scholarly journals Exploring mechanisms responsible for tidal modulation in flow of the Filchner-Ronne Ice Shelf

2019 ◽  
Author(s):  
Sebastian H. R. Rosier ◽  
G. Hilmar Gudmundsson
Keyword(s):  
Large Scale ◽  
Elastic Response ◽  
Lateral Migration ◽  
Ice Shelf ◽  
Ice Flow ◽  
Flow Perturbation ◽  
Tidal Motion ◽  
Shelf Slope ◽  
Shelf Flow ◽  
Tidal Response

Abstract. An extensive network of GPS sites on the Filchner-Ronne Ice Shelf and adjoining ice streams show strong tidal modulation of horizontal ice flow at a range of frequencies. A particularly strong (horizontal) response is found at the fortnightly (Msf) frequency. Since this tidal constituent is absent in the (vertical) tidal forcing, this observation implies the action of some nonlinear mechanism. Another striking aspect is the strong amplitude of the flow perturbation, causing a periodic reversal in the direction of ice shelf flow in some areas, and a 10–20 % change in speed at grounding lines. No model has yet been able to reproduce the quantitative aspects of the observed tidal modulation on the Filchner-Ronne Ice Shelf. The cause of the tidal response has therefore remained an enigma, indicating a serious limitation in our current understanding of the mechanics of large-scale ice flow. A further limitation of previous studies is that they have all focused on isolated regions and interactions between different areas have, therefore, not been fully accounted for. Here, we conduct the first large-scale ice-flow modelling study to explore these processes using a viscoelastic rheology and realistic geometry of the entire Filchner-Ronne ice shelf, where the best observations of tidal response are available. We evaluate all the relevant mechanisms that have hitherto been put forward to explain how tides might affect ice-shelf flow and compare our results with observational data. We conclude that, while some are able to generate the correct general qualitative aspects of the tidally-induced perturbations in ice flow, most of these mechanisms must be ruled out as being the primary cause of the large observed nonlinear response. We find that only tidally-induced lateral migration of grounding lines can generate a sufficiently strong long-periodic Msf response on the ice shelf to match observations. Furthermore, we show that the observed short-periodic diurnal tidal motion, causing twice-daily flow reversals at the ice front, can be generated through a purely elastic response to basin-wide tidal perturbations in the ice shelf slope. This model also allows us to quantify the effect of tides on mean ice flow and we find that the Filchner-Ronne Ice Shelf flows on average ~ 21 % faster than it would in the absence of large ocean tides.

scholarly journals Exploring mechanisms responsible for tidal modulation in flow of the Filchner–Ronne Ice Shelf

2020 ◽  
Vol 14 (1) ◽  
pp. 17-37 ◽  
Author(s):  
Sebastian H. R. Rosier ◽  
G. Hilmar Gudmundsson
Keyword(s):  
Large Scale ◽  
Elastic Response ◽  
Lateral Migration ◽  
Ice Shelf ◽  
Ice Flow ◽  
Flow Perturbation ◽  
Long Period ◽  
Shelf Slope ◽  
Short Period ◽  
Shelf Flow

Abstract. An extensive network of GPS sites on the Filchner–Ronne Ice Shelf and adjoining ice streams shows strong tidal modulation of horizontal ice flow at a range of frequencies. A particularly strong (horizontal) response is found at the fortnightly (Msf) frequency. Since this tidal constituent is absent in the (vertical) tidal forcing, this observation implies the action of some non-linear mechanism. Another striking aspect is the strong amplitude of the flow perturbation, causing a periodic reversal in the direction of ice shelf flow in some areas and a 10 %–20 % change in speed at grounding lines. No model has yet been able to reproduce the quantitative aspects of the observed tidal modulation across the entire Filchner–Ronne Ice Shelf. The cause of the tidal ice flow response has, therefore, remained an enigma, indicating a serious limitation in our current understanding of the mechanics of large-scale ice flow. A further limitation of previous studies is that they have all focused on isolated regions and interactions between different areas have, therefore, not been fully accounted for. Here, we conduct the first large-scale ice flow modelling study to explore these processes using a viscoelastic rheology and realistic geometry of the entire Filchner–Ronne Ice Shelf, where the best observations of tidal response are available. We evaluate all relevant mechanisms that have hitherto been put forward to explain how tides might affect ice shelf flow and compare our results with observational data. We conclude that, while some are able to generate the correct general qualitative aspects of the tidally induced perturbations in ice flow, most of these mechanisms must be ruled out as being the primary cause of the observed long-period response. We find that only tidally induced lateral migration of grounding lines can generate a sufficiently strong long-period Msf response on the ice shelf to match observations. Furthermore, we show that the observed horizontal short-period semidiurnal tidal motion, causing twice-daily flow reversals at the ice front, can be generated through a purely elastic response to basin-wide tidal perturbations in the ice shelf slope. This model also allows us to quantify the effect of tides on mean ice flow and we find that the Filchner–Ronne Ice Shelf flows, on average, ∼ 21 % faster than it would in the absence of large ocean tides.

scholarly journals Tidal bending of ice shelves as a mechanism for large-scale temporal variations in ice flow

2017 ◽  
Author(s):  
Sebastian H. R. Rosier ◽  
G. Hilmar Gudmundsson
Keyword(s):  
Large Scale ◽  
Temporal Variations ◽  
Mean Flow ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Glacier Ice ◽  
Bending Stresses ◽  
Viscoelastic Modelling ◽  
Shelf Flow

Abstract. GPS measurements reveal strong modulation of horizontal ice-shelf and ice-stream flow at a variety of tidal frequencies, most notably a fortnightly (Msf) frequency not present in the vertical tides themselves. Current theories largely fail to explain the strength and prevalence of this signal over floating ice shelves. We propose that tidal bending stresses, through the nonlinear rheology of glacier ice, can have a sufficiently large impact on the effective viscosity of ice along its floating margins to give rise to significant and widespread temporal variations in the horizontal velocity of ice shelves. Using full-Stokes viscoelastic modelling, we show that inclusion of tidal bending within the model accounts for much of the observed tidal modulation of ice-shelf flow. Furthermore, our model shows that, in the absence of vertical tidal forcing, the mean flow of the ice shelf is reduced by almost 30 % for the geometry that we consider.

scholarly journals Tidal bending of ice shelves as a mechanism for large-scale temporal variations in ice flow

2018 ◽  
Vol 12 (5) ◽  
pp. 1699-1713 ◽  
Author(s):  
Sebastian H. R. Rosier ◽  
G. Hilmar Gudmundsson
Keyword(s):  
Stream Flow ◽  
Large Scale ◽  
Temporal Variations ◽  
Mean Flow ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
The Mean ◽  
Viscoelastic Modelling ◽  
Shelf Flow

Abstract. GPS measurements reveal strong modulation of horizontal ice shelf and ice stream flow at a variety of tidal frequencies, most notably a fortnightly (Msf) frequency not present in the vertical tides themselves. Current theories largely fail to explain the strength and prevalence of this signal over floating ice shelves. We show how well-known non-linear aspects of ice rheology can give rise to widespread, long-periodic tidal modulation in ice shelf flow, generated within ice shelves themselves through tidal flexure acting at diurnal and semidiurnal frequencies. Using full-Stokes viscoelastic modelling, we show that inclusion of tidal bending within the model accounts for much of the observed tidal modulation of ice shelf flow. Furthermore, our model shows that, in the absence of vertical tidal forcing, the mean flow of the ice shelf is reduced by almost 30 % for the geometry that we consider.

scholarly journals Implementing an empirical scalar constitutive relation for ice with flow-induced polycrystalline anisotropy in large-scale ice sheet models

2018 ◽  
Vol 12 (3) ◽  
pp. 1047-1067 ◽  
Author(s):  
Felicity S. Graham ◽  
Mathieu Morlighem ◽  
Roland C. Warner ◽  
Adam Treverrow
Keyword(s):  
Constitutive Relation ◽  
Enhancement Factor ◽  
Large Scale ◽  
Flow Line ◽  
Ice Sheet ◽  
Computationally Efficient ◽  
Ice Shelf ◽  
Stress Regime ◽  
Ice Flow ◽  
Polycrystalline Ice

Abstract. The microstructure of polycrystalline ice evolves under prolonged deformation, leading to anisotropic patterns of crystal orientations. The response of this material to applied stresses is not adequately described by the ice flow relation most commonly used in large-scale ice sheet models – the Glen flow relation. We present a preliminary assessment of the implementation in the Ice Sheet System Model (ISSM) of a computationally efficient, empirical, scalar, constitutive relation which addresses the influence of the dynamically steady-state flow-compatible induced anisotropic crystal orientation patterns that develop when ice is subjected to the same stress regime for a prolonged period – sometimes termed tertiary flow. We call this the ESTAR flow relation. The effect on ice flow dynamics is investigated by comparing idealised simulations using ESTAR and Glen flow relations, where we include in the latter an overall flow enhancement factor. For an idealised embayed ice shelf, the Glen flow relation overestimates velocities by up to 17 % when using an enhancement factor equivalent to the maximum value prescribed in the ESTAR relation. Importantly, no single Glen enhancement factor can accurately capture the spatial variations in flow across the ice shelf generated by the ESTAR flow relation. For flow line studies of idealised grounded flow over varying topography or variable basal friction – both scenarios dominated at depth by bed-parallel shear – the differences between simulated velocities using ESTAR and Glen flow relations depend on the value of the enhancement factor used to calibrate the Glen flow relation. These results demonstrate the importance of describing the deformation of anisotropic ice in a physically realistic manner, and have implications for simulations of ice sheet evolution used to reconstruct paleo-ice sheet extent and predict future ice sheet contributions to sea level.

scholarly journals Ice-shelf dynamics near the front of the Filchner-Ronne Ice Shelf, Antaretica, revealed by SAR interferometry: model/interferogram comparison

1998 ◽  
Vol 44 (147) ◽  
pp. 419-428 ◽  
Author(s):  
Douglas R MacAyeal ◽  
Eric Rignot ◽  
Christina L Hulbe
Keyword(s):  
Sea Ice ◽  
Large Scale ◽  
Sar Interferometry ◽  
Deformation Pattern ◽  
Ice Shelf ◽  
Mechanical Coupling ◽  
Iceberg Calving ◽  
Shelf Dynamics ◽  
Coastal Boundary ◽  
Shelf Flow

AbstractWe compare European remote-sensing satellite (ERS) synthetic aperture radar interferograms with artificial interferograms constructed using output of a finite-element ice-shelf flow model to study the dynamics of Filchner-Ronne Ice Shelf (FRIS), Antaretica, near Hemmen Ice Rise (HIR) where the iceberg-calving front intersects Berkner Island. We find that the model must account for rifts, mechanically competent sea ice which fills rifts, and ice softening in coastal boundary layers in order to agree with the ice-deformation pattern implied by observed interferograms. Analysis of the stress field in the model experiment that best matches the observed interferograms suggests that: (1) HIR introduces weakness into the ice shelf through the generation of large-scale rifts, and (2) the melange of sea ice and ice-shelf fragments that fills the rifts stabilizes the shelf front by providing mechanical coupling between the fractured shelf front and the adjacent coast. The rift-filling melange could melt more easily than the surrounding ice shelf and thus could represent a vulnerability of the FRIS to climate warming.

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 Calving cycle of the Brunt Ice Shelf, Antarctica, driven by changes in ice shelf geometry

2019 ◽  
Vol 13 (10) ◽  
pp. 2771-2787 ◽  
Author(s):  
Jan De Rydt ◽  
Gudmundur Hilmar Gudmundsson ◽  
Thomas Nagler ◽  
Jan Wuite
Keyword(s):  
Large Scale ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Flow Models ◽  
Speed Up ◽  
The Antarctic ◽  
Detrimental Impact ◽  
Natural Laboratory

Abstract. Despite the potentially detrimental impact of large-scale calving events on the geometry and ice flow of the Antarctic Ice Sheet, little is known about the processes that drive rift formation prior to calving, or what controls the timing of these events. The Brunt Ice Shelf in East Antarctica presents a rare natural laboratory to study these processes, following the recent formation of two rifts, each now exceeding 50 km in length. Here we use 2 decades of in situ and remote sensing observations, together with numerical modelling, to reveal how slow changes in ice shelf geometry over time caused build-up of mechanical tension far upstream of the ice front, and culminated in rift formation and a significant speed-up of the ice shelf. These internal feedbacks, whereby ice shelves generate the very conditions that lead to their own (partial) disintegration, are currently missing from ice flow models, which severely limits their ability to accurately predict future sea level rise.

scholarly journals Ice shelf fracture parameterization in an ice sheet model

2017 ◽  
Vol 11 (6) ◽  
pp. 2543-2554 ◽  
Author(s):  
Sainan Sun ◽  
Stephen L. Cornford ◽  
John C. Moore ◽  
Rupert Gladstone ◽  
Liyun Zhao
Keyword(s):  
Enhancement Factor ◽  
Large Scale ◽  
Damage Model ◽  
Ice Sheet ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Order Of Magnitude ◽  
Eventual Loss ◽  
Spatially Varying

Abstract. Floating ice shelves exert a stabilizing force onto the inland ice sheet. However, this buttressing effect is diminished by the fracture process, which on large scales effectively softens the ice, accelerating its flow, increasing calving, and potentially leading to ice shelf breakup. We add a continuum damage model (CDM) to the BISICLES ice sheet model, which is intended to model the localized opening of crevasses under stress, the transport of those crevasses through the ice sheet, and the coupling between crevasse depth and the ice flow field and to carry out idealized numerical experiments examining the broad impact on large-scale ice sheet and shelf dynamics. In each case we see a complex pattern of damage evolve over time, with an eventual loss of buttressing approximately equivalent to halving the thickness of the ice shelf. We find that it is possible to achieve a similar ice flow pattern using a simple rule of thumb: introducing an enhancement factor ∼ 10 everywhere in the model domain. However, spatially varying damage (or equivalently, enhancement factor) fields set at the start of prognostic calculations to match velocity observations, as is widely done in ice sheet simulations, ought to evolve in time, or grounding line retreat can be slowed by an order of magnitude.

scholarly journals Calving cycle of the Brunt Ice Shelf, Antarctica, driven by changes in ice-shelf geometry

2019 ◽  
Author(s):  
Jan De Rydt ◽  
G. Hilmar Gudmundsson ◽  
Thomas Nagler ◽  
Jan Wuite
Keyword(s):  
Large Scale ◽  
Ice Shelf ◽  
Ice Flow ◽  
Ice Shelves ◽  
Flow Models ◽  
Speed Up ◽  
The Antarctic ◽  
Detrimental Impact ◽  
Natural Laboratory

Abstract. Despite the potentially detrimental impact of large-scale calving events on the geometry and ice flow of the Antarctic Ice Sheet, little is known about the processes that drive rift formation prior to calving, or what controls the timing of these events. The Brunt Ice Shelf in East Antarctica presents a rare natural laboratory to study these processes, following the recent formation of two rifts, each now exceeding 50 km in length. Here we use a unique 50-years' time series of in-situ and remote sensing observations, together with numerical modelling, to reveal how slow changes in ice shelf geometry over time caused build-up of mechanical tension far upstream of the ice front, and culminated in rift formation and a significant speed-up of the ice shelf. These internal feedbacks, whereby ice shelves generate the very conditions that lead to their own (partial) disintegration are currently missing from ice flow models, which severely limits their ability to accurately predict future sea level rise.

scholarly journals Implementing an empirical scalar tertiary anisotropic rheology (ESTAR) into large-scale ice sheet models

2017 ◽  
Author(s):  
Felicity S. Graham ◽  
Mathieu Morlighem ◽  
Roland C. Warner ◽  
Adam Treverrow
Keyword(s):  
Enhancement Factor ◽  
Large Scale ◽  
Flow Line ◽  
Ice Sheet ◽  
Computationally Efficient ◽  
Ice Shelf ◽  
Ice Flow ◽  
Maximum Value ◽  
Polycrystalline Ice ◽  
Flow Enhancement

Abstract. The microstructural evolution that occurs in polycrystalline ice during deformation leads to the development of anisotropic rheological properties that are not adequately described by the most common, isotropic, ice flow relation used in large-scale ice sheet models – the Glen flow relation. We present a preliminary assessment of the implementation in the Ice Sheet System Model (ISSM) of a computationally-efficient, empirical, scalar, tertiary, anisotropic rheology (ESTAR). The effect of this anisotropic rheology on ice flow dynamics is investigated by comparing idealised simulations using ESTAR with those using the isotropic Glen flow relation, where the latter includes a flow enhancement factor. For an idealised embayed ice shelf, the Glen flow relation overestimates velocities by up to 17 % when using an enhancement factor equivalent to the maximum value prescribed by ESTAR. Importantly, no single Glen enhancement factor can accurately capture the spatial variations in flow over the ice shelf. For flow-line studies of idealised grounded flow over a bumpy topography or a sticky base – both scenarios dominated at depth by bed-parallel shear – the differences between simulated velocities using ESTAR and the Glen flow relation vary according to the value of the enhancement factor used to calibrate the Glen flow relation. These results demonstrate the importance of describing the anisotropic rheology of ice in a physically realistic manner, and have implications for simulations of ice sheet evolution used to reconstruct paleo-ice sheet extent and predict future ice sheet contributions to sea level.

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