scholarly journals Bed topography and marine ice-sheet stability

2021 ◽  
pp. 1-15
Author(s):  
Olga V. Sergienko ◽  
Duncan J. Wingham
Keyword(s):  
Accumulation Rate ◽  
Stress Gradient ◽  
Ice Sheet ◽  
Transcendental Equation ◽  
Line Position ◽  
Line Flux ◽  
Ice Streams ◽  
Bed Topography ◽  
Grounding Line ◽  
The Stability

Abstract This paper examines the effect of basal topography and strength on the grounding-line position, flux and stability of rapidly-sliding ice streams. It does so by supposing that the buoyancy of the ice stream is small, and of the same order as the longitudinal stress gradient. Making this scaling assumption makes the role of the basal gradient and accumulation rate explicit in the lowest order expression for the ice flux at the grounding line and also provides the transcendental equation for the grounding-line position. It also introduces into the stability condition terms in the basal curvature and accumulation-rate gradient. These expressions revert to well-established expressions in circumstances in which the thickness gradient is large at the grounding line, a result which is shown to be the consequence of the non-linearity of the flow. The behaviour of the grounding-line flux is illustrated for a range of bed topographies and strengths. We show that, when bed topography at a horizontal scale of several tens of ice thicknesses is present, the grounding-line flux and stability have more complex dependencies on bed gradient than that associated with the ‘marine ice-sheet instability hypothesis’, and that unstable grounding-line positions can occur on prograde beds as well as stable positions on retrograde beds.

scholarly journals Grounding line stability in a regime of low driving and basal stresses

2019 ◽  
Vol 65 (253) ◽  
pp. 833-849 ◽  
Author(s):  
O. V. Sergienko ◽  
D. J. Wingham
Keyword(s):  
Accumulation Rate ◽  
Complex Function ◽  
Ice Sheet ◽  
Steady States ◽  
Climate System ◽  
Ice Streams ◽  
Ice Shelves ◽  
Grounding Line ◽  
Ice Flows ◽  
The Stability

AbstractThe dynamics of a marine ice sheet's grounding lines determine the rate of ice discharge from the grounded part of ice sheet into surrounding oceans. In many locations in West Antarctica ice flows into ice shelves through ice streams experiencing low driving stress. However, existing simple theories of marine ice sheets are developed under the assumption of high basal and driving stress. Here we analyze the grounding line behavior of marine ice streams experiencing low basal shear and driving stress. We find that in this regime, the ice flux at the grounding line is a complex function of the geometry of the ice-stream bed, net accumulation rate and gradient of the net accumulation rate. Our analysis shows that the stability of distinct steady states is determined by the same parameters, suggesting a more complex (in)stability criterion than what is commonly referred to within the context of the ‘marine ice-sheet instability hypothesis’. We also determine characteristic timescales (e-folding time) of ice-sheet configurations perturbed from their steady states. These timescales can be used to determine whether particular configurations can be considered in isolation from other components of the climate system or whether their effects and feedbacks between the ice sheet and the rest of the climate system have to be taken into account.

The effects of bed topography and strength on stability of marine ice sheets

2021 ◽  
Author(s):  
Olga Sergienko ◽  
Duncan Wingham
Keyword(s):  
Accumulation Rate ◽  
Ice Sheets ◽  
Ice Sheet ◽  
West Antarctica ◽  
Amundsen Sea ◽  
Bed Topography ◽  
Wide Range ◽  
Grounding Line ◽  
Bed Slope ◽  
Conditions Of Stability

<p>The "marine ice-sheet instability hypothesis", which states that unconfined marine ice sheets are unconditionally unstable on retrograde slopes, was developed under assumptions of negligible bed slopes. Realistic ice sheets, however, flow over beds which topographies have a wide range of bed slopes (for example, Thwaites Glacier in the Amundsen Sea sector, West Antarctica). Reexamining the original model of marine ice sheets proposed by Schoof (2007), and relaxing an assumption of negligible bed slopes, we find that a steady-state ice flux at the grounding line is an implicit function of the grounding-line ice thickness, bed slope and accumulation rate. Depending on the sliding conditions, the magnitudes of the ice flux at the grounding line differ by one-to-three orders of magnitudes from that computed with a power-law expression derived by Schoof (2007) under assumptions of the negligible bed slopes. Non-negligible bed slopes also result in conditions of stability of the grounding line that are significantly more complex than those associated with the "marine ice sheet instability hypothesis". Bed slopes are no longer the sole determinant of whether the grounding line is stable or unstable. We find that the grounding line can be stable on beds with retrograde slopes and unstable on beds with prograde slopes. </p>

scholarly journals Of isbræ and ice streams

2003 ◽  
Vol 36 ◽  
pp. 66-72 ◽  
Author(s):  
Martin Truffer ◽  
Keith A. Echelmeyer
Keyword(s):  
Ice Sheet ◽  
Shear Zones ◽  
West Antarctica ◽  
Ice Streams ◽  
Ice Flow ◽  
Bed Topography ◽  
Ice Stream ◽  
Fast Ice ◽  
Fast Flow ◽  
End Members

AbstractFast-flowing ice streams and outlet glaciers provide the major avenues for ice flow from past and present ice sheets. These ice streams move faster than the surrounding ice sheet by a factor of 100 or more. Several mechanisms for fast ice-stream flow have been identified, leading to a spectrum of different ice-stream types. In this paper we discuss the two end members of this spectrum, which we term the “ice-stream” type (represented by the Siple Coast ice streams in West Antarctica) and the “isbræ” type (represented by Jakobshavn Isbræ in Greenland). The typical ice stream is wide, relatively shallow (∼1000 m), has a low surface slope and driving stress (∼10 kPa), and ice-stream location is not strongly controlled by bed topography. Fast flow is possible because the ice stream has a slippery bed, possibly underlain by weak, actively deforming sediments. The marginal shear zones are narrow and support most of the driving stress, and the ice deforms almost exclusively by transverse shear. The margins seem to be inherently unstable; they migrate, and there are plausible mechanisms for such ice streams to shut down. The isbræ type of ice stream is characterized by very high driving stresses, often exceeding 200 kPa. They flow through deep bedrock channels that are significantly deeper than the surrounding ice, and have steep surface slopes. Ice deformation includes vertical as well as lateral shear, and basal motion need not contribute significantly to the overall motion. The marginal shear zone stend to be wide relative to the isbræ width, and the location of isbræ and its margins is strongly controlled by bedrock topography. They are stable features, and can only shut down if the high ice flux cannot be supplied from the adjacent ice sheet. Isbræs occur in Greenland and East Antarctica, and possibly parts of Pine Island and Thwaites Glaciers, West Antarctica. In this paper, we compare and contrast the two types of ice streams, addressing questions such as ice deformation, basal motion, subglacial hydrology, seasonality of ice flow, and stability of the ice streams.

scholarly journals Dynamical processes involved in the retreat of marine ice sheets

2001 ◽  
Vol 47 (157) ◽  
pp. 271-282 ◽  
Author(s):  
Richard C.A. Hindmarsh ◽  
E. Le Meur
Keyword(s):  
Ice Sheets ◽  
Ice Sheet ◽  
Ice Shelf ◽  
Ice Streams ◽  
Antarctic Ice Sheet ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Grounding Line ◽  
Neutral Equilibrium ◽  
Generally True

AbstractMarine ice sheets with mechanics described by the shallow-ice approximation by definition do not couple mechanically with the shelf. Such ice sheets are known to have neutral equilibria. We consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice-sheet retreat. The removal of ice-shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds. Sea-level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial West Antarctic ice-sheet retreat but is inconsistent with borehole evidence. Warming thins the ice sheet by reducing the average viscosity but does not lead to grounding-line retreat. Internal oscillations either specified or generated via a MacAyeal–Payne thermal mechanism promote migration. This is a noise-induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets. This migration occurs at quite slow rates, but these are sufficiently large to have possibly played a role in the dynamics of the West Antarctic ice sheet after the glacial maximum. Numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes, spatial variability coupled with dynamical variability is needed to cause margin movement.

scholarly journals Suppression of marine ice sheet instability

2018 ◽  
Vol 857 ◽  
pp. 648-680 ◽  
Author(s):  
Samuel S. Pegler
Keyword(s):  
Steady State ◽  
Rapid Assessment ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Stable Steady State ◽  
Ice Shelf ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Grounding Line ◽  
The Stability

A long-standing open question in glaciology concerns the propensity for ice sheets that lie predominantly submerged in the ocean (marine ice sheets) to destabilise under buoyancy. This paper addresses the processes by which a buoyancy-driven mechanism for the retreat and ultimate collapse of such ice sheets – the marine ice sheet instability – is suppressed by lateral stresses acting on its floating component (the ice shelf). The key results are to demonstrate the transition between a mode of stable (easily reversible) retreat along a stable steady-state branch created by ice-shelf buttressing to tipped (almost irreversible) retreat across a critical parametric threshold. The conditions for triggering tipped retreat can be controlled by the calving position and other properties of the ice-shelf profile and can be largely independent of basal stress, in contrast to principles established from studies of unbuttressed grounding-line dynamics. The stability and recovery conditions introduced by lateral stresses are analysed by developing a method of constructing grounding-line stability (bifurcation) diagrams, which provide a rapid assessment of the steady-state positions, their natures and the conditions for secondary grounding, giving clear visualisations of global stabilisation conditions. A further result is to reveal the possibility of a third structural component of a marine ice sheet that lies intermediate to the fully grounded and floating components. The region forms an extended grounding area in which the ice sheet lies very close to flotation, and there is no clearly distinguished grounding line. The formation of this region generates an upsurge in buttressing that provides the most feasible mechanism for reversal of a tipped grounding line. The results of this paper provide conceptual insight into the phenomena controlling the stability of the West Antarctic Ice Sheet, the collapse of which has the potential to dominate future contributions to global sea-level rise.

scholarly journals Hydrostatic grounding line parameterization in ice sheet models

2014 ◽  
Vol 8 (6) ◽  
pp. 2075-2087 ◽  
Author(s):  
H. Seroussi ◽  
M. Morlighem ◽  
E. Larour ◽  
E. Rignot ◽  
A. Khazendar
Keyword(s):  
Steady State ◽  
High Spatial Resolution ◽  
Numerical Models ◽  
Ice Sheet ◽  
System Model ◽  
Line Position ◽  
Grounding Line ◽  
Mesh Resolution ◽  
Averaged Model ◽  
Line Positions

Abstract. Modeling of grounding line migration is essential to accurately simulate the behavior of marine ice sheets and investigate their stability. Here, we assess the sensitivity of numerical models to the parameterization of the grounding line position. We run the MISMIP3D benchmark experiments using the Ice Sheet System Model (ISSM) and a two-dimensional shelfy-stream approximation (SSA) model with different mesh resolutions and different sub-element parameterizations of grounding line position. Results show that different grounding line parameterizations lead to different steady state grounding line positions as well as different retreat/advance rates. Our simulations explain why some vertically depth-averaged model simulations deviate significantly from the vast majority of simulations based on SSA in the MISMIP3D benchmark. The results reveal that differences between simulations performed with and without sub-element parameterization are as large as those performed with different approximations of the stress balance equations in this configuration. They also demonstrate that the reversibility test is passed at relatively coarse resolution while much finer resolutions are needed to accurately capture the steady-state grounding line position. We conclude that fixed grid SSA models that do not employ such a parameterization should be avoided, as they do not provide accurate estimates of grounding line dynamics, even at high spatial resolution. For models that include sub-element grounding line parameterization, in the MISMIP3D configuration, a mesh resolution finer than 2 km should be employed.

scholarly journals Actively surging West Antarctic ice streams and their response characteristics

1997 ◽  
Vol 24 ◽  
pp. 409-414 ◽  
Author(s):  
Robert Bindschadler
Keyword(s):  
Behavior Pattern ◽  
Accumulation Rate ◽  
Ice Sheet ◽  
Ice Streams ◽  
The West ◽  
Antarctic Ice Sheet ◽  
Thickness Change ◽  
West Antarctic Ice Sheet ◽  
Response Characteristics ◽  
West Antarctic

Ice Streams B, D and E, West Antarctica, all show a longitudinal pattern of ice thickness change that is consistent with ongoing surge behavior modeled for glaciers. The measured pattern is not consistent with model response of any other scenario such as accumulation-rate change or changes on the ice shelf. Inland migration of the ice-stream onset is a requirement of this behavior pattern. If such a surge is presently taking place, the remaining lifetime of the West Antarctic ice sheet is 1200–6000 years. A complete surge period lasting 50 000–120 000 years is hypothesized, with a relatively brief surge phase (lasting 16000–21 000 years) required to completely remove the West Antarctic ice sheet from its maximum extent. Applying classic glacier response theory demonstrates that the diffusive component of response is much faster for ice streams than for glaciers, making the identification of either kinematic waves or localized responses on ice streams unlikely.

scholarly journals Stochastic marine ice sheet variability

2018 ◽  
Vol 843 ◽  
pp. 748-777 ◽  
Author(s):  
T. E. Mulder ◽  
S. Baars ◽  
F. W. Wubs ◽  
H. A. Dijkstra
Keyword(s):  
Transition Probabilities ◽  
Accumulation Rate ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Field Measurements ◽  
Multiple Equilibrium ◽  
Short Time Scale ◽  
Two Dimensional ◽  
Grounding Line ◽  
Short Time

It is well known that deterministic two-dimensional marine ice sheets can only be stable if the grounding line is positioned at a sufficiently steep, downward sloping bedrock. When bedrock conditions favour instabilities, multiple stable ice sheet profiles may occur. Here, we employ continuation techniques to examine the sensitivity of a two-dimensional marine ice sheet to stochastic noise representing short time scale variability, either in the accumulation rate or in the sea level height. We find that in unique regimes, the position of the grounding line is most sensitive to noise in the accumulation rate and can explain excursions observed in field measurements. In the multiple equilibrium regime, there is a strong asymmetry in transition probabilities between the different ice sheet states, with a strong preference to switch to the branch with a steeper bedrock slope.

scholarly journals Isostatic Equilibrium Grounding Line Between The West Antarctic Ice Sheet And The Ross Ice Shelf

1979 ◽  
Vol 24 (90) ◽  
pp. 500 ◽  
Author(s):  
C. R. Bentley ◽  
L. Greischar
Keyword(s):  
Ice Sheet ◽  
Shelf Area ◽  
Ice Shelf ◽  
Ice Streams ◽  
West Antarctic Ice Sheet ◽  
Ross Ice Shelf ◽  
Isostatic Rebound ◽  
West Antarctic ◽  
Grounding Line ◽  
Eastern Half

Abstract Taking various retreat-rates for the presumed grounded ice sheet in the Ross embayment during Wisconsin time, as calculated by Thomas (Thomas and Bentley, 1978), and assuming a time constant of 4400 years for isostatic rebound, a sea-floor uplift of 100±50 m still to be expected in the grid western part of the Ross Ice Shelf can be calculated. The expected uplift diminishes from grid west to grid east, and is probably negligible in the eastern half of the shelf area. There are extensive areas near the present grounding line where the water depth beneath the shelf is less than 100 m, so that uplift would lead to grounding. As grounding occurred, the neighboring ice shelf would thicken, causing grounding to advance farther. This process would probably extend the grounding line to a position running grid north-eastward across the shelf from the seaward end of Roosevelt Island, deeply indented by the extensions of the present ice streams. Floating ice would remain in the grid south-eastern half of the shelf.

scholarly journals Resolution of ice streams and outlet glaciers in large-scale simulations of the Greenland ice sheet

2013 ◽  
Vol 54 (63) ◽  
pp. 209-220 ◽  
Author(s):  
Ralf Greve ◽  
Ute C. Herzfeld
Keyword(s):  
Sea Level ◽  
Large Scale ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Future Climate ◽  
Climate Control ◽  
Ice Streams ◽  
Bed Topography ◽  
The Future ◽  
Community Effort

AbstractThe dynamic/thermodynamic shallow-ice model SICOPOLIS is applied to the Greenland ice sheet. Paleoclimatic spin-ups from 125 ka BP until today, as well as future-climate experiments 500 years into the future, are carried out with three different grid spacings, namely 20, 10 and 5 km. The scenarios are a subset of those specified by the SeaRISE (Sea-level Response to Ice Sheet Evolution) community effort. The bed topography includes improved troughs for Jakobshavn Isbræ, Helheim, Kangerdlugssuaq and Petermann glaciers, processed by an algorithm that preserves shape, orientation and continuity of the troughs on the 5 km scale. Comparison of simulated and observed present-day surface velocities shows that these ice streams and outlet glaciers are resolved with different accuracies, ranging from poor (20 km grid) to reasonably good (5 km grid). In the future-climate experiments, the simulated absolute ice volumes depend significantly on the resolution, while the sensitivities (ice volumes relative to the constant-climate control run) vary only by a few centimeters of sea-level equivalent.

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