Summer surface melt thins Petermann Gletscher Ice Shelf by enhancing channelized basal melt

ABSTRACTIncreasing ocean and air temperatures have contributed to the removal of floating ice shelves from several Greenland outlet glaciers; however, the specific contribution of these external forcings remains poorly understood. Here we use atmospheric, oceanographic and glaciological time series data from the ice shelf of Petermann Gletscher, NW Greenland to quantify the forcing of the ocean and atmosphere on the ice shelf at a site ~16 km from the grounding line within a large sub-ice-shelf channel. Basal melt rates here indicate a strong seasonality, rising from a winter mean of 2 m a−1 to a maximum of 80 m a−1 during the summer melt season. This increase in basal melt rates confirms the direct link between summer atmospheric warming around Greenland and enhanced ocean-forced melting of its remaining ice shelves. We attribute this enhanced melting to increased discharge of subglacial runoff into the ocean at the grounding line, which strengthens under-ice currents and drives a greater ocean heat flux toward the ice base.

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Twenty-first century response of Petermann Glacier, northwest Greenland to ice shelf loss

Journal of Glaciology ◽

10.1017/jog.2020.97 ◽

2020 ◽

pp. 1-11

Author(s):

Emily A. Hill ◽

G. Hilmar Gudmundsson ◽

J. Rachel Carr ◽

Chris R. Stokes ◽

Helen M. King

Keyword(s):

Sea Level ◽

First Century ◽

Ice Shelf ◽

Ice Shelves ◽

Sea Levels ◽

Grounding Line ◽

Global Mean Sea Level ◽

Near Future ◽

Global Mean ◽

Large Sections

Abstract Ice shelves restrain flow from the Greenland and Antarctic ice sheets. Climate-ocean warming could force thinning or collapse of floating ice shelves and subsequently accelerate flow, increase ice discharge and raise global mean sea levels. Petermann Glacier (PG), northwest Greenland, recently lost large sections of its ice shelf, but its response to total ice shelf loss in the future remains uncertain. Here, we use the ice flow model Úa to assess the sensitivity of PG to changes in ice shelf extent, and to estimate the resultant loss of grounded ice and contribution to sea level rise. Our results have shown that under several scenarios of ice shelf thinning and retreat, removal of the shelf will not contribute substantially to global mean sea level (<1 mm). We hypothesize that grounded ice loss was limited by the stabilization of the grounding line at a topographic high ~12 km inland of its current grounding line position. Further inland, the likelihood of a narrow fjord that slopes seawards suggests that PG is likely to remain insensitive to terminus changes in the near future.

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The effect of Antarctic ice-shelf extent on ice discharge

10.5194/egusphere-egu21-2650 ◽

2021 ◽

Author(s):

Jim Jordan ◽

HIlmar Gudmundsson ◽

Adrian Jenkins ◽

Chris Stokes ◽

Stewart Jamiesson ◽

...

Keyword(s):

Mass Loss ◽

Recent Work ◽

Flow Model ◽

Ice Shelf ◽

Geophysical Research ◽

Ice Flow ◽

Ice Shelves ◽

Grounding Line ◽

Natural Variance ◽

Observational Record

<div>The buttressing strength of Antarctic ice shelves directly effects the amount of ice discharge across the grounding line, with buttressing strength affected by both the thickness and extent of an ice shelf. Recent work has shown that a reduction in ice-shelf buttressing due to ocean induced ice-shelf thinning is responsible for a significant portion of increased Antarctic ice discharge (Gudmundsson et al., 2019, but few studies have attempted to show the effect of variability in ice-shelf extent on ice discharge. This variability arises due to ice-shelf calving following a cycle of long periods of slow, continuous calving interposed with calving of large, discrete sections. &#160;These discrete calving events tend to occur on a comparative timeframe to that of the observational record. As such, when determining observed changes in ice discharge it is crucial that this natural variability is separated from any observed trends. &#160;</div><div>&#160;</div><div>In this work we use the numerical ice-flow model &#218;a in combination with observations of ice shelf extent to diagnostically calculate Antarctic ice discharge. These observations primarily date back to the 1970s, though for some ice shelves records exist back to the 1940s. We assemble an Antarctic wide model for two scenarios: 1) with ice shelves at their maximum observed extent and 2) with ice shelves at their minimum observed extent. We then compare these two scenarios to differences in the observed changes in Antarctic ice-discharge to determine how much can be attributed to natural variance .</div><p>&#160;</p><p><span>Gudmundsson, G. H.</span><span>,&#160;Paolo, F. S.,&#160;Adusumilli, S., &&#160;Fricker, H. A.&#160;(2019).&#160;</span>Instantaneous Antarctic ice&#8208;&#160;sheet mass loss driven by thinning ice shelves.&#160;<em>Geophysical Research Letters</em>,&#160;46,&#160;13903&#8211;&#160;13909.&#160;</p>

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Simulating detailed spatial patterns of ice shelf basal melt

10.5194/egusphere-egu21-4750 ◽

2021 ◽

Author(s):

Erwin Lambert ◽

André Jüling ◽

Paul Holland ◽

Roderik van de Wal

Keyword(s):

Flow Direction ◽

Ice Sheet ◽

Realistic Model ◽

Western Boundary ◽

Layer Model ◽

Ice Shelf ◽

Amundsen Sea ◽

Ice Shelves ◽

Grounding Line ◽

Ocean Properties

<p>The contact between ice shelves and relatively warm ocean waters causes basal melt, ice shelf thinning, and ultimately ice sheet mass loss. This basal melt, and its dependence on ocean properties, is poorly understood due to an overall lack of direct observations and a difficulty in explicit simulation of the circulation in sub-shelf cavities. In this study, we compare a number of parameterisations and models of increasing complexity, up to a 2D &#8216;Layer&#8217; model. Each model is aimed at quantifying basal melt rates as a function of offshore temperature and salinity. We test these models in an idealised setting (ISOMIP+) and in a realistic setting for the Amundsen Sea Embayment. All models show a comparable non-linear sensitivity of ice-shelf average basal melt to ocean warming, indicating a positive feedback between melt and circulation. However, the Layer model is the only one which explicitly resolves the flow direction of the buoyant melt plumes, which is primarily governed by rotation and by the basal topography of the ice shelves. At 500m resolution, this model simulates locally enhanced basal melt near the grounding line, in topographical channels, and near the western boundary. The simulated melt patterns for the Amundsen Sea ice shelves are compared to satellite observations of ice shelf thinning and to 3D numerical simulations of the sub-shelf cavity circulation. As detailed melt rates near the grounding line are essential for the stability of ice sheets, spatially realistic melt rates are crucial for future projections of ice sheet dynamics. We conclude that the Layer model can function as a relatively cheap yet realistic model to downscale 3D ocean simulations of ocean properties to sub-kilometer scale basal melt fields to provide detailed forcing fields to ice sheet models.</p>

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Damage accelerates ice shelf instability and mass loss in Amundsen Sea Embayment

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1912890117 ◽

2020 ◽

Vol 117 (40) ◽

pp. 24735-24741 ◽

Cited By ~ 1

Author(s):

Stef Lhermitte ◽

Sainan Sun ◽

Christopher Shuman ◽

Bert Wouters ◽

Frank Pattyn ◽

...

Keyword(s):

Sea Level ◽

Rapid Development ◽

Shear Zones ◽

West Antarctica ◽

Ice Shelf ◽

Damage Development ◽

Amundsen Sea ◽

Ice Shelves ◽

Grounding Line ◽

Global Sea Level

Pine Island Glacier and Thwaites Glacier in the Amundsen Sea Embayment are among the fastest changing outlet glaciers in West Antarctica with large consequences for global sea level. Yet, assessing how much and how fast both glaciers will weaken if these changes continue remains a major uncertainty as many of the processes that control their ice shelf weakening and grounding line retreat are not well understood. Here, we combine multisource satellite imagery with modeling to uncover the rapid development of damage areas in the shear zones of Pine Island and Thwaites ice shelves. These damage areas consist of highly crevassed areas and open fractures and are first signs that the shear zones of both ice shelves have structurally weakened over the past decade. Idealized model results reveal moreover that the damage initiates a feedback process where initial ice shelf weakening triggers the development of damage in their shear zones, which results in further speedup, shearing, and weakening, hence promoting additional damage development. This damage feedback potentially preconditions these ice shelves for disintegration and enhances grounding line retreat. The results of this study suggest that damage feedback processes are key to future ice shelf stability, grounding line retreat, and sea level contributions from Antarctica. Moreover, they underline the need for incorporating these feedback processes, which are currently not accounted for in most ice sheet models, to improve sea level rise projections.

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On the Disintegration of Ice Shelves: the Role of Thinning

Annals of Glaciology ◽

10.1017/s0260305500002676 ◽

1982 ◽

Vol 3 ◽

pp. 146-151 ◽

Cited By ~ 11

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.

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Numerical simulations of the ice flow dynamics of George VI Ice Shelf, Antarctica

Journal of Glaciology ◽

10.3189/002214307784409180 ◽

2007 ◽

Vol 53 (183) ◽

pp. 659-664 ◽

Cited By ~ 11

Author(s):

Angelika Humbert

Keyword(s):

High Spatial Resolution ◽

Thickness Distribution ◽

Satellite System ◽

Flow Dynamics ◽

Ice Thickness ◽

Ice Shelf ◽

Ice Flow ◽

Ice Shelves ◽

Flow Geometry ◽

Grounding Line

A diagnostic, dynamic/thermodynamic ice-shelf model is applied to the George VI Ice Shelf, situated in the Bellinghausen Sea, Antarctica. The George VI Ice Shelf has a peculiar flow geometry which sets it apart from other ice shelves. Inflow occurs along the two longest, and almost parallel, sides, whereas outflow occurs on the two ice fronts that are relatively short and situated at opposite ends of the ice shelf. Two data sources were used to derive the ice thickness distribution: conventional radioecho sounding from the British Antarctic Survey was combined with thickness inferred from surface elevation obtained by the NASA GLAS satellite system assuming hydrostatic equilibrium. We simulate the present ice flow over the ice shelf that results from the ice thickness distribution, the inflow at the grounding line and the flow rate factor. The high spatial resolution of the ice thickness distribution leads to very detailed simulations. The flow field has some extraordinary elements (e.g. the stagnation point characteristics resulting from the unusual ice-shelf geometry).

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Channelization of plumes beneath ice shelves

Journal of Fluid Mechanics ◽

10.1017/jfm.2015.609 ◽

2015 ◽

Vol 785 ◽

pp. 109-134 ◽

Cited By ~ 7

Author(s):

M. C. Dallaston ◽

I. J. Hewitt ◽

A. J. Wells

Keyword(s):

Turbulent Mixing ◽

Simplified Model ◽

Ice Thickness ◽

Ice Shelf ◽

Ice Flow ◽

Ice Shelves ◽

Narrow Channels ◽

One Dimensional ◽

Grounding Line ◽

The One

We study a simplified model of ice–ocean interaction beneath a floating ice shelf, and investigate the possibility for channels to form in the ice shelf base due to spatial variations in conditions at the grounding line. The model combines an extensional thin-film description of viscous ice flow in the shelf, with melting at its base driven by a turbulent ocean plume. Small transverse perturbations to the one-dimensional steady state are considered, driven either by ice thickness or subglacial discharge variations across the grounding line. Either forcing leads to the growth of channels downstream, with melting driven by locally enhanced ocean velocities, and thus heat transfer. Narrow channels are smoothed out due to turbulent mixing in the ocean plume, leading to a preferred wavelength for channel growth. In the absence of perturbations at the grounding line, linear stability analysis suggests that the one-dimensional state is stable to initial perturbations, chiefly due to the background ice advection.

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Modelling present-day basal melt rates for Antarctic ice shelves using a parametrization of buoyant meltwater plumes

The Cryosphere ◽

10.5194/tc-12-49-2018 ◽

2018 ◽

Vol 12 (1) ◽

pp. 49-70 ◽

Cited By ~ 28

Author(s):

Werner M. J. Lazeroms ◽

Adrian Jenkins ◽

G. Hilmar Gudmundsson ◽

Roderik S. W. van de Wal

Keyword(s):

Temperature Field ◽

Ice Sheet ◽

Detailed Knowledge ◽

Ocean Temperature ◽

Ice Shelf ◽

Antarctic Ice Sheet ◽

Ice Shelves ◽

Grounding Line ◽

The Antarctic ◽

Line Depth

Abstract. Basal melting below ice shelves is a major factor in mass loss from the Antarctic Ice Sheet, which can contribute significantly to possible future sea-level rise. Therefore, it is important to have an adequate description of the basal melt rates for use in ice-dynamical models. Most current ice models use rather simple parametrizations based on the local balance of heat between ice and ocean. In this work, however, we use a recently derived parametrization of the melt rates based on a buoyant meltwater plume travelling upward beneath an ice shelf. This plume parametrization combines a non-linear ocean temperature sensitivity with an inherent geometry dependence, which is mainly described by the grounding-line depth and the local slope of the ice-shelf base. For the first time, this type of parametrization is evaluated on a two-dimensional grid covering the entire Antarctic continent. In order to apply the essentially one-dimensional parametrization to realistic ice-shelf geometries, we present an algorithm that determines effective values for the grounding-line depth and basal slope in any point beneath an ice shelf. Furthermore, since detailed knowledge of temperatures and circulation patterns in the ice-shelf cavities is sparse or absent, we construct an effective ocean temperature field from observational data with the purpose of matching (area-averaged) melt rates from the model with observed present-day melt rates. Our results qualitatively replicate large-scale observed features in basal melt rates around Antarctica, not only in terms of average values, but also in terms of the spatial pattern, with high melt rates typically occurring near the grounding line. The plume parametrization and the effective temperature field presented here are therefore promising tools for future simulations of the Antarctic Ice Sheet requiring a more realistic oceanic forcing.

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Dynamic response of an Arctic epishelf lake to seasonal and long-term forcing: implications for ice shelf thickness

10.5194/tc-2017-19 ◽

2017 ◽

Cited By ~ 1

Author(s):

Andrew K. Hamilton ◽

Bernard E. Laval ◽

Derek R. Mueller ◽

Warwick F. Vincent ◽

Luke Copland

Keyword(s):

Seasonal Variation ◽

Canadian Arctic ◽

The Arctic ◽

Ice Shelf ◽

Ice Shelves ◽

Air Temperatures ◽

Long Term Changes ◽

Episodic Mixing ◽

Outflow Pathway

Abstract. Changes in the depth of the freshwater-seawater interface in epishelf lakes have been used to infer long-term changes in the thickness of ice shelves, however, little is known about the dynamics of epishelf lakes and what other factors may influence their depth. Continuous observations collected between 2011 and 2014 in the Milne Fiord epishelf lake, in the Canadian Arctic, showed that the depth of the halocline varied seasonally by up to 3.3 m, which was comparable to interannual variability. The seasonal depth variation was controlled by the magnitude of surface meltwater inflow and the hydraulics of the inferred outflow pathway, a narrow basal channel in the Milne Ice Shelf. When seasonal variation and an episodic mixing of the halocline were accounted for, long-term records of depth indicated there was no significant change in thickness of ice along the basal channel from 1983 to 2004, followed by a period of steady thinning at 0.50 m a-1 between 2004 and 2011. Rapid thinning at 1.15 m a-1 then occurred from 2011 to 2014, corresponded to a period of warming regional air temperatures. Continued warming is expected to lead to the breakup of the ice shelf and the imminent loss of the last known epishelf lake in the Arctic.

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Basal crevasses in Larsen C Ice Shelf and implications for their global abundance

The Cryosphere Discussions ◽

10.5194/tcd-5-2035-2011 ◽

2011 ◽

Vol 5 (4) ◽

pp. 2035-2060 ◽

Cited By ~ 5

Author(s):

A. Luckman ◽

D. Jansen ◽

B. Kulessa ◽

E. C. King ◽

P. Sammonds ◽

...

Keyword(s):

Snow Accumulation ◽

Ice Accretion ◽

Ice Shelf ◽

Ice Shelves ◽

Shelf Stability ◽

Geometrical Properties ◽

Linear Elastic ◽

Grounding Line ◽

Modis Imagery ◽

Elastic Fracture

Abstract. Basal crevasses extend upwards from the base of ice bodies and can penetrate more than halfway through the ice column under conditions found commonly on ice shelves. As a result, they may locally modify the exchange of mass and energy between ice shelf and ocean, and by altering the shelf's mechanical properties could play a fundamental role in ice shelf stability. Although early studies revealed that such features may be abundant on Antarctic ice shelves, their geometrical properties and spatial distribution has gained little attention. We investigate basal crevasses in Larsen C Ice Shelf using field radar survey, remote sensing and numerical modelling. We demonstrate that a group of features visible in MODIS imagery are the surface expressions of basal crevasses in the form of surface troughs, and find that basal crevasses can be generated as a result of stresses well downstream of the grounding line. We show that linear elastic fracture mechanics modelling is a good predictor of basal crevasse penetration height where stresses are predominantly tensile, and that measured surface trough depth does not always reflect this height, probably because of snow accumulation in the trough, marine ice accretion in the crevasse, or stress bridging from the surrounding ice. We conclude that all features visible in MODIS imagery of ice shelves and previously labelled simply as "crevasses", where they are not full thickness rifts, must be basal crevasse troughs, highlighting a fundamental structural property of many ice shelves that may have been previously overlooked.

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