C31D-0528
Grounding-line dynamics in a shallow-ice model in two horizontal dimensions
The changing mass balance of continental ice sheets has implications for global sea-level rise and is hence
of great societal relevance. Predicting the evolution of ice sheets depends, in part, on an understanding of
the dynamics of the grounding line, where an ice sheet floats off its bed to become an ice shelf. We extend
the grounding-line theory of Robinson et al. (2008) into two horizontal dimensions in the context of a
Newtonian, shallow-ice model. (The work of Robison et al. (2008) builds on that of Schoof (2007) to derive a
time-dependent partial differential equation for grounding line position based on a balance of forces between
the ice sheet and ice shelf.) We present numerical results that simulate the flow of ice over a dipping sea-
bed with periodic variations in slope angle. The simulated ice sheets exhibit lateral flux variations and
buttressing. We report on steady-state and dynamic behavior for a variety of density ratios, slope angles
and slope perturbations.
Robison, R.A. and H.E. Huppert and M.G. Worster (2008) Dynamics of viscous grounding lines. Under
consideration at J. Fluid Mech.
Schoof, C (2007) Marine ice-sheet dynamics. Part I. The case of rapid sliding. J. Fluid Mech. v. 573
http://www.earth.ox.ac.uk/~richardk/
C31D-0529
Experiments using a two-dimensional, ice-sheet-stream-shelf model, fully coupled to an ocean-circulation model
We have coupled a two-dimensional, ice sheet/shelf model to the Los Alamos National Laboratory HYPOP ocean-circulation model. The ice sheet/shelf model, which is based on the 1st-order shallow-ice approximation and accounts for both horizontal and vertical stress gradients (Blatter, J. Glac. 41, 1995), treats the ice flow continuously throughout the sheet, stream, and shelf flow regimes. Within the ice sheet, basal motion occurs through specified basal traction or specified basal yield stress. Basal motion at the grounding line is parameterized similar to Pattyn et al. (JGR 111, 2006) and grounding line advance and retreat is comparable to that from preliminary Marine Ice Sheet Model Intercomparison benchmarks. The ocean circulation model is a modified and improved version of POP that uses a hybrid-vertical coordinate, allowing for a Lagrangian treatment of the ice shelf/ocean boundary. Here, we report on initial experiments exploring the effects of ocean temperature and circulation on the evolution of the ice sheet/shelf system, and vice versa.
C31D-0530
Ice-Internal and Sedimenetary Structures in the Ekströmisen Grounding Line Region Detected With Multi-offset Seismics.
Ekströmisen is a small catchment area in coastal Dronning Maud Land, Antarctica, terminating in the Ekström ice shelf, which is bounded by a narrow embayment formed by two ice ridges. A seismic survey has been performed along a flow line on Ekströmisen over about 22 km, crossing the grounding line between ice sheet and shelf approximately midway of the profile. The measurements were performed with explosives in shallow firn holes as seismic sources and a 60 channel 1.5 km snow streamer for data recording. The data has been resorted to form a virtual 120 channel 3 km streamer, consisting of 150 shots. The maximum shot-receiver offset is thus about three times larger then the ice thickness, yielding wide angle information for intra-ice and bedrock reflections. Standard seismic data processing yields 862 common depth points in total, with an increment of 25~m. This provides a 20-fold coverage of each common depth point. In addition to yielding the distribution of seismic velocity within the firn, ice and sediment, the data clearly images ice and sedimentary layers. Within the bottom part of the ice, a number of continuous internal layers are visible upstream of the grounding line. Currently, our favorite explanation is abrupt changes in the crystal orientation fabric caused by a combination of laterally compressional flow and vertical shear, as observed with radio-echo sounding at other places in Antarctica. Upstream of and at the grounding line, structures are visible within the bedrock, which we interpret as sedimentary deposits related to glacial activity. Downstream of the grounding line the seismic data suggests the existence of a pinning point under the ice shelf, formed by sedimentary deposits. The data provide the base for interpretations of the ice-dynamic and sedimentary processes occurring in the basal ice layer and at the ice-bedrock boundary, of relevance for further understanding details of the ice sheet-to-shelf transition area.
C31D-0531
Numerical modelling of interaction between ice shelf calving and dynamics of Crane Glacier, a former tributary of the Larsen-B ice shelf, Antarctic Peninsula.
The retreat or removal of an ice shelf reduces its buttressing effect onto the upstream grounded tributary ice stream and thereby potentially leads to rapid inland thinning, acceleration and grounding line retreat. The process of calving, although essential in controlling the ice shelf dynamics, is poorly understood and its representation in existing numerical models is very crude. We include a recently developed physically based calving criteria into a numerical flowline model. This model is applied to Crane Glacier, a former tributary of the Larsen B ice shelf, Antarctic Peninsula. We investigate the interaction between the grounded tributary glacier and the retreat and disintegration of the Larsen-B ice shelf. We compare the model results to retreat positions, flow velocities and thinning rates from remote sensing. The freely moving grounding line included in our model further allows us to investigate the effect of ice-shelf change onto grounding line retreat (retreat-thinning feedback) and thereby assess the future stability of such ice-shelf bound tributaries.
C31D-0532
Effects of Basal-Melting Distribution on the Retreat of Ice-Shelf Grounding Lines
The stability of marine ice streams depends on the distribution as well as the magnitude of melting beneath the adjacent ice shelf, as shown by new model results. Recent observations of rapid retreat of ice-shelf grounding lines in the Amundsen Sea sector of West Antarctica have highlighted the need for understanding how basal melting of ice shelves by warm ocean waters affects ice dynamics and potentially contributes indirectly to sea-level rise. We apply two ice stream-ice shelf flowline models to investigate the effects of varying the spatial distribution of basal melting on grounding-line dynamics. For experiments with identical average melting, we find that retreat increases significantly as melting is concentrated near the grounding line, indicating that knowledge of the basal-melting distribution is likely necessary for accurate prediction of grounding-line migration.
C31D-0533
Where Does an Ice Shelf Begin?
Accurate determination of the position of the grounding line remains one of the challenges faced by current marine ice sheet models. Traditionally, the grounding line has been prescribed by a floatation condition (hydrostatic equilibrium) or from a migration rate based upon differentiation of the floatation condition. More recently, a boundary layer theory for marine ice sheets proposed by Schoof (2007), suggest that in the case a of rapidly sliding ice sheet flowing into an unconstrained ice shelf, grounding line migration rate should be replaced by a flux condition at the grounding line. Here we consider the situation where shearing in the grounded ice is no longer negligible, and use contact conditions to show that the floatation condition is not always appropriate, as it can result in steady state solutions that are not physically acceptable. The contact conditions reflect that upstream from the grounding line, the compressive normal stress at the base of the grounded ice should exceed equivalent water pressure, while downstream from the grounding line, the shelf cannot get into contact with the bedrock. Violation of either condition would result in grounding line migration. These conclusions are drawn from the numerical simulations of a full Stokes finite element model, which also solves for the position of free surfaces (ice-air and ice-water interfaces).
C31D-0534
Combined active/passive microwave wavelet-based approach for snowmelt detection over Antarctica ice shelves
Monitoring the geographic extent of melting snow and its temporal persistence is crucial as it can be used to study variables of climatological interest. The presence of wet snow on Antarctica ice shelves can be monitored using Earth orbiting microwave sensors, such as Seawinds on QuikSCAT and the Special Sensor Microwave Imager (SSM/I), at very high temporal resolution. The introduction, through a change of phase, of liquid water into a volume of snow will dramatically alter its microwave properties, leading to a marked decrease in the observed microwave backscatter signal and an increase in the microwave brightness temperature. Most algorithms found in literature detect wet snow when microwave brightness temperature (backscattering) are above (below) a certain threshold value. More sophisticated and elegant approaches that make use of wavelet transform have been applied to passive microwave data. However, to our knowledge, a wavelet- transform based edge detection approach has never been applied to active microwave observations of Antarctica. In this study we report results regarding melting derived from combined active and passive microwave data using a wavelet-transform edge detection technique. QuikSCAT data are available at a much higher resolution (~ 2.5 km) than passive data (~ tens of kilometers) from the NASA Scatterometer Climate Record Pathfinder (SCP) project. This allows the study of melting over ice shelves at a relatively high resolution and understanding of how the spatial distribution within each passive cell grid can affect snowmelt detection algorithms. Snowmelt extent and timing over the Antarctica ice shelves, as derived from both active and passive datasets, are compared with each other and with the outputs of literature approaches and ground observations.
C31D-0535
Ice shelf morphology and the efficiency of basal melting
In the Amundsen Sea sector of Antarctica, changes in basal melting have triggered rapid ice shelf thinning;
increasing subsurface ocean temperatures may have initiated this process. A scaling law relating melting to
ocean temperature will aid the analysis of observed thinning as well as predictions of ice sheet behavior. Yet
it is unclear whether relationships found in prior studies are valid across different ice shelf shapes and a
large range of temperatures, particularly for the small ice shelves in the Amundsen Sea. Here, we use a
numerical ocean model (the Hallberg Isopycnal Model) to investigate how ice shelf morphology influences the
response of ice shelves to changes in ocean temperature.
We analyze simulations of six idealized ice shelves, forced with subsurface ocean temperatures ranging from
-2°C to 2°C. In the sub-ice shelf mixed layer, three spatially distinct dynamic regimes are
apparent. Near grounding lines, steep basal slopes enhance entrainment of heat in the "initiation" region.
Heat is advected upslope and used to melt ice in the "maintenance" region; however, flow convergence in the
"outflow" region limits heat flux to flatter portions of the ice shelf. Because advected heat is trapped in
regions of low slope, ice shelf thickness gradients act to limit the area-integrated efficiency (the fraction of
heat entrained into the mixed layer that is used to melt or heat ice). Mixed layer detrainment and interior
stratification have a similar effect; previously entrained heat is unable to access the ice shelf.
For all modeled ice shelves, advective heat loss increases with ocean temperature. This morphology-
dependent decrease of efficiency may explain previous divergence from a simple scaling law.
http://www.princeton.edu/~cmlittle
C31D-0536
Flexural-gravity wave phenomena on ice shelves.
Large amplitude ocean waves generated along the seaward edge of an ice shelf could excite vibrational motions of the ice shelf that propagate as flexural-gravity waves (i.e., where the restoring force of gravity acting on the water beneath the ice shelf as well as the restoring force of elastic flexure of the ice shelf both influence the wave propagation) throughout the ice-shelf interior. To investigate the possible effects of these ocean wave induced motions on the ice-shelf interior, an effort is made to model the free-oscillations of various Antarctic ice shelves in their realistic geometries. The collection of eigenmodes and eigenfrequencies resulting from the model analysis is inspected to see what features might influence stress distributions around features of interest (e.g., ice-shelf rifts, ice rises, etc).
C31D-0537
Tidal Effects on the Amundsen Ice Shelf
Recent observations have indicated that ice shelf melting in the Amundsen Sea is an order of magnitude higher than other Antarctic ice shelves. Melting and decay of the ice shelf allows glacial flow to accelerate and sea level to rise even though melting or even collapse of the ice shelf itself does not affect sea level. The Amundsen Ice Shelf was found not to flow smoothly, but to move in spurts which coincided with tidal periods as tides lift the ice shelves off the grounding islands. To identify tidal effects on the ice shelf, barotropic and baroclinic tides in the Amundsen Sea including the cavities under the ice shelves were simulated and evaluated against simulations without tides. Tidal elevation estimates were produced to provide estimates of the tidal excursions. Estimates of the tidally induced flow of the warmer deep waters into the ice shelf cavities were calculated along with the heat flux into the ice shelf, which contributes to melting. Mixing due to baroclinic tides was also investigated and estimates made of the outflow of ice shelf water.
C31D-0538
Using Tidal Signals in ICESat Data to Characterize the Grounding Zones of Siple Coast Ice Streams
Tidal motion modulates the horizontal ice velocities of the Ross Ice Shelf and surrounding ice sheet, as seen in GPS records from near the ice-shelf front and on Siple Coast ice streams. Motion is usually smoothly varying and closely correlated with the vertical signal of the ocean tide. However, GPS records from Whillans Ice Stream (formerly Ice Stream B) have shown remarkable stick-slip behavior. We seek an understanding of how the tidal motion of grounded and floating ice is coupled across the grounding zone and how that coupling may differ between ice streams. As a preliminary approach we use repeat-track analysis of ICESat laser altimetry data to investigate the geometry of the grounding zone, focusing on the lateral extent of the limits of ice flexure in response to the tide.
C31D-0539
Thirty years of change on Wilkins and Larsen ice shelves from multi-mission satellite altimetry
We use data from satellite radar (SEASAT, ERS-1 and -2, Envisat) and laser (ICESat) altimeters from 1978 to 2008 to determine multidecadal elevation changes for major Antarctic Peninsula ice shelves. SEASAT data (1978) approximately double the length of the altimetry record, and the longer length of the new height record allows us to determine changes in elevation trends over the 30-year period. However, there are several problems associated with synthesizing multi-mission altimeter data that must be addressed before the data can be used to quantify longer-term trends. We review some of these error sources, including orbit error, firn compaction, and surface penetration. We use data from the open ocean to estimate the inter- satellite biases and apply this to the ice shelf data. We also describe new DEMs for Wilkins and Larsen-B developed from SEASAT radar altimetry for the 1978 epoch. The Larsen-B DEM is combined with the new EIGEN-04C geoid and a firn model to generate ice thickness maps for Larsen-B 24 years before this shelf disintegrated in 2002.
C31D-0540
The Flow of Buoyant Meltwater Next to Ice Shelves and Icebergs
Melting at the base of an ice shelf can play a significant role in the polar oceans, contributing to the mass balance of the ice shelf and leading to the formation of Ice Shelf Water. Fresh meltwater is relatively buoyant compared to the surrounding ocean and can rise along the ice surface, with the strength of this flow depending critically on the heat and salt fluxes from the ocean to the ice. We justify a simplified theoretical model that describes the coupling of heat and salt fluxes with the buoyancy- driven flow of meltwater, next to both vertical and sloping ice surfaces. The flow develops with distance along the ice surface, and different flow regimes can be obtained depending on the length and the slope of the ice surface. Both the heat and salt fluxes differ between the two regimes. On moderate scales the flow is controlled by buoyancy in a narrow region close to the ice surface. This predicts that the melting rate is independent of distance along the ice surface, consistent with previous laboratory scale measurements of heat transfer. This regime may be important for ablation at the sides of tabular icebergs, and under some regions of ice shelves. Further downstream, the flow is dominated by buoyancy located further from the wall, and can be described by a model similar to those often used to model ice-shelf-water plumes. This predicts that the melting rate increases with distance along the ice surface. Simple analytic solutions are also derived for flow in an unstratified fluid, which indicate the possible sensitivity of the ablation rate to changes in ocean temperature. The predicted variation of the heat and salt fluxes with distance along the ice surface may have important consequences for more complex models of ice-shelf-water flow.
C31D-0541
Mass Balance Implications of Wind-Transported Snow Loss From Antarctic Ice Shelves
Some fraction of the snow that falls as precipitation over the Antarctic ice sheet is transported across the coastline by the wind. This is a long-recognized but poorly constrained problem. If recent projections of increasing coastal wind speeds are correct, wind-blown snow transport will also intensify, as the relationship between mass transport and wind speed is strongly nonlinear. The large-scale importance of wind- transported snow to coastal ocean freshening or ice sheet mass balance depends on unknowns including details of the transport of snow by the wind, the net precipitation over Antarctica, and the effective length of its coastline. Prior estimates of snow loss into the ocean from Antarctica range over two orders of magnitude, from less than 2 to more than 200 Gt / year. Modeled annual snow transport based on measured winds at an automatic weather station site on the northern edge of the Ross Ice Shelf is in good agreement with measured values from Halley Station. When extrapolated around the coastline, these values fall between the reported extremes. Because most of Antarctica's coastal areas experience higher winds and greater snow supply than its ice shelves, this data provides a lower limit on the mass of snow removed from the ice sheet by the wind. From this lower bound we estimate the probable range of values for present-day wind blown snow export to the Southern Ocean, and explore the implications of projected rising winds for increases in wind-blown snow transport.
C31D-0542
Flow changes, rheology and stability of the Larsen C Ice Shelf
The continued thinning and possible destabilization of the Larsen C Ice Shelf present a valuable setting to investigate the many connected processes involved in the evolution of ice shelves in a warming climate, especially following the disintegration of Larsen B and other Peninsular ice shelves, and the consequent increase in continental ice flow to the ocean. We are addressing this complex question, as part of a multi- disciplinary effort that also includes fieldwork, by a combination of remote sensing and numerical modeling. Thus, we analyze satellite InSAR observations of Larsen C obtained in the years 2000 and 2007 to detect changes that might have occurred in the flow patterns and speeds of the ice shelf in the interval. We then present the outcome of applying inverse modeling, combining the observed velocity field with numerical flow models, to infer a spatial distribution of the flow parameter for the ice shelf, which is indispensable for the accurate modeling of ice shelf flow and evolution. Furthermore, the inferred rheology field allows us to recognize zones of weakness in the ice shelf, and the locations of possible future calving events. Our similar, previous analysis of Larsen B at the eve of its disintegration in 2002 emphasizes the significant interaction and interdependence among frontal calving, flow acceleration, variable rheology including fracture zones, and the ultimate destabilization of the ice shelf. Therefore, this work, by measuring any recent acceleration in the flow of Larsen C and inferring its rheology provides essential tools to evaluate its stability and long-term prospects. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration, Cryospheric Sciences Program.
C31D-0543
Comparison of the side drag and buttressing generated by flow-line and plan-view models of ice shelves.
Where grounded ice flows into an ice shelf, the ice shelf can buttress the outflow through drag generated both along the sides of the shelf and over locally grounded areas such as ice rises. Flow line models of stream/shelf systems require that side drag be parameterized. Side-drag parameterizations usually relate width-averaged or center-line flow speed to shear stress at the lateral margin, and are derived through either a boundary-layer treatment, or by treating lateral shear as separable from longitudinal stress equilibrium. Here we compare results from a 1-d flow line model with those from a 2-d (plan view) model, in an effort to assess the utility of one such parameterization. Depth-integrated stream/shelf flow is modeled in both cases, using the MacAyeal/Morland equations. The side drag and buttressing profiles generated by the models for various scenarios are compared, as is the relationship between the center-line speed and side drag. This latter exercise provides an assessment of the quality of the side drag parameterization adopted, wherein side drag is proportional to the nth root of the center-line speed, n being the flow-law exponent for Glen's Law. Preliminary results suggest that the parameterization is at least of heuristic value, with the caveat that its performance is weaker in the immediate vicinity of an ice front.
C31D-0544
Surface Melt Magnitude Retrieval Over Ross Ice Shelf, Antarctica Using Coupled MODIS Optical and Thermal Satellite Measurements During the 2002-03 Melt Season
Ice shelf stability is of crucial importance in the Antarctic because shelves serve as buttresses to glacial ice advancing from the Antarctic Ice Sheet. Surface melt has been increasing over recent years, especially over the Antarctic Peninsula, contributing to disintegration of shelves such as Larsen. Unfortunately, we are not realistically able to quantify surface snowmelt from ground-based methods because there is sparse coverage in automatic weather stations. Satellite based assessments of melt from passive microwave systems are limited in that they only provide an indication of melt occurrence and have coarse resolution. Though this is useful in tracking the duration of melt, melt amount of magnitude is still unknown. Coupled optical/thermal surface measurements from MODIS were calibrated by estimates of liquid water fraction (LWF) in the upper 1cm of the firn derived from a one-dimensional thermal snowmelt model (SNTHERM). SNTHERM was forced by hourly meteorological data from automatic weather station data at reference sites spanning a range of melt conditions across the Ross Ice Shelf during a particularly intense melt season. Melt intensities or LWF were derived for satellite composite periods covering the Antarctic summer months at a 4km resolution over the entire Ross Ice Shelf, ranging from 0-2 percent LWF in early December to areas along the coast with upwards of 10 percent LWF during the time of peak surface melt. Spatial and temporal variations in the amount of surface melt are seen to be related to both katabatic wind strength and wind shifts due to the progression of cyclones along the circumpolar vortex. Sea ice concentration along the ice shelf front, specifically the formation of polynyas, are also thought to be driving factors for surface melt as latent and sensible heat fluxes increase by one to three orders of magnitude as polynyas form. A future application of surface melt mapping using this empirical retrieval model is to determine melt magnitude over other Antarctic Ice Shelves, such as Larsen, where surface melt has been well documented in contributing to the disintegration of the ice shelf.