scholarly journals Stochastic marine ice sheet variability

2018 ◽  
Vol 843 ◽  
pp. 748-777 ◽  
Author(s):  
T. E. Mulder ◽  
S. Baars ◽  
F. W. Wubs ◽  
H. A. Dijkstra
Keyword(s):  
Transition Probabilities ◽  
Accumulation Rate ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Field Measurements ◽  
Multiple Equilibrium ◽  
Short Time Scale ◽  
Two Dimensional ◽  
Grounding Line ◽  
Short Time

It is well known that deterministic two-dimensional marine ice sheets can only be stable if the grounding line is positioned at a sufficiently steep, downward sloping bedrock. When bedrock conditions favour instabilities, multiple stable ice sheet profiles may occur. Here, we employ continuation techniques to examine the sensitivity of a two-dimensional marine ice sheet to stochastic noise representing short time scale variability, either in the accumulation rate or in the sea level height. We find that in unique regimes, the position of the grounding line is most sensitive to noise in the accumulation rate and can explain excursions observed in field measurements. In the multiple equilibrium regime, there is a strong asymmetry in transition probabilities between the different ice sheet states, with a strong preference to switch to the branch with a steeper bedrock slope.

The effects of bed topography and strength on stability of marine ice sheets

2021 ◽  
Author(s):  
Olga Sergienko ◽  
Duncan Wingham
Keyword(s):  
Accumulation Rate ◽  
Ice Sheets ◽  
Ice Sheet ◽  
West Antarctica ◽  
Amundsen Sea ◽  
Bed Topography ◽  
Wide Range ◽  
Grounding Line ◽  
Bed Slope ◽  
Conditions Of Stability

<p>The "marine ice-sheet instability hypothesis", which states that unconfined marine ice sheets are unconditionally unstable on retrograde slopes, was developed under assumptions of negligible bed slopes. Realistic ice sheets, however, flow over beds which topographies have a wide range of bed slopes (for example, Thwaites Glacier in the Amundsen Sea sector, West Antarctica). Reexamining the original model of marine ice sheets proposed by Schoof (2007), and relaxing an assumption of negligible bed slopes, we find that a steady-state ice flux at the grounding line is an implicit function of the grounding-line ice thickness, bed slope and accumulation rate. Depending on the sliding conditions, the magnitudes of the ice flux at the grounding line differ by one-to-three orders of magnitudes from that computed with a power-law expression derived by Schoof (2007) under assumptions of the negligible bed slopes. Non-negligible bed slopes also result in conditions of stability of the grounding line that are significantly more complex than those associated with the "marine ice sheet instability hypothesis". Bed slopes are no longer the sole determinant of whether the grounding line is stable or unstable. We find that the grounding line can be stable on beds with retrograde slopes and unstable on beds with prograde slopes. </p>

scholarly journals Dynamical processes involved in the retreat of marine ice sheets

2001 ◽  
Vol 47 (157) ◽  
pp. 271-282 ◽  
Author(s):  
Richard C.A. Hindmarsh ◽  
E. Le Meur
Keyword(s):  
Ice Sheets ◽  
Ice Sheet ◽  
Ice Shelf ◽  
Ice Streams ◽  
Antarctic Ice Sheet ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Grounding Line ◽  
Neutral Equilibrium ◽  
Generally True

AbstractMarine ice sheets with mechanics described by the shallow-ice approximation by definition do not couple mechanically with the shelf. Such ice sheets are known to have neutral equilibria. We consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice-sheet retreat. The removal of ice-shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds. Sea-level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial West Antarctic ice-sheet retreat but is inconsistent with borehole evidence. Warming thins the ice sheet by reducing the average viscosity but does not lead to grounding-line retreat. Internal oscillations either specified or generated via a MacAyeal–Payne thermal mechanism promote migration. This is a noise-induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets. This migration occurs at quite slow rates, but these are sufficiently large to have possibly played a role in the dynamics of the West Antarctic ice sheet after the glacial maximum. Numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes, spatial variability coupled with dynamical variability is needed to cause margin movement.

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 Modeling precipitation over ice sheets: an assessment using Greenland

2002 ◽  
Vol 48 (160) ◽  
pp. 70-80 ◽  
Author(s):  
Gerard H. Roe
Keyword(s):  
Accumulation Rate ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Climate Forcing ◽  
Steady State Response ◽  
Ice Dynamics ◽  
Accumulation Pattern ◽  
State Response ◽  
Long Time

AbstractThe interaction between ice sheets and the rest of the climate system at long time-scales is not well understood, and studies of the ice ages typically employ simplified parameterizations of the climate forcing on an ice sheet. It is important therefore to understand how an ice sheet responds to climate forcing, and whether the reduced approaches used in modeling studies are capable of providing robust and realistic answers. This work focuses on the accumulation distribution, and in particular considers what features of the accumulation pattern are necessary to model the steady-state response of an ice sheet. We examine the response of a model of the Greenland ice sheet to a variety of accumulation distributions, both observational datasets and simplified parameterizations. The predicted shape of the ice sheet is found to be quite insensitive to changes in the accumulation. The model only differs significantly from the observed ice sheet for a spatially uniform accumulation rate, and the most important factor for the successful simulation of the ice sheet’s shape is that the accumulation decreases with height according to the ability of the atmosphere to hold moisture. However, the internal ice dynamics strongly reflects the influence of the atmospheric circulation on the accumulation distribution.

scholarly journals Marine ice-sheet profiles and stability under Coulomb basal conditions

2015 ◽  
Vol 61 (226) ◽  
pp. 205-215 ◽  
Author(s):  
Victor C. Tsai ◽  
Andrew L. Stewart ◽  
Andrew F. Thompson
Keyword(s):  
Power Law ◽  
Coulomb Friction ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Friction Behavior ◽  
West Antarctic Ice Sheet ◽  
West Antarctic ◽  
Grounding Line ◽  
Catastrophic Loss ◽  
Bed Slope

AbstractThe behavior of marine-terminating ice sheets, such as the West Antarctic ice sheet, is of interest due to the possibility of rapid grounding-line retreat and consequent catastrophic loss of ice. Critical to modeling this behavior is a choice of basal rheology, where the most popular approach is to relate the ice-sheet velocity to a power-law function of basal stress. Recent experiments, however, suggest that near-grounding line tills exhibit Coulomb friction behavior. Here we address how Coulomb conditions modify ice-sheet profiles and stability criteria. The basal rheology necessarily transitions to Coulomb friction near the grounding line, due to low effective stresses, leading to changes in ice-sheet properties within a narrow boundary layer. Ice-sheet profiles ‘taper off’ towards a flatter upper surface, compared with the power-law case, and basal stresses vanish at the grounding line, consistent with observations. In the Coulomb case, the grounding-line ice flux also depends more strongly on flotation ice thickness, which implies that ice sheets are more sensitive to climate perturbations. Furthermore, with Coulomb friction, the ice sheet grounds stably in shallower water than with a power-law rheology. This implies that smaller perturbations are required to push the grounding line into regions of negative bed slope, where it would become unstable. These results have important implications for ice-sheet stability in a warming climate.

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 Modelling dynamic ice-sheet boundaries and grounding line migration using the level set method

2020 ◽  
Vol 66 (259) ◽  
pp. 766-776
Author(s):  
M. Alamgir Hossain ◽  
Sam Pimentel ◽  
John M. Stockie
Keyword(s):  
Level Set Method ◽  
Level Set ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Numerical Approach ◽  
Ice Surface ◽  
Grounding Line ◽  
High Degree ◽  
Ice Water ◽  
Benchmark Solutions

AbstractComputing predictions of future sea level that include well-defined uncertainty bounds requires models that are capable of robustly simulating the evolution of ice sheets and glaciers. Ice flow behaviour is known to be sensitive to the location and geometry of dynamic ice boundaries such as the grounding line (GRL), terminus position and ice surface elevation, so that any such model should track these interfaces with a high degree of accuracy. To address this challenge, we implement a numerical approach that uses the level-set method (LSM) that accurately models the evolution of the ice–air and ice–water interface as well as capturing topological changes in ice-sheet geometry. This approach is evaluated by comparing simulations of grounded and marine-terminating ice sheets to various analytical and numerical benchmark solutions. A particular advantage of the LSM is its ability to explicitly track the moving margin and GRL while using a fixed grid finite-difference scheme. Our results demonstrate that the LSM is an accurate and robust approach for tracking the ice surface interface and terminus for advancing and retreating ice sheets, including the transient marine ice-sheet interface and GRL positions.

scholarly journals Position of Ice Divides and Ice Centers on Ice Sheets

1973 ◽  
Vol 12 (66) ◽  
pp. 353-360 ◽  
Author(s):  
J. Weertman
Keyword(s):  
Ice Sheets ◽  
Ice Sheet ◽  
Two Dimensional ◽  
Model Calculations ◽  
Accumulation Pattern ◽  
Rate Of Accumulation

Model calculations are made of the magnitude of the shift of the portion of an ice divide on a two dimensional ice sheet and of the “center” (the position of highest elevation) of a circular ice sheet when the rate of accumulation is different on different sides or in different sectors of an ice sheet. It is concluded that gross changes in the accumulation pattern are required to cause an appreciable shift of the position of ice divides or ice centers if the positions of the edge of the ice sheet are fixed.

scholarly journals Seasonal Variation of Soil Respiration in the Mongolian Oak (Quercus mongolica Fisch. Ex Ledeb.) Forests at the Cool Temperate Zone in Korea

Forests ◽  
2020 ◽  
Vol 11 (9) ◽  
pp. 984
Author(s):  
Gyung Soon Kim ◽  
Seung Jin Joo ◽  
Chang Seok Lee
Keyword(s):  
Soil Respiration ◽  
Environmental Parameters ◽  
Field Measurements ◽  
Temperate Zone ◽  
Short Time Scale ◽  
Strongly Correlated ◽  
Quercus Mongolica ◽  
Cool Temperate ◽  
Mongolian Oak ◽  
Short Time

To investigate the variation in seasonal soil respiration (SR) as a function of soil temperature (Ts) and soil water content (SWC) in Mongolian oak (Quercus mongolica) forests in urban (Mt. Nam) and well-reserved (Mt. Jeombong) areas in South Korea, we conducted continuous field measurements of SR and other environmental parameters (Ts and SWC) using an automated chamber system. Overall, the SR rates in both stands were strongly correlated with the Ts variable during all seasons. However, abrupt fluctuations in SR were significantly related to episodic increases in SWC on a short time scale during the growing season. The integrated optimal regression models for SR using Ts at a depth of 5 cm and SWC at a depth of 15 cm yielded the following: the SR rate in Mt. Nam = SR(Ts) + ΔSR(Ts) = 104.87 exp(0.1108Ts) − 10.09(SWC)2 + 604.2(SWC) − 8627.7 for Ts ≥ 0 °C, and the SR rate in Mt. Jeombong = SR(Ts) + ΔSR(Ts) = 95.608 exp(0.1304Ts) − 33.086(SWC)2 + 1949.2(SWC) − 28499 for Ts ≥ 0 °C. In both cases, SR = 0 for Ts < 0 °C. As per these equations, the estimated annual total SRs were 1339.4 g C m−2 for Mt. Nam and 1003.0 g C m−2 for Mt. Jeombong. These values were quite similar to the measured values in field. Our results demonstrate that the improved empirical equation is an effective tool for estimating and predicting SR variability and provide evidence that the SR of Q. mongolica forests in the cool temperate zone of Korean Peninsula depends on Ts and SWC variables.

Abrupt last glacial loess-dust deposition over Southeast England coupled with dynamics of the British-Irish Ice Sheet

2021 ◽  
Author(s):  
Thomas Stevens ◽  
Daniele Sechi ◽  
Balázs Bradák ◽  
Ragna Orbe ◽  
Yunus Baykal ◽  
...  
Keyword(s):  
North Sea ◽  
Accumulation Rate ◽  
Dust Emission ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Last Glacial ◽  
The North ◽  
The North Sea ◽  
Dust Dynamics ◽  
Dust Accumulation

&lt;p&gt;Loess deposits are globally important dust archives but are often limited by imprecise chronological control. In particular, loess records adjacent to former ice sheets seldom have detailed, independent age models yet have the potential to elucidate the causes of past high latitude (&gt;50&amp;#176; N in Northern Hemisphere) coarse dust emission close to former ice sheets, a relatively poorly known aspect of past dust dynamics. Loess deposits in southern Britain were formed in close proximity to western parts of the last glacial Eurasian ice sheets. However, currently their age and accumulation rate remain poorly known, limiting interpretation of the controls on last glacial coarse dust emission and deposition in the region.&lt;/p&gt;&lt;p&gt;Here we apply high sampling resolution quartz optically stimulated luminescence (OSL) to constrain the timing of dust accumulation and loess formation at the Pegwell Bay site in east Kent, SE England. The OSL ages and Bayesian (Bacon) age modelling results are the most detailed to date for western European loess, and show that loess began to accumulate around c. 25 ka, coinciding with Heinrich event 2 and the coupling of Fennoscandian and British-Irish ice sheets. There were two phases of greatly enhanced dust accumulation at the site, at 25-23.5 ka and 20-19 ka, separated by a lower accumulation rate period. Loess accumulation appears to have stopped or been dramatically reduced after 19-18 ka. We propose that the dynamics of the British-Irish and Fennoscandian Ice Sheets, associated glacial lake drainage, and linked reorganisations of atmospheric circulation, act to control loess accumulation at the site. In particular, we argue that both periods of enhanced dust accumulation were caused by advance-retreat phases of the North Sea ice lobe, and associated drainage of Dogger Lake. These events would have led to abrupt input of sediment-rich ice dammed lake and melt water from northern and eastern England and the North Sea into the exposed southern North Sea basin. This would have dramatically increased sediment availability for transport and deposition as loess in SE England. Easterly and north-easterly winds that could have transported this dust to SE England would have been enhanced by presence of an ice sheet anticyclone, enlarged during Fennoscandian and British-Irish ice sheet coalescence, as well as katabatic winds and easterly flow occurring on the northern side of Atlantic cyclones forced south of southern Britain by the extended western British-Irish ice sheet. As such, last glacial dust dynamics and loess accumulation in Britain is highly influenced by the interaction of the British-Irish and Fennoscandian ice sheets, Atlantic storm tracks, and the topography and drainage of the exposed North Sea basin.&lt;/p&gt;

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