scholarly journals Diagnosing the sensitivity of grounding line flux to changes in sub-ice shelf melting

2020 ◽  
Author(s):  
Tong Zhang ◽  
Stephen F. Price ◽  
Matthew J. Hoffman ◽  
Mauro Perego ◽  
Xylar Asay-Davis
Keyword(s):  
Principal Stress ◽  
Flow Model ◽  
Ice Thickness ◽  
Diagnostic Model ◽  
Line Flux ◽  
Ice Shelf ◽  
Ice Flow ◽  
Causal Connections ◽  
Alternative Approach ◽  
Grounding Line

Abstract. We seek to understand causal connections between changes in sub-ice shelf melting, ice shelf buttressing, and grounding-line flux. Using a numerical ice flow model, we study changes in ice shelf buttressing and grounding line flux due to localized ice thickness perturbations – a proxy for changes in sub-ice shelf melting – applied to idealized (MISMIP+) and realistic (Larsen C) domains. From our experiments, we identify a correlation between a locally derived buttressing number on the ice shelf, based on the first principal stress, and changes in the integrated grounding line flux. The origin of this correlation, however, remains elusive from a physical perspective; while local thickness perturbations on the ice shelf (thinning) generally correspond to local increases in buttressing, their integrated impact on changes at the grounding line are exactly the opposite (buttressing at the grounding line decreases and ice flux at the grounding line increases). This and additional complications encountered when examining realistic domains motivates us to seek an alternative approach, an adjoint-based method for calculating the sensitivity of the integrated grounding line flux to local changes in ice shelf geometry. We show that the adjoint-based sensitivity is identical to that deduced from pointwise, diagnostic model perturbation experiments. Based on its much wider applicability and the significant computational savings, we propose that the adjoint-based method is ideally suited for assessing grounding line flux sensitivity to changes in sub-ice shelf melting.

scholarly journals Diagnosing the sensitivity of grounding-line flux to changes in sub-ice-shelf melting

2020 ◽  
Vol 14 (10) ◽  
pp. 3407-3424
Author(s):  
Tong Zhang ◽  
Stephen F. Price ◽  
Matthew J. Hoffman ◽  
Mauro Perego ◽  
Xylar Asay-Davis
Keyword(s):  
Principal Stress ◽  
Flow Model ◽  
Ice Thickness ◽  
Diagnostic Model ◽  
Line Flux ◽  
Ice Shelf ◽  
Ice Flow ◽  
Alternative Approach ◽  
Grounding Line ◽  
Physically Based

Abstract. Using a numerical ice flow model, we study changes in ice shelf buttressing and grounding-line flux due to localized ice thickness perturbations, a proxy for localized changes in sub-ice-shelf melting. From our experiments, applied to idealized (MISMIP+) and realistic (Larsen C) ice shelf domains, we identify a correlation between a locally derived buttressing number on the ice shelf, based on the first principal stress, and changes in the integrated grounding-line flux. The origin of this correlation, however, remains elusive from the perspective of a theoretical or physically based understanding. This and the fact that the correlation is generally much poorer when applied to realistic ice shelf domains motivate us to seek an alternative approach for predicting changes in grounding-line flux. We therefore propose an adjoint-based method for calculating the sensitivity of the integrated grounding-line flux to local changes in ice shelf geometry. We show that the adjoint-based sensitivity is identical to that deduced from pointwise, diagnostic model perturbation experiments. Based on its much wider applicability and the significant computational savings, we propose that the adjoint-based method is ideally suited for assessing grounding-line flux sensitivity to changes in sub-ice-shelf melting.

The effect of Antarctic ice-shelf extent on ice discharge

2021 ◽  
Author(s):  
Jim Jordan ◽  
HIlmar Gudmundsson ◽  
Adrian Jenkins ◽  
Chris Stokes ◽  
Stewart Jamiesson ◽  
...  
Keyword(s):  
Mass Loss ◽  
Recent Work ◽  
Flow Model ◽  
Ice Shelf ◽  
Geophysical Research ◽  
Ice Flow ◽  
Ice Shelves ◽  
Grounding Line ◽  
Natural Variance ◽  
Observational Record

<div>The buttressing strength of Antarctic ice shelves directly effects the amount of ice discharge across the grounding line, with buttressing strength affected by both the thickness and extent of an ice shelf. Recent work has shown that a reduction in ice-shelf buttressing due to ocean induced ice-shelf thinning is responsible for a significant portion of increased Antarctic ice discharge (Gudmundsson et al., 2019, but few studies have attempted to show the effect of variability in ice-shelf extent on ice discharge. This variability arises due to ice-shelf calving following a cycle of long periods of slow, continuous calving interposed with calving of large, discrete sections.  These discrete calving events tend to occur on a comparative timeframe to that of the observational record. As such, when determining observed changes in ice discharge it is crucial that this natural variability is separated from any observed trends.  </div><div> </div><div>In this work we use the numerical ice-flow model Úa in combination with observations of ice shelf extent to diagnostically calculate Antarctic ice discharge. These observations primarily date back to the 1970s, though for some ice shelves records exist back to the 1940s. We assemble an Antarctic wide model for two scenarios: 1) with ice shelves at their maximum observed extent and 2) with ice shelves at their minimum observed extent. We then compare these two scenarios to differences in the observed changes in Antarctic ice-discharge to determine how much can be attributed to natural variance .</div><p> </p><p><span>Gudmundsson, G. H.</span><span>, Paolo, F. S., Adusumilli, S., & Fricker, H. A. (2019). </span>Instantaneous Antarctic ice‐ sheet mass loss driven by thinning ice shelves. <em>Geophysical Research Letters</em>, 46, 13903– 13909. </p>

scholarly journals Numerical simulations of the ice flow dynamics of George VI Ice Shelf, Antarctica

2007 ◽  
Vol 53 (183) ◽  
pp. 659-664 ◽  
Author(s):  
Angelika Humbert
Keyword(s):  
High Spatial Resolution ◽  
Thickness Distribution ◽  
Satellite System ◽  
Flow Dynamics ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Flow Geometry ◽  
Grounding Line

A diagnostic, dynamic/thermodynamic ice-shelf model is applied to the George VI Ice Shelf, situated in the Bellinghausen Sea, Antarctica. The George VI Ice Shelf has a peculiar flow geometry which sets it apart from other ice shelves. Inflow occurs along the two longest, and almost parallel, sides, whereas outflow occurs on the two ice fronts that are relatively short and situated at opposite ends of the ice shelf. Two data sources were used to derive the ice thickness distribution: conventional radioecho sounding from the British Antarctic Survey was combined with thickness inferred from surface elevation obtained by the NASA GLAS satellite system assuming hydrostatic equilibrium. We simulate the present ice flow over the ice shelf that results from the ice thickness distribution, the inflow at the grounding line and the flow rate factor. The high spatial resolution of the ice thickness distribution leads to very detailed simulations. The flow field has some extraordinary elements (e.g. the stagnation point characteristics resulting from the unusual ice-shelf geometry).

scholarly journals Channelization of plumes beneath ice shelves

2015 ◽  
Vol 785 ◽  
pp. 109-134 ◽  
Author(s):  
M. C. Dallaston ◽  
I. J. Hewitt ◽  
A. J. Wells
Keyword(s):  
Turbulent Mixing ◽  
Simplified Model ◽  
Ice Thickness ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Narrow Channels ◽  
One Dimensional ◽  
Grounding Line ◽  
The One

We study a simplified model of ice–ocean interaction beneath a floating ice shelf, and investigate the possibility for channels to form in the ice shelf base due to spatial variations in conditions at the grounding line. The model combines an extensional thin-film description of viscous ice flow in the shelf, with melting at its base driven by a turbulent ocean plume. Small transverse perturbations to the one-dimensional steady state are considered, driven either by ice thickness or subglacial discharge variations across the grounding line. Either forcing leads to the growth of channels downstream, with melting driven by locally enhanced ocean velocities, and thus heat transfer. Narrow channels are smoothed out due to turbulent mixing in the ocean plume, leading to a preferred wavelength for channel growth. In the absence of perturbations at the grounding line, linear stability analysis suggests that the one-dimensional state is stable to initial perturbations, chiefly due to the background ice advection.

scholarly journals Force-perturbation analysis of Pine Island Glacier, Antarctica, suggests cause for recent acceleration

2004 ◽  
Vol 39 ◽  
pp. 133-138 ◽  
Author(s):  
Robert Thomas ◽  
Eric Rignot ◽  
Pannirselvam Kanagaratnam ◽  
William Krabill ◽  
Gino Casassa
Keyword(s):  
Surface Strain ◽  
Ice Thickness ◽  
Strain Rates ◽  
West Antarctica ◽  
Ice Shelf ◽  
Amundsen Sea ◽  
Ice Flow ◽  
Velocity Increase ◽  
Grounding Line ◽  
Force Perturbation

AbstractPine Island Glacier, flowing into the Amundsen Sea from West Antarctica, thinned substantially during the 1990s, its grounding line receded by several km, and its velocity increased by >10% to values approaching 3 km a–1. Here, we use these observations, together with estimates of ice thickness and surface strain rates, to estimate the perturbation in forces resisting ice flow compatible with the observations. The analysis assumes that such perturbations are transmitted far upstream from where they originate, and that creep response to the perturbations can be described by equations similar to those that govern ice-shelf creep. It indicates that observed acceleration between 1996 and 2000 could have been caused by progressive ungrounding within the most seaward 25 km ‘ice plain’ of the grounded glacier. Earlier retreat and thinning of the glacier’s floating ice shelf may have provided the conditions that initiated ungrounding of the ice plain. Our analysis indicates that continued ice-plain thinning at the current rate of about 2 ma–1 will result in a velocity increase by 1 km a–1 within the next 11 years as the ice plain becomes totally ungrounded.

scholarly journals The sensitivity of Cook Glacier, East Antarctica, to changes in ice-shelf extent and grounding-line position

2021 ◽  
pp. 1-13
Author(s):  
James R. Jordan ◽  
G. Hilmar Gudmundsson ◽  
Adrian Jenkins ◽  
Chris R. Stokes ◽  
Bertie W. J. Miles ◽  
...  
Keyword(s):  
Flow Model ◽  
East Antarctica ◽  
Line Position ◽  
Ice Shelf ◽  
Ice Flow ◽  
Mean Sea Level ◽  
Front Position ◽  
Grounding Line ◽  
Global Mean Sea Level ◽  
Global Mean

Abstract The Wilkes Subglacial Basin in East Antarctica contains ice equivalent to 3–4 m of global mean sea level rise and is primarily drained by Cook Glacier. Of concern is that recent observations (since the 1970s) show an acceleration in ice speed over the grounding line of both the Eastern and Western portions of Cook Glacier. Here, we use a numerical ice-flow model (Úa) to simulate the instantaneous effects of observed changes at the terminus of Cook Glacier in order to understand the link between these changes and recently observed ice acceleration. Simulations suggest that the acceleration of Cook West was caused by a retreat in calving-front position in the 1970s, potentially enhanced by grounding-line retreat, while acceleration of Cook East was likely caused by ice-shelf thinning and grounding-line retreat in the mid-1990s. Moreover, we show that the instantaneous ice discharge at Cook East would increase by up to 85% if the whole ice shelf is removed and it ungrounds from a pinning point; and that the discharge at Cook West could increase by ~300% if its grounding line retreated by 10 km.

scholarly journals Evidence for rapid retreat and mass loss of Thwaites Glacier, West Antarctica

2001 ◽  
Vol 47 (157) ◽  
pp. 213-222 ◽  
Author(s):  
Eric Rignot
Keyword(s):  
Ice Thickness ◽  
Interferometric Synthetic Aperture Radar ◽  
West Antarctica ◽  
Ice Shelf ◽  
Amundsen Sea ◽  
Ice Flow ◽  
Grounding Line ◽  
Satellite Radar ◽  
Inland Ice ◽  
Satellite Radar Altimetry

AbstractThwaites Glacier, the second largest ice stream in West Antarctica, drains an area of 166 500 ± 2000 km2 which accumulates 55 ± 5 Gt a−1 (or 60 ± 6 km3 ice a−1) into the Amundsen Sea, unrestrained by an ice shelf. Using interferometric synthetic-aperture radar (InSAR) data collected by the European Remote-sensing Satellites (ERS-1 and -2) in 1996, an output flux of 71 ±7 Gt a−1 (or 77 ± 8 km3 ice a−1) is estimated at the grounding line, where ice thickness is deduced from hydrostatic equilibrium. A similar flux, 70 ± 7 Gt a−1 (or 76 ± 8 km3 ice a−1), is obtained at a gate located 20 km upstream, where ice thickness was measured in 1978 by ice-sounding radar. Total accumulation in between the two gates is 1.6 Gt a−1, or 1.8 km3 ice a−1. Ice discharge therefore exceeds mass accumulation by 30 ± 15%, and Thwaites Glacier must be thinning and retreating at present. The InSAR data show that the glacier floating ice tongue exerts no back pressure on the inland ice, calves into tabular icebergs along a significant fraction of its grounding line, and has a grounding-line thickness which exceeds a prior-calculated limit for stability. Glacier thinning is confirmed at the coast by the detection of a 1.4 ± 0.2 km retreat of its grounding line between 1992 and 1996 with InSAR, which implies 3.2 ± 0.6 m ice a−1 thinning at the glacier center and less near the sides. These results complement the decimeter-scale annual surface lowering observed with satellite radar altimetry several hundred km inland of the grounding line. The magnitude of ice thinning estimated at the coast, however, rules out temporal changes in accumulation as the explanation for surface lowering. Ice thinning must be due to changes in ice flow.

scholarly journals Ice-shelf retreat drives recent Pine Island Glacier speedup

2021 ◽  
Vol 7 (24) ◽  
pp. eabg3080
Author(s):  
Ian Joughin ◽  
Daniel Shapero ◽  
Ben Smith ◽  
Pierre Dutrieux ◽  
Mark Barham
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Flow Model ◽  
Velocity Data ◽  
Ice Shelf ◽  
Ice Flow ◽  
The Past ◽  
Grounding Line

Speedup of Pine Island Glacier over the past several decades has made it Antarctica’s largest contributor to sea-level rise. The past speedup is largely due to grounding-line retreat in response to ocean-induced thinning that reduced ice-shelf buttressing. While speeds remained fairly steady from 2009 to late 2017, our Copernicus Sentinel 1A/B–derived velocity data show a >12% speedup over the past 3 years, coincident with a 19-km retreat of the ice shelf. We use an ice-flow model to simulate this loss, finding that accelerated calving can explain the recent speedup, independent of the grounding-line, melt-driven processes responsible for past speedups. If the ice shelf’s rapid retreat continues, it could further destabilize the glacier far sooner than would be expected due to surface- or ocean-melting processes.

scholarly journals On the substantial influence of the treatment of friction at the grounding line

2015 ◽  
Vol 9 (3) ◽  
pp. 3475-3501 ◽  
Author(s):  
O. Gagliardini ◽  
J. Brondex ◽  
F. Gillet-Chaulet ◽  
L. Tavard ◽  
V. Peyaud ◽  
...  
Keyword(s):  
Flow Model ◽  
Ice Sheets ◽  
Velocity Fields ◽  
Practical Implementation ◽  
Total Force ◽  
Model Intercomparison ◽  
Fe Method ◽  
Ice Flow ◽  
Grounding Line ◽  
Diagnostic Experiment

Abstract. The dynamical contribution of marine ice sheets to sea level rise is largely controlled by grounding line (GL) dynamics. Seroussi et al. (2014) emphasised the sensitivity of numerical ice flow model results to the practical implementation of the friction of the ice on its bed in the very close vicinity of the GL. Elmer/Ice is a reference finite element (FE) ice flow model used in recent marine ice sheet model intercomparison (MISMIP) exercises. In the model, the GL is defined as the nodes where the ice is in contact with the bedrock but belong to both grounded and floating elements. Inherently to the FE method, computing the contribution of the friction by element requires evaluating the friction at the integration points. In Elmer/Ice, this is done by interpolating the values of the friction parameter C prescribed at the nodes. In this brief communication, we discuss and compare three alternative ways to prescribe the friction at the GL: (i) C is prescribed and non null at the GL nodes, (ii) C is set to zero at the GL nodes, and (iii) C is discontinuous at the GL nodes (i.e. is prescribed and non null for grounded elements and otherwise null). So far, all published results using Elmer/Ice were obtained with the first method. Using the MISMIP3d diagnostic experiment, we first show that, although the change in the total force at the base is insignificant, the three methods lead to significantly different velocity fields. We then show that these methods also lead to different steady state GL positions and different transient behaviours. Such model sensitivity to the methods discussed here is certainly specific to the high friction prescribed in the MISMIP experiments and should be smaller in real setups where friction in the vicinity of the GL would be expected to be lower. Results obtained with the three methods are available as Supplement for future comparisons.

scholarly journals Ice-shelf basal channels in a coupled ice/ocean model

2012 ◽  
Vol 58 (212) ◽  
pp. 1227-1244 ◽  
Author(s):  
Carl V. Gladish ◽  
David M. Holland ◽  
Paul R. Holland ◽  
Stephen F. Price
Keyword(s):  
Ocean Circulation ◽  
Coupled System ◽  
Ocean Model ◽  
Subsurface Water ◽  
Ice Thickness ◽  
Ice Shelf ◽  
First Approximation ◽  
Grounding Line ◽  
History Of ◽  
Basal Melting

AbstractA numerical model for an interacting ice shelf and ocean is presented in which the ice- shelf base exhibits a channelized morphology similar to that observed beneath Petermann Gletscher’s (Greenland) floating ice shelf. Channels are initiated by irregularities in the ice along the grounding line and then enlarged by ocean melting. To a first approximation, spatially variable basal melting seaward of the grounding line acts as a steel-rule die or a stencil, imparting a channelized form to the ice base as it passes by. Ocean circulation in the region of high melt is inertial in the along-channel direction and geostrophically balanced in the transverse direction. Melt rates depend on the wavelength of imposed variations in ice thickness where it enters the shelf, with shorter wavelengths reducing overall melting. Petermann Gletscher’s narrow basal channels may therefore act to preserve the ice shelf against excessive melting. Overall melting in the model increases for a warming of the subsurface water. The same sensitivity holds for very slight cooling, but for cooling of a few tenths of a degree a reorganization of the spatial pattern of melting leads, surprisingly, to catastrophic thinning of the ice shelf 12 km from the grounding line. Subglacial discharge of fresh water along the grounding line increases overall melting. The eventual steady state depends on when discharge is initiated in the transient history of the ice, showing that multiple steady states of the coupled system exist in general.

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