C41D-01
Acceleration of Jakobshavn Isbrae Triggered by Warm, Subsurface Irminger Waters
Observations over the past decade show a rapid acceleration of several outlet glaciers in Greenland and
Antarctica. Among the largest changes seen, Jakobshavn Isbrae(JI), an outlet glacier feeding a deep ocean
fjord on the west coast of Greenland, recently, and suddenly, switched its behavior from slow thickening prior
to 1997 to subsequent rapid thinning and a doubling in glacier velocity. Suggested reasons for the JI
speedup range from increased lubrication of the ice-bedrock interface as more meltwater drains to the bed
during recently warmer summers, to weakening and breakup of the floating ice tongue. Here, we present
evidence that the changes of the JI were in fact triggered by an increase in subsurface ocean temperature,
based on hydrographic data showing a sudden jump during 1997 along the entire west coast of Greenland.
This arrival of upstream, Irminger Sea warm water, originating near Iceland, was driven by changes in
atmospheric circulation in the North Atlantic.
http://efdl.cims.nyu.edu/project_oisi/realistic/jakobshavn/
C41D-02
Ice-shelf melting around Antarctica
The traditional view on the mass balance of Antarctic ice shelves is that they loose mass principally from iceberg calving with bottom melting a much lower contributing factor. Because ice shelves are now known to play a fundamental role in ice sheet evolution, it is important to re-evaluate their wastage processes from a circumpolar perspective using a combination of remote sensing techniques. We present area average rates deduced from grounding line discharge, snow accumulation, firn depth correction and ice shelf topography. We find that ice shelf melting accounts for roughly half of ice-shelf ablation, with a total melt water production of 1027 Gt/yr. The attrition fraction due to in-situ melting varies from 9 to 90 percent around Antarctica. High melt producers include the Ronne, Ross, Getz, Totten, Amery, George VI, Pine Island, Abbot, Dotson/Crosson, Shackleton, Thwaites and Moscow University Ice Shelves. Low producers include the Larsen C, Princess Astrid and Ragnhild coast, Fimbul, Brunt and Filchner. Correlation between melt water production and grounding line discharge is low (R2 = 0.65). Correlation with thermal ocean forcing from the ocean are highest in the northern parts of West Antarctica where regressions yield R2 of 0.93-0.97. Melt rates in the Amundsen Sea exhibit a quadratic sensitivity to thermal ocean forcing. We conclude that ice shelf melting plays a dominant role in ice shelf mass balance, with a potential to change rapidly in response to altered ocean heat transport onto the Antarctic continental shelf.
C41D-03
Modeling Iceberg Calving From Ice Shelves Using a Stress Based Calving Law: The Stabilizing Effect of Vertical Compression
Iceberg calving from ice shelves and ice tongues provides an efficient mechanism to transfer larges amounts of ice to the ocean in a near-instantaneous fashion. This not only drastically changes the mass balance of the ice shelf, but the geometry of the ice-sheet-shelf system. The potential for a positive feedback between the ice shelf and the flow of inland ice ('buttressing') combined with the sensitive of ice shelves to forcing from both the atmosphere and the ocean suggests that iceberg calving is an important process. However, iceberg calving remains a poorly understood process. As a first step, we attempt to construct a calving law for ice shelves, based on a combination of scaling and physical arguments, which yields appropriate steady states. This leads us to postulate that calving rate is primarily a function of the ratio of tensile stress to vertical compressive stress, i.e., the calving rate is a function of the ratio of the largest to smallest principle stress. Implementing this calving law in a numerical model, we show that this law predicts that (i) embayed ice shelves are stable; (ii) ice shelves do not protrude (much) beyond their embayment. More importantly, this calving law provides a criterion for ice shelf instability: if a small calving event causes the tensile stress to increase faster than the compressive stress, the ice shelf will retreat and vice versa. Using this stability criterion we map out the 'phase-space' of parameters (basal melting/freezing, lateral shear, divergence of embayment walls) that allow a steady-state ice shelf to exist as opposed to those combinations of parameters that do not permit a steady-state ice shelf. Comparing our predictions of the parameter regime where ice shelves are stable with the observed parameter regime of healthy ice shelves can provide an independent estimate showing where our (and other) calving laws fails.
C41D-04 INVITED
Modeling the Contribution of Tides to the Larsen-C Ice Shelf Basal Melt Rate
Processes affecting the evolution of the Larsen-C Ice Shelf are still too poorly understood to predict whether the shelf will ultimately collapse like the neighboring Larsen-A and Larsen-B; however, evidence that portions of Larsen-C have recently thinned significantly suggests that future collapse is possible. We hypothesize that basal melt under Larsen-C is an important component of the ice shelf mass balance, with tidal currents providing most of the turbulent kinetic energy responsible for mixing warmer ocean water up to the ice shelf base to facilitate melt. This hypothesis is explored through modeling of basal melt rates with the Regional Ocean Modeling System (ROMS 2.2). The model uses simplified ice shelf geometry and initial stratification. The tide is modeled as an idealized coastal-trapped Kelvin wave forcing at the M2 tidal frequency. Basal melt rate is strongly affected by tidal strength. Varying the Kelvin wave amplitude over a range representing likely current speeds under the shelf gives shelf-averaged melt rates in the range 0.5-5 m/yr, encompassing the value of ~2 m/yr implied by trends in ERS satellite radar altimetry over much of the northern shelf. These results demonstrate the need to include tides in coupled ocean/ice-shelf models for predicting ice shelf thinning and water mass modification. The variability of melt rate with tidal current speed also highlights the importance of mapping water column thickness (wct) under ice shelves, since wct helps determine the relationship between tidal currents and sea surface height variability.
C41D-05 INVITED
The retreating ice shelves of the Antarctic Peninsula
The recent episodes of collapse of Larsen-B and Wilkins ice shelves on the Antarctic Peninsula were dramatic, and were widely-reported as compelling examples of the impact of rapid climate change. However, to focus exclusively on just these two examples, would be to ignore a much wider catalogue of events and potential insight into the mechanisms and impact of ice shelf loss. Ice shelf retreat has been more widespread along both coasts of the Antarctic Peninsula for more than 50 years. During this period, ten glaciologically distinct ice shelves have shown signs of climate-driven retreat, with a total loss of around 27,000 km2. Retreating ice shelves occupied diverse climatic settings, and their changes were conditioned by marked differences in their sensitivity to changing atmospheric and oceanic conditions. Thus the sequence in which ice shelves were lost, and the differences in individual histories of retreat and collapse, provide a valuable tool for understanding the processes involved. Here we present a new dataset containing consistent area calculations for ice shelves around the Antarctic Peninsula over 5 decades. We discuss the implications of the individual retreat histories, for the identification of dominant processes. We highlight some apparently anomalous behaviour. We consider whether recent rates of climate change in this area could continue as present rates in future decades and whether more ice shelves are threatened. We also present new observational evidence for the influence of ice shelves on the velocity of the glaciers that feed them. We conclude, that while a major limitation on the development of models of ice-sheet evolution is the lack of real-world observations of ice-sheet change that are available for verification and testing, the 50-year retreat of ice shelves of the Antarctic Peninsula now provide an extraordinarily valuable dataset for improving our capability for prediction for this important component of the ice sheet system.
C41D-06
Mounting evidence for intense ocean interaction with the Pine Island Glacier Ice Shelf
The spatial signature of thinning and acceleration of the Pine Island Glacier has led to the inference that these changes originate at the seaward end of the glacier, possibly within or under the ice shelf (Payne et al., 2004; Shepherd et al., 2004). We present new analyses resulting from both new and archived satellite imagery of the ice shelf that supports this inference and provides new insights into strong seasonal and intra- annual characters of ocean-ice shelf interaction. Strong longitudinal variations in both thickness and surface elevation measured by British Antarctic Survey airborne radars (Vaughan et al., 2006) have wavelengths that correspond roughly to the annual motion of the ice shelf. These could be caused by seasonal variations in flow speed, but such variations of flow speed have never been reported and are not seen in the most recent continuous GPS observations of the ice shelf. We suggest that these strong variations in ice thickness, as large as 200 meters in an average thickness of 600 meters, are caused by seasonal variations in the properties of the water circulating underneath the ice shelf. One likely explanation is that the dominant water mass reaching the deepest parts of the ice shelf alternates between cold High Salinity Shelf Water in the winter and warm Circumpolar Deep Water in the summer. Evidence for recent strengthening of the sub- shelf circulation is the sudden occurrence of three persistent polynyas immediately adjacent to the ice front. These are located in precisely the locations expected from modeled sub-shelf circulation (Payne et al., 2007). This mode was never observed in any satellite imagery prior to the 1999-2000 austral summer (data of 7 summers since 1973 were available), but has occurred in 7 of the 9 summers since and persists throughout the summer. Payne, A.J., A. Vieli, A.P. Shepherd, D.J. Wingham and E. Rignot, 2004. Recent dramatic thinning of largest West Antarctic ice stream triggered by oceans, Geophysical Research Letters, Vol. 31, No. 23: Art. No. L23401 DEC 9 2004 Payne, A.J., P.R. Holland, A.P. Shepherd, I.C. Rutt, A. Jenkins and I. Joughin, 2007. Numerical modeling of ocean-ice interactions under Pine Island Bay's ice shelf, Journal of Geophysical Research, Vol. 112, C10019, doi:10.1029/2006JC003733. Shepherd, A., D.J. Wingham and E. Rignot, 2004. Warm ocean is eroding West Antarctic Ice Sheet, Geophysical Research Letters, Vol. 31, Art. No. L23402 DEC 9 2004. Vaughan, D.G., H.F.J. Corr, F. Ferraccioli, N. Frearson, A. O'Hare, D. Mach, J.W. Holt, D.D. Blankenship, D. Morse, and D.A. Young, 2006. New boundary conditions for the West Antarctic ice sheet: Subglacial topography beneath Pine Island Glacier. Geophys. Res. Let., Vol. 33, No. 9, Art. No. L09501, May 3, 2006.
C41D-07 INVITED
Weddell Sea and Dronning Maud Land ice shelf mass balance from satellite altimetry and velocity measurements.
The Weddell Sea basin and the coast of Dronning Maud Land contain two large ice shelves (Filchner and Ronne) and several smaller ice shelves (). We present data that constrain the main processes that can alter the mass balance of these ice shelves: ice flux divergence, basal melt, and surface accumulation. ERS and ICESat altimetric measurements, together with tide and surface-accumulation models, allow us to estimate patterns of ice thickness and thickness changes under the assumption of hydrostatic equilibrium. Combined with our thickness estimates, RADARSAT velocity measurements give the pattern of ice flux divergence. The combination of these estimates allows estimation of the steady-state pattern of basal melt and freeze-on needed to balance the ice flux divergence, and of deviations from this steady-state pattern producing temporal changes in ice shelf thickness. The spatial pattern of basal ice balance is diagnostic of basal processes, including tidal advection of surface water under the ice front, and buoyancy-driven flows that erode ice near the grounding line and produce freeze-on at middle depth, and we use these patterns to infer the magnitude of these processes and their deviations from steady state.
C41D-08
Ocean wave generation by collapsing ice shelves
The 28-29 February, 2008, break-up of the Wilkins Ice Shelf, Antarctica, exemplifies the now-familiar, yet largely unexplained pattern of explosive ice-shelf break-up. While environmental warming is a likely ultimate cause of explosive break-up, several key aspects of their short-term behavior need to be explained: (1) The abrupt, near-simultaneous onset of iceberg calving across long spans of the ice front margin; (2) High outward drift velocity (about 0.3 m/s) of a leading phalanx of tabular icebergs that originate from the seaward edge of the intact ice shelf prior to break-up; (3) Rapid coverage of the ocean surface in the wake of this leading phalanx by small, capsized and dismembered tabular icebergs; (4) Extremely large gravitational potential energy release rates, e.g., up to 3 × 1010 W; (5) Lack of proximal iceberg-calving triggers that control the timing of break-up onset and that maintain the high break-up calving rates through to the conclusion of the event. Motivated by seismic records obtained from icebergs and the Ross Ice Shelf that show hundreds of micro- tsunamis emanating from near the ice shelf front, we re-examine the basic dynamic features of ice- shelf/ocean-wave interaction and, in particular, examine the possibility that collapsing ice shelves themselves are a source of waves that stimulate the disintegration process. We propose that ice-shelf generated surface-gravity waves associated with initial calving at an arbitrary seed location produce stress perturbations capable of triggering the onset of calving on the entire ice front. Waves generated by parting detachment rifts, iceberg capsize and break-up act next to stimulate an inverted submarine landslide (ice- slide) process, where gravitational potential energy released by upward movement of buoyant ice is radiated as surface gravity waves in the wake of the advancing phalanx of tabular icebergs. We conclude by describing how field research and remote sensing can be used to test the various conjectures about ice- shelf/wave interaction that appear to be at play during ice-shelf disintegration.