evolving subglacial drainage, a high-fidelity record of basal traction is needed to constrain the timing and magnitude of traction changes that modulate summer ice flow.  We present a continuous summertime record of basal traction, basal ice deformation, and surface velocity measured at a densely instrumented field site in western Greenland. We use a five-station GPS network and englacial measurements of shearing and ice temperature to directly estimate the basal traction using the force balance method at the site-scale (100s of meters). Localized traction variations (10s of meters) are inferred via variations in the near-basal deformation field recorded by inclinometers installed directly above the basal interface. Combined, the data give a multi-scale perspective on how the basal traction changes during summer and relates to the conceptual model of melt season flow. Our results show the basal traction migrates between extremes during the melt season, with magnitudes greater than three times the average winter traction and near zero. The basal traction extremes correspond with the spring event, the inferred transition to efficient drainage, and the late summer velocity decline. The rapid strengthening and weakening of the basal interface show the complicated interaction of local and regional forcing that modulate melt season sliding. The near-basal deformation variations allow us to constrain the stress configuration and drainage state during each extreme traction period. Overall, the results allow us to refine the conceptual model for melt season traction changes and provide measured estimates of traction variations which can be used as quantitative targets for coupled drainage – ice dynamic models.</p>

Seasonal to multi-annual speedup and slowdown of Greenland outlet glaciers inferred from time-dependent remote sensing observations

2020 ◽  
Author(s):  
Lizz Ultee ◽  
Bryan Riel ◽  
Brent Minchew
Keyword(s):  
Time Series ◽  
Flow Velocity ◽  
Ice Sheet ◽  
Surface Air Temperature ◽  
Greenland Ice Sheet ◽  
Time Dependent ◽  
Surface Velocity ◽  
Short Term ◽  
Ice Flow

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

scholarly journals Ice flow around large obstacles as indicated by basal ice exposed at the margin of the Greenland ice sheet

1994 ◽  
Vol 40 (135) ◽  
pp. 359-367 ◽  
Author(s):  
Peter G. Knight ◽  
David E. Sugden ◽  
Christopher D. Minty
Keyword(s):  
Empirical Model ◽  
Basal Layer ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Flow ◽  
Subglacial Environment ◽  
Basal Ice ◽  
Convergence And Divergence ◽  
Basal Melting ◽  
Marginal Environment

AbstractSpatial variations in the debris-bearing basal ice layer exposed at the ice-sheet margin in West Greenland reflect the geography of basal melting and ice flow around large obstacles close to the margin. This paper demonstrates the character of the basal ice layer, which comprises fine material incorporated in an interior, subglacial environment and coarser material entrained in an ice-marginal environment. We develop an empirical model of ice flow close to a lobate margin of the ice sheet in which ice convergence and divergence, and limited subglacial melting affect the character and distribution of the basal ice at the margin. There is a tendency for the convergence and divergence to thicken the basal layer in lobate areas and to thin it in inter-lobate areas. Under certain circumstances, basal melting may remove much of the layer from beneath the snouts of larger lobes, thus causing the basal layer to be thickest in an intermediate location.

scholarly journals Sustained high basal motion of the Greenland ice sheet revealed by borehole deformation

2014 ◽  
Vol 60 (222) ◽  
pp. 647-660 ◽  
Author(s):  
Claudia Ryser ◽  
Martin P. Lüthi ◽  
Lauren C. Andrews ◽  
Matthew J. Hoffman ◽  
Ginny A. Catania ◽  
...  
Keyword(s):  
Dynamical Behavior ◽  
Ice Sheet ◽  
Stress Transfer ◽  
Greenland Ice Sheet ◽  
Good Choice ◽  
Surface Velocity ◽  
Ice Streams ◽  
Ice Flow ◽  
Flow Models ◽  
Dependent Flow

AbstractIce deformation and basal motion characterize the dynamical behavior of the Greenland ice sheet (GrIS). We evaluate the contribution of basal motion from ice deformation measurements in boreholes drilled to the bed at two sites in the western marginal zone of the GrIS. We find a sustained high amount of basal motion contribution to surface velocity of 44–73% in winter, and up to 90% in summer. Measured ice deformation rates show an unexpected variation with depth that can be explained with the help of an ice-flow model as a consequence of stress transfer from slippery to sticky areas. This effect necessitates the use of high-order ice-flow models, not only in regions of fast-flowing ice streams but in all temperate-based areas of the GrIS. The agreement between modeled and measured deformation rates confirms that the recommended values of the temperature-dependent flow rate factor A are a good choice for ice-sheet models.

scholarly journals Mass balance and ice flow along the north-northwest ridge of the Greenland ice sheet at NorthGRIP

2002 ◽  
Vol 35 ◽  
pp. 521-526 ◽  
Author(s):  
Christine Schøtt Hvidberg ◽  
Kristian Keller ◽  
Niels S. Gundestrup
Keyword(s):  
Mass Balance ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
High Elevation ◽  
Surface Strain ◽  
Surface Velocity ◽  
Ice Flow ◽  
The North ◽  
Ice Flows ◽  
Drilling Site

AbstractThe North Greenland Icecore Project (NorthGRIP) deep drilling site (75˚05’47’’N, 42˚19’42’’ W) is located at the north-northwest ridge of the Greenland ice sheet, 320 km from Summit. A strain net has been established around the NorthGRIP site and surveyed with global positioning system. Our results show that ice flows with a horizontal surface velocity of 1.329 ±0.015ma–1 along the ridge. Estimated principal surface strain rates at NorthGRIP are and in the directions along and transverse to the north-northwest ridge, respectively, i.e. ice is compressed along the ridge but stretched transverse to the ridge. Possible implications of the observed flow pattern for the stratigraphy are discussed. the average thickening rate in the strain-net area is found to be ∂H/∂t = 0.00 ±0.04ma– 1, in agreement with previous estimates of mass balance in high-elevation areas of Greenland.

scholarly journals Ice flow around large obstacles as indicated by basal ice exposed at the margin of the Greenland ice sheet

1994 ◽  
Vol 40 (135) ◽  
pp. 359-367 ◽  
Author(s):  
Peter G. Knight ◽  
David E. Sugden ◽  
Christopher D. Minty
Keyword(s):  
Empirical Model ◽  
Basal Layer ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Flow ◽  
Subglacial Environment ◽  
Basal Ice ◽  
Convergence And Divergence ◽  
Basal Melting ◽  
Marginal Environment

AbstractSpatial variations in the debris-bearing basal ice layer exposed at the ice-sheet margin in West Greenland reflect the geography of basal melting and ice flow around large obstacles close to the margin. This paper demonstrates the character of the basal ice layer, which comprises fine material incorporated in an interior, subglacial environment and coarser material entrained in an ice-marginal environment. We develop an empirical model of ice flow close to a lobate margin of the ice sheet in which ice convergence and divergence, and limited subglacial melting affect the character and distribution of the basal ice at the margin. There is a tendency for the convergence and divergence to thicken the basal layer in lobate areas and to thin it in inter-lobate areas. Under certain circumstances, basal melting may remove much of the layer from beneath the snouts of larger lobes, thus causing the basal layer to be thickest in an intermediate location.

scholarly journals Uppermost crustal structure regulates the flow of the Greenland Ice Sheet

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
G. A. Jones ◽  
A. M. G. Ferreira ◽  
B. Kulessa ◽  
M. Schimmel ◽  
A. Berbellini ◽  
...  
Keyword(s):  
Basal Slip ◽  
Seismic Velocity ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Velocity Model ◽  
Upper Crust ◽  
Geothermal Heat ◽  
Ice Flow ◽  
The Past ◽  
Basal Ice

AbstractThe flow of the Greenland Ice Sheet is controlled by subglacial processes and conditions that depend on the geological provenance and temperature of the crust beneath it, neither of which are adequately known. Here we present a seismic velocity model of the uppermost 5 km of the Greenlandic crust. We show that slow velocities in the upper crust tend to be associated with major outlet glaciers along the ice-sheet margin, and elevated geothermal heat flux along the Iceland hotspot track inland. Outlet glaciers particularly susceptible to basal slip over deformable subglacial sediments include Jakobshavn, Helheim and Kangerdlussuaq, while geothermal warming and softening of basal ice may affect the onset of faster ice flow at Petermann Glacier and the Northeast Greenland Ice Stream. Interactions with the solid earth therefore control the past, present and future dynamics of the Greenland Ice Sheet and must be adequately explored and implemented in ice sheet models.

scholarly journals Influence of supraglacial lakes and ice-sheet geometry on seasonal ice-flow variability

2013 ◽  
Vol 7 (2) ◽  
pp. 1101-1118 ◽  
Author(s):  
I. Joughin ◽  
S. B. Das ◽  
G. E. Flowers ◽  
M. D. Behn ◽  
R. B. Alley ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Velocity ◽  
Ice Flow ◽  
Bed Topography ◽  
Supraglacial Lakes ◽  
Melt Water ◽  
Flow Enhancement ◽  
Lake Drainage ◽  
Existing Data

Abstract. Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal melt water to reach the base of the Greenland Ice Sheet. Here we use new surface velocity observations to examine the influence of supraglacial lake drainages and surface melt rate on ice flow. We find large, spatially extensive speedups concurrent with times of lake drainage, showing that lakes play a key role in modulating regional ice flow. While surface meltwater is supplied to the bed via a geographically sparse network of moulins, the observed ice-flow enhancement suggests that this meltwater spreads widely over the ice-sheet bed. We also find that the complex spatial pattern of speedup is strongly determined by the combined influence of bed and surface topography on subglacial water flow. Thus, modeling of ice-sheet basal hydrology likely will require knowledge of bed topography resolved at scales (sub-kilometer) far finer than existing data (several km).

scholarly journals Subglacial roughness of the Greenland Ice Sheet: relationship with contemporary ice velocity and geology

2019 ◽  
Vol 13 (11) ◽  
pp. 3093-3115 ◽  
Author(s):  
Michael A. Cooper ◽  
Thomas M. Jordan ◽  
Dustin M. Schroeder ◽  
Martin J. Siegert ◽  
Christopher N. Williams ◽  
...  
Keyword(s):  
Flow Direction ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Flow Speed ◽  
Surface Velocity ◽  
Ice Flow ◽  
Systematic Analysis ◽  
Subglacial Environment ◽  
Topographic Roughness ◽  
Ice Surface

Abstract. The subglacial environment of the Greenland Ice Sheet (GrIS) is poorly constrained both in its bulk properties, for example geology, the presence of sediment, and the presence of water, and interfacial conditions, such as roughness and bed rheology. There is, therefore, limited understanding of how spatially heterogeneous subglacial properties relate to ice-sheet motion. Here, via analysis of 2 decades of radio-echo sounding data, we present a new systematic analysis of subglacial roughness beneath the GrIS. We use two independent methods to quantify subglacial roughness: first, the variability in along-track topography – enabling an assessment of roughness anisotropy from pairs of orthogonal transects aligned perpendicular and parallel to ice flow and, second, from bed-echo scattering – enabling assessment of fine-scale bed characteristics. We establish the spatial distribution of subglacial roughness and quantify its relationship with ice flow speed and direction. Overall, the beds of fast-flowing regions are observed to be rougher than the slow-flowing interior. Topographic roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing regions, and the degree of anisotropy is correlated with ice surface speed. In many slow-flowing regions both roughness methods indicate spatially coherent regions of smooth beds, which, through combination with analyses of underlying geology, we conclude is likely due to the presence of a hard flat bed. Consequently, the study provides scope for a spatially variable hard- or soft-bed boundary constraint for ice-sheet models.

scholarly journals Influence of ice-sheet geometry and supraglacial lakes on seasonal ice-flow variability

2013 ◽  
Vol 7 (4) ◽  
pp. 1185-1192 ◽  
Author(s):  
I. Joughin ◽  
S. B. Das ◽  
G. E. Flowers ◽  
M. D. Behn ◽  
R. B. Alley ◽  
...  
Keyword(s):  
Spatial Pattern ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Velocity ◽  
Ice Flow ◽  
Bed Topography ◽  
Supraglacial Lakes ◽  
Flow Enhancement ◽  
Lake Drainage ◽  
Existing Data

Abstract. Supraglacial lakes play an important role in establishing hydrological connections that allow lubricating seasonal meltwater to reach the base of the Greenland Ice Sheet. Here we use new surface velocity observations to examine the influence of supraglacial lake drainages and surface melt rate on ice flow. We find large, spatially extensive speedups concurrent with times of lake drainage, showing that lakes play a key role in modulating regional ice flow. While surface meltwater is supplied to the bed via a geographically sparse network of moulins, the observed ice-flow enhancement suggests that this meltwater spreads widely over the ice-sheet bed. We also find that the complex spatial pattern of speedup is strongly determined by the combined influence of bed and surface topography on subglacial water flow. Thus, modeling of ice-sheet basal hydrology likely will require knowledge of bed topography resolved at scales (sub-kilometer) far finer than existing data (several km).

scholarly journals Subglacial roughness of the Greenland Ice Sheet: relationship with contemporary ice velocity and geology

2019 ◽  
Author(s):  
Michael A. Cooper ◽  
Thomas M. Jordan ◽  
Dustin M. Schroeder ◽  
Martin J. Siegert ◽  
Christopher N. Williams ◽  
...  
Keyword(s):  
Flow Direction ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Flow Speed ◽  
Surface Velocity ◽  
Ice Flow ◽  
Systematic Analysis ◽  
Subglacial Environment ◽  
Topographic Roughness ◽  
Ice Surface

Abstract. The subglacial environment of the Greenland Ice Sheet (GrIS) is poorly constrained, both in its bulk properties, for example geology, presence of sediment, and of water, and interfacial conditions, such as roughness and bed rheology. There is, therefore, limited understanding of how spatially heterogeneous subglacial properties relate to ice-sheet motion. Here, via analysis of two decades worth of radio-echo sounding data, we present a new systematic analysis of subglacial roughness beneath the GrIS. We use two independent methods to quantify subglacial roughness: first, the variability of along- track topography—enabling an assessment of roughness anisotropy from pairs of orthogonal transects aligned perpendicular and parallel to ice flow; and second, from bed-echo scattering—enabling assessment of fine-scale bed characteristics. We establish the spatial distribution of subglacial roughness and quantify its relationship with ice flow speed and direction. Overall, the beds of fast-flowing regions are observed to be rougher than the slow-flowing interior. Topographic roughness exhibits an exponential scaling relationship with ice surface velocity parallel, but not perpendicular, to flow direction in fast-flowing regions, and the degree of anisotropy is correlated with ice surface speed. In many slow-flowing regions both roughness methods indicate spatially coherent regions of smooth bed, which, through combination with analyses of underlying geology, we conclude is likely due to the presence of a hard flat bed. Consequently, the study provides scope for a spatially variable hard bed/soft bed boundary constraint for ice-sheet models.

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