scholarly journals Sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions: a model study

2002 ◽  
Vol 48 (163) ◽  
pp. 552-558 ◽  
Author(s):  
Marjorie Schmeltz ◽  
Eric Rignot ◽  
Todd K. Dupont ◽  
Douglas R. MacAyeal
Keyword(s):  
Shear Stress ◽  
Element Model ◽  
Shelf Area ◽  
West Antarctica ◽  
Ice Shelf ◽  
Ice Stream ◽  
Model Study ◽  
Grounding Line ◽  
Glacier Velocity ◽  
Basal Shear

AbstractWe use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5–13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5–10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.

scholarly journals Numerical Modelling Of Rutford Ice Stream, Antarctica

1990 ◽  
Vol 14 ◽  
pp. 336-336 ◽  
Author(s):  
R.M Frolich ◽  
D.R. MacAyeal
Keyword(s):  
Finite Element ◽  
Surface Morphology ◽  
Finite Element Model ◽  
Element Model ◽  
Back Stress ◽  
Vertical Shear ◽  
Ice Shelf ◽  
Ice Stream ◽  
Grounding Line ◽  
Basal Shear

A two-dimensional finite element model has been applied to Rutford Ice Stream, Antarctica, and part of Ronne Ice Shelf into which the ice stream flows. The model is an extension of one describing ice-shelf flow, and relies on vertical shear in the ice stream being small in some mathematically defined sense. This is equivalent to requiring the vertical shear to be confined to a basal layer or a deformable substrate.Although there is no direct observational evidence for such a layer beneath Rutford Ice Stream, extensive surface surveys and estimates of the strength of the overlying ice show that some dynamically equivalent mechanism must occur. If basal shear stress is parameterised in terms of the thickness and viscosity of a linearly viscous substrate, as may be the case beneath Ice Stream Β in Antarctica, then going upstream from the grounding line, the thickness of this layer must decrease, and the viscosity increase (to retain a realistic thickness at the upstream limit), in order to reproduce the observed surface velocities. This physically reasonable picture is currently adopted as a working hypothesis.Vertical shear in the body of the ice stream appears to be negligible for approximately 70 km above the grounding line. Sensitivity tests show that, in this lower section, ice-shelf back stress is an important restraining influence. A 10% reduction in back stress would produce an immediate 15% increase in grounding line flux. Further upstream, however, higher surface slopes and slightly lower surface velocities suggest that the neglect of vertical shear may be less appropriate. The effect of a reduction in ice-shelf back stream is not felt in this region immediately, as the gravitational driving force is almost balanced by local basal shear traction.A complex surface morphology has been revealed by satellite imagery below the grounding line of Rutford Ice Stream. On the basis that this may be evidence of time dependent behaviour, the finite element model is being used to investigate the origin of the pattern. Ice-shelf back stress, basal melting, mass flux from tributary glaciers and substrate properties can all be varied in physically realistic ways to try to reproduce, qualitatively, the observed surface morphology.

scholarly journals Numerical Modelling Of Rutford Ice Stream, Antarctica

1990 ◽  
Vol 14 ◽  
pp. 336 ◽  
Author(s):  
R.M Frolich ◽  
D.R. MacAyeal
Keyword(s):  
Finite Element ◽  
Surface Morphology ◽  
Finite Element Model ◽  
Element Model ◽  
Back Stress ◽  
Vertical Shear ◽  
Ice Shelf ◽  
Ice Stream ◽  
Grounding Line ◽  
Basal Shear

A two-dimensional finite element model has been applied to Rutford Ice Stream, Antarctica, and part of Ronne Ice Shelf into which the ice stream flows. The model is an extension of one describing ice-shelf flow, and relies on vertical shear in the ice stream being small in some mathematically defined sense. This is equivalent to requiring the vertical shear to be confined to a basal layer or a deformable substrate. Although there is no direct observational evidence for such a layer beneath Rutford Ice Stream, extensive surface surveys and estimates of the strength of the overlying ice show that some dynamically equivalent mechanism must occur. If basal shear stress is parameterised in terms of the thickness and viscosity of a linearly viscous substrate, as may be the case beneath Ice Stream Β in Antarctica, then going upstream from the grounding line, the thickness of this layer must decrease, and the viscosity increase (to retain a realistic thickness at the upstream limit), in order to reproduce the observed surface velocities. This physically reasonable picture is currently adopted as a working hypothesis. Vertical shear in the body of the ice stream appears to be negligible for approximately 70 km above the grounding line. Sensitivity tests show that, in this lower section, ice-shelf back stress is an important restraining influence. A 10% reduction in back stress would produce an immediate 15% increase in grounding line flux. Further upstream, however, higher surface slopes and slightly lower surface velocities suggest that the neglect of vertical shear may be less appropriate. The effect of a reduction in ice-shelf back stream is not felt in this region immediately, as the gravitational driving force is almost balanced by local basal shear traction. A complex surface morphology has been revealed by satellite imagery below the grounding line of Rutford Ice Stream. On the basis that this may be evidence of time dependent behaviour, the finite element model is being used to investigate the origin of the pattern. Ice-shelf back stress, basal melting, mass flux from tributary glaciers and substrate properties can all be varied in physically realistic ways to try to reproduce, qualitatively, the observed surface morphology.

scholarly journals In search of ice-stream sticky spots

1993 ◽  
Vol 39 (133) ◽  
pp. 447-454 ◽  
Author(s):  
Richard B. Alley
Keyword(s):  
Shear Stress ◽  
Large Scale ◽  
Water Pressure ◽  
Pressure Measurements ◽  
West Antarctica ◽  
Ice Surface ◽  
Ice Stream ◽  
Bed Roughness ◽  
Basal Shear

AbstractThe basal shear stress of an ice stream may be supported disproportionately on localized regions or “sticky spots”. The drag induced by large bedrock bumps sticking into the base of an ice stream is the most likely cause of sticky spots. Discontinuity of lubricating till can cause sticky spots, but they will collect lubricating water and therefore are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps. Raised regions on the ice-air surface can also cause moderate increases in the shear stress supported on the bed beneath. Surveys of large-scale bed roughness would identify sticky spots caused by bedrock bumps, water-pressure measurements in regions of thin or zero till might reveal whether they were sticky spots, and strain grids across the margins of ice-surface highs would show whether the highs were causing sticky spots. Sticky spots probably are not dominant in controlling Ice Stream Β near the Upstream Β camp, West Antarctica.

Ice-stream-ice-shelf transition: theoretical analysis of two-dimensional flow

2000 ◽  
Vol 30 ◽  
pp. 153-162 ◽  
Author(s):  
Alexander V. Wilchinsky ◽  
Vladimir A. Chugunov
Keyword(s):  
Shear Stress ◽  
Transition Zone ◽  
Stokes Equations ◽  
Stress Deviator ◽  
Two Dimensional ◽  
Ice Shelf ◽  
Full System ◽  
Ice Stream ◽  
Grounding Line ◽  
Two Dimensional Flow

AbstractTwo-dimensional steady isothermal flow of a marine ice stream is studied. Gases of different relations between shear stress and longitudinal deviatoric stress in the ice stream are considered. Analysis of the ice-stream-ice-shelf transition zone shows that even if the longitudinal stress deviator in the ice stream is much larger than the shear stress (as it is in the ice shelf), the ice-stream-ice-shelf transition zone if singular and the full system of Stokes equations must be solved in it. Scales of fields in the transition zone and the relation between the ice thickness and the horizontal mass flux at the grounding line are found.

scholarly journals Changes in the ice plain of Whillans Ice Stream, West Antarctica

2005 ◽  
Vol 51 (175) ◽  
pp. 620-636 ◽  
Author(s):  
Robert Bindschadler ◽  
Patricia Vornberger ◽  
Laurence Gray
Keyword(s):  
West Antarctica ◽  
Ice Shelf ◽  
Ross Ice Shelf ◽  
Ice Stream ◽  
Whillans Ice Stream ◽  
Grounding Line ◽  
Local Areas ◽  
The Mean ◽  
Geometric Changes ◽  
Thinning Rate

AbstractData from the mouth of the decelerating Whillans Ice Stream (WIS), West Antarctica, spanning 42 years are reviewed. Deceleration has continued, with local areas of both thinning and thickening occurring. The mean thinning rate is 0.48 ± 0.77 ma–1. No consistent overall pattern is observed. Ice thickens immediately upstream of Crary Ice Rise where deceleration and divergence are strongest, suggesting expanded upstream influence of the ice rise. Thinning is prevalent on the Ross Ice Shelf. Grounding-line advance at a rate of 0.3 km a–1 is detected in a few locations. Basal stresses vary across an ice-stream transect with a zone of enhanced flow at the margin. Marginal shear is felt at the ice-stream center. Mass-balance values are less negative, but larger errors of earlier measurements mask any possible temporal pattern. Comparisons of the recent flow field with flow stripes suggest WIS contributes less ice to the deep subglacial channel carved by Mercer Ice Stream and now flows straighter. The general lack of geometric changes suggests that the regional velocity decrease is due to changing basal conditions.

scholarly journals Simulated dynamic regrounding during marine ice sheet retreat

2018 ◽  
Vol 12 (7) ◽  
pp. 2425-2436
Author(s):  
Lenneke M. Jong ◽  
Rupert M. Gladstone ◽  
Benjamin K. Galton-Fenzi ◽  
Matt A. King
Keyword(s):  
Shear Stress ◽  
Ad Hoc ◽  
Ice Sheet ◽  
Ice Shelf ◽  
Ice Streams ◽  
Overburden Pressure ◽  
Grounding Line ◽  
Ice Velocity ◽  
Basal Shear ◽  
The Relationship

Abstract. Marine-terminating ice sheets are of interest due to their potential instability, making them vulnerable to rapid retreat. Modelling the evolution of glaciers and ice streams in such regions is key to understanding their possible contribution to sea level rise. The friction caused by the sliding of ice over bedrock and the resultant shear stress are important factors in determining the velocity of sliding ice. Many models use simple power-law expressions for the relationship between the basal shear stress and ice velocity or introduce an effective-pressure dependence into the sliding relation in an ad hoc manner. Sliding relations based on water-filled subglacial cavities are more physically motivated, with the overburden pressure of the ice included. Here we show that using a cavitation-based sliding relation allows for the temporary regrounding of an ice shelf at a point downstream of the main grounding line of a marine ice sheet undergoing retreat across a retrograde bedrock slope. This suggests that the choice of sliding relation is especially important when modelling grounding line behaviour of regions where potential ice rises and pinning points are present and regrounding could occur.

scholarly journals In search of ice-stream sticky spots

1993 ◽  
Vol 39 (133) ◽  
pp. 447-454 ◽  
Author(s):  
Richard B. Alley
Keyword(s):  
Shear Stress ◽  
Large Scale ◽  
Water Pressure ◽  
Pressure Measurements ◽  
West Antarctica ◽  
Ice Surface ◽  
Ice Stream ◽  
Bed Roughness ◽  
Basal Shear

Abstract The basal shear stress of an ice stream may be supported disproportionately on localized regions or “sticky spots”. The drag induced by large bedrock bumps sticking into the base of an ice stream is the most likely cause of sticky spots. Discontinuity of lubricating till can cause sticky spots, but they will collect lubricating water and therefore are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps. Raised regions on the ice-air surface can also cause moderate increases in the shear stress supported on the bed beneath. Surveys of large-scale bed roughness would identify sticky spots caused by bedrock bumps, water-pressure measurements in regions of thin or zero till might reveal whether they were sticky spots, and strain grids across the margins of ice-surface highs would show whether the highs were causing sticky spots. Sticky spots probably are not dominant in controlling Ice Stream Β near the Upstream Β camp, West Antarctica.

scholarly journals Reconstruction and Disintegration of Ice Sheets for the Climap 18000 and 125000 Years B.P. Experiments: Theory

1979 ◽  
Vol 24 (90) ◽  
pp. 493-495
Author(s):  
T. J. Hughes
Keyword(s):  
Shear Stress ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Frictional Heat ◽  
Ice Streams ◽  
Ice Stream ◽  
Grounding Line ◽  
Flow Law ◽  
Basal Shear ◽  
Stress Variations

AbstractSize, shape, and surface albedo of former ice sheets are needed in order to model atmospheric circulation for the CLIMAP 18000 years B.P. experiment. Both the size and shape of an ice sheet depend on the hardness of ice and its coupling to bedrock. Ice hardness is controlled by ice temperature and fabric, which are not adequately described by any ice flow law. Ice–bed coupling is controlled by bed roughness and basal melt water, which are not adequately described by any ice sliding law. With these inadequacies in mind, we assumed equilibrium ice-sheet conditions 18000 years ago and combined the standard steady-state flow and sliding laws of ice with the equation of mass balance to obtain separate basal shear-stress variations along ice-sheet flow lines for a frozen bed when the flow law dominates and for a melted bed when the sliding law dominates. Theoretical basal shear-stress variations were then derived for freezing and melting beds on the assumption that separate melted areas of the bed had water films of constant thickness which expanded and merged for a melting bed but contracted and separated for a freezing bed. Theoretical basal shear-stress variations were also derived for ice streams along marine ice-sheet margins and ice lobes along terrestrial ice-sheet margins on the assumption that the entire area of their bed was wet so that further melting increased the water-layer thickness, which would then be decreased by freezing. Melting was assumed to continue to the grounding line of an ice stream and the minimum-slope surface inflection line of an ice lobe, where freezing began and continued to the ice-lobe terminus. Ice–bed uncoupling is complete at an ice-stream grounding line and maximized at an ice-lobe minimum-slope inflection line, so ice velocity and consequent generation of frictional heat were assumed to reach maxima across these lines. Theoretical basal shear-stress variations were derived for the zone of converging flow at the heads of ice streams and ice lobes, and from domes to saddles along the ice divide for both frozen and melted beds.

scholarly journals Basal drag of Fleming Glacier, Antarctica, Part B: implications of evolution from 2008 to 2015

2018 ◽  
Author(s):  
Chen Zhao ◽  
Rupert M. Gladstone ◽  
Roland C. Warner ◽  
Matt A. King ◽  
Thomas Zwinger
Keyword(s):  
Shear Stress ◽  
Frictional Heating ◽  
Shear Stresses ◽  
Ice Shelf ◽  
Ice Dynamics ◽  
The Past ◽  
Glacier System ◽  
Grounding Line ◽  
Glacier Front ◽  
Basal Shear

Abstract. The Wordie Ice Shelf-Fleming Glacier system in the southern Antarctic Peninsula has experienced a long-term retreat and disintegration of its ice shelf in the past 50 years. Upstream glacier acceleration and dynamic thinning have been observed over the past two decades, especially after 2008 when only a little constraining ice shelf remained at the Fleming Glacier front. It is important to know whether the substantial speed up and surface draw-down of the glacier since 2008 is a direct response to increasing ocean forcing or driven by the feedback within an unstable marine-based glacier system or both. To explore the mechanism underlying the changes, we use a Stokes (full stress) model to simulate the basal shear stress of the Fleming system in 2008 and 2015. Recent observational studies have suggested the 2008–2015 velocity change was due to the ungrounding of the Fleming Glacier front. Our modelling shows that the fast flowing region of the Fleming Glacier shows a very low basal shear stress in 2008 but with a band of higher basal shear stress along the ice front. It indicates that the ungrounding process might have not started in 2008, which is consistent with the height above buoyancy calculation in 2008. Comparison of our inversions for basal shear stresses for 2008 and 2015 suggests the migration of the grounding line by ~ 9 km upstream from the grounding line position in 1996, a shift which is consistent with the change in floating area deduced from the height above buoyancy in 2015. The southern branch of the Fleming Glacier and the Prospect Glacier apparently have retreated by ~ 1–3 km from 2008 to 2015. The retrograde bed underneath the Fleming Glacier has promoted migration of the grounding line, which we suggest may be triggered by subglacial drainage as a response to the increased basal water supply through greater frictional heating at the ice-bedrock interface further upstream in the fast-flowing region. Improved knowledge of bed topography near the grounding line and further transient simulation is required to predict the future grounding line movement of the Fleming Glacier system precisely and subsequently understand better the ice dynamics and the its future contribution to sea level.

scholarly journals Basal sliding of Ice Stream B, West Antarctica

1998 ◽  
Vol 44 (147) ◽  
pp. 223-230 ◽  
Author(s):  
Engelhardt Hermann ◽  
Kamb Barclay
Keyword(s):  
West Antarctica ◽  
Ice Shelf ◽  
The West ◽  
Ross Ice Shelf ◽  
Sliding Motion ◽  
Ice Stream ◽  
Grounding Line ◽  
Basal Sliding ◽  
Ice Stream B ◽  
Streaming Motion

AbstractA “tethered stake” apparatus is used to measure basal sliding in a borehole on Ice Stream B, West Antaretica, about 300 km upstream (east) from its grounding line near the head of the Ross Ice Shelf. A metal stake, emplaced at the top of a laver of unfrozen till underlying the ice, is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding. The measured sliding motion includes any actual slip across the ice–till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface. This 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment, based on features of the record of apparent sliding. Alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm. In any case it is small compared to the seismically inferred till thickness of 9 m. Measured apparent sliding averages 69% of the total motion of 1.2 m d−1over 26 days of observation if a 3.5 day period of slow apparent sliding (8% of the total motion) is included in the average. The occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c.80% and c. 8% of the total motion, on a time-scale of a few days. However, it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole. If the slow period is therefore omitted, the indicated average basal sliding rate is 83% of the total motion. In either case, basal sliding predominates as the cause of the rapid ice-stream motion. In the last 2 days of observation the average apparent sliding rate reached 1.17 m d−1, essentially 100% of the motion of the ice stream. If till deformation contributes significantly to the ice-stream motion, the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer. These observations, if applicable to the West Antaretic ice sheet in general, pose complications in modeling the rapid ice-streaming motion.

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