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

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.

Download Full-text

Contrasting the modelled sensitivity of the Amundsen Sea Embayment ice streams

Journal of Glaciology ◽

10.1017/jog.2016.40 ◽

2016 ◽

Vol 62 (233) ◽

pp. 552-562 ◽

Cited By ~ 31

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.

Download Full-text

The Dynamics of Marine Ice Sheets

Journal of Glaciology ◽

10.3189/s0022143000014726 ◽

1979 ◽

Vol 24 (90) ◽

pp. 167-177 ◽

Cited By ~ 13

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.

Download Full-text

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

The Cryosphere Discussions ◽

10.5194/tcd-9-1887-2015 ◽

2015 ◽

Vol 9 (2) ◽

pp. 1887-1942 ◽

Cited By ~ 3

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.

Download Full-text

The Dynamics of Marine Ice Sheets

Journal of Glaciology ◽

10.1017/s0022143000014726 ◽

1979 ◽

Vol 24 (90) ◽

pp. 167-177 ◽

Cited By ~ 145

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.

Download Full-text

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

10.5194/tc-2019-156 ◽

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.

Download Full-text

Sensitivity of the Weddell Sea sector ice streams to sub-shelf melting and surface accumulation

The Cryosphere Discussions ◽

10.5194/tcd-7-5475-2013 ◽

2013 ◽

Vol 7 (6) ◽

pp. 5475-5508 ◽

Cited By ~ 2

Author(s):

A. P. Wright ◽

A. M. Le Brocq ◽

S. L. Cornford ◽

M. J. Siegert ◽

R. G. Bingham ◽

...

Keyword(s):

Adaptive Mesh Refinement ◽

Reference Model ◽

Deep Ocean ◽

Adaptive Mesh ◽

Weddell Sea ◽

Ice Shelf ◽

Ice Streams ◽

Deep Ocean Water ◽

Grounding Line ◽

The Impact

Abstract. A recent ocean modelling study indicates that possible changes in circulation may bring warm deep ocean water into direct contact with the grounding lines of the Filchner-Ronne ice streams, suggesting the potential for future ice losses from this sector equivalent to ~ 0.3 m of sea-level rise. Significant advancements have been made in our knowledge of both the basal topography and ice velocity in the Weddell Sea sector, thus enabling an assessment to be made of the relative sensitivities of the diverse collection of ice streams feeding the Filchner-Ronne Ice Shelf. Here we use the BISICLES ice sheet model, which employs adaptive-mesh refinement to resolve grounding line dynamics, to carry out such an assessment. The impact of perturbations to the surface and sub-shelf mass balance forcing fields from our 2000 yr "reference" model run indicate that both the Institute and Möller Ice Streams are highly sensitive to changes in basal melting either near to their respective grounding lines, or in the region of the ice rises within the Filchner-Ronne Ice Shelf. These same perturbations have little impact, however, on Rutford, Carlson or Foundation ice streams, while Evans Ice Stream is found to enter a phase of unstable retreat only after melt at its grounding line has increased by an order-of-magnitude from likely present-day values.

Download Full-text

Brief communication: PICOP, a new ocean melt parameterization under ice shelves combining PICO and a plume model

The Cryosphere ◽

10.5194/tc-13-1043-2019 ◽

2019 ◽

Vol 13 (3) ◽

pp. 1043-1049 ◽

Cited By ~ 7

Author(s):

Tyler Pelle ◽

Mathieu Morlighem ◽

Johannes H. Bondzio

Keyword(s):

Ocean Circulation ◽

Numerical Models ◽

Three Dimensional ◽

Ice Sheet ◽

Ice Shelf ◽

Ice Streams ◽

Amundsen Sea ◽

Ice Shelves ◽

Grounding Line ◽

Basal Melting

Abstract. Basal melting at the bottom of Antarctic ice shelves is a major control on glacier dynamics, as it modulates the amount of buttressing that floating ice shelves exert onto the ice streams feeding them. Three-dimensional ocean circulation numerical models provide reliable estimates of basal melt rates but remain too computationally expensive for century-scale projections. Ice sheet modelers therefore routinely rely on simplified parameterizations based on either ice shelf depth or more sophisticated box models. However, existing parameterizations do not accurately resolve the complex spatial patterns of sub-shelf melt rates that have been observed over Antarctica's ice shelves, especially in the vicinity of the grounding line, where basal melting is one of the primary drivers of grounding line migration. In this study, we couple the Potsdam Ice-shelf Cavity mOdel (PICO, Reese et al., 2018) to a buoyant plume melt rate parameterization (Lazeroms et al., 2018) to create PICOP, a novel basal melt rate parameterization that is easy to implement in transient ice sheet numerical models and produces a melt rate field that is in excellent agreement with the spatial distribution and magnitude of observations for several ocean basins. We test PICOP on the Amundsen Sea sector of West Antarctica, Totten, and Moscow University ice shelves in East Antarctica and the Filchner-Ronne Ice Shelf and compare the results to PICO. We find that PICOP is able to reproduce inferred high melt rates beneath Pine Island, Thwaites, and Totten glaciers (on the order of 100 m yr−1) and removes the “banding” pattern observed in melt rates produced by PICO over the Filchner-Ronne Ice Shelf. PICOP resolves many of the issues contemporary basal melt rate parameterizations face and is therefore a valuable tool for those looking to make future projections of Antarctic glaciers.

Download Full-text

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

Journal of Glaciology ◽

10.3189/2014jog13j117 ◽

2014 ◽

Vol 60 (220) ◽

pp. 305-313 ◽

Cited By ~ 11

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.

Download Full-text

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

Annals of Glaciology ◽

10.1017/aog.2016.13 ◽

2016 ◽

Vol 57 (73) ◽

pp. 1-9 ◽

Cited By ~ 25

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.

Download Full-text