scholarly journals Contrasting the modelled sensitivity of the Amundsen Sea Embayment ice streams

2016 ◽  
Vol 62 (233) ◽  
pp. 552-562 ◽  
Author(s):  
ISABEL J. NIAS ◽  
STEPHEN L. CORNFORD ◽  
ANTONY J. PAYNE
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Adaptive Mesh Refinement ◽  
Adaptive Mesh ◽  
Mass Conservation ◽  
Physical Parameters ◽  
Ice Streams ◽  
Amundsen Sea ◽  
West Antarctic ◽  
Grounding Line

AbstractPresent-day mass loss from the West Antarctic ice sheet is centred on the Amundsen Sea Embayment (ASE), primarily through ice streams, including Pine Island, Thwaites and Smith glaciers. To understand the differences in response of these ice streams, we ran a perturbed parameter ensemble, using a vertically-integrated ice flow model with adaptive mesh refinement. We generated 71 sets of three physical parameters (basal traction coefficient, ice viscosity stiffening factor and sub-shelf melt rate), which we used to simulate the ASE for 50 years. We also explored the effects of different bed geometries and basal sliding laws. The mean rate of sea-level rise across the ensemble of simulations is comparable with current observed rates for the ASE. We found evidence that grounding line dynamics are sensitive to features in the bed geometry: simulations using BedMap2 geometry resulted in a higher rate of sea-level rise than simulations using a rougher geometry, created using mass conservation. Modelled grounding-line retreat of all the three ice streams was sensitive to viscosity and basal traction, while the melt rate was more important in Pine Island and Smith glaciers, which flow through more confined ice shelves than Thwaites, which has a relatively unconfined shelf.

scholarly journals Ocean-forced evolution of the Amundsen Sea catchment, West Antarctica, by 2100

2020 ◽  
Vol 14 (4) ◽  
pp. 1245-1258
Author(s):  
Alanna V. Alevropoulos-Borrill ◽  
Isabel J. Nias ◽  
Antony J. Payne ◽  
Nicholas R. Golledge ◽  
Rory J. Bingham
Keyword(s):  
Sea Level ◽  
21St Century ◽  
Adaptive Mesh Refinement ◽  
Ice Sheet ◽  
Adaptive Mesh ◽  
Ocean Temperature ◽  
Ice Shelf ◽  
Ice Streams ◽  
Amundsen Sea ◽  
Grounding Line

Abstract. The response of ice streams in the Amundsen Sea Embayment (ASE) to future climate forcing is highly uncertain. Here we present projections of 21st century response of ASE ice streams to modelled local ocean temperature change using a subset of Coupled Model Intercomparison Project (CMIP5) simulations. We use the BISICLES adaptive mesh refinement (AMR) ice sheet model, with high-resolution grounding line resolving capabilities, to explore grounding line migration in response to projected sub-ice-shelf basal melting. We find a contribution to sea level rise of between 2.0 and 4.5 cm by 2100 under RCP8.5 conditions from the CMIP5 subset, where the mass loss response is linearly related to the mean ocean temperature anomaly. To account for uncertainty associated with model initialization, we perform three further sets of CMIP5-forced experiments using different parameterizations that explore perturbations to the prescription of initial basal melt, the basal traction coefficient and the ice stiffening factor. We find that the response of the ASE to ocean temperature forcing is highly dependent on the parameter fields obtained in the initialization procedure, where the sensitivity of the ASE ice streams to the sub-ice-shelf melt forcing is dependent on the choice of parameter set. Accounting for ice sheet model parameter uncertainty results in a projected range in sea level equivalent contribution from the ASE of between −0.02 and 12.1 cm by the end of the 21st century.

scholarly journals Ocean forced evolution of the Amundsen Sea catchment, West Antarctica, by 2100

2019 ◽  
Author(s):  
Alanna V. Alevropoulos-Borrill ◽  
Isabel J. Nias ◽  
Antony J. Payne ◽  
Nicholas R. Golledge ◽  
Rory J. Bingham
Keyword(s):  
Sea Level ◽  
21St Century ◽  
Adaptive Mesh Refinement ◽  
Ice Sheet ◽  
Adaptive Mesh ◽  
Ocean Temperature ◽  
Ice Shelf ◽  
Ice Streams ◽  
Amundsen Sea ◽  
Grounding Line

Abstract. The response of ice streams in the Amundsen Sea Embayment (ASE) to future climate forcing is highly uncertain. Here we present projections of 21st century response of ASE ice streams to modelled local ocean temperature change using a subset of Coupled Model Intercomparison Project (CMIP5) simulations. We use the BISICLES adaptive mesh refinement (AMR) ice sheet model, with high resolution grounding line resolving capabilities, to explore grounding line migration in response to projected sub-ice shelf basal melting. We find a contribution to sea level rise of between 2.0 cm and 4.5 cm by 2100 under RCP8.5 conditions from the CMIP5 subset, where the mass loss response is linearly related to the mean ocean temperature anomaly. To account for uncertainty associated with model initialisation, we perform three further sets of CMIP5 forced experiments using different parameterisations that explore perturbations to the prescription of initial basal melt, the basal traction coefficient, and the ice stiffening factor. We find that the response of the ASE to ocean temperature forcing is highly dependent on the parameter fields obtained in the initialisation procedure, where the sensitivity of the ASE ice streams to the sub-ice shelf melt forcing is dependent on the choice of parameter set. Accounting for ice sheet model parameter uncertainty results in a projected range in sea level equivalent contribution from the ASE of between −0.02 cm and 12.1 cm by the end of the 21st century.

scholarly journals Thwaites Glacier grounding-line retreat: influence of width and buttressing parameterizations

2014 ◽  
Vol 60 (220) ◽  
pp. 305-313 ◽  
Author(s):  
David Docquier ◽  
David Pollard ◽  
Frank Pattyn
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Ice Shelf ◽  
Amundsen Sea ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Ice Stream ◽  
Grounding Line ◽  
Flow Convergence

AbstractMajor ice loss has recently been observed along coastal outlet glaciers of the West Antarctic ice sheet, mainly due to increased melting below the ice shelves. However, the behavior of this marine ice sheet is poorly understood, leading to significant shortcomings in ice-sheet models attempting to predict future sea-level rise. The stability of a marine ice sheet is controlled by the dynamics at the grounding line, the boundary between the grounded ice stream and the floating ice shelf. One of the largest contributors to current sea-level rise is the fast-flowing Thwaites Glacier, which flows into the Amundsen Sea. Here we use an ice-stream/ice-shelf model and perform a number of experiments along a central flowline to analyze the sensitivity of its grounding line on centennial timescales. In the absence of width and buttressing effects, we find that the grounding line retreats by ˜300 km in 200 years from the present day (rate of 1.5 km a–1). With variable glacier width implemented in the model, flow convergence slows the retreat of Thwaites grounding line at 0.3–1.2 km a–1. The parameterization of ice-shelf buttressing according to different observed scenarios further reduces the glacier retreat and can even lead to a slight advance in the most buttressed case.

scholarly journals Adaptive mesh refinement versus subgrid friction interpolation in simulations of Antarctic ice dynamics

2016 ◽  
Vol 57 (73) ◽  
pp. 1-9 ◽  
Author(s):  
S. L. Cornford ◽  
D. F. Martin ◽  
V. Lee ◽  
A. J. Payne ◽  
E. G. Ng
Keyword(s):  
Adaptive Mesh Refinement ◽  
Ice Sheet ◽  
Adaptive Mesh ◽  
Ice Streams ◽  
Order Error ◽  
Ice Dynamics ◽  
West Antarctic ◽  
Grounding Line ◽  
Improve Accuracy ◽  
Mesh Spacing

ABSTRACTAt least in conventional hydrostatic ice-sheet models, the numerical error associated with grounding line dynamics can be reduced by modifications to the discretization scheme. These involve altering the integration formulae for the basal traction and/or driving stress close to the grounding line and exhibit lower – if still first-order – error in the MISMIP3d experiments. MISMIP3d may not represent the variety of real ice streams, in that it lacks strong lateral stresses, and imposes a large basal traction at the grounding line. We study resolution sensitivity in the context of extreme forcing simulations of the entire Antarctic ice sheet, using the BISICLES adaptive mesh ice-sheet model with two schemes: the original treatment, and a scheme, which modifies the discretization of the basal traction. The second scheme does indeed improve accuracy – by around a factor of two – for a given mesh spacing, but $\lesssim 1$ km resolution is still necessary. For example, in coarser resolution simulations Thwaites Glacier retreats so slowly that other ice streams divert its trunk. In contrast, with $\lesssim 1$ km meshes, the same glacier retreats far more quickly and triggers the final phase of West Antarctic collapse a century before any such diversion can take place.

scholarly journals Subglacial thermal balance permits ongoing grounding-line retreat along the Siple Coast of West Antarctica

2003 ◽  
Vol 36 ◽  
pp. 251-256 ◽  
Author(s):  
Byron R. Parizek ◽  
Richard B. Alley ◽  
Christina L. Hulbe
Keyword(s):  
Sea Level ◽  
Water Distribution ◽  
Regulation Mechanism ◽  
Ice Streams ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Ice Stream ◽  
Grounding Line ◽  
History Of ◽  
Global Sea Level

AbstractChanges in the discharge of West Antarctic ice streams are of potential concern with respect to global sea level. The six relatively thin, fast-flowing Ross ice streams are of interest as low-slope end-members among Antarctic ice streams. Extensive research has demonstrated that these “rivers of ice” have a history of relatively high-frequency , asynchronous discharge variations with evolving lateral boundaries. Amidst this variability, a ∼1300 km grounding-line retreat has occurred since the Last Glacial Maximum. Numerical studies of Ice Stream D (Parizek and others, 2002) indicate that a proposed thermal-regulation mechanism (Clarke and Marshall, 1998; Hulbe and MacAyeal, 1999; Tulaczyk and others, 2000a, b), which could buffer the West Antarctic ice sheet against complete collapse, may be over-ridden by latent-heat transport within melt-water from beneath inland ice. Extending these studies to Ice Stream A, Whillans Ice Stream and Ice Stream C suggests that further grounding-line retreat contributing to sea-level rise is possible thermodynamically However, the efficiency of basal water distribution may be a constraint on the system. Because local thermal deficits promote basal freeze-on (especially on topographic highs), observed short-term variability is likely to persist.

scholarly journals The Dynamics of Marine Ice Sheets

1979 ◽  
Vol 24 (90) ◽  
pp. 167-177 ◽  
Author(s):  
Robert H. Thomas
Keyword(s):  
Sea Level ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ice Shelves ◽  
The North ◽  
West Antarctic ◽  
Grounding Line ◽  
Ice Sheet Dynamics

AbstractMarine ice sheets rest on land that, for the most part, is below sea-level. Ice that flows across the grounding line, where the ice sheet becomes afloat, either calves into icebergs or forms a floating ice shelf joined to the ice sheet. At the grounding line there is a transition from ice-sheet dynamics to ice-shelf dynamics, and the creep-thinning rate in this region is very sensitive to sea depth; rising sea-level causes increased thinning-rates and grounding-line retreat, falling sea-level has the reverse effect. If the bedrock slopes down towards the centre of the ice sheet there may be only two stable modes: a freely-floating ice shelf or a marine ice sheet that extends to the edge of the continental shelf. Once started, collapse of such an ice sheet to form an ice shelf may take place extremely rapidly. Ice shelves which form in embayments of a marine ice sheet, or which are partially grounded, have a stabilizing influence since ice flowing across the grounding line has to push the ice shelf past its sides. Retreat of the grounding line tends to enlarge the ice shelf, which ultimately may become large enough to prevent excessive outflow from the ice sheet so that a new equilibrium grounding line is established; removal of the ice shelf would allow retreat to continue. During the late-Wisconsin glacial maximum there may have been marine ice sheets in the northern hemisphere but the only current example is the West Antarctic ice sheet. This is buttressed by the Ross and Ronne Ice Shelves, and if climatic warming were to prohibit the existence of these ice shelves then the ice sheet would collapse. Field observations suggest that, at present, the ice sheet may be advancing into parts of the Ross Ice Shelf. Such advance, however, would not ensure the security of the ice sheet since ice streams that drain to the north appear to flow directly into the sea with little or no ice shelf to buttress them. If these ice streams do not flow over a sufficiently high bedrock sill then they provide the most likely avenues for ice-sheet retreat.

scholarly journals Century-scale simulations of the response of the West Antarctic Ice Sheet to a warming climate

2015 ◽  
Vol 9 (2) ◽  
pp. 1887-1942 ◽  
Author(s):  
S. L. Cornford ◽  
D. F. Martin ◽  
A. J. Payne ◽  
E. G. Ng ◽  
A. M. Le Brocq ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Ice Shelf ◽  
Ice Streams ◽  
The West ◽  
Amundsen Sea ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Ice Accumulation ◽  
Grounding Line ◽  
Mesh Resolution

Abstract. We use the BISICLES adaptive mesh ice sheet model to carry out one, two, and three century simulations of the fast-flowing ice streams of the West Antarctic Ice Sheet. Each of the simulations begins with a geometry and velocity close to present day observations, and evolves according to variation in meteoric ice accumulation, ice shelf melting, and mesh resolution. Future changes in accumulation and melt rates range from no change, through anomalies computed by atmosphere and ocean models driven by the E1 and A1B emissions scenarios, to spatially uniform melt rates anomalies that remove most of the ice shelves over a few centuries. We find that variation in the resulting ice dynamics is dominated by the choice of initial conditions, ice shelf melt rate and mesh resolution, although ice accumulation affects the net change in volume above flotation to a similar degree. Given sufficient melt rates, we compute grounding line retreat over hundreds of kilometers in every major ice stream, but the ocean models do not predict such melt rates outside of the Amundsen Sea Embayment until after 2100. Sensitivity to mesh resolution is spurious, and we find that sub-kilometer resolution is needed along most regions of the grounding line to avoid systematic under-estimates of the retreat rate, although resolution requirements are more stringent in some regions – for example the Amundsen Sea Embayment – than others – such as the Möller and Institute ice streams.

scholarly journals The Dynamics of Marine Ice Sheets

1979 ◽  
Vol 24 (90) ◽  
pp. 167-177 ◽  
Author(s):  
Robert H. Thomas
Keyword(s):  
Sea Level ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Ice Shelf ◽  
Ice Streams ◽  
Ice Shelves ◽  
The North ◽  
West Antarctic ◽  
Grounding Line ◽  
Ice Sheet Dynamics

AbstractMarine ice sheets rest on land that, for the most part, is below sea-level. Ice that flows across the grounding line, where the ice sheet becomes afloat, either calves into icebergs or forms a floating ice shelf joined to the ice sheet. At the grounding line there is a transition from ice-sheet dynamics to ice-shelf dynamics, and the creep-thinning rate in this region is very sensitive to sea depth; rising sea-level causes increased thinning-rates and grounding-line retreat, falling sea-level has the reverse effect. If the bedrock slopes down towards the centre of the ice sheet there may be only two stable modes: a freely-floating ice shelf or a marine ice sheet that extends to the edge of the continental shelf. Once started, collapse of such an ice sheet to form an ice shelf may take place extremely rapidly. Ice shelves which form in embayments of a marine ice sheet, or which are partially grounded, have a stabilizing influence since ice flowing across the grounding line has to push the ice shelf past its sides. Retreat of the grounding line tends to enlarge the ice shelf, which ultimately may become large enough to prevent excessive outflow from the ice sheet so that a new equilibrium grounding line is established; removal of the ice shelf would allow retreat to continue. During the late-Wisconsin glacial maximum there may have been marine ice sheets in the northern hemisphere but the only current example is the West Antarctic ice sheet. This is buttressed by the Ross and Ronne Ice Shelves, and if climatic warming were to prohibit the existence of these ice shelves then the ice sheet would collapse. Field observations suggest that, at present, the ice sheet may be advancing into parts of the Ross Ice Shelf. Such advance, however, would not ensure the security of the ice sheet since ice streams that drain to the north appear to flow directly into the sea with little or no ice shelf to buttress them. If these ice streams do not flow over a sufficiently high bedrock sill then they provide the most likely avenues for ice-sheet retreat.

scholarly journals Spatial probabilistic calibration of a high-resolution Amundsen Sea Embayment ice-sheet model with satellite altimeter data

2019 ◽  
Author(s):  
Andreas Wernecke ◽  
Tamsin L. Edwards ◽  
Isabel J. Nias ◽  
Philip B. Holden ◽  
Neil R. Edwards
Keyword(s):  
Sea Level ◽  
Statistical Treatment ◽  
Substantial Reduction ◽  
Ice Sheet ◽  
Adaptive Mesh ◽  
Altimeter Data ◽  
Amundsen Sea ◽  
Spatial And Temporal Scales ◽  
Grounding Line ◽  
Global Mean Sea Level

Abstract. Probabilistic predictions of the sea level contribution from Antarctica often have large uncertainty intervals. Calibration with observations can reduce uncertainties and improve confidence in projections, particularly if this exploits as much of the available information as possible (such as spatial characteristics), but the necessary statistical treatment is often challenging and can be computationally prohibitive. Ice sheet models with sufficient spatial resolution to resolve grounding line evolution are also computationally expensive. Here we address these challenges by adopting a novel dimension-reduced approach to calibration combined with statistical emulation of the adaptive mesh model BISICLES. We find the most likely contribution to global mean sea level rise from the Amundsen Sea Embayment (ASE) over the next 50 years is 10.4 [0.6, 23.3] mm (mode and 5–95 % probability interval), a substantial reduction in uncertainty from the uncalibrated estimates of 9.6 [−5.9, 78.2] mm. We predict retreat of the grounding line along most parts of the ASE coast with high confidence, with a maximum inland extent of around 28 km at Smith Glacier. The model behaviour is much more consistent with observations if, instead of Bedmap2, a modified bedrock topography is used that most notably removes a topographic rise near the initial grounding line of Pine Island Glacier, though this does influence the future mass loss less than basal traction and viscosity scaling parameters. The ASE dominates the current Antarctic sea level contribution, but other regions have the potential to become more important on centennial scales. These larger spatial and temporal scales would benefit even more from methods of fast but exhaustive model calibration. Our approach therefore has the potential to improve projections for the Antarctic ice sheet on continental and centennial scales by efficiently improving our understanding of model behaviour, and substantiating and reducing projection uncertainties.

scholarly journals Comparing ice discharge through West Antarctic Gateways: Weddell vs. Amundsen Sea warming

2015 ◽  
Vol 9 (2) ◽  
pp. 1705-1733 ◽  
Author(s):  
M. A. Martin ◽  
A. Levermann ◽  
R. Winkelmann
Keyword(s):  
Sea Level ◽  
Ice Sheet ◽  
Back Stress ◽  
Step Function ◽  
Ocean Warming ◽  
Weddell Sea ◽  
Amundsen Sea ◽  
Function Type ◽  
West Antarctic ◽  
Grounding Line

Abstract. Future changes in Antarctic ice discharge will be largely controlled by the fate of the floating ice shelves, which exert a back-stress onto Antarctica's marine outlet glaciers. Ice loss in response to warming of the Amundsen Sea has been observed and investigated as a potential trigger for the marine ice-sheet instability. Recent observations and simulations suggest that the Amundsen Sea Sector might already be unstable which would have strong implications for global sea-level rise. At the same time, regional ocean projections show much stronger warm-water intrusion into ice-shelf cavities in the Weddell Sea compared to the observed Amundsen warming. Here we present results of numerical ice sheet modelling with the Parallel Ice Sheet Model (PISM) which show that idealized, step-function type ocean warming in the Weddell Sea leads to more immediate ice discharge with a higher sensitivity to small warming levels than the same warming in the Amundsen Sea. This is consistent with the specific combination of bedrock and ice topography in the Weddell Sea Sector which results in an ice sheet close to floatation. In response to even slight ocean warming, ice loss increases rapidly, peaks and declines within one century. While the cumulative ice loss in the Amundsen Sea Sector is of similar magnitude after five centuries of continued warming, ice loss increases at a slower pace and only for significantly higher warming levels. Although there is more marine ice stored above sea level in close vicinity of the grounding line compared to the Weddell Sea Sector, the ice sheet is farther from floatation and the grounding line initially retreats more slowly.

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