scholarly journals A Decade of Ocean Changes Impacting the Ice Shelf of Petermann Gletscher, Greenland

2018 ◽  
Vol 48 (10) ◽  
pp. 2477-2493 ◽  
Author(s):  
P. Washam ◽  
A. Münchow ◽  
K. W. Nicholls
Keyword(s):  
Heat Flux ◽  
Steady State ◽  
Freshwater Flux ◽  
Ice Shelf ◽  
Hydrographic Data ◽  
Ocean Stratification ◽  
Freshwater Fluxes ◽  
Subsurface Ocean ◽  
Grounding Line ◽  
Seasonal Signal

AbstractHydrographic data collected during five summer surveys between 2002 and 2015 reveal that the subsurface ocean near Petermann Gletscher, Greenland, warmed by 0.015° ± 0.013°C yr−1. New 2015–16 mooring data from beneath Petermann Gletscher’s ice shelf imply a continued warming of 0.025° ± 0.013°C yr−1 with a modest seasonal signal. In 2015, we measured ocean temperatures of 0.28°C near the grounding line of Petermann Gletscher’s ice shelf, which drove submarine melting along the base of the glacier. The resultant meltwater contributed to ocean stratification, which forced a stronger geostrophic circulation at the ice shelf terminus compared with previous years. This increased both the freshwater flux away from the sub–ice shelf cavity and the heat flux into it. Net summertime geostrophic heat flux estimates into the sub–ice shelf cavity exceed the requirement for steady-state melting of Petermann Gletscher’s ice shelf. Likewise, freshwater fluxes away from the glacier exceed the expected steady-state meltwater discharge. These results suggest that the warmer, more active ocean surrounding Petermann Gletscher forces “non steady state” melting of its ice shelf. When sustained, such melting thins the ice shelf.

scholarly journals Suppression of marine ice sheet instability

2018 ◽  
Vol 857 ◽  
pp. 648-680 ◽  
Author(s):  
Samuel S. Pegler
Keyword(s):  
Steady State ◽  
Rapid Assessment ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Stable Steady State ◽  
Ice Shelf ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Grounding Line ◽  
The Stability

A long-standing open question in glaciology concerns the propensity for ice sheets that lie predominantly submerged in the ocean (marine ice sheets) to destabilise under buoyancy. This paper addresses the processes by which a buoyancy-driven mechanism for the retreat and ultimate collapse of such ice sheets – the marine ice sheet instability – is suppressed by lateral stresses acting on its floating component (the ice shelf). The key results are to demonstrate the transition between a mode of stable (easily reversible) retreat along a stable steady-state branch created by ice-shelf buttressing to tipped (almost irreversible) retreat across a critical parametric threshold. The conditions for triggering tipped retreat can be controlled by the calving position and other properties of the ice-shelf profile and can be largely independent of basal stress, in contrast to principles established from studies of unbuttressed grounding-line dynamics. The stability and recovery conditions introduced by lateral stresses are analysed by developing a method of constructing grounding-line stability (bifurcation) diagrams, which provide a rapid assessment of the steady-state positions, their natures and the conditions for secondary grounding, giving clear visualisations of global stabilisation conditions. A further result is to reveal the possibility of a third structural component of a marine ice sheet that lies intermediate to the fully grounded and floating components. The region forms an extended grounding area in which the ice sheet lies very close to flotation, and there is no clearly distinguished grounding line. The formation of this region generates an upsurge in buttressing that provides the most feasible mechanism for reversal of a tipped grounding line. The results of this paper provide conceptual insight into the phenomena controlling the stability of the West Antarctic Ice Sheet, the collapse of which has the potential to dominate future contributions to global sea-level rise.

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.

scholarly journals Basal Mass Balance Along a Flow Line on Ronne Ice Shelf, Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 201
Author(s):  
A. Jenkins ◽  
C.S.M. Doake
Keyword(s):  
Steady State ◽  
Mass Balance ◽  
Flow Line ◽  
Average Rate ◽  
Ice Shelf ◽  
Grounding Line ◽  
Radio Echo Sounding ◽  
State Models ◽  
Basal Melting ◽  
Echo Sounding

Recent glaciological work on Ronne Ice Shelf has focused on an assumed flow line which extends from Rutford Ice Stream grounding line to the ice front. Results from doppler satellite surveying and radio echo-sounding are used in kinematic calculations to determine the basal mass balance, assuming the flow line to be in a steady state. Models suggest that basal melting dominates over most of the flow line and is most pronounced at the extremities. In the region within 300 km of the grounding line and over the final 45 km before the ice front, at least 1 m/a on average must melt away to maintain the observed velocity and thickness profile. More gentle melting occurs over about half the remaining distance, but in a region between 130 and 300 km in from the ice front, basal freezing must occur at an average rate of about 0.1 m/a to maintain a steady state. The existence of a thin layer of saline ice underlying the ice shelf, which persists for a further 80 km down-stream before being melted away entirely, is consistent with the weak returns observed during both airborne and ground-based radio echo-sounding in this region.

scholarly journals Bottom Melting under George VI Ice Shelf, Antarctica

1981 ◽  
Vol 27 (97) ◽  
pp. 429-447 ◽  
Author(s):  
J. F. Bishop ◽  
J. L. W. Walton
Keyword(s):  
Steady State ◽  
Spatial Variation ◽  
Complex Pattern ◽  
Simple Explanation ◽  
Ice Shelf ◽  
Flow Lines ◽  
Grounding Line ◽  
Kinematic Behaviour ◽  
Oceanographic Conditions

AbstractBottom melting rates have been calculated for a large number of sites on George VI Ice Shelf from measurements of its kinematic behaviour. No simple explanation for the melt-rate pattern was found in terms of ice-shelf parameters, assuming steady-state conditions. Values of apparent melt rates varied from 1 to 8 m a−1of ice. Along different flow lines the melt rate would sometimes increase with distance from the grounding line and sometimes the melt rate would decrease with distance. Large melt rates were found both where ice flowed off Palmer Land and where the ice shelf butted against Alexander Island. Although oceanographic conditions probably control bottom melting rates the complex pattern with large spatial variation seems to indicate that some areas of ice shelf are changing in thickness.

scholarly journals Grounding line transient response in marine ice sheet models

2012 ◽  
Vol 6 (5) ◽  
pp. 3903-3935 ◽  
Author(s):  
A. S. Drouet ◽  
D. Docquier ◽  
G. Durand ◽  
R. Hindmarsh ◽  
F. Pattyn ◽  
...  
Keyword(s):  
Steady State ◽  
Sea Level ◽  
Ice Sheet ◽  
Numerical Approach ◽  
Surface Velocity ◽  
Ice Shelf ◽  
Transient Behaviour ◽  
Surface Elevation Change ◽  
Grounding Line ◽  
Intercomparison Project

Abstract. Marine ice sheet stability is mostly controlled by the dynamics of the grounding line, i.e., the junction between the grounded ice sheet and the floating ice shelf. Grounding line migration has been investigated in the framework of MISMIP (Marine Ice Sheet Model Intercomparison Project), which mainly aimed at investigating steady state solutions. Here we focus on transient behaviour, executing short-term simulations (200 yr) of a steady ice sheet perturbed by the release of the buttressing restraint exerted by the ice shelf on the grounded ice upstream. The transient grounding line behaviour of four different flowline ice sheet models has been compared. The models differ in the physics implemented (full-Stokes and Shallow Shelf Approximation), the numerical approach, as well as the grounding line treatment. Their overall response to the loss of buttressing is found to be consistent in terms of grounding line position, rate of surface elevation change and surface velocity. However, large discrepancies (>100%) are observed in terms of ice sheet contribution to sea level. Despite the recent important improvements of marine ice sheet models in their ability to compute steady-state configurations, our results question models' capacity to compute reliable sea-level rise projections.

scholarly journals Basal Mass Balance Along a Flow Line on Ronne Ice Shelf, Antarctica (Abstract)

1988 ◽  
Vol 11 ◽  
pp. 201-201
Author(s):  
A. Jenkins ◽  
C.S.M. Doake
Keyword(s):  
Steady State ◽  
Mass Balance ◽  
Flow Line ◽  
Average Rate ◽  
Ice Shelf ◽  
Grounding Line ◽  
Radio Echo Sounding ◽  
State Models ◽  
Basal Melting ◽  
Echo Sounding

Recent glaciological work on Ronne Ice Shelf has focused on an assumed flow line which extends from Rutford Ice Stream grounding line to the ice front. Results from doppler satellite surveying and radio echo-sounding are used in kinematic calculations to determine the basal mass balance, assuming the flow line to be in a steady state. Models suggest that basal melting dominates over most of the flow line and is most pronounced at the extremities. In the region within 300 km of the grounding line and over the final 45 km before the ice front, at least 1 m/a on average must melt away to maintain the observed velocity and thickness profile. More gentle melting occurs over about half the remaining distance, but in a region between 130 and 300 km in from the ice front, basal freezing must occur at an average rate of about 0.1 m/a to maintain a steady state. The existence of a thin layer of saline ice underlying the ice shelf, which persists for a further 80 km down-stream before being melted away entirely, is consistent with the weak returns observed during both airborne and ground-based radio echo-sounding in this region.

scholarly journals Bottom Melting under George VI Ice Shelf, Antarctica

1981 ◽  
Vol 27 (97) ◽  
pp. 429-447 ◽  
Author(s):  
J. F. Bishop ◽  
J. L. W. Walton
Keyword(s):  
Steady State ◽  
Spatial Variation ◽  
Complex Pattern ◽  
Simple Explanation ◽  
Ice Shelf ◽  
Flow Lines ◽  
Grounding Line ◽  
Kinematic Behaviour ◽  
Oceanographic Conditions

AbstractBottom melting rates have been calculated for a large number of sites on George VI Ice Shelf from measurements of its kinematic behaviour. No simple explanation for the melt-rate pattern was found in terms of ice-shelf parameters, assuming steady-state conditions. Values of apparent melt rates varied from 1 to 8 m a−1 of ice. Along different flow lines the melt rate would sometimes increase with distance from the grounding line and sometimes the melt rate would decrease with distance. Large melt rates were found both where ice flowed off Palmer Land and where the ice shelf butted against Alexander Island. Although oceanographic conditions probably control bottom melting rates the complex pattern with large spatial variation seems to indicate that some areas of ice shelf are changing in thickness.

scholarly journals A Conceptual Model of Polar Overturning Circulations

2020 ◽  
Author(s):  
Thomas W. N. Haine
Keyword(s):  
Heat Flux ◽  
Sea Ice ◽  
Conceptual Model ◽  
The Arctic ◽  
Global Ocean ◽  
Freshwater Flux ◽  
Ice Surface ◽  
Freshwater Fluxes ◽  
Weak Constraints ◽  
Polar Water

AbstractThe global ocean overturning circulation carries warm, salty water to high latitudes, both in the Arctic and Antarctic. Interaction with the atmosphere transforms this inflow into three distinct products: sea ice, surface Polar Water, and deep Overflow Water. The Polar Water and OverflowWater formestuarine and thermal overturning cells, stratified by salinity and temperature, respectively. A conceptual model specifies the characteristics of these water masses and cells given the inflow and air/sea/land fluxes of heat and freshwater. The model includes budgets of mass, salt, and heat, and parametrizations of Polar Water and Overflow Water formation, which include exchange with continental shelves. Model solutions are mainly controlled by a linear combination of air/sea/ice heat and freshwater fluxes and inflow heat flux that approximates the meteoric freshwater flux plus the sea ice export flux. The model shows that for the Arctic, the thermal overturning is likely robust, but the estuarine cell appears vulnerable to collapse via a so-called heat crisis that violates the budget equations. The system is pushed towards this crisis by increasing AtlanticWater inflow heat flux, increasing meteoric freshwater flux, and/or decreasing heat loss to the atmosphere. The Antarctic appears close to a so-called Overflow Water emergency with weak constraints on the strengths of the estuarine and thermal cells, uncertain sensitivity to parameters, and possibility of collapse of the thermal cell.

scholarly journals Sensitivity of submarine melting on North East Greenland towards ocean forcing

2019 ◽  
Author(s):  
Philipp Anhaus ◽  
Lars H. Smedsrud ◽  
Marius Årthun ◽  
Fiammetta Straneo
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Rate Sensitivity ◽  
Atlantic Water ◽  
Maximum Temperature ◽  
Freshwater Flux ◽  
Ice Shelf ◽  
North East ◽  
Grounding Line ◽  
Ice Shelf Water

Abstract. The Nioghalvfjerdsbræ (79NG) is a floating ice tongue on Northeast Greenland draining a large part of the Greenland Ice Sheet. A CTD profile from a rift on the ice tongue close to the northern front shows that Atlantic Water (AW) is present in the cavity below, with maximum temperature of approximately 1 °C at 610 m depth. The AW present in the cavity thus has the potential to drive submarine melting along the ice base. Here, we simulate melt rates from the 79NG with a 1D numerical Ice Shelf Water (ISW) plume model. A meltwater plume is initiated at the grounding line depth (600 m) and rises along the ice base as a result of buoyancy contrast to the underlying AW. Ice melts as the plume entrains the warm AW. Maximum simulated melt rates are 50–76 m yr−1 within 10 km of the grounding line. Within a zone of rapid decay between 10 km and 20 km melt rates drop to roughly 6 m yr−1. Further downstream, melt rates are between 15 m yr−1 and 6 m yr−1. The melt-rate sensitivity to variations in AW temperatures is assessed by forcing the model with AW temperatures between 0.1–1.4 °C, as identified from the ECCOv4 ocean state estimate. The melt rates increase linearly with rising AW temperature, ranging from 10 m yr−1 to 21 m yr−1 along the centerline. The corresponding freshwater flux ranges between 11 km3 yr−1 (0.4 mSv) and 30 km3 yr−1 (1.0 mSv), which is 5 % and 12 % of the total freshwater flux from the Greenland Ice Sheet since 1995, respectively. Our results improve the understanding of processes driving submarine melting of marine-terminating glaciers around Greenland, and its sensitivity to changing ocean conditions.

scholarly journals Grounding line transient response in marine ice sheet models

2013 ◽  
Vol 7 (2) ◽  
pp. 395-406 ◽  
Author(s):  
A. S. Drouet ◽  
D. Docquier ◽  
G. Durand ◽  
R. Hindmarsh ◽  
F. Pattyn ◽  
...  
Keyword(s):  
Steady State ◽  
Sea Level ◽  
Ice Sheet ◽  
Numerical Approach ◽  
Surface Velocity ◽  
Ice Shelf ◽  
Short Term ◽  
Transient Behaviour ◽  
Grounding Line ◽  
Intercomparison Project

Abstract. Marine ice-sheet stability is mostly controlled by the dynamics of the grounding line, i.e. the junction between the grounded ice sheet and the floating ice shelf. Grounding line migration has been investigated within the framework of MISMIP (Marine Ice Sheet Model Intercomparison Project), which mainly aimed at investigating steady state solutions. Here we focus on transient behaviour, executing short-term simulations (200 yr) of a steady ice sheet perturbed by the release of the buttressing restraint exerted by the ice shelf on the grounded ice upstream. The transient grounding line behaviour of four different flowline ice-sheet models has been compared. The models differ in the physics implemented (full Stokes and shallow shelf approximation), the numerical approach, as well as the grounding line treatment. Their overall response to the loss of buttressing is found to be broadly consistent in terms of grounding line position, rate of surface elevation change and surface velocity. However, still small differences appear for these latter variables, and they can lead to large discrepancies (> 100%) observed in terms of ice sheet contribution to sea level when cumulated over time. Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections.

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