285 research outputs found
neXtSIM: a new Lagrangian sea ice model
The Arctic sea ice cover has changed drastically over the last
decades. Associated with these changes is a shift in dynamical regime seen by
an increase of extreme fracturing events and an acceleration of sea ice
drift. The highly non-linear dynamical response of sea ice to external
forcing makes modelling these changes and the future evolution of Arctic sea
ice a challenge for current models. It is, however, increasingly important
that this challenge be better met, both because of the important role of sea
ice in the climate system and because of the steady increase of industrial
operations in the Arctic. In this paper we present a new
dynamical/thermodynamical sea ice model called neXtSIM that is designed to
address this challenge. neXtSIM is a continuous and fully Lagrangian model,
whose momentum equation is discretised with the finite-element method. In
this model, sea ice physics are driven by the combination of two core
components: a model for sea ice dynamics built on a mechanical framework
using an elasto-brittle rheology, and a model for sea ice thermodynamics
providing damage healing for the mechanical framework. The evaluation of the
model performance for the Arctic is presented for the period September 2007
to October 2008 and shows that observed multi-scale statistical properties of
sea ice drift and deformation are well captured as well as the seasonal
cycles of ice volume, area, and extent. These results show that neXtSIM is an
appropriate tool for simulating sea ice over a wide range of spatial and
temporal scales
Fluctuations of a Greenlandic tidewater glacier driven by changes in atmospheric forcing : observations and modelling of Kangiata Nunaata Sermia, 1859–present
Acknowledgements. The authors wish to thank Stephen Price, Mauri Pelto, and the anonymous reviewer for their reviews and comments that helped to improve the manuscript. RACMO2.1 data were provided by Jan van Angelen and Michiel van den Broeke, IMAU, Utrecht University. MAR v3.2 data used for runoff calculations were provided by Xavier Fettweis, Department of Geography, University of Liège. The photogrammetric DEM used in Figs. 1 and 3 was provided by Kurt H. Kjær, Centre for GeoGenetics, University of Copenhagen. This research was financially supported by J. M. Lea’s PhD funding, NERC grant number NE/I528742/1. Support for F. M. Nick was provided through the Conoco-Phillips/Lundin Northern Area Program CRIOS project (Calving Rates and Impact on Sea Level).Peer reviewedPublisher PD
Modeling the response of northwest Greenland to enhanced ocean thermal forcing and subglacial discharge
Calving-front dynamics is an important
control on Greenland's ice mass balance. Ice front retreat of
marine-terminating glaciers may, for example, lead to a loss in resistive
stress, which ultimately results in glacier acceleration and thinning. Over
the past decade, it has been suggested that such retreats may be triggered by
warm and salty Atlantic Water, which is typically found at a depth below
200–300 m. An increase in subglacial water discharge at glacier ice fronts
due to enhanced surface runoff may also be responsible for an intensification
of undercutting and calving. An increase in ocean thermal forcing or
subglacial discharge therefore has the potential to destabilize
marine-terminating glaciers along the coast of Greenland. It remains unclear
which glaciers are currently stable but may retreat in the future and how far
inland and how fast they will retreat. Here, we quantify the sensitivity and
vulnerability of marine-terminating glaciers along the northwest coast of
Greenland (from 72.5 to 76∘ N) to ocean forcing and subglacial
discharge using the Ice Sheet System Model (ISSM). We rely on a
parameterization of undercutting based on ocean thermal forcing and
subglacial discharge and use ocean temperature and salinity from
high-resolution ECCO2 (Estimating the Circulation and Climate of the Ocean,
Phase II) simulations at the fjord mouth to constrain the ocean thermal
forcing. The ice flow model includes a calving law based on a tensile von
Mises criterion. We find that some glaciers, such as Dietrichson Gletscher or
Alison Glacier, are sensitive to small increases in ocean thermal forcing,
while others, such as Illullip Sermia or Cornell
Gletscher, are remarkably stable,
even in a +3 ∘C ocean warming scenario. Under the most intense
experiment, we find that Hayes Gletscher retreats by more than 50 km inland by 2100 into a deep trough,
and its velocity increases by a factor of 3 over only 23 years. The model
confirms that ice–ocean interactions can trigger extensive and rapid glacier
retreat, but the bed controls the rate and magnitude of the retreat. Under
current oceanic and atmospheric conditions, we find that this sector of the
Greenland ice sheet alone will contribute more than 1 cm to sea level rise
and up to 3 cm by 2100 under the most extreme scenario.</p
Future Antarctic bed topography and its implications for ice sheet dynamics
The Antarctic bedrock is evolving as the solid Earth responds to the past and ongoing evolution of the ice sheet. A recently improved ice loading history suggests that the Antarctic Ice Sheet (AIS) has generally been losing its mass since the Last Glacial Maximum. In a sustained warming climate, the AIS is predicted to retreat at a greater pace, primarily via melting beneath the ice shelves. We employ the glacial isostatic adjustment (GIA) capability of the Ice Sheet System Model (ISSM) to combine these past and future ice loadings and provide the new solid Earth computations for the AIS. We find that past loading is relatively less important than future loading for the evolution of the future bed topography. Our computations predict that the West Antarctic Ice Sheet (WAIS) may uplift by a few meters and a few tens of meters at years AD 2100 and 2500, respectively, and that the East Antarctic Ice Sheet is likely to remain unchanged or subside minimally except around the Amery Ice Shelf. The Amundsen Sea Sector in particular is predicted to rise at the greatest rate; one hundred years of ice evolution in this region, for example, predicts that the coastline of Pine Island Bay will approach roughly 45 mm yr−1 in viscoelastic vertical motion. Of particular importance, we systematically demonstrate that the effect of a pervasive and large GIA uplift in the WAIS is generally associated with the flattening of reverse bed slope, reduction of local sea depth, and thus the extension of grounding line (GL) towards the continental shelf. Using the 3-D higher-order ice flow capability of ISSM, such a migration of GL is shown to inhibit the ice flow. This negative feedback between the ice sheet and the solid Earth may promote stability in marine portions of the ice sheet in the future
Representation of sharp rifts and faults mechanics in modeling ice shelf flow dynamics: Application to Brunt/Stancomb-Wills Ice Shelf, Antarctica
Ice shelves play a major role in buttressing ice sheet flow into the ocean, hence the importance of accurate numerical modeling of their stress regime. Commonly used ice flow models assume a continuous medium and are therefore complicated by the presence of rupture features (crevasses, rifts, and faults) that significantly affect the overall flow patterns. Here we apply contact mechanics and penalty methods to develop a new ice shelf flow model that captures the impact of rifts and faults on the rheology and stress distribution of ice shelves. The model achieves a best fit solution to satellite observations of ice shelf velocities to infer the following: (1) a spatial distribution of contact and friction points along detected faults and rifts, (2) a more realistic spatial pattern of ice shelf rheology, and (3) a better representation of the stress balance in the immediate vicinity of faults and rifts. Thus, applying the model to the Brunt/Stancomb-Wills Ice Shelf, Antarctica, we quantify the state of friction inside faults and the opening rates of rifts and obtain an ice shelf rheology that remains relatively constant everywhere else on the ice shelf. We further demonstrate that better stress representation has widespread application in examining aspects affecting ice shelf structure and dynamics including the extent of ice mélange in rifts and the change in fracture configurations. All are major applications for better insight into the important question of ice shelf stability
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Abstract not availabl
SHAKTI: Subglacial Hydrology and Kinetic, Transient Interactions v1.0
Subglacial hydrology has a strong influence on glacier and ice
sheet dynamics, particularly through the dependence of sliding velocity on
subglacial water pressure. Significant challenges are involved in modeling
subglacial hydrology, as the drainage geometry and flow mechanics are
constantly changing, with complex feedbacks that play out between water and
ice. A clear tradition has been established in the subglacial hydrology
modeling literature of distinguishing between channelized (efficient) and
sheetlike (inefficient or distributed) drainage systems or components and
using slightly different forms of the governing equations in each subsystem
to represent the dominant physics. Specifically, many previous subglacial
hydrology models disregard opening by melt in the sheetlike system or
redistribute it to adjacent channel elements in order to avoid runaway
growth that occurs when it is included in the sheetlike system. We present a
new subglacial hydrology model, SHAKTI (Subglacial Hydrology and Kinetic,
Transient Interactions), in which a single set of governing equations is used
everywhere, including opening by melt in the entire domain. SHAKTI employs a
generalized relationship between the subglacial water flux and the hydraulic
gradient that allows for the representation of laminar, turbulent, and transitional
regimes depending on the local Reynolds number. This formulation allows for
the
coexistence of these flow regimes in different regions, and the configuration
and geometry of the subglacial system evolves naturally to represent
sheetlike drainage as well as systematic channelized drainage under
appropriate conditions. We present steady and transient example simulations
to illustrate the features and capabilities of the model and to examine
sensitivity to mesh size and time step size. The model is implemented as part
of the Ice Sheet System Model (ISSM).</p
Drivers of Change of Thwaites Glacier, West Antarctica, Between 1995 and 2015
We run several transient numerical simulations applying these three perturbations individually. Our results show that ocean-induced ice-shelf thinning generates most of the observed grounding line retreat, inland speed-up, and mass loss, in agreement with previous work. We improve the agreement with observed inland speed-up and thinning by prescribing changes in ice-shelf geometry and a reduction in basal traction over areas that became ungrounded since 1995, suggesting that shelf breakups and thinning-induced reduction in basal traction play a critical role on Thwaites's dynamics, as pointed out by previous studies. These findings suggest that modeling Thwaites's future requires reliable ocean-induced melt estimates in models that respond accurately to downstream perturbations
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