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Overview articles
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Ice-sheet response to oceanic forcing
Abstract: The ice sheets of Greenland and Antarctica are losing ice at accelerating rates, much of which is a response to oceanic forcing, especially of the floating ice shelves. Recent observations establish a clear correspondence between the increased delivery of oceanic heat to the ice-sheet margin and increased ice loss. In Antarctica, most of these processes are reasonably well understood but have not been rigorously quantified. In Greenland, an understanding of the processes by which warmer ocean temperatures drive the observed retreat remains elusive. Experiments designed to identify the relevant processes are confounded by the logistical difficulties of instrumenting ice-choked fjords with actively calving glaciers. For both ice sheets, multiple challenges remain before the fully coupled ice-ocean-atmosphere models needed for rigorous sea-level projection are available.
Citation: Joughin, I., R. B. Alley, and D. M. Holland (2012), Ice-Sheet Response to Oceanic Forcing, Science, 338, 1172-1176, doi:10.1126/science.1226481.
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Recent progress in understanding marine-terminating Arctic outlet glacier response to climatic and oceanic forcing: Twenty years of rapid change
Abstract: Until relatively recently, it was assumed that Arctic ice masses would respond to climatic/oceanic forcing over millennia, but observations made during the past two decades have radically altered this viewpoint and have demonstrated that marine-terminating outlet glaciers can undergo dramatic dynamic change at annual timescales. This paper reviews the substantial progress made in our understanding of the links between marine-terminating Arctic outlet glacier behaviour and the ocean-climate system during the past 20 years, when many ice masses have rapidly lost mass. Specifically, we assess three primary climatic/oceanic controls on outlet glacier dynamics, namely air temperature, ocean temperature and sea ice concentrations, and discuss key linkages between them. Despite recent progress, significant uncertainty remains over the response of marine-terminating outlet glaciers to these forcings, most notably: (1) the spatial variation in the relative importance of each factor; (2), the contribution of glacier-specific factors to glacier dynamics; and (3) the limitations in our ability to accurately model marine-terminating outlet glacier behaviour. Our present understanding precludes us from identifying patterns of outlet glacier response to forcing that are applicable across the Arctic and we underscore the potential danger of extrapolating rates of mass loss from a small sample of study glaciers.
Citation: Carr, J. R., C. R. Stokes, and A. Vieli (2013), Recent progress in understanding marine-terminating Arctic outlet glacier response to climatic and oceanic forcing: Twenty years of rapid change, Progress in Physical Geography, DOI: 10.1177/0309133313483163.
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Challenges to understanding the dynamic response of Greenland's marine terminating glaciers to oceanic and atmospheric forcing
Abstract: An interdisciplinary and multifaceted 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.
Citation: Straneo, F., P. Heimbach, O. Sergienko, G. Hamilton, G. Catania, S. Griffies, R. Hallberg, A. Jenkins, I. Joughin, R. Motyka, W. T. Pfeffer, S. F. Price, E. Rignot, T. Scambos, M. Truffer, and A. Vieli (2013), Challenges to understanding the dynamic response of Greenland's marine terminating glaciers to oceanic and atmospheric forcing, Bulletin of the American Meteorological Society, 1131-1144.
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North Atlantic warming and the retreat of Greenland's outlet glaciers
Abstract: Mass loss from the Greenland ice sheet quadrupled over the past two decades, contributing a quarter of the observed global sea-level rise. Increased submarine melting is thought to have triggered the retreat of Greenland's outlet glaciers, which is partly responsible for the ice loss. However, the chain of events and physical processes remain elusive. Recent evidence suggests that an anomalous inflow of subtropical waters driven by atmospheric changes, multidecadal natural ocean variability and a long-term increase in the North Atlantic's upper ocean heat content since the 1950s all contrib¬uted to a warming of the subpolar North Atlantic. This led, in conjunction with increased runoff, to enhanced submarine glacier melting. Future climate projections raise the potential for continued increases in warming and ice-mass loss, with implications for sea level and climate.
Citation: Straneo, F., and P. Heimbach (2013), North Atlantic warming and the retreat of Greenland's outlet glaciers, Nature, 504, 36-43, doi:10.1038/nature12854.
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Modeling Studies
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Diverse calving patterns linked to glacier geometry
Abstract: Iceberg calving has been implicated in the retreat and acceleration of glaciers and ice shelves along the margins of the Greenland and Antarctic ice sheets. Accurate projections of sea-level rise therefore require an understanding of how and why calving occurs. Unfortunately, calving is a complex process and previous models of the phenomenon have not reproduced the diverse patterns of iceberg calving observed in nature. Here we present a numerical model that simulates the disparate calving regimes observed, including the detachment of large tabular bergs from floating ice tongues, the disintegration of ice shelves and the capsizing of smaller bergs from grounded glaciers that terminate in deep water. Our model treats glacier ice as a granular material made of interacting boulders of ice that are bonded together. Simulations suggest that different calving regimes are controlled by glacier geometry, which controls the stress state within the glacier. We also find that calving is a two-stage process that requires both ice fracture and transport of detached icebergs away from the calving front. We suggest that, as a result, rapid iceberg discharge is possible in regions where highly crevassed glaciers are grounded deep beneath sea level, indicating portions of Greenland and Antarctica that may be vulnerable to rapid ice loss through catastrophic disintegration.
Citation: Bassis, J. N. and S. Jacobs (2013), Diverse calving patterns linked to glacier geometry, Nature Geoscience, 6, 833-836, doi: 10.1038/ngeo1887.
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Future sea-level rise from Greenland's main outlet glaciers in a warming climate
Abstract: Over the past decade, ice loss from the Greenland Ice Sheet increased as a result of both increased surface melting and ice discharge to the ocean. The latter is controlled by the acceleration of ice flow and subsequent thinning of fast-flowing marine-terminating outlet glaciers. Quantifying the future dynamic contribution of such glaciers to sea-level rise (SLR) remains a major challenge because outlet glacier dynamics are poorly understood. Here we present a glacier flow model that includes a fully dynamic treatment of marine termini. We use this model to simulate behaviour of four major marine-terminating outlet glaciers, which collectively drain about 22 per cent of the Greenland Ice Sheet. Using atmospheric and oceanic forcing from a mid-range future warming scenario that predicts warming by 2.8 degrees Celsius by 2100, we project a contribution of 19 to 30 millimetres to SLR from these glaciers by 2200. This contribution is largely (80 percent) dynamic in origin and is caused by several episodic retreats past overdeepenings in outlet glacier troughs. After initial increases, however, dynamic losses from these four outlets remain relatively constant and contribute to SLR individually at rates of about 0.01 to 0.06 millimetres per year. These rates correspond to ice fluxes that are less than twice those of the late 1990s, well below previous upper bounds. For a more extreme future warming scenario (warming by 4.5 degrees Celsius by 2100), the projected losses increase by more than 50 percent, producing a cumulative SLR of 29 to 49 millimetres by 2200.
Citation: Nick, F. M., A. Vieli, M. L. Andersen, I. Joughin, A. Payne, T. L. Edwards, F. Pattyn, and R. S. W. van de Wal (2013), Future sea-level rise from Greenland's main outlet glaciers in a warming climate, Nature, 497, 235-238, doi: 10.1038/nature12068.
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Calving on tidewater glaciers amplified by submarine frontal melting
Abstract: While it has been shown repeatedly that ocean conditions exhibit an important control on the behaviour of grounded tidewater glaciers, modelling studies have focused largely on the effects of basal and surface melting. Here, a finite-element model of stresses near the front of a tidewater glacier is used to investigate the effects of frontal melting on calving, independently of the calving criterion used. Applications of the stress model to idealized scenarios reveal that undercutting of the ice front due to frontal melting can drive calving at up to ten times the mean melt rate. Factors which cause increased frontal melt-driven calving include a strong thermal gradient in the ice, and a concentration of frontal melt at the base of the glacier. These properties are typical of both Arctic and Antarctic tidewater glaciers. The finding that frontal melt near the base is a strong driver of calving leads to the conclusion that water temperatures near the bed of the glacier are critically important to the glacier front, and thus the flow of the glacier. These conclusions are robust against changes in the basal boundary condition and the choice of calving criterion, as well as variations in the glacier size or level of crevassing.
Citation: O'Leary, M., and P. Christoffersen (2013), Calving on tidewater glaciers amplified by submarine frontal melting, The Cryosphere, 7, 119-128, doi:10.5194/tc-7-119-2013.
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Understanding and Modelling Rapid Dynamic Changes of Tidewater Outlet Glaciers: Issues and Implications
Abstract: Recent dramatic acceleration, thinning and retreat of tidewater outlet glaciers in Greenland raises concern regarding their contribution to future sea-level rise. These dynamic changes seem to be parallel to oceanic and climatic warming but the linking mechanisms and forcings are poorly understood and, furthermore, large-scale ice sheet models are currently unable to realistically simulate such changes which provides a major limitation in our ability to predict dynamic mass losses. In this paper we apply a specifically designed numerical flowband model to Jakobshavn Isbrae (JIB), a major marine outlet glacier of the Greenland ice sheet, and we explore and discuss the basic concepts and emerging issues in our understanding and modelling ability of the dynamics of tidewater outlet glaciers. The modelling demonstrates that enhanced ocean melt is able to trigger the observed dynamic changes of JIB but it heavily relies on the feedback between calving and terminus retreat and therefore the loss of buttressing. Through the same feedback, other forcings such as reduced winter sea-ice duration can produce similar rapid retreat. This highlights the need for a robust representation of the calving process and for improvements in the understanding and implementation of forcings at the marine boundary in predictive ice sheet models. Furthermore, the modelling uncovers high sensitivity and rapid adjustment of marine outlet glaciers to perturbations at their marine boundary implying that care should be taken in interpreting or extrapolating such rapid dynamic changes as recently observed in Greenland.
Citation: Vieli, A., and F. M. Nick (2011), Understanding and Modelling Rapid Dynamic Changes of Tidewater Outlet Glaciers: Issues and Implications, Surveys in Geophysics, 32, 437-458, doi: 10.1007/s10712-011-9132-4.
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Numerical experiments on subaqueous melting of Greenland tidewater glaciers in response to ocean warming and enhanced subglacial discharge
Abstract: The largest dischargers of ice in Greenland are glaciers that terminate in the ocean and melt in contact with sea water. Studies of ice-sheet/ocean interactions have mostly focused on melting beneath near-horizontal floating ice shelves. For tidewater glaciers, melting instead takes place along the vertical face of the calving front. Here we modify the Massachusetts Institute of Technology general circulation model (MITgcm) to include ice melting from a calving face with the freshwater outflow at the glacier grounding line. We use the model to predict melt rates and their sensitivity to ocean thermal forcing and to subglacial discharge. We find that melt rates increase with approximately the one-third power of the subglacial water flux, and increase linearly with ocean thermal forcing. Our simulations indicate that, consistent with limited field data, melting ceases when subglacial discharge is shut off, and reaches several meters per day when subglacial discharge is high in the summer. These results are a first step toward a more realistic representation of subglacial discharge and of ocean thermal forcing on the subaqueous melting of tidewater glaciers in a numerical ocean model. Our results illustrate that the ice-front melting process is both complex and strongly time-dependent.
Citation: Xu, Y., E. Rignot, D. Menemenlis, and M. Koppes (2012), Numerical experiments on subaqueous melting of Greenland tidewater glaciers in response to ocean warming and enhanced subglacial discharge, Annals of Glaciology, 53, 229-234, doi: 10.3189/2012AoG60A139.
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Ice and ocean observations (in situ and/or remote)
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Ice mélange dynamics and implications for terminus stability, Jakobshavn Isbræ, Greenland
Abstract: We used time-lapse imagery, seismic and audio recordings, iceberg and glacier velocities, ocean wave measurements, and simple theoretical considerations to investigate the interactions between Jakobshavn Isbræ and its proglacial ice mélange. The mélange behaves as a weak, granular ice shelf whose rheology varies seasonally. Sea ice growth in winter stiffens the mélange matrix by binding iceberg clasts together, ultimately preventing the calving of full-glacier-thickness icebergs (the dominant style of calving) and enabling a several kilometer terminus advance. Each summer the mélange weakens and the terminus retreats. The mélange remains strong enough, however, to be largely unaffected by ocean currents (except during calving events) and to influence the timing and sequence of calving events. Furthermore, motion of the mélange is highly episodic: between calving events, including the entire winter, it is pushed down fjord by the advancing terminus (at ~40 m d-1), whereas during calving events it can move in excess of 50×103 m d-1 for more than 10 min. By influencing the timing of calving events, the mélange contributes to the glacier's several kilometer seasonal advance and retreat; the associated geometric changes of the terminus area affect glacier flow. Furthermore, a force balance analysis shows that large-scale calving is only possible from a terminus that is near floatation, especially in the presence of a resistive ice mélange. The net annual retreat of the glacier is therefore limited by its proximity to floatation, potentially providing a physical mechanism for a previously described near-floatation criterion for calving.
Citation: Amundson, J. M., M. Fahnestock, M. Truffer, J. Brown, M. P. Lüthi, and R. J. Motyka (2010), Ice mélange dynamics and implications for terminus stability, Jakobshavn Isbræ, Greenland, Journal of Geophysical Research, 115, F01005, doi:10.1029/2009JF001405.
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Warming of waters in an East Greenland fjord prior to glacier retreat: mechanisms and connection to large-scale atmospheric conditions
Abstract: Hydrographic data acquired in Kangerdlugssuaq Fjord and adjacent seas in 1993 and 2004 are used together with reanalysis from the NEMO ocean modelling framework to elucidate water-mass change and ice-ocean-atmosphere interactions in East Greenland. The hydrographic data show that the fjord contains warm subtropical waters and that fjord waters in 2004 were considerably warmer than in 1993. The ocean reanalysis shows that the warm properties of fjord waters in 2004 are related to a major peak in oceanic shoreward heat flux into a cross-shelf trough on the outer continental shelf. The heat flux into this trough varies according to seasonal exchanges with the atmosphere as well as from deep seasonal intrusions of subtropical waters. Both mechanisms contribute to high (low) shoreward heat flux when winds from the northeast are weak (strong). The combined effect of surface heating and inflow of subtropical waters is seen in the hydrographic data, which were collected after periods when along-shore coastal winds from the north were strong (1993) and weak (2004). The latter data were furthermore acquired during the early phase of a prolonged retreat of Kangerdlugssuaq Glacier. We show that coastal winds vary according to the pressure gradient defined by a semi-permanent atmospheric high-pressure system over Greenland and a persistent atmospheric low situated near Iceland. The magnitude of this pressure gradient is controlled by longitudinal variability in the position of the Icelandic Low.
Citation: Christoffersen, P., R. I. Mugford, K. J. Heywood, I. Joughin, J. A. Dowdeswell, J. P. M. Syvitski, A. Luckman, and T. J. Benham (2011), Warming of waters in an East Greenland fjord prior to glacier retreat: mechanisms and connection to large-scale atmospheric conditions, The Cryosphere, 5, 701–714, doi:10.5194/tc-5-701-2011.
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Submarine melt rate estimates for floating termini of Greenland outlet glaciers (2000–2010)
Abstract: The rate of mass loss from the Greenland ice sheet has increased over the past decade due, in large part, to changes in marine-terminating outlet glacier dynamics. These changes are attributed to increased submarine melt rates of floating ice tongues and submerged calving faces resulting from increased coastal ocean heat transport. We use remotely sensed data to calculate submarine melt rates for 13 marine-terminating outlet glaciers in Greenland on a semi-annual basis between 2000 and 2010. We assess temporal and spatial variability in the calculated submarine melt rates and compare those variabilities to concurrent glacier change and offshore ocean temperatures. Over the period of study, average melt rates ranged from 0.03 to 2.98m d–1 and account for 5–85% of the total volume loss from the floating ice tongue, with no clear spatial pattern. Only four glaciers show substantial interannual variability in melt rate during the decade. Melt rates were uncorrelated with front retreat, speed and changes in ocean temperature. Although the small sample size limits our analysis of the relationship between oceanographic forcing and glacier response, these data suggest that the calving rate may vary with discharge but that submarine melt rates are independent of grounding line discharge.
Citation: Enderlin, E. M., and I. M. Howat (2013), Submarine melt rate estimates for floating termini of Greenland outlet glaciers (2000–2010), Journal of Glaciology, 59, 213, doi: 10.3189/2013JoG12J049.
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Two years of oceanic observations below the Fimbul Ice Shelf, Antarctica
Abstract: The mechanisms by which heat is delivered to Antarctic ice shelves are a major source of uncertainty when assessing the response of the Antarctic ice sheet to climate change. Direct observations of the ice shelf-ocean interaction are extremely scarce and in many regions melt rates from ice shelf-ocean models are not constrained by measurements. Our two years of data (2010 and 2011) from three oceanic moorings below the Fimbul Ice Shelf in the Eastern Weddell Sea show cold cavity waters, with average temperatures of less than 0.1 °C above the surface freezing point. This suggests low basal melt rates, consistent with remote sensing based, steady-state mass balance estimates for this sector of the Antarctic coast. Oceanic heat for basal melting is found to be supplied by two sources of warm water entering below the ice: (i) eddy-like bursts of Modified Warm Deep Water that access the cavity at depth for eight months of the record; and (ii) fresh surface water that flushes parts of the ice base with temperatures above freezing during late summer and fall. This interplay of processes implies that basal melting at the Fimbul Ice Shelf cannot simply be parameterized by coastal deep ocean temperatures, but instead appears directly linked to both solar forcing at the surface as well as to the dynamics of the coastal current system.
Citation: Hattermann, T., O. A. Nøst, J. M. Lilly, and L .H. Smedsrud (2012), Two years of oceanic observations below the Fimbul Ice Shelf, Antarctica, Geophysical Research Letters, 39, L12605, doi:10.1029/2012GL051012.
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Acceleration of Jakobshavn Isbræ triggered by warm subsurface ocean waters
Abstract: Observations over the past decades show a rapid acceleration of several outlet glaciers in Greenland and Antarctica. One of the largest changes is a sudden switch of Jakobshavn Isbræ, a large outlet glacier feeding a deep-ocean fjord on Greenland's west coast, from slow thickening to rapid thinning in 1997, associated with a doubling in glacier velocity. Suggested explanations for the speed-up of Jakobshavn Isbræ include increased lubrication of the ice–bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers and weakening and break-up of the floating ice tongue that buttressed the glacier. Here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of Greenland, suggesting that the changes in Jakobshavn Isbræ were instead triggered by the arrival of relatively warm water originating from the Irminger Sea near Iceland. We trace these oceanic changes back to changes in the atmospheric circulation in the North Atlantic region. We conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets.
Citation: Holland, D. M., R. H. Thomas, B. De Young, M. H. Ribergaard, and B. Lyberth (2008), Acceleration of Jakobshavn Isbræ triggered by warm subsurface ocean waters, Nature Geoscience, 1, 659-664.
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Estuaries beneath ice sheets
Abstract: Interactions between subglacial hydrology and the ocean make the existence of estuaries at the grounding zones of ice sheets likely. Here we present geophysical observations of an estuary at the downstream end of the hydrologic system that links the active subglacial lakes beneath Whillans Ice Stream to the ocean beneath the Ross Ice Shelf, Antarctica. This subglacial estuary consists of a hydropotential low upstream of the grounding zone, which is linked to the ocean by a hydropotential trough and a large subglacial channel. This subglacial channel, which is imaged using active source seismic methods, has an apparent width of 1 km and a maximum depth of 7 m. The hydropotential trough continues upstream of the grounding zone and results from an along-flow depression in surface elevations. Pressure differences along the trough axis are within a range that can be overcome by tidally induced processes, making the interaction of subglacial and ocean water likely.
Citation: Horgan, H. J., R. B. Alley, K. Christianson, R. W. Jacobel, S. Anandakrishnan, A. Muto, L. H. Beem, and M. R. Siegfried (2013), Estuaries beneath ice sheets, Geology, G34654, doi:10.1130/G34654.1.
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Seasonal variability in the dynamics of marine-terminating outlet glaciers in Greenland
Abstract: Recent studies indicate that the dynamics of fast-flowing, marine-terminating outlet glaciers of the Greenland ice sheet may be sensitive to climate and ocean forcing on sub-annual timescales. Observations of seasonal behavior of these glaciers at such high temporal resolution, however, are currently few. Here we present observations of front position, flow speed, near-surface air temperature and ocean conditions for six large marine-terminating glaciers in the Uummannaq region of West Greenland, to investigate controls on short-term glacier dynamics. As proposed by other studies, we find that seasonal front advance and retreat correlates with the formation and disappearance of an ice melange. Our data suggest that high sea-surface temperature, anomalously low sea-ice concentration and reduced melange formation in early 2003 have triggered multi-year retreat of several glaciers in the study area, which is consistent with other regions in Greenland. Of the stable glaciers, only Rink Isbræ exhibits a seasonal speed variation that correlates with variations in front position, with the others undergoing mid-summer deceleration that indicates the effects of subglacial meltwater discharge and drainage system evolution. Drainage of supraglacial lakes and water-filled crevasses results in substantial decreases in speed (40–60%) on fast-flowing glaciers. Our results demonstrate that attempts to model ice-sheet evolution must take into account short-timescale flow dynamics resulting from drainage events and oceanographic conditions.
Citation: Howat, I. M., J. E. Box, Y. Ahn, A. Herrington, E. M. McFadden (2010), Seasonal variability in the dynamics of marine-terminating outlet glaciers in Greenland, Journal of Glaciology, 56, 198, 601-613.
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Stability of the West Antarctic ice sheet in a warming world
Abstract: Ice sheets are expected to shrink in size as the world warms, which in turn will raise sea level. The West Antarctic ice sheet is of particular concern, because it was probably much smaller at times during the past million years when temperatures were com¬parable to levels that might be reached or exceeded within the next few centuries. Much of the grounded ice in West Antarctica lies on a bed that deepens inland and extends well below sea level. Oceanic and atmospheric warming threaten to reduce or eliminate the floating ice shelves that buttress the ice sheet at present. Loss of the ice shelves would accelerate the flow of non-floating ice near the coast. Because of the slope of the sea bed, the consequent thinning could ultimately float much of the ice sheet's interior. In this scenario, global sea level would rise by more than three metres, at an unknown rate. Simplified analyses suggest that much of the ice sheet will survive beyond this century. We do not know how likely or inevitable eventual collapse of the West Antarctic ice sheet is at this stage, but the possibility cannot be discarded. For confident projections of the fate of the ice sheet and the rate of any collapse, further work including the development of well-validated physical models will be required.
Citation: Joughin, I., and R. B. Alley (2011), Stability of the West Antarctic ice sheet in a warming world, Nature Geoscience, 4, 506-513, doi:10.1038/ngeo1194.
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Submarine melting of the 1985 Jakobshavn Isbræ floating tongue and the triggering of the current retreat
Abstract: Photogrammetric reanalysis of 1985 aerial photos has revealed substantial submarine melting of the floating ice tongue of Jakobshavn Isbræ, west Greenland. The thickness of the floating tongue determined from hydrostatic equilibrium tapers from ∼940 m near the grounding zone to ∼600 m near the terminus. Feature tracking on orthophotos shows speeds on the July 1985 ice tongue to be nearly constant (∼18.5 m d−1), indicating negligible dynamic thinning. The thinning of the ice tongue is mostly due to submarine melting with average rates of 228 ± 49 m yr−1 (0.62 ± 0.13 m d−1) between the summers of 1984 and 1985. The cause of the high melt rate is the circulation of warm seawater (thermal forcing of up to 4.2°C) beneath the tongue with convection driven by the substantial discharge of subglacial freshwater from the grounding zone. We believe that this buoyancy-driven convection is responsible for a deep channel incised into the sole of the floating tongue. A dramatic thinning, retreat, and speedup began in 1998 and continues today. The timing of the change is coincident with a 1.1°C warming of deep ocean waters entering the fjord after 1997. Assuming a linear relationship between thermal forcing and submarine melt rate, average melt rates should have increased by ∼25% (∼57 m yr−1), sufficient to destabilize the ice tongue and initiate the ice thinning and the retreat that followed.
Citation: Motyka, R. J., M. Truffer, M. Fahnestock, J. Mortensen, S. Rysgaard, and I. Howat (2011), Submarine melting of the 1985 Jakobshavn Isbræ floating tongue and the triggering of the current retreat, Journal of Geophysical Research, 116, F01007, doi:10.1029/2009JF001632.
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From circumpolar deep water to the glacial meltwater plume on the eastern Amundsen Shelf
Abstract: The melting of Pine Island Ice Shelf (PIIS) has increased since the 1990s, which may have a large impact on ice sheet dynamics, sea level rise, and changes in water mass properties of surrounding oceans. The reason for the PIIS melting is the relatively warm (∼1.2 °C) Circumpolar Deep Water (CDW) that penetrates into the PIIS cavity through two submarine glacial troughs located on the Amundsen Sea continental shelf. In this study, we mainly analyze the hydrographic data obtained during ANTXXVI/3 in 2010 with the focus on pathways of the intruding CDW, PIIS melt rates, and the fate of glacial meltwater. We analyze the data by dividing CTD profiles into 6 groups according to intruding CDW properties and meltwater content. From this analysis, it is seen that CDWwarmer than 1.23 °C (colder than 1.23 °C) intrudes via the eastern (central) trough. The temperature is controlled by the thickness of the intruding CDW layer. The eastern trough supports a denser CDW layer than the water mass in Pine Island Trough (PIT). The eastern intrusion is modified on the way into PIT through mixing with the lighter and colder CDW from the central trough. Using ocean transport and tracer transport calculations from the ice shelf front CTD section, the estimated melt rate in 2010 is ∼30 m yr−1, which is comparable to published values. From spatial distributions of meltwater content, meltwater flows along the bathymetry towards the west. When compared with earlier (2000) observations, a warmer and thicker CDW layer is observed in Pine Island Trough for the period 2007–2010, indicating a recent thickening of the CDW intrusion.
Citation: Nakayama, Y., M. Schröder, and H. H. Hellmer (2013), From circumpolar deep water to the glacial meltwater plume on the eastern Amundsen Shelf, Deep-Sea Research, I, 77, 50–62.
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Rapid submarine melting of the calving faces of West Greenland glaciers
Abstract: Widespread glacier acceleration has been observed in Greenland in the past few years associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean. These glaciers thin both at the surface, from warm air temperatures, and along their submerged faces in contact with warm ocean waters8. Little is known about the rates of submarine melting and how they may affect glacier dynamics. Here we present measurements of ocean currents, temperature and salinity near the calving fronts of the Eqip Sermia, Kangilerngata Sermia, Sermeq Kujatdleq and Sermeq Avangnardleq glaciers in central West Greenland, as well as ice-front bathymetry and geographical positions. We calculate water-mass and heat budgets that reveal summer submarine melt rates ranging from 0.7 ± 0.2 to 3.9 ± 0.8 m d−1. These rates of submarine melting are two orders of magnitude larger than surface melt rates, but comparable to rates of iceberg discharge. We conclude that ocean waters melt a considerable, but highly variable, fraction of the calving fronts of glaciers before they disintegrate into icebergs, and suggest that submarine melting must have a profound influence on grounding-line stability and ice-flow dynamics.
Citation: Rignot, E., M. Koppes, and I. Velicogna (2010), Rapid submarine melting of the calving faces of West Greenland glaciers, Nature Geoscience, 3, 3, 141-218, doi: 10.1038/ngeo765.
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Ocean forcing of the Greenland Ice Sheet: Calving fronts and patterns of retreat identified by automatic satellite monitoring of eastern outlet glaciers
Abstract: We have developed an automatic method to identify changes in the position of calving glacier margins using daily MODIS imagery. Application of the method to 32 ocean-terminating glaciers in East Greenland produced 26,802 margin positions for a 10 year long period (2000–2009). We report these high-resolution data and show that the glaciers exhibit seasonal cycles with magnitudes of advance and retreat proportional to glacier width. Despite similar seasonality there is a distinct difference between the interannual trends of calving front positions north and south of 69°N. All glaciers above this latitude showed very limited or no change when seasonality was excluded, while glaciers south of 69°N retreated significantly between 2001 and 2005 (~2.3 km on average). Approximately 26% of the retreat of southern glaciers was regained by readvance from 2005 to 2009. To explain the latitudinal boundary of glacier dynamics, we review basic climatic factors, including summer and winter atmospheric forcing, sea ice conditions, and ocean temperature. We conclude that the southern retreats were strongly influenced by warm oceanic conditions associated with increased transport of subtropical waters to the Irminger Sea and to fjords and coastal regions south of 69°N. Northern glaciers remained stable despite significant increase in runoff in this region because fjords at latitudes higher than 69°N are less exposed to subtropical waters. The southern retreats illustrate sensitive behavior of calving glaciers, and we hypothesize that the calving fronts retreated because they were exposed to rapid ice-front melting.
Citation: Seale, A., P. Christoffersen, R. I. Mugford, and M. O'Leary (2011), Ocean forcing of the Greenland Ice Sheet: Calving fronts and patterns of retreat identified by automatic satellite monitoring of eastern outlet glaciers, Journal of Geophysical Research, 116, F03013, doi:10.1029/2010JF001847.
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Characteristics of ocean waters reaching Greenland's glaciers
Abstract: Interaction of Greenland's marine-terminating glaciers with the ocean has emerged as a key term in the ice-sheet mass balance and a plausible trigger for their recent acceleration. Our knowledge of the dynamics, however, is limited by scarcity of ocean measurements at the glacier/ocean boundary. Here data collected near six marine-terminating glaciers (79 North, Kangerdlugssuaq, Helheim and Petermann glaciers, Jakobshavn Isbræ, and the combined Sermeq Kujatdleq and Akangnardleq) are compared to investigate the water masses and the circulation at the ice/ocean boundary. PolarWater, of Arctic origin, and Atlantic Water, from the subtropical North Atlantic, are found near all the glaciers. Property analysis indicates melting by AtlanticWater (AW; found at the grounding line depth near all the glaciers) and the influence of subglacial discharge at depth in summer. AW temperatures near the glaciers range from 4.5°C in the southeast, to 0.16°C in northwest Greenland, consistent with the distance from the subtropical North Atlantic and cooling across the continental shelf. A review of its offshore variability suggests that AW temperature changes in the fjords will be largest in southern and smallest in northwest Greenland, consistent with the regional distribution of the recent glacier acceleration.
Citation: Straneo, F., D. A. Sutherland, D. Holland, C. Gladish, G. S. Hamilton, H. L. Johnson, E. Rignot, Y. Xu, and M. Koppes (2012), Characteristics of ocean waters reaching Greenland's glaciers, Annals of Glaciology, 53, 60, doi:10.3189/2012AoG60A059.
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Atlantic water variability on the SE Greenland continental shelf and its relationship to SST and bathymetry
Abstract: Interaction of warm, Atlantic-origin water (AW) and colder, polar origin water (PW) advecting southward in the East Greenland Current (EGC) influences the heat content of water entering Greenland's outlet glacial fjords. Here we use depth and temperature data derived from deep-diving seals to map out water mass variability across the continental shelf and to augment existing bathymetric products. We compare depths derived from the seal dives with the IBCAO Version 3 bathymetric database over the shelf and find differences up to 300m near several large submarine canyons. In the vertical temperature structure, we find two dominant modes: a cold mode, with the typical AW/PW layering observed in the EGC, and a warm mode, where AW is present throughout the water column. The prevalence of these modes varies seasonally and spatially across the continental shelf, implying distinct AW pathways. In addition, we find that satellite sea surface temperatures (SST) correlate significantly with temperatures in the upper 50 m (R= 0.54), but this correlation decreases with depth (R = 0.22 at 200 m), and becomes insignificant below 250 m. Thus, care must be taken in using SST as a proxy for heat content, as AW mainly resides in these deeper layers.
Citation: Sutherland, D. A., F. Straneo, G. B. Stenson, F. J. M. Davidson, M. O. Hammill, and A. Rosing-Asvid (2013), Atlantic water variability on the SE Greenland continental shelf and its relationship to SST and bathymetry, Journal of Geophysical Research-Oceans, 118, 847–855, doi:10.1029/2012JC008354.
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Biological/Ecological Impacts
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Megafaunal Communities in Rapidly Warming Fjords along the West Antarctic Peninsula: Hotspots of Abundance and Beta Diversity
Abstract: Glacio-marine fjords occur widely at high latitudes and have been extensively studied in the Arctic, where heavy meltwater inputs and sedimentation yield low benthic faunal abundance and biodiversity in inner-middle fjords. Fjord benthic ecosystems remain poorly studied in the subpolar Antarctic, including those in extensive fjords along the West Antarctic Peninsula (WAP). Here we test ecosystem predictions from Arctic fjords on three subpolar, glacio-marine fjords along the WAP. With seafloor photographic surveys we evaluate benthic megafaunal abundance, community structure, and species diversity, as well as the abundance of demersal nekton and macroalgal detritus, in soft-sediment basins of Andvord, Flandres and Barilari Bays at depths of 436–725 m. We then contrast these fjord sites with three open shelf stations of similar depths. Contrary to Arctic predictions, WAP fjord basins exhibited 3 to 38-fold greater benthic megafaunal abundance than the open shelf, and local species diversity and trophic complexity remained high from outer to inner fjord basins. Furthermore, WAP fjords contained distinct species composition, substantially contributing to beta and gamma diversity at 400–700 m depths along the WAP. The abundance of demersal nekton and macroalgal detritus was also substantially higher in WAP fjords compared to the open shelf. We conclude that WAP fjords are important hotspots of benthic abundance and biodiversity as a consequence of weak meltwater influences, low sedimentation disturbance, and high, varied food inputs. We postulate that WAP fjords differ markedly from their Arctic counterparts because they are in earlier stages of climate warming, and that rapid warming along the WAP will increase meltwater and sediment inputs, deleteriously impacting these biodiversity hotspots. Because WAP fjords also provide important habitat and foraging areas for Antarctic krill and baleen whales, there is an urgent need to develop better understanding of the structure, dynamics and climate-sensitivity of WAP subpolar fjord ecosystems.
Citation: Grange, L. J., and C. R. Smith (2013), Megafaunal Communities in Rapidly Warming Fjords along the West Antarctic Peninsula: Hotspots of Abundance and Beta Diversity, PLoS ONE, 8, 11, doi:10.1371/journal.pone.0077917.
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The importance of tidewater glaciers for marine mammals and seabirds in Svalbard, Norway
Abstract: Approximately 60 % of Svalbard's land areas are glaciated at the present time. The Archipelago has more than 1,100 glaciers (> 1 km2) and 163 of these are "tidewater glaciers"– that is glaciers that terminate (with their calving front) at the sea. It has been known for a long time that these glacier front areas are important feeding areas for seabirds and marine mammals. Herein, we review current knowledge regarding the importance of these areas for these animals and reflect upon the processes that create these apparent "hotspots". Kittiwakes Rissa tridactyla, routinely dominate avian assemblages in front of glaciers in Svalbard, but fulmars Fulmarus glacialis, ivory gulls Pagophila eburnea and glaucous gulls Larus hyperboreus also contribute to aggregations, which can sometimes comprise many thousands of individuals. The birds are often found in the so-called "brown zone", which is an area in front of tidewater glaciers that is ice-free due to currents and muddy due to suspended sediments. Animals at these sites typically have their stomachs full of large zooplankton or fish. These brown zones are also foraging hot-spots for Svalbard's ringed seals (Pusa hispida) and white whales (Delphinapterus leucas). Prime breeding habitat for ringed seals in Svalbard occurs deep in the fjords where ice pieces calved from the glacier fronts become frozen into land-fast sea-ice, promoting the accumulation of snow to a depth suitable for ringed seal females to dig out birth lairs above breathing holes in the ice. These pupping areas are important hunting areas for polar bears (Ursus maritimus) in spring, especially female bears with cubs of the year during the period following emergence from the winter/birthing den. Glacier-ice pieces floating in coastal areas are also important for all seal species in the region as dry platforms during moulting and also as general resting platforms for both birds and seals. During the last decade there have been several years with a complete lack of spring sea ice in many of the fjords along the west coast of Spitsbergen. During the spring periods in these years, bearded seals (Erignathus barbatus) have replaced their regular sea-ice platform with glacier ice, using it as a solid substrate for both birthing and nursing as well as general resting. The mechanisms that create foraging hotspots at the fronts of tidewater glaciers are related to the massive subsurface plumes of freshwater discharged from the glacier fronts. As these plumes rise towards the surface they entrain large volumes of ambient water, tens to hundreds of times the original discharge volume. This water is drawn from all depth levels as the plume ascends. This entrainment ensures a continuous resupply of intermediate depth waters from the outer parts of the fjords towards the glacier front and greatly amplifies the general estuarine circulation. The intermediate water masses carry plankton from a broad area, including the outer fjord, into the glacier front area, where they get entrained in the plume rising towards the surface, and often become stunned or die from freshwater osmotic shock. These small animals fall easy prey to the surface feeding predators. Large, strong swimming marine zooplankton species can sometimes escape by swimming below the inflow of marine water. But, they then become concentrated in a water layer near the bottom, making them of interest and susceptible to predators. The intermediate water masses also bring nutrients towards the glacier fronts where they are transported up to the surface layer where they can subsequently be utilized for post-bloom primary production. However, this tends to have greatest influence some distance away from the glacier front, when much of the outflow sediment has settled out. Currently, the mass balance for Svalbard glaciers is negative and climate change predictions for the future suggest continued warming, and hence continued glacial retreat. This will result in a reduction in both the number of glaciers calving into the ocean in Svalbard, and also a reduction in the total length of calving fronts around the Archipelago. Similar to the retraction of the northern sea-ice edge (which is another diminishing foraging hot-spot for these same arctic vertebrates), the climate-warming-induced changes in glaciers will likely lead to substantial distributional shifts and abundance reductions for many arctic species.
Citation: Lydersen, C., P. Assmy, S. Falk-Petersen, J. Kohler, K. M. Kovacs, M. Reigstad, H. Steen, H. Strøm, A. Sundfjord, Ø. Varpe, W. Walczowski, J. M. Weslawski, and M. Zajaczkowski (2013), The importance of tidewater glaciers for marine mammals and seabirds in Svalbard, Norway, Journal of Marine Systems, doi: 10.1016/j.jmarsys.2013.09.006.
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