Huth, Alexander, Ravindra Duddu, Benjamin Smith, and Olga V Sergienko, September 2023: Simulating the processes controlling ice-shelf rift paths using damage mechanics. Journal of Glaciology, DOI:10.1017/jog.2023.71. Abstract
Rifts are full-thickness fractures that propagate laterally across an ice shelf. They cause ice-shelf weakening and calving of tabular icebergs, and control the initial size of calved icebergs. Here, we present a joint inverse and forward computational modeling framework to capture rifting by combining the vertically integrated momentum balance and anisotropic continuum damage mechanics formulations. We incorporate rift–flank boundary processes to investigate how the rift path is influenced by the pressure on rift–flank walls from seawater, contact between flanks, and ice mélange that may also transmit stress between flanks. To illustrate the viability of the framework, we simulate the final 2 years of rift propagation associated with the calving of tabular iceberg A68 in 2017. We find that the rift path can change with varying ice mélange conditions and the extent of contact between rift flanks. Combinations of parameters associated with slower rift widening rates yield simulated rift paths that best match observations. Our modeling framework lays the foundation for robust simulation of rifting and tabular calving processes, which can enable future studies on ice-sheet–climate interactions, and the effects of ice-shelf buttressing on land ice flow.
Sergienko, Olga V., and Marianne Haseloff, October 2023: ‘Stable’ and ‘unstable’ are not useful descriptions of marine ice sheets in the Earth's climate system. Journal of Glaciology, 69(277), DOI:10.1017/jog.2023.401483-1499. Abstract
Investigations of the time-dependent behavior of marine ice sheets and their sensitivity to basal conditions require numerical models because existing theoretical analyses focus only on steady-state configurations primarily with a power-law basal shear stress. Numerical results indicate that the choice of the sliding law strongly affects ice-sheet dynamic behavior. Although observed or simulated grounding-line retreat is typically interpreted as an indication of marine ice sheet instability introduced by Weertman (1974), this (in)stability is a characteristic of the ice sheet's steady states – not time-variant behavior. To bridge the gap between theoretical and numerical results, we develop a framework to investigate grounding line dynamics with generalized basal and lateral stresses (i.e. the functional dependencies are not specified). Motivated by observations of internal variability of the Southern Ocean conditions we explore the grounding-line response to stochastic variability. We find that adding stochastic variability to submarine melt rates that produced stable steady-state configurations leads to intermittently advancing and retreating grounding lines. They can also retreat in an unstoppable manner on time-scales significantly longer than the stochastic correlation time-scales. These results suggest that at any given time of their evolution, the transient behavior of marine ice sheets cannot be described in terms of ‘stable’ or ‘unstable’.
Coffey, Niall B., Douglas R MacAyeal, Luke Copland, Derek R Mueller, Olga V Sergienko, Alison F Banwell, and Ching-Yao Lai, February 2022: Enigmatic surface rolls of the Ellesmere Ice Shelf. Journal of Glaciology, DOI:10.1017/jog.2022.3. Abstract
The once-contiguous Ellesmere Ice Shelf, first reported in writing by European explorers in 1876, and now almost completely disintegrated, has rolling, wave-like surface topography, the origin of which we investigate using a viscous buckling instability analysis. We show that rolls can develop during a winter season (~ 100 d) if sea-ice pressure (depth-integrated horizontal stress applied to the seaward front of the Ellesmere Ice Shelf) is sufficiently large (1 MPa m) and ice thickness sufficiently low (1–10 m). Roll wavelength initially depends only on sea-ice pressure, but evolves over time depending on amplitude growth rate. This implies that a thinner ice shelf, with its faster amplitude growth rate, will have a shorter wavelength compared to a thicker ice shelf when sea-ice pressure is equal. A drawback of the viscous buckling mechanism is that roll amplitude decays once sea-ice pressure is removed. However, non-Newtonian ice rheology, where effective viscosity, and thus roll change rate, depends on total applied stress may constrain roll decay rate to be much slower than growth rate and allow roll persistence from year to year. Whether the viscous-buckling mechanism we explore here ultimately can be confirmed as the origin of the Ellesmere Ice Shelf rolls remains for future research.
Forced global ocean/sea-ice hindcast simulations are subject to persistent surface mass flux estimation biases, for example, configurations with an explicit-free surface may not take into account the seasonal storage of water on land when constraining sea level. We present a physically motivated surface mass flux closure, that results in: reduced watermass drift from initialization; improved Atlantic meridional overturning cirulation intensity; and more realistic rates of ocean heat uptake, in simulations using global ocean/sea-ice/land (MOM6/SIS2/LM3) model configurations, forced with atmospheric reanalysis data. In addition to accounting for the land storage, the area-integrated subpolar-to-polar (40°–90°N/S) surface mass fluxes are constrained, using a climatological estimate derived from the the CMIP6 historical ensemble, which helps to further improve hindcast performance. Simulations using MERRA-2 and JRA55-do forcing, subject to identical hydrologic constraints, exhibit similar reductions in drift.
Haseloff, Marianne, and Olga V Sergienko, May 2022: Effects of calving and submarine melting on steady states and stability of buttressed marine ice sheets. Journal of Glaciology, DOI:10.1017/jog.2022.29. Abstract
Mass loss from ice shelves is a strong control on grounding-line dynamics. Here we investigate how calving and submarine melt parameterizations affect steady-state grounding-line positions and their stability. Our results indicate that different calving laws with the same melt parameterization result in more diverse steady-state ice-sheet configurations than different melt parameterizations with the same calving law. We show that the backstress at the grounding line depends on the integrated ice-shelf mass flux. Consequently, ice shelves are most sensitive to high melt rates in the vicinity of their grounding lines. For the same shelf-averaged melt rates, different melt parameterizations can lead to very different ice-shelf configurations and grounding-line positions. If the melt rate depends on the slope of the ice-shelf draft, then the positive feedback between increased melting and steepening of the slope can lead to singular melt rates at the ice-shelf front, producing an apparent lower limit of the shelf front thickness as the ice thickness vanishes over a small boundary layer. Our results illustrate that the evolution of marine ice sheets is highly dependent on ice-shelf mass loss mechanisms, and that existing parameterizations can lead to a wide range of modelled grounding-line behaviours.
Large tabular icebergs account for the majority of ice mass calved from Antarctic ice shelves, but are omitted from climate models. Specifically, these models do not account for iceberg breakup and as a result, modeled large icebergs could drift to low latitudes. Here, we develop a physically based parameterization of iceberg breakup based on the “footloose mechanism” suitable for climate models. This mechanism describes breakup of ice pieces from the iceberg edges triggered by buoyancy forces associated with a submerged ice foot fringing the iceberg. This foot develops as a result of ocean-induced melt and erosion of the iceberg freeboard explicitly parameterized in the model. We then use an elastic beam model to determine when the foot is large enough to trigger calving, as well as the size of each child iceberg, which is controlled with the ice stiffness parameter. We test the breakup parameterization with a realistic large iceberg calving-size distribution in the Geophysical Fluid Dynamics Laboratory OM4 ocean/sea-ice model and obtain simulated iceberg trajectories and areas that closely match observations. Thus, the footloose mechanism appears to play a major role in iceberg decay that was previously unaccounted for in iceberg models. We also find that varying the size of the broken ice bits can influence the iceberg meltwater distribution more than physically realistic variations to the footloose decay rate.
In December 2020, giant tabular iceberg A68a (surface area 3900 km2) broke up in open ocean much deeper than its keel, indicating that the breakage was not immediately caused by collision with the seafloor. Giant icebergs with lengths exceeding 18.5 km account for most of the calved ice mass from the Antarctic Ice Sheet. Upon calving, they drift away and transport freshwater into the Southern Ocean, modifying ocean circulation, disrupting sea ice and the marine biosphere, and potentially triggering changes in climate. Here, we demonstrate that the A68a breakup event may have been triggered by ocean-current shear, a new breakup mechanism not previously reported. We also introduce methods to represent giant icebergs within climate models that currently do not have any representation of them. These methods open opportunities to explore the interactions between icebergs and other components of the climate system and will improve the fidelity of global climate simulations.
This paper examines the effect of basal topography and strength on the grounding-line position, flux and stability of rapidly-sliding ice streams. It does so by supposing that the buoyancy of the ice stream is small, and of the same order as the longitudinal stress gradient. Making this scaling assumption makes the role of the basal gradient and accumulation rate explicit in the lowest order expression for the ice flux at the grounding line and also provides the transcendental equation for the grounding-line position. It also introduces into the stability condition terms in the basal curvature and accumulation-rate gradient. These expressions revert to well-established expressions in circumstances in which the thickness gradient is large at the grounding line, a result which is shown to be the consequence of the non-linearity of the flow. The behaviour of the grounding-line flux is illustrated for a range of bed topographies and strengths. We show that, when bed topography at a horizontal scale of several tens of ice thicknesses is present, the grounding-line flux and stability have more complex dependencies on bed gradient than that associated with the ‘marine ice-sheet instability hypothesis’, and that unstable grounding-line positions can occur on prograde beds as well as stable positions on retrograde beds.
The “marine ice-sheet instability” hypothesis continues to be used to interpret the observed mass loss from the Antarctic and Greenland ice sheets. This hypothesis has been developed for conditions that do not account for feedbacks between ice sheets and environmental conditions. However, snow accumulation and the ice-sheet surface melting depend on the surface temperature, which is a strong function of elevation. Consequently, there is a feedback between precipitation, atmospheric surface temperature and ice-sheet surface elevation. Here, we investigate stability conditions of a marine-based ice sheet in the presence of such a feedback. Our results show that no general stability condition similar to one associated with the “marine ice-sheet instability” hypothesis can be determined. Stability of individual configurations can be established only on a case-by-case basis. These results apply to a wide range of feedbacks between marine ice sheets and atmosphere, ocean and lithosphere.
Laterally confined marine outlet glaciers exhibit a diverse range of behaviours. This study investigates time-evolving and steady configurations of such glaciers. Using simplified analytic models, it determines conditions for steady states, their stability and expressions for the rate of the calving-front migration for three widely used calving rules. It also investigates the effects of ice mélange when it is present. The results show that ice flux at the terminus is an implicit function of ice thickness that depends on the glacier geometric and dynamic parameters. As a consequence, stability of steady-state configurations is determined by a complex combination of these parameters, specifics of the calving rule and the details of mélange stress conditions. The derived expressions of the rate of terminus migration suggest a non-linear feedback between the migration rate and the calving-front position. A close agreement between the obtained analytic expressions and numerical simulations suggests that these expressions can be used to gain insights into the observed behaviour of the glaciers and also to use observations to improve understanding of calving conditions.
The mechanical interactions between ice floes in the polar sea-ice packs play an important role in the state and predictability of the sea-ice cover. We use a Lagrangian-based numerical model to investigate such floe-floe interactions. Our simulations show that elastic and reversible deformation offers significant resistance to compression before ice floes yield with brittle failure. Compressional strength dramatically decreases once pressure ridges start to form, which implies that thicker sea ice is not necessarily stronger than thinner ice. The mechanical transition is not accounted for in most current sea-ice models that describe ice strength by thickness alone. We propose a parameterization that describes failure mechanics from fracture toughness and Coulomb sliding, improving the representation of ridge building dynamics in particle-based and continuum sea-ice models.
MacAyeal, Douglas R., and Olga V Sergienko, et al., October 2021: Treatment of ice-shelf evolution combining flow and flexure. Journal of Glaciology, 67(265), DOI:10.1017/jog.2021.39885-902. Abstract
We develop a two-dimensional, plan-view formulation of ice-shelf flow and viscoelastic ice-shelf flexure. This formulation combines, for the first time, the shallow-shelf approximation for horizontal ice-shelf flow (and shallow-stream approximation for flow on lubricated beds such as where ice rises and rumples form), with the treatment of a thin-plate flexure. We demonstrate the treatment by performing two finite-element simulations: one of the relict pedestalled lake features that exist on some debris-covered ice shelves due to strong heterogeneity in surface ablation, and the other of ice rumpling in the grounding zone of an ice rise. The proposed treatment opens new venues to investigate physical processes that require coupling between the longitudinal deformation and vertical flexure, for instance, the effects of surface melting and supraglacial lakes on ice shelves, interactions with the sea swell, and many others.
Sergienko, Olga V., and Duncan J Wingham, October 2019: Grounding line stability in a regime of low driving and basal stresses. Journal of Glaciology, 65(253), DOI:10.1017/jog.2019.53. Abstract
The dynamics of a marine ice sheet's grounding lines determine the rate of ice discharge from the grounded part of ice sheet into surrounding oceans. In many locations in West Antarctica ice flows into ice shelves through ice streams experiencing low driving stress. However, existing simple theories of marine ice sheets are developed under the assumption of high basal and driving stress. Here we analyze the grounding line behavior of marine ice streams experiencing low basal shear and driving stress. We find that in this regime, the ice flux at the grounding line is a complex function of the geometry of the ice-stream bed, net accumulation rate and gradient of the net accumulation rate. Our analysis shows that the stability of distinct steady states is determined by the same parameters, suggesting a more complex (in)stability criterion than what is commonly referred to within the context of the ‘marine ice-sheet instability hypothesis’. We also determine characteristic timescales (e-folding time) of ice-sheet configurations perturbed from their steady states. These timescales can be used to determine whether particular configurations can be considered in isolation from other components of the climate system or whether their effects and feedbacks between the ice sheet and the rest of the climate system have to be taken into account.
Most ocean climate models do not represent ice shelf calving in a physically realistic way, even though the calving of icebergs is a major component of the mass balance for Antarctic ice shelves. The infrequency of large calving events together with the difficulty of placing observational instruments around icebergs means that little is known about how calving icebergs affect the ocean. In this study we present a novel model of an ice shelf coupled to an ocean circulation model, where the ice shelf is constructed of Lagrangian elements that allow simulation of iceberg calving. The Lagrangian ice shelf model is used to simulate the flow beneath a static idealized ice shelf, to verify that it can reproduce the results of an existing Eulerian model simulation with an identical configuration. The Lagrangian model is then used to simulate the ocean's response to a calved iceberg drifting away from the ice shelf. The results show how a calving event and subsequent iceberg drift affect the ocean. At the ice front, the calving event leads to a warming of the ocean surface and cooling of the water column at depth, allowing cooler waters to enter the ice shelf cavity, leading to reduced melt rates within the cavity. A Taylor column is observed below the iceberg, which moves with the iceberg as it drifts into the open ocean. As the iceberg drifts further from the ice shelf, the circulation within the ice shelf cavity tends toward a new steady state, consistent with the new ice shelf geometry.
Meltwater from the Antarctic Ice Sheet is projected to cause up to one metre of sea-level rise by 2100 under the highest greenhouse gas concentration trajectory (RCP8.5) considered by the Intergovernmental Panel on Climate Change (IPCC). However, the effects of meltwater from the ice sheets and ice shelves of Antarctica are not included in the widely used CMIP5 climate models, which introduces bias into IPCC climate projections. Here we assess a large ensemble simulation of the CMIP5 model ‘GFDL ESM2M’ that accounts for RCP8.5-projected Antarctic Ice Sheet meltwater. We find that, relative to the standard RCP8.5 scenario, accounting for meltwater delays the exceedance of the maximum global-mean atmospheric warming targets of 1.5 and 2 degrees Celsius by more than a decade, enhances drying of the Southern Hemisphere and reduces drying of the Northern Hemisphere, increases the formation of Antarctic sea ice (consistent with recent observations of increasing Antarctic sea-ice area) and warms the subsurface ocean around the Antarctic coast. Moreover, the meltwater-induced subsurface ocean warming could lead to further ice-sheet and ice-shelf melting through a positive feedback mechanism, highlighting the importance of including meltwater effects in simulations of future climate.
Lagrangian models of sea‐ice dynamics have several advantages over Eulerian continuum models. Spatial discretization on the ice‐floe scale is natural for Lagrangian models and offers exact solutions for mechanical non‐linearities with arbitrary sea‐ice concentrations. This allows for improved model performance in ice‐marginal zones, where sea ice is fragmented. Furthermore, Lagrangian models can explicitly simulate jamming processes that occur when sea ice moves through narrow confinements. While difficult to parameterize in continuum formulations, jamming emerges spontaneously in dense granular systems simulated in a Lagrangian framework. Here, we present a flexible discrete‐element framework for approximating Lagrangian sea‐ice mechanics at the ice‐floe scale, forced by ocean and atmosphere velocity fields. Our goal is to evaluate the potential of simpler models than the traditional discrete‐element methods for granular dynamics. We demonstrate that frictionless contact models based on compressive stiffness alone are unlikely to produce jamming, and describe two different approaches based on Coulomb‐friction and cohesion which both result in increased bulk shear strength of the granular assemblage. The frictionless but cohesive contact model displays jamming behavior which is similar to the more complex model with Coulomb friction and ice‐floe rotation at larger scales, and has significantly lower computational cost.
Fyke, J, and Olga V Sergienko, et al., June 2018: An overview of interactions and feedbacks between ice sheets and the Earth system. Reviews of Geophysics, 56(2), DOI:10.1029/2018RG000600. Abstract
Ice sheet response to forced changes ‐ such as that from anthropogenic climate forcing ‐ is closely regulated by two‐way interactions with other components of the Earth system. These interactions encompass the ice sheet response to Earth system forcing, the Earth system response to ice sheet change, and feedbacks resulting from coupled ice‐sheet/Earth system evolution. Motivated by the impact of Antarctic and Greenland ice sheet change on future sea level rise, here we review the state of knowledge of ice‐sheet/Earth system interactions and feedbacks. We also describe emerging observation and model‐based methods that can improve understanding of ice‐sheet/Earth system interactions and feedbacks. We particularly focus on the development of Earth System Models that incorporate current understanding of Earth system processes, ice dynamics and ice‐sheet/Earth system couplings. Such models will be critical tools for projecting future sea level rise from anthropogenically forced ice sheet mass loss.
Determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice-sheet models. Here, we investigate the role of lateral shear and ice-shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets. We derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non-local ice-shelf properties. Application of these results to a laterally confined version of the MISMIP 1a experiment shows that the boundary condition at the ice-shelf front (i.e. the calving law) is a major control on the location and stability of the grounding line in the presence of buttressing, allowing for both stable and unstable grounding line positions on downwards sloping beds. These results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice-shelf properties, in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed. Consequently, the marine ice-sheet instability hypothesis may not apply to buttressed marine ice sheets.
Greenland Ice Sheet (GIS) might have lost a large amount of its volume during the last interglacial and may do so again in the future due to climate warming. In this study, we test whether the climate response to the glacial meltwater is sensitive to its discharging location. Two fully coupled atmosphere–ocean general circulation models, CM2G and CM2M, which have completely different ocean components are employed to do the test. In each experiment, a prescribed freshwater flux of 0.1 Sv is discharged from one of the four locations around Greenland—Petermann, 79 North, Jacobshavn and Helheim glaciers. The results from both models show that the AMOC weakens more when the freshwater is discharged from the northern GIS (Petermann and 79 North) than when it is discharged from the southern GIS (Jacobshavn and Helheim), by 15% (CM2G) and 31% (CM2M) averaged over model year 50–300 (CM2G) and 70–300 (CM2M), respectively. This is due to easier access of the freshwater from northern GIS to the deepwater formation site in the Nordic Seas. In the long term (> 300 year), however, the AMOC change is nearly the same for freshwater discharged from any location of the GIS. The East Greenland current accelerates with time and eventually becomes significantly faster when the freshwater is discharged from the north than from the south. Therefore, freshwater from the north is transported efficiently towards the south first and then circulates back to the Nordic Seas, making its impact to the deepwater formation there similar to the freshwater discharged from the south. The results indicate that the details of the location of meltwater discharge matter if the short-term (< 300 years) climate response is concerned, but may not be critical if the long-term (> 300 years) climate response is focused upon.
We develop a formal thin-plate treatment of the viscoelastic flexure of floating ice shelves as an initial step in treating various problems relevant to ice-shelf response to sudden changes of surface loads and applied bending moments (e.g. draining supraglacial lakes, iceberg calving, surface and basal crevassing). Our analysis is based on the assumption that total deformation is the sum of elastic and viscous (or power-law creep) deformations (i.e. akin to a Maxwell model of viscoelasticity, having a spring and dashpot in series). The treatment follows the assumptions of well-known thin-plate approximation, but is presented in a manner familiar to glaciologists and with Glen’s flow law. We present an analysis of the viscoelastic evolution of an ice shelf subject to a filling and draining supraglacial lake. This demonstration is motivated by the proposition that flexure in response to the filling/drainage of meltwater features on the Larsen B ice shelf, Antarctica, contributed to the fragmentation process that accompanied its collapse in 2002.
Sergienko, Olga V., August 2017: Behavior of flexural gravity waves on ice shelves: Application to the Ross Ice Shelf. Journal of Geophysical Research: Oceans, 122(8), DOI:10.1002/2017JC012947. Abstract
Ocean waves continuously impact floating ice shelves and affect their stress regime. Low-frequency, long-period (75-400 s), ocean waves are able to reach ice-shelf cavities from distant sources and excite flexural gravity waves that represent coupled motion in the water of the cavity and the ice covering above. Analytic treatment of simplified geometric configuration and three-dimensional numerical simulations of these flexural gravity waves applied to the Ross Ice Shelf show that propagation and ice-shelf flexural stresses are strongly controlled by the geometry of the system, bathymetry of the ice-shelf cavity, and ice-shelf cavity thickness. The derived dispersion relationships, group and phase velocities of these waves can be used to infer poorly constrained characteristics of ice shelves from field observations. The results of numerical simulations show that the flexural gravity waves propagate as beams. The orientation of these beams is determined by the direction of the open ocean waves incident on the ice-shelf front. The higher frequency ocean waves cause larger flexural stresses, while lower frequency waves can propagate farther away from the ice-shelf front and cause flexural stresses in the vicinity of the grounding line.
Large tabular icebergs calved from Antarctic ice shelves have long lifetimes (due to their large size), during which they drift across large distances, altering ambient ocean circulation, bottom-water formation, sea-ice formation, and biological primary productivity in the icebergs' vicinity. However, despite their importance, the current generation of ocean circulation models usually do not represent large tabular icebergs. In this study we develop a novel framework to model large tabular icebergs submerged in the ocean. In this framework, tabular icebergs are represented by pressure-exerting Lagrangian elements that drift in the ocean. The elements are held together and interact with each other via bonds. A breaking of these bonds allows the model to emulate calving events (i.e. detachment of a tabular iceberg from an ice shelf) and tabular icebergs breaking up into smaller pieces. Idealized simulations of a calving tabular iceberg, its drift, and its breakup demonstrate capabilities of the developed framework.
Hiester, J, Olga V Sergienko, and C L Hulbe, February 2016: Topographically Mediated Ice Stream Subglacial Drainage Networks. Journal of Geophysical Research: Earth Surface, 121(2), DOI:10.1002/2015JF003660. Abstract
Satellite laser altimetry reveals short time scale changes in Antarctic ice sheet surface elevation that are suggested to be driven by subglacial water transport and storage. Here, details of the interaction between the dynamics of ice stream flow, subglacial water system and bed elevation relief are examined in the context of idealized, heterogeneous bed geometries. Using a two-way coupled model of ice and subglacial water flow, we show that basal topography controls the temporal and spatial variability of the sub-ice-stream hydraulic system. The orientation and characteristic dimensions of the topographic undulations determine the morphology (connected subglacial ponds or channel-like subglacial water features) and time-scales of the sub-ice-stream drainage system. The short-term (several years to decades) variability of the simulated coupled ice-stream/subglacial-water system suggest that the short-term surface variations detected in remote-sensing observations may be indicative of a rapidly evolving subglacial water system. Our simulations also show that interaction between ice flow and the highly dynamic subglacial water system has a strong effects on effective stress in the ice. Large effective stress magnitudes arise over areas where the basal traction is characterized by strong spatial gradients, that is, transitions from high to low basal traction or vise versa. These transitions migrate on multiyear time scales, and thus cause large effective stress variability on the same temporal scales.
Icebergs calved from the Antarctic continent act as moving sources of freshwater while drifting in the Southern Ocean. The lifespan of these icebergs strongly depends on their original size during calving. In order to investigate the effects (if any) of the calving size of icebergs on the Southern Ocean, we use a coupled general circulation model with an iceberg component. Iceberg calving length is varied from 62 m up to 2.3 km, which is the typical range used in climate models. Results show that increasing the size of calving icebergs leads to an increase in the westward iceberg freshwater transport around Antarctica. In simulations using larger icebergs, the reduced availability of meltwater in the Amundsen and Bellingshausen Seas suppresses the sea-ice growth in the region. In contrast, the increased iceberg freshwater transport leads to increased sea-ice growth around much of the East Antarctic coastline. These results suggest that the absence of large tabular icebergs with horizontal extent of tens of kilometers in climate models may introduces systematic biases in sea-ice formation, ocean temperatures and salinities around Antarctica.
Ice streams transport ice rapidly from the interior of the Antarctic ice sheet to the coast. An analysis of surface flow convergence suggests that ice flow and geometry are intricately linked within these ice streams.
Goldberg, D N., C Schoof, and Olga V Sergienko, July 2014: Stick‐slip motion of an Antarctic Ice Stream: The effects of viscoelasticity. Journal of Geophysical Research: Earth Surface, 119(7), DOI:10.1002/2014JF003132. Abstract
Stick‐slip behavior is a distinguishing characteristic of the flow of Whillans Ice Stream (Siple Coast, Antarctica). Distinct from stick slip on Northern Hemisphere glaciers, which is generally attributed to supraglacial melt, the behavior is thought be controlled by basal processes and by tidally induced stress. However, the connection between stick‐slip behavior and flow of the ice stream on long time scales, if any, is not clear. To address this question we develop a new ice flow model capable of reproducing stick‐slip cycles similar to ones observed on the Whillans Ice Plain. The model treats ice as a viscoelastic material and emulates the weakening and healing that are suggested to take place at the ice‐till interface. The model results suggest the long‐term ice stream flow that controls ice discharge to surrounding oceans is somewhat insensitive to certain aspects of stick‐slip behavior, such as velocity magnitude during the slip phase and factors that regulate it (e.g., elastic modulus). Furthermore, it is found that factors controlling purely viscous flow, such as temperature, influence stick‐slip contribution to long‐term flow in much the same way. Additionally, we show that viscous ice deformation, traditionally disregarded in analysis of stick‐slip behavior, has a strong effect on the timing of slip events and therefore should not be ignored in efforts to deduce bed properties from stick‐slip observations.
Sergienko, Olga V., T T Creyts, and R C A Hindmarsh, June 2014: Similarity of organized patterns in driving and basal stresses of Antarctic and Greenland ice sheets beneath extensive areas of basal sliding. Geophysical Research Letters, 41(11), DOI:10.1002/2014GL059976. Abstract
The rate of ice transport from the interior of ice sheets to their margins, and hence the rate with which it contributes to sea level, is determined by the balance of driving stress, basal resistance and ice internal deformation. Using recent high resolution observations of the Antarctic and Greenland ice sheets, we compute driving stress and ice deformation velocities, inferring basal traction by inverse techniques. The results reveal broad scale organization in 5–20 km band-like patterns inboth the driving and basal shear stresses located in zones with substantial basal sliding. Both ice sheets experience basal sliding over areas substantially larger than previously recognized. The likely cause of the spatial patterns is the development of a band-like structure in the basal shear stress distribution that is the results of pattern-forming instabilities related to subglacial water. The similarity of patterns on the Greenland and Antarctic Ice Sheets suggests the flow of ice sheets is controlled by the same fundamental processes operating at their base, which control ice-sheet sliding and are highly variable on relatively short spatial and temporal scales, with poor predictability. This has far-reaching implications for understanding of the current and projection of the future ice sheets’ evolution.
Sergienko, Olga V., April 2014: A vertically integrated treatment of ice stream and ice shelf thermodynamics. Journal of Geophysical Research: Earth Surface, 119(4), DOI:10.1002/2013JF002908. Abstract
The extremely small vertical shear in ice stream and ice shelf flow simplifies the equations, which govern their thermodynamic evolution. Complemented by the widely used shallow shelf approximation used to simplify the ice flow momentum balance, a vertically integrated formulation of heat transfer presented here reduces the dimensionality of the thermodynamic problem from three to two (plan view) dimensions and thus significantly reduces the computational cost of treating ice stream and ice shelf thermodynamics in models. For realistic conditions, errors in ice stiffness parameter, ice thickness, and speed caused by the vertically integrated treatment of heat transfer are less than 5% of magnitudes of these values compared to the standard three-dimensional thermomechanical computations. In addition, for the specific case of ice shelves with strong bottom melting, the governing equation describing evolution of the vertically integrated ice stiffness parameter is derived, which further reduces computational cost. The presented error analysis and formulations of ice stream and ice shelf thermodynamics in terms of the vertically integrated temperature allow the thermodynamic effects on ice deformation to be easily incorporated into studies that traditionally disregard them.
Banwell, Alison F., Douglas R MacAyeal, and Olga V Sergienko, November 2013: Break-up of the Larsen B Ice Shelf Triggered by Chain-Reaction Drainage of Supraglacial Lakes. Geophysical Research Letters, 40(22), DOI:10.1002/2013GL057694. Abstract
The explosive disintegration of the Larsen B Ice Shelf poses two unresolved questions: What process (1) set a horizontal fracture spacing sufficiently small to pre-dispose the subsequent ice-shelf fragments to capsize, and (2) synchronized the widespread drainage of >2750 supraglacial meltwater lakes observed in the days prior to break-up? We answer both questions through analysis of the ice shelf's elastic-flexure response to the supraglacial lakes on the ice shelf prior to break-up. By expanding the previously articulated role of lakes beyond mere water-reservoirs supporting hydrofracture, we show that lake-induced flexural stresses produce a fracture network with appropriate horizontal spacing toinduce capsize-driven break-up. The analysis of flexural stresses suggests that drainage of a single lake can cause neighboring lakes to drain, which, in turn, cause farther removed lakes to drain. Such self-stimulating behavior can account for the sudden, widespread appearance of a fracture system capable of driving explosive break-up.
A conspicuous precursor of catastrophic ice-shelf break-up along the Antarctic Peninsula, reported widely in the literature, is the gradual increase in surface melting and consequent proliferation of supraglacial lakes and dolines. Here we present analytical and numerical solutions for the flexure stresses within an ice shelf covered by lakes and dolines, both isolated and arrayed. We conclude that surface water promotes ice-shelf instability in two ways: (1) by water-assisted crevasse penetration, as previously noted, and (2) by the inducement of strong tensile flexure stresses (exceeding background spreading stress by 10-100 times) in response to surface water mass loads and 'hydrostatic rebound' occurring when meltwater lakes drain.
Ice shelves and ice tongues are dynamically coupled to their cavities. Here we compute normal modes (eigenfrequencies and eigenfunctions) of this coupled system using a thin-plate approximation for the ice shelf and potential water flow in the ice-shelf cavity. Our results show that normal modes depend not only on the ice-shelf parameters (length, thickness, Young's modulus, etc.) but also on the cavity depth. The dominant eigenmodes are higher for ice shelves floating over deeper cavities; they are also higher for shorter ice shelves and ice tongues (< 50 km long). The high-eigenfrequency eigenmodes are primarily controlled by the ice flexure and have similar periods to sea swell. These results suggest that both long ocean waves with periods of 100-400 s and shorter sea swell with periods of 10-20 s can have strong impacts on relatively short ice shelves and ice tongues by exciting oscillations with their eigenfrequencies, which can lead to iceberg calving and, in some circumstances, ice-shelf disintegration.
Subglacial lakes beneath ice streams of Antarctica and supraglacial lakes observed on the flanks of the Greenland ice sheet may seem to be unrelated. The former derive their water from energy dissipation associated with basal friction, the latter from atmospherically driven surface melting. However, using numerical models of ice and water flow, it is shown here that they share a common relationship to basal conditions that implies that surface lakes (or depressions that could host lakes under warmer atmospheric conditions) and basal lakes might exist in tandem.
Sergienko, Olga V., D N Goldberg, and Christopher M Little, June 2013: Alternative ice shelf equilibria determined by ocean environment. Journal of Geophysical Research, 118(2), DOI:10.1002/jgrf.20054. Abstract
Dynamic and thermodynamic regimes of ice shelves experiencing weak (
≲
1 m year
1
)
to strong (~10myear
1
) basal melting in cold (bottom temperature close to the in situ
freezing point) and warm oceans (bottom temperature more than half of a degree warmer
than the in situ freezing point) are investigated using a 1-D coupled ice/ocean model
complemented with a newly derived analytic expression for the steady state temperature
distribution in ice shelves. This expression suggests the existence of a basal thermal
boundary layer with thickness inversely proportional to the basal melt rate. Model
simulations show that ice shelves a
fl
oat in warm ocean waters have signi
fi
cantly colder
internal ice temperatures than those that
fl
oat in cold waters. Our results indicate that in
steady states, the mass balance of ice shelves experiencing strong and weak melting is
controlled by different processes: in ice shelves with strong melting, it is a balance between
ice advection and basal melting, and in ice shelves with weak melting, it is a balance
between ice advection and deformation. Sensitivity simulations show that ice shelves in
cold and warm oceans respond differently to increase of the ocean heat content. Ice shelves
in cold waters are more sensitive to warming of the ocean bottom waters, while ice shelves
in warm waters are more sensitive to shallowing of the depth of the thermocline.
Sergienko, Olga V., and R C A Hindmarsh, November 2013: Regular patterns in frictional resistance of ice-stream beds seen by surface data inversion. Science, 342(6162), DOI:10.1126/science.1243903. Abstract
Fast-flowing glaciers and ice streams are pathways for ice discharge from the interior of the Antarctic Ice Sheet to ice shelves, at rates controlled by conditions at the ice-bed interface. Using recently compiled high-resolution data sets and a standard inverse method, we computed basal shear stress distributions beneath Pine Island and Thwaites Glaciers, which are currently losing mass at an accelerating rate. The inversions reveal the presence of rib-like patterns of very high basal shear stress embedded within much larger areas with zero basal shear stress. Their colocation with highs in the gradient of hydraulic potential suggests that subglacial water may control the evolution of these high shear stress ribs, potentially causing migration of the grounding line by changes in basal resistance in its vicinity.
Recent surveys of floating ice shelves associated with Pine Island Glacier
(Antarctica) and Petermann Glacier (Greenland) indicate that there are channels incised
upward into their bottoms that may serve as the conduits of meltwater outflow from the
sub-ice-shelf cavity. The formation of the channels, their evolution over time, and their
impact on ice-shelf flow are investigated using a fully-coupled ice-shelf/sub-ice-shelf
ocean model. The model simulations suggest that channels may form spontaneously in
response to meltwater plume flow initiated at the grounding line if there are relatively
high melt rates and if there is transverse to ice-flow variability in ice-shelf thickness.
Typical channels formed in the simulations have a width of about 1–3 km and a vertical
relief of about 100–200 m. Melt rates and sea-water transport in the channels are
significantly higher than on the smooth flat ice bottom between the channels. The melt
channels develop through melting, deformation, and advection with ice-shelf flow.
Simulations suggest that both steady state and cyclic state solutions are possible
depending on conditions along the lateral ice-shelf boundaries. This peculiar dynamics of
the system has strong implications on the interpretation of observations. The richness of
channel morphology and evolution seen in this study suggests that further observations
and theoretical analysis are imperative for understanding ice-shelf behavior in warm
oceanic conditions.
Straneo, F, P Heimbach, Olga V Sergienko, G Hamilton, G Catania, Stephen M Griffies, and Robert Hallberg, et al., August 2013: Challenges to Understand the Dynamic Response of Greenland's Marine Terminating Glaciers to Oceanic and Atmospheric Forcing. Bulletin of the American Meteorological Society, 94(8), DOI:10.1175/BAMS-D-12-00100.1. Abstract
The recent retreat and speedup of outlet glaciers, as well as enhanced surface melting around the ice sheet margin, have increased Greenland's contribution to sea level rise to 0.6±0.1 mm/yr and its discharge of freshwater into the North Atlantic. The widespread, near-synchronous glacier retreat, and its coincidence with a period of oceanic and atmospheric warming, suggest a common climate driver. Evidence points to the marine margins of these glaciers as the region from which changes propagated inland. Yet the forcings and mechanisms behind these dynamic responses are poorly understood and either missing or crudely parameterized in climate and ice sheet models. Resulting projected sea level rise contributions from Greenland by 2100 remain highly uncertain.
This paper summarizes current state of knowledge and highlights key physical aspects of Greenland's coupled ice-sheet/ocean/atmosphere system. Three research thrusts are identified to yield fundamental insights into ice sheet, ocean, sea ice and atmosphere interactions, their role in Earth's climate system, and probable trajectories of future changes: (1) focused process studies addressing critical glacier, ocean, atmosphere and coupled dynamics; (2) sustained observations at key sites; and (3) inclusion of relevant dynamics in Earth System Models.
Understanding the dynamic response of Greenland's glaciers to climate forcing constitutes both a scientific and technological frontier given the challenges of obtaining the appropriate measurements from the glaciers' marine termini and the complexity of the dynamics involved, including the coupling of the ocean, atmosphere, glacier and sea ice systems. Interdisciplinary and international cooperation are crucial to making progress on this novel and complex problem.
Capsule: An interdisciplinary and multi-faceted approach is needed to understand the forcings and mechanisms behind the recent retreat and acceleration of Greenland's glaciers and its implications for future sea level rise
Goldberg, D N., Christopher M Little, Olga V Sergienko, Anand Gnanadesikan, Robert Hallberg, and M Oppenheimer, June 2012: Investigation of land ice-ocean interaction with a fully coupled ice-ocean model, Part 2: Sensitivity to external forcings. Journal of Geophysical Research: Earth Surface, 117, F02038, DOI:10.1029/2011JF002247. Abstract
A coupled ice stream-ice shelf-ocean cavity model is used to assess the sensitivity of the coupled system to far-field ocean temperatures, varying from 0.0 to 1.80C, as well as sensitivity to the parameters controlling grounded ice flow. A response to warming is seen in grounding line retreat and grounded ice loss that cannot be inferred from the response of integrated melt rates alone. This is due to concentrated thinning at the ice shelf lateral margin, and to processes that contribute to this thinning. Parameters controlling the flow of grounded ice have a strong influence on the response to sub-ice shelf melting, but this influence is not seen until several years after an initial perturbation in temperatures. The simulated melt rates are on the order of that observed for Pine Island Glacier in the 1990s. However, retreat rates are much slower, possibly due to unrepresented bedrock features.
Goldberg, D N., Christopher M Little, Olga V Sergienko, Anand Gnanadesikan, Robert Hallberg, and M Oppenheimer, June 2012: Investigation of land ice-ocean interaction with a fully coupled ice-ocean model, Part 1: Model description and behavior. Journal of Geophysical Research: Earth Surface, 117, F02037, DOI:10.1029/2011JF002246. Abstract
Antarctic ice shelves interact closely with the ocean cavities beneath them, with ice shelf geometry influencing ocean cavity circulation, and heat from the ocean driving changes in the ice shelves, as well as the grounded ice streams that feed them. We present a new coupled model of an ice stream-ice shelf-ocean system that is used to study this interaction. The model is capable of representing a moving grounding line and dynamically responding ocean circulation within the ice shelf cavity. Idealized experiments designed to investigate the response of the coupled system to instantaneous increases in ocean temperature show ice-ocean system responses on multiple timescales. Melt rates and ice shelf basal slopes near the grounding line adjust in 1-2 years, and downstream advection of the resulting ice shelf thinning takes place on decadal timescales. Retreat of the grounding line and adjustment of grounded ice takes place on a much longer timescale, and the system takes several centuries to reach a new steady state. During this slow retreat, and in the absence of either an upward-or downward-sloping bed or long-term trends in ocean heat content, the ice shelf and melt rates maintain a characteristic pattern relative to the grounding line.
Sergienko, Olga V., August 2012: The effects of transverse bed topography variations in ice-flow models. Journal of Geophysical Research: Earth Surface, 117, F03011, DOI:10.1029/2011JF002203. Abstract
A suite of ice flow models is subject to a performance test designed to investigate treatment of flow over variable basal topography. Using transfer functions developed by Gudmundsson [2003] and numerical models of various complexity, it is demonstrated that a widely used flowband model has strong limitations associated with its underlying assumptions, and thus should be applied only in specific geometrical settings. Its performance can be significantly improved by using the laterally averaged basal topography instead of centerline topography. In applications, where spatial variability of flow fields is important, hybrid ice flow models can be a viable alternative to flowband models. In addition, analysis of horizontal distributions of the various ice flow characteristic (e.g., surface elevation, velocity and horizontal stress-components) shows that field observations focused on assessing these parameters only along the centerline of ice flow could be misleading. These results also suggest that such spatial variability needs to be taken into account in designing field surveys of ice flow.
Hybrid models, or depth-integrated flow models
that include the effect of both longitudinal stresses and vertical
shearing, are becoming more prevalent in dynamical ice
modeling. Under a wide range of conditions they closely approximate
the well-known First Order stress balance, yet are
of computationally lower dimension, and thus require less
intensive resources. Concomitant with the development and
use of these models is the need to perform inversions of observed
data. Here, an inverse control method is extended to
use a hybrid flow model as a forward model. We derive an
adjoint of a hybrid model and use it for inversion of icestream
basal traction from observed surface velocities. A
novel aspect of the adjoint derivation is a retention of nonlinearities
in Glen’s flow law. Experiments show that in some
cases, including those nonlinearities is advantageous in minimization
of the cost function, yielding a more efficient inversion
procedure.
MacAyeal, Douglas R., D S Abbot, and Olga V Sergienko, August 2011: Iceberg-capsize tsunamigenesis. Annals of Glaciology, 52(58), 51-56. Abstract
Calving from the floating termini of outlet glaciers and ice shelves is just the beginning
of an interesting chain of events that can subsequently have important impacts on human life and
property. Immediately after calving, many icebergs capsize (roll over by 90◦) due to the instability
of their initial geometry. As icebergs melt and respond to the cumulative effects of ocean swell, they
can also reorient their mass distribution by further capsize and fragmentation. These processes release
gravitational potential energy and can produce impulsive large-amplitude surface-gravity waves known
as tsunamis (a term derived from the Japanese language). Iceberg-capsize tsunamis in Greenland fjords
can be of sufficient amplitude to threaten human life and cause destruction of property in settlements.
Iceberg-capsize tsunamis may also have a role in determining why some ice shelves along the Antarctic
Peninsula disintegrate ‘explosively’ in response to general environmental warming. To quantify iceberg
tsunami hazards we investigate iceberg-capsize energetics, and develop a rule relating tsunami height
to iceberg thickness. This rule suggests that the open-water tsunami height (located far from the iceberg
and from shorelines where the height can be amplified) has an upper limit of 0.01H where H is the
initial vertical dimension of the iceberg.
Locations of subglacial lakes discovered under fast-moving West Antarctic ice streams tend
to be associated with topographic features of the subglacial bed or with areas that have strong variations
in basal conditions. Inversion of ice-stream surface velocity indicates that basal conditions under ice
streams can be highly variable and that there can be widespread regions where basal traction is high. To
seek an explanation for why lakes appear to be sited near areas with high basal traction, we use
numerical models to simulate ice-stream dynamics, thermodynamics and subglacial water flow. We
demonstrate that the ice flow over high basal traction areas produces favourable conditions for the
ponding of meltwater. Energy dissipation associated with ice sliding over a region with high basal
traction constitutes a water source supplying a lake, and ice-thickness perturbations induced by ice flow
over variable traction create local minima in hydraulic potential. Variations in thermodynamic
processes caused by such ice flow could be responsible for limiting the horizontal extent of the
subglacial lakes.
Bromirski, P D., Olga V Sergienko, and Douglas R MacAyeal, January 2010: Transoceanic infragravity waves impacting Antarctic ice shelves. Geophysical Research Letters, 37, L02502, DOI:10.1029/2009GL041488. Abstract
Long-period oceanic infragravity (IG) waves (ca. [250, 50] s period) are generated along continental coastlines by nonlinear wave interactions of storm-forced shoreward propagating swell. Seismic observations on the Ross Ice Shelf show that free IG waves generated along the Pacific coast of North America propagate transoceanically to Antarctica, where they induce a much higher amplitude shelf response than ocean swell (ca. [30, 12] s period). Additionally, unlike ocean swell, IG waves are not significantly damped by sea ice, and thus impact the ice shelf throughout the year. The response of the Ross Ice Shelf to IG-wave induced flexural stresses is more than 60 dB greater than concurrent ground motions measured at nearby Scott Base. This strong coupling suggests that IG-wave forcing may produce ice-shelf fractures that enable abrupt disintegration of ice shelves that are also affected by strong surface melting. Bolstering this hypothesis, each of the 2008 breakup events of the Wilkins Ice Shelf coincides with wave-model-estimated arrival of IG-wave energy from the Patagonian coast.
Sergienko, Olga V., December 2010: Elastic response of floating glacier ice to impact of long-period ocean waves. Journal of Geophysical Research: Earth Surface, 115, F04028, DOI:10.1029/2010JF001721. Abstract
Disintegration of ice shelves along the Antarctic Peninsula over the past two decades has clearly demonstrated their high sensitivity to recent changes in the local thermal regime of the atmosphere and ocean and has given rise to the question of whether mechanical coupling with waves in the ocean may provide the triggering mechanism that starts collapse events. Motivated by these events, this study considers a more general question: how ocean waves affect the stress regime of floating ice, and in particular, how ocean waves can influence the creation of fractures and its fatiguing that may lead to breakup and collapse. A new treatment of ice shelf/ocean wave interaction in which the typical “thin plate” approximation is relaxed is presented here, and exact, analytic solutions describing ice shelf stresses induced by long (>60 s period) ocean waves in various idealized ice/ocean geometries are derived. The numerical calculations demonstrate that the amplitudes of the wave-induced stresses are sufficiently large to initiate top to bottom crevasse penetration through the depth of the ice shelf. The cyclic nature of the wave-induced stresses contributes to ice fatigue and damage that is also a precursor to ice shelf disintegration. Although primarily theoretical, the results of the present analysis suggest that ocean waves could be a potential trigger of ice shelf collapse as well as less dramatic, but equally important, episodic calving.
