scholarly journals The shape of change: an EOF approach to identifying sources of transient thickness change in an ice shelf

2017 ◽  
Vol 58 (74) ◽  
pp. 21-27 ◽  
Author(s):  
Adam J. Campbell ◽  
Christina L. Hulbe ◽  
Choon-Ki Lee
Keyword(s):  
Statistical Approach ◽  
Flow Model ◽  
Response Surfaces ◽  
Ice Shelf ◽  
Ice Streams ◽  
Thickness Change ◽  
Ross Ice Shelf ◽  
Spatial Response ◽  
Shelf Flow ◽  
Flux Perturbation

ABSTRACT Ross Ice Shelf (RIS) is known to experience transient thickness change due to changes in the flow of its tributary ice streams and glaciers and this may complicate identification of external, climate-forced signals in contemporary observations of ice shelf thinning and thickening. Flux changes at the lateral boundaries produce both instantaneous and longer timescale adjustments in the coupled velocity and thickness fields. Here, we adapt a statistical approach to output from a numerical model of ice shelf flow to identify characteristic patterns (spatial response surfaces) associated with stepped and cyclic perturbations to boundary fluxes. Once known, characteristic patterns identified in observational data may be attributed to specific sources. An example involving discharge of Byrd Glacier into RIS is described. We find that spatial response surfaces for thickness and velocity generated in individual flow model experiments appear to be independent of flux perturbation shape and magnitude. Additionally, recent acceleration of Byrd Glacier is apparent in ICESat-detected change in RIS thickness.

scholarly journals The Effects of Basal Melting on the Present Flow of the Ross Ice Shelf, Antarctica

1986 ◽  
Vol 32 (110) ◽  
pp. 72-86 ◽  
Author(s):  
D.R. MacAyeal ◽  
R.H. Thomas
Keyword(s):  
Large Scale ◽  
Oceanic Circulation ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ross Ice Shelf ◽  
Flow And Heat Transfer ◽  
Improve Model ◽  
Dependent Flow ◽  
Basal Melting ◽  
Shelf Flow

AbstractWe use a hybrid finite-element/finite-difference model of ice-shelf flow and heat transfer to investigate the effects of basal melting on the present observed flow of the Ross Ice Shelf, Two hypothetical basal melting scenarios are compared: (i) zero melting everywhere and (ii) melting sufficient to balance any large-scale patterns of ice-shelf thickening that would otherwise occur. As a result of the temperature-dependent flow law (which we idealize as having a constant activation energy of 120 kJ mol−1, a scaling coefficient of 1.3 N m−2s1/3, and an exponent of 3), simulated ice-shelf velocities for the second scenario are reduced by up to 20% below those of the first. Our results support the hypothesis that melting patterns presently maintain ice thickness in steady state and conform to patterns of oceanic circulation presently thought to ventilate the sub-ice cavity. Differences between the simulated and observed velocities are too large in the extreme south-eastern quarter of the ice shelf to permit verification of either basal melting scenario. These differences highlight the need to improve model boundary conditions at points where ice streams feed the ice shelf and where the ice shelf meets stagnant grounded ice.

scholarly journals The Effects of Basal Melting on the Present Flow of the Ross Ice Shelf, Antarctica

1986 ◽  
Vol 32 (110) ◽  
pp. 72-86 ◽  
Author(s):  
D.R. MacAyeal ◽  
R.H. Thomas
Keyword(s):  
Large Scale ◽  
Oceanic Circulation ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ross Ice Shelf ◽  
Flow And Heat Transfer ◽  
Improve Model ◽  
Dependent Flow ◽  
Basal Melting ◽  
Shelf Flow

AbstractWe use a hybrid finite-element/finite-difference model of ice-shelf flow and heat transfer to investigate the effects of basal melting on the present observed flow of the Ross Ice Shelf, Two hypothetical basal melting scenarios are compared: (i) zero melting everywhere and (ii) melting sufficient to balance any large-scale patterns of ice-shelf thickening that would otherwise occur. As a result of the temperature-dependent flow law (which we idealize as having a constant activation energy of 120 kJ mol−1, a scaling coefficient of 1.3 N m−2 s1/3, and an exponent of 3), simulated ice-shelf velocities for the second scenario are reduced by up to 20% below those of the first. Our results support the hypothesis that melting patterns presently maintain ice thickness in steady state and conform to patterns of oceanic circulation presently thought to ventilate the sub-ice cavity. Differences between the simulated and observed velocities are too large in the extreme south-eastern quarter of the ice shelf to permit verification of either basal melting scenario. These differences highlight the need to improve model boundary conditions at points where ice streams feed the ice shelf and where the ice shelf meets stagnant grounded ice.

scholarly journals Numerical Modeling of Ice-Shelf Motion

1982 ◽  
Vol 3 ◽  
pp. 189-194 ◽  
Author(s):  
D. R. MacAyeal ◽  
R. H. Thomas
Keyword(s):  
Ice Thickness ◽  
Subgrid Scale ◽  
Good Qualitative Agreement ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ross Ice Shelf ◽  
Sea Bed ◽  
Flow Law ◽  
Stress Variability ◽  
Shelf Flow

Finite-element simulations of the Ross Ice Shelf flow yield good qualitative agreement with observation, but velocity magnitudes tend to be too low up-stream of ice rises and too high near the ice front. These errors will be significantly reduced by better model convergence to the flow law, more accurate values of the flow-law parameters, better resolution of the ice streams, and inclusion of subgrid scale stress variability produced by crevassing and ice-fabric orientation. A model run featuring a hypothetical ice rise down-stream of Crary Ice Rise indicates that the distinctive strain-rate patterns produced by an isolated ice rise can be cancelled by neighboring ice rises. This suggests that an isolated ice rise might migrate up-stream along sea-bed ridges unless there are neighboring ice rises acting to stabilize the surrounding ice-thickness tendencies.

scholarly journals Numerical Modeling of Ice-Shelf Motion

1982 ◽  
Vol 3 ◽  
pp. 189-194 ◽  
Author(s):  
D. R. MacAyeal ◽  
R. H. Thomas
Keyword(s):  
Ice Thickness ◽  
Subgrid Scale ◽  
Good Qualitative Agreement ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ross Ice Shelf ◽  
Sea Bed ◽  
Flow Law ◽  
Stress Variability ◽  
Shelf Flow

Finite-element simulations of the Ross Ice Shelf flow yield good qualitative agreement with observation, but velocity magnitudes tend to be too low up-stream of ice rises and too high near the ice front. These errors will be significantly reduced by better model convergence to the flow law, more accurate values of the flow-law parameters, better resolution of the ice streams, and inclusion of subgrid scale stress variability produced by crevassing and ice-fabric orientation. A model run featuring a hypothetical ice rise down-stream of Crary Ice Rise indicates that the distinctive strain-rate patterns produced by an isolated ice rise can be cancelled by neighboring ice rises. This suggests that an isolated ice rise might migrate up-stream along sea-bed ridges unless there are neighboring ice rises acting to stabilize the surrounding ice-thickness tendencies.

scholarly journals Tidal modulation of ice-shelf flow: a viscous model of the Ross Ice Shelf

2014 ◽  
Vol 60 (221) ◽  
pp. 500-508 ◽  
Author(s):  
Kelly M. Brunt ◽  
Douglas R. MacAyeal
Keyword(s):  
Ross Sea ◽  
Element Model ◽  
Tidal Effects ◽  
Ice Shelf ◽  
Ice Streams ◽  
Indirect Response ◽  
Ross Ice Shelf ◽  
Creep Flow ◽  
Sea Surface Slopes ◽  
Shelf Flow

AbstractThree stations near the calving front of the Ross Ice Shelf, Antarctica, recorded GPS data through a full spring–neap tidal cycle in November 2005. The data revealed a diurnal horizontal motion that varied both along and transverse to the long-term average velocity direction, similar to tidal signals observed in other ice shelves and ice streams. Based on its periodicity, it was hypothesized that the signal represents a flow response of the Ross Ice Shelf to the diurnal tides of the Ross Sea. To assess the influence of the tide on the ice-shelf motion, two hypotheses were developed. The first addressed the direct response of the ice shelf to tidal forcing, such as forces due to sea-surface slopes or forces due to sub-ice-shelf currents. The second involved the indirect response of ice-shelf flow to the tidal signals observed in the ice streams that source the ice shelf. A finite-element model, based on viscous creep flow, was developed to test these hypotheses, but succeeded only in falsifying both hypotheses, i.e. showing that direct tidal effects produce too small a response, and indirect tidal effects produce a response that is not smooth in time. This nullification suggests that a combination of viscous and elastic deformation is required to explain the observations.

scholarly journals A Time-Dependent Simulation of the Ross Ice Shelf Flow (Abstract)

1984 ◽  
Vol 5 ◽  
pp. 217-219 ◽  
Author(s):  
Douglas R. MacAyeal ◽  
Robert H. Thomas
Keyword(s):  
Element Model ◽  
Momentum Balance ◽  
Ice Shelf ◽  
Time Step ◽  
Ross Ice Shelf ◽  
Time Marching ◽  
Accelerate Convergence ◽  
Spreading Rates ◽  
Shelf Flow ◽  
The Finite Element Model

The finite-element model discussed by MacAyeal and Thomas (1982) has been improved to include solution of the heat equation within each element, and to accelerate convergence to solution of the momentum-balance equations in terms of ice-shelf spreading rates. The model is now sufficiently rapid to permit both snapshot and time-marching simulations of a large ice shelf at high spatial and temporal resolution (grid size 10 km; time step 0.1 a). Here, we describe three model simulations of Ross Ice Shelf behavior.

scholarly journals The Unsteady State of The Ross Ice Shelf (Abstract only)

1982 ◽  
Vol 3 ◽  
pp. 339
Author(s):  
Charles R. Bentley
Keyword(s):  
Ice Cores ◽  
Secular Change ◽  
Supporting Evidence ◽  
Ice Shelf ◽  
Outlet Glacier ◽  
Thickness Change ◽  
Ross Ice Shelf ◽  
Input Flux ◽  
Entire Flow ◽  
Flow Band

According to our present picture, the Ross Ice Shelf is subject to relatively rapid changes, perhaps constantly out of steady state, but not undergoing a long-term secular change. Recent supporting evidence comes from a flow band of ice extending from the edges of Beardmore Glacier as far as Nimrod Glacier. The boundaries of that band and of ice stemming from several individual glaciers within it have been traced on airborne radar records. Using measurements made as part of the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS) program, mass-flux variations along the bands have been calculated. The band from Nimrod Glacier, a major outlet glacier from the East Antarctic inland ice sheet, shows no significant deviations from zero for the sum of thence thickness change rate ∂H/∂t and bottom melt rate. ḃH. We interpret this to mean that ∂H/∂t and ḃH are separately small. Significant flux variations in the entire flow band are then attributed to relatively large variations in input flux from the alpine glaciers of the Transantarctic Mountains, and from zones between the glaciers. Although flux variations are not coherent between individual glacier bands, the average strengths of internal reflections (from bottom crevasses and/or included moraine), exhibit a semi-coherent variation with a period of 400 a that correlates with 180 variations in ice cores from Dome C and Byrd station.

scholarly journals On the Disintegration of Ice Shelves: the Role of Thinning

1982 ◽  
Vol 3 ◽  
pp. 146-151 ◽  
Author(s):  
T. J. Hughes
Keyword(s):  
Weak Links ◽  
Ice Melting ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ice Shelves ◽  
Ross Ice Shelf ◽  
Tidal Pumping ◽  
Grounding Line ◽  
Converging Flow

It is proposed that an ice shelf disintegrates when its calving front retreats faster than its grounding line. This paper examines the role of ice thinning in grounding-line retreat. Thinning occurs as a result of creep spreading and ice melting. Thinning by creep is examined for the general regime of bending converging flow in an ice shelf lying in a confined embayment, and at the grounding lines of ice streams that supply the ice shelf and ice rises where the ice shelf is grounded on bedrock. Thinning by melting is examined at these grounding lines for tidal pumping and for descent of surface melt water into strandline crevasses, where concentrated melting is focused at the supposed weak links that connect the ice shelf to its embayment, its ice streams, and its ice rises. Applications are made to the Ross Ice Shelf.

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 The supply of subglacial meltwater to the grounding line of the Siple Coast, West Antarctica

2012 ◽  
Vol 53 (60) ◽  
pp. 267-280 ◽  
Author(s):  
S.P. Carter ◽  
H.A. Fricker
Keyword(s):  
Temporal Variability ◽  
Freshwater Flux ◽  
Spatial And Temporal Variability ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ross Ice Shelf ◽  
Lake Volume ◽  
Grounding Line ◽  
Subglacial Lakes

AbstractRecent satellite studies have shown that active subglacial lakes exist under the Antarctic ice streams and persist almost to their grounding lines. When the lowest-lying lakes flood, the water crosses the grounding line and enters the sub-ice-shelf cavity. Modeling results suggest that this additional freshwater influx may significantly enhance melting at the ice-shelf base. We examine the spatial and temporal variability in subglacial water supply to the grounding lines of the Siple Coast ice streams, by combining estimates for lake volume change derived from Ice, Cloud and land Elevation Satellite (ICESat) data with a model for subglacial water transport. Our results suggest that subglacial outflow tends to concentrate towards six embayments in the Siple Coast grounding line. Although mean grounding line outflow is ~60m3 s–1 for the entire Siple Coast, maximum local grounding line outflow may temporarily exceed 300 m3 s–1 during the synchronized flooding of multiple lakes in a hydrologic basin. Variability in subglacial outflow due to subglacial lake drainage may account for a substantial portion of the observed variability in freshwater flux out of the Ross Ice Shelf cavity. The temporal variability in grounding line outflow results in a net reduction in long-term average melt rate, but temporary peak melting rates may exceed the long-term average by a factor of three.

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