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Glacier ice-ocean interactions
Glaciers that terminate in the ocean (i.e., marine-terminating or tidewater glaciers) lose mass at their marine margins through iceberg calving, meltwater runoff and submarine melting. The stability of a glacier is dependent on a balance between mass entering and exiting a catchment. In contrast with land-terminating glaciers, marine-terminating glaciers are able to undergo rapid changes in mass because changes in ocean forcing can lead to rapid changes in the calving and/or submarine melt rates, which can influence glacier flow dynamics by controlling the location and shape of the glacier terminus (which can be grounded or floating). The location and shape of the terminus influences the balance of stresses controlling ice flow; thus, changes in ice-ocean interactions have the potential to strongly influence a glacier’s mass balance.
The magnitude and timing of iceberg calving can be quantified using remote sensing and in situ observations (e.g., satellite imagery, time lapse photography, scanning lidar, and seismic measurements), however the processes controlling calving are poorly understood. Observations and modeling suggest that calving is a two-stage process: (1) fracture/detachment and (2) seaward transport. Transport of the detached ice can be influenced by both the thickness of the glacier relative to the depth of the neighboring ocean and the rigidity of ice mélange (a mixture of sea ice and icebergs).
Due to the difficulty in predicting the magnitude and timing of calving events, the proglacial environment of a tidewater glacier is not easily accessible and direct measurements of submarine melting and subglacial runoff are difficult to collect. Numerical and physical models used to look at this relationship have found that the enhancement of submarine melting occurs when cold fresh buoyant meltwater from the subglacial system entrains warm saline ocean water as it moves up the glacier terminus towards the ocean surface. As such, the magnitude of submarine melting will vary with both the ocean water temperature and the strength of the rising subglacial meltwater plume. Estimated melt rates suggest that submarine melting can be on the order of meters per day for some glaciers but vary widely between glaciers. It has also been suggested that an increase in submarine melt rates in the 1990s and 2000s may have triggered the recent rapid changes in ice flow at numerous outlet glaciers draining the Greenland and West Antarctic ice sheets. However, the construction of submarine melt rate time series is hindered by the scarcity of in situ hydrographic observations and the limitations of remote sensing techniques, preventing a thorough analysis of temporal changes in submarine melting with respect to changes in glacier behavior.
Tidewater glacier stability and the impact of tidewater glaciers on their environment is complex due to the numerous feedbacks occurring in the glacier ice-ocean system. The impact of changes in the glacier-ocean system is not only limited to sea level rise, but also smaller scale changes such as local biologic communities that rely on calved icebergs for breeding or nutrient-rich meltwater plumes for food. Thus, it is imperative that research efforts continue to focus on developing a better understanding of glacier ice-ocean interactions.
A great overview of the current understanding of glacier ice-ocean interactions, critical knowledge gaps, and recommended research plans/objectives can be found in the report published by the US CLIVAR Greenland Ice Sheet-Ocean Interactions Working Group found by clicking here.
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Sea ice-ocean interactions
Sea ice is a key indicator of the global climate change. Recent decades have been marked by rapid sea ice decline in the Arctic Ocean. In contrast, no significant decrease in Southern Ocean sea ice has been observed. Although the widespread changes in sea ice are concurrent with changing atmospheric and oceanographic conditions, sea ice-climate models are generally not capable of properly simulating the observed variability of Arctic and Antarctic sea ice cover. The failure of these models is likely due to the poor understanding of processes governing sea ice-ocean interactions. As such, a better understanding of sea ice-ocean interactions and an improved parameterization in climate and sea ice in Earth system models must be developed.
An important aspect of sea ice-ocean interactions that warrants further exploration and model development is the interaction of sea ice and the ocean with the atmospheric boundary layer. The formation and melting of sea ice in the polar regions are critical processes that must be included in Earth system models because the associated heat, moisture, momentum and gas exchanges at the ocean-sea ice-atmosphere interface are strongly influenced by changes in sea ice cover. Additionally, changes in sea ice cover influence the penetration of solar radiation and wind-induced turbulent mixing of the upper ocean layer, which will influence the biogeochemical cycling and ecosystem functioning in the upper ocean layer and lower atmosphere.
It is important to remember that the current state of knowledge regarding sea ice-ocean interactions was developed over the past several decades; a time period marked by a shift from relatively stable sea ice cover to the most recent period of declining sea ice extent and thickness. As such, the recent changes in sea ice extent, thickness, and distribution (particularly in the Arctic Ocean) have revealed knowledge gaps that must be addressed by the scientific community in a timely manner. For example, the recent transition from widespread multi-year sea ice towards predominantly first-year ice in the Arctic may enable the penetration of a more solar radiation beneath the ice than observed in the past. Consequently, the upper-ocean warming associated with increased penetration of solar radiation can contribute to enhanced melting of sea ice, further enhancing absorption of solar radiation by the upper ocean layer (i.e., positive feedback loop). Changes in sea ice cover and associated changes in the upper ocean temperature and salinity can lead to changes in sea ice algae/phytoplankton productivity, which will in turn influence the marine food web and carbon cycling. Interconnections such as these make the study of sea ice-ocean interactions an inherently multidisciplinary task.
Ice-ocean interactions
Within the past few decades, global climate change has lead to widespread changes in the mass and aerial extent of the Earth's glaciers, ice sheets, and sea ice as well as changes in the temperature and circulation of the oceans. This research feature focuses on the interactions between glaciers, ice sheets, sea ice and the oceans, the feedbacks inherent to the coupled systems, and the broader impacts of changes in ice-ocean interactions. The following two sections provide a brief overview of ice-ocean interactions at/near glacier margins and beneath sea ice.
Three webinars on ice-ocean interactions were held during February 2014 and recordings of the webinars can be found below. These webinars focused on the current understanding of ice-ocean interactions, the governing processes, and recent observations of rapid change, with a specific focus on the following three subject areas: sea ice in the Arctic, marine-terminating glaciers, and glacial fjords. Background information on glacier ice- and sea ice-ocean interactions can also be found in the overview sections included below. A more detailed understanding of ice-ocean interactions can be gained from the journal articles listed beneath the overviews. The abstracts and full citation information can be used to easily locate the full-length articles using either the journal links provided near the bottom of the page or electronic library resources.
If you have any questions regarding the content of this research feature or you would like to recommend another article for the list below, please contact This email address is being protected from spambots. You need JavaScript enabled to view it..
This page was put together by Dr. Ellyn M. Enderlin and Dr. Alexey Pavlov.
Webinars
Video recordings are posted on the APECS Vimeo site (site link). Direct links to each video recording can also be found with their descriptions below.
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Ice-Ocean Interactions Part 1: Sea ice in the Arctic
Panelist: Dr. Geli Renner (This email address is being protected from spambots. You need JavaScript enabled to view it.)
Dr. Angelika Renner from the Norwegian Polar Institute provided an overview of sea ice-ocean interactions, with a focus on sea ice in the Arctic. Following the presentation of introductory/background information, she presented the methodology and results for her recent research on turbulent heat flux between Arctic sea ice and the underlying upper ocean layer. -
Ice-Ocean Interactions Part 2: Marine-terminating Glaciers
Panelist: Dr. Gordon Hamilton (This email address is being protected from spambots. You need JavaScript enabled to view it.)
Dr. Gordon Hamilton from the University of Maine provided background on recent changes in marine-terminating glacier behavior and potentional ocean triggering mechanisms. The presentation focused on research performed at the margins of the Greenland Ice Sheet, where the majority of the research on glacier ice-ocean interactions has been executed and where Dr. Hamilton has conducted a suite of research projects. -
Ice-Ocean Interactions Part 3: Glacial fjord circulation
Panelist: Dave Sutherland (This email address is being protected from spambots. You need JavaScript enabled to view it.)
Dr. David Sutherland from the University of Oregon provided an overview of ice-ocean interactions in glacial fjords, with a fcous on the controls of fjord circulation and temperature structure. Current techniques used to study glacial fjords and their limitations were also presented in detail.
Glacier ice-ocean interactions: Journal articles
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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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Sea ice-ocean interactions: Journal articles
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Absorption, Reflection, Transmission of Solar Radiation
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Radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008
Abstract: The extent of snow cover and sea ice in the Northern Hemisphere has declined since 1979, coincident with hemispheric warming and indicative of a positive feedback of surface reflectivity on climate. This albedo feedback of snow on land has been quantified from observations at seasonal timescales, and century-scale feedback has been assessed using climate models. However, the total impact of the cryosphere on radiative forcing and albedo feedback has yet to be determined from measurements. Here we assess the influence of the Northern Hemisphere cryosphere on Earth's radiation budget at the top of the atmosphere—termed Cryosphere radiative forcing—by synthesizing a variety of remote sensing and field measurements. We estimate mean Northern Hemisphere forcing at -4.6 to -2.2 W m-2, with a peak in May of -9.0 ± 2.7 W m-2. We find that cyrospheric cooling declined by 0.45 W m-2 from 1979 to 2008, with nearly equal contributions from changes in land snow cover and sea ice. On the basis of these observations, we conclude that the albedo feedback from the Northern Hemisphere Cryosphere falls between 0.3 and 1.1 W m-2 K-1, substantially larger than comparable estimates obtained from 18 climate models.
Citation: Flanner, M. G., K. M. Shell, M. Barlage, D. K. Perovich, and M. A. Tschudi (2011), Radiative forcing and albedo feedback from the Northern Hemisphere cryosphere between 1979 and 2008, Nature Geoscience, 4, 151-155, doi: 10.1038/ngeo1062.
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The spatial distribution of solar radiation under a melting Arctic sea ice cover
Abstract: The sea ice cover of the Chukchi and Beaufort Seas is currently undergoing a fundamental shift from multiyear ice to first-year ice. Field observations of sea ice physical and optical properties were collected in this region during June–July 2010, revealing unexpectedly complex spatial distributions of solar radiation under the melt-season ice cover. Based on our optical measurements of first-year ice, we found the under-ice light field in the upper ocean to be spatially heterogeneous and dependent on wavelength, ice thickness, and the areal and geometric distribution of melt ponded and bare ice surfaces. Much of the observed complexity in radiation fields arose because the transmission of light through ponded ice was generally an order of magnitude greater than through bare, unponded ice. Furthermore, while many sites exhibited a consistent, exponential decay in light transmission through both ponded and bare ice surfaces, light transmission under bare ice was also observed to increase with depth (reaching maximum values ∼5–10 m below the bottom of the ice). A simple geometric model shows these transmission peaks are a result of scattering in the ice and the interspersion of bare and ponded sea ice surfaces. These new observations of complex radiation fields beneath melt-season first-year sea ice have significant implications for biological production, biogeochemical processes, and the heat balance of sea ice and under-ice ocean waters and should be carefully considered when modeling these sea ice-related phenomena.
Citation: Frey, K. E., D. K. Perovich, and B. Light (2011), The spatial distribution of solar radiation under a melting Arctic sea ice cover, Geophysical Research Letters, 38, L22501, doi:10.1029/2011GL049421.
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Energy budget of first-year Arctic sea ice in advanced stages of melt
Abstract: During an 8 day drift in July–August 2012 in the Nansen Basin, all components of the energy budget of melting first-year sea ice were observed. Absorption of solar radiation by the ice and ponds was the largest source of energy to the ice at almost all times during the drift. However, oceanic heat flux also provided significant heating and dominated during one wind event. Longwave fluxes provided a relatively small cooling effect, and atmospheric heat fluxes were negligible. The aggregate scale albedo of this younger, thinner ice was significantly lower than at Surface Heat Budget of the Arctic Ocean (SHEBA), and the transmittance was significantly higher here, despite similar pond and open water fractions. The oceanic heat flux was only half of the solar flux through the ice to the water, producing warm water near the surface that might delay ice growth in autumn, an important effect of the transition to thinner first-year ice in the high Arctic.
Citation: Hudson, S. R., M. A. Granskog, A. Sundfjord, A. Randelhoff, A. H. H. Renner, and D. V. Divine (2013), Energy budget of first-year Arctic sea ice in advanced stages of melt, Geophysical Research Letters, 40, 2679–2683, doi:10.1002/grl.50517.
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Changes in Arctic sea ice result in increasing light transmittance and absorption
Abstract: Arctic sea ice has declines and become thinner and younger (more seasonal) during the last decade. Once consequence of this is that the surface energy budget of the Arctic Ocean is changing. While the role of surface albedo has been studied intensively, it is still widely unknown how much light penetrates through sea ice into the upper ocean, affecting sea-ice mass balance, ecosystems, and geochemical processes. HEre we present the first large-scale under-ice light measurement, operating spectral radiometers on a remotely operated vehicle (ROV) under Arctic sea ice in summer. This data set is used to produce an Arctic-wide map of light distribution under summer sea ice. Our results show that transmittance through first-year ice (FYI, 0.11) was almost thee times larger than through multi-year ice (MYI, 0.04), and that this is mothly caused by the larger melt-pond covergae of FYI (42 vs 23%). Also, energy absorption was 50% larger in FYI than in MYI. Thus, a continuation of the observed sea-ice changes will increase the amount of light penetrating into the Arctic Ocean, enhancing sea-ice melt and affecting sea-ice and upper-ocean ecosystems.
Citation: Nicolaus, M., C. Katlein, J. Maslanik, and S. Hendricks (2012), Changes in Arctic sea ice result in increasing light transmittance and absorption, Geophysical Research Letters, 39, L24501, doi:10.1029/2012GL053738.
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Variability of light transmission through Arctic land-fast sea ice during spring
Abstract: The amount of solar radiation transmitted through Arctic sea ice is determined by the thickness and physical properties of snow and sea ice. Light transmittance is highly variable in space and time since thickness and physical properties of snow and sea ice are highly heterogeneous on variable time and length scales. We present field measurements of under-ice irradiance along transects under undeformed land-fast sea ice at Barrow, Alaska (March, May, and June 2010). The measurements were performed with a spectral radiometer mounted on a floating under-ice sled. The objective was to quantify the spatial variability of light transmittance through snow and sea ice, and to compare this variability along its seasonal evolution. Along with optical measurements, snow depth, sea ice thickness, and freeboard were recorded, and ice cores were analyzed for chlorophyll a and particulate matter. Our results show that snow cover variability prior to onset of snow melt causes as much relative spatial variability of light transmittance as the contrast of ponded and white ice during summer. Both before and after melt onset, measured transmittances fell in a range from one third to three times the mean value. In addition, we found a twentyfold increase of light transmittance as a result of partial snowmelt, showing the seasonal evolution of transmittance through sea ice far exceeds the spatial variability. However, prior melt onset, light transmittance was time invariant and differences in under-ice irradiance were directly related to the spatial variability of the snow cover.
Citation: Nicolaus, M., C. Petrich, S. R. Hudson, and M. A. Granskog (2013), Variability of light transmission through Arctic land-fast sea ice during spring, The Cryosphere, 7, 977-986, doi:10.5194/tc-7-977-2013.
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Methylmercury photodegradation influenced by sea-ice cover in Arctic marine ecosystems
Abstract: Atmospheric deposition of mercury to remote areas has increased threefold since pre-industrial times. Mercury deposition is particularly pronounced in the Arctic. Following deposition to surface oceans and sea ice, mercury can be converted into methylmercury, a biologically accessible form of the toxin, which biomagnifies along the marine food chain. Mass-independent fractionation of mercury isotopes accompanies the photochemical breakdown of methylmercury to less bioavailable forms in surface waters. Here we examine the isotopic composition of mercury in seabird eggs collected from colonies in the North Pacific Ocean, the Bering Sea and the western Arctic Ocean, to determine geographical variations in methylmercury breakdown at northern latitudes. We find evidence for mass-independent fractionation of mercury isotopes. The degree of mass-independent fractionation declines with latitude. Foraging behaviour and geographic variations in mercury sources and solar radiation fluxes were unable to explain the latitudinal gradient. However, mass-independent fractionation was negatively correlated with sea-ice cover. We conclude that sea-ice cover impedes the photochemical breakdown of methylmercury in surface waters, and suggest that further loss of Arctic sea ice this century will accelerate sunlight-induced breakdown of methylmercury in northern surface waters.
Citation: Point, D., J. E. Sonke, R. D. Day, D. G. Roseneau, K. A. Hobson, S. S. Vander Pol, A. J. Moors, R. S. Pugh, O. F. X. Donard, and P. R. Becker (2011), Methylmercury photodegradation influenced by sea-ice cover in Arctic marine ecosystems, Nature Geoscience, 4, 188-194, doi: 10.1038/ngeo1049.
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Sea Ice Extent & Thickness
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Estimation of sea ice thickness in Ross and Weddell Seas from SSMI/I brightness temperatures
Abstract: In polar regions, ocean–atmosphere interactions are strongly influenced by sea ice and its thickness. Since satellite passive microwave observations became available in the 1970s, significant progress has been made in the study of snow depth and sea ice concentration and extent in these regions. Estimating sea-ice thickness (SIT), instead, turned out to be considerably more difficult. We present a new empirical algorithm to estimate SIT in the Ross and Weddell Seas from Special Sensor Microwave/Imager brightness temperatures. This algorithm combines brightness temperature polarization difference and ratio values to obtain SIT for seasonal ice up to a thickness of about 90 cm during freezing conditions. A series of filters accounts for open water, new ice, and snow on sea ice. Our SIT estimates are consistent with colocated visual ship-based SIT observations made according to the Antarctic Sea Ice Processes and Climate project, showing linear correlation values between 0.73 and 0.96 and root-mean-square-error values between 14 and 24 cm. The seasonal development of the region average SIT derived with our approach agrees with the corresponding values derived from U.S. National Ice Center ice charts. Comparison with collocated polynya distribution maps suggests that the algorithm could be optimized for its performance with regard to SIT values around 50 cm and that a closer investigation of the snow impact on the SIT retrieval is required.
Citation: Aulicino G., G. Fusco, S. Kern, and G. Budillon (2013), Estimation of sea ice thickness in Ross and Weddell Seas from SSM/I brightness temperatures, TGRS, 52, 8, doi: 10.1109/TGRS.2013.2279799.
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On the 2012 record low Arctic sea ice cover: Combined impact of preconditioning and an August storm
Abstract: A new record low Arctic sea ice extent for the satellite era, 3.4 × 106 km2, was reached on 13 September 2012; and a new record low sea ice area, 3.0 × 106 km2, was reached on the same date. Preconditioning through decades of overall ice reductions made the ice pack more vulnerable to a strong storm that entered the central Arctic in early August 2012. The storm caused the separation of an expanse of 0.4 × 106 km2 of ice that melted in total, while its removal left the main pack more exposed to wind and waves, facilitating the main pack's further decay. Future summer storms could lead to a further acceleration of the decline in the Arctic sea ice cover and should be carefully monitored.
Citation: Parkinson, C. L., and J. C. Comiso (2013), On the 2012 record low Arctic sea ice cover: Combined impact of preconditioning and an August storm, Geophysical Research Letters, 40, 1356-1361, doi:10.1002/grl.50349.
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The impact of an intense summer cyclone on 2012 Arctic sea ice retreat
Abstract: This model study examines the impact of an intense early August cyclone of the 2012 record low Arctic sea ice extent. The cyclone passed when Arctic sea ice was thin and the simulated Arctic ice volumes had already declined ~40% from the 2007-2011 mean. The thin sea ice pack and the presence of ocean heat in the near surface temperature maximum layer created conditions that made the ice particularly vulnerable to storms. During the storm, ice volume decreased about twice as fast as usual, owing largely to a quadrupling in bottom melt caused by increased upward ocean heat transport. This increased ocean heat flux was due to enhanced mixing in the oceanic boundary layer, driven by strong winds and rapid ice movement. A comparison with a sensitivity simulation driven by reduced wind speeds during the cyclone indicates that cyclone-enhanced bottom melt strongly reduces ice extent for about 2 weeks, with a declining effect afterward. The simulated Arctic sea ice extent minimum in 2012 is reduced by the cyclone but only by 0.15× 106 km2 (4.4%). Thus, without the storm, 2012 would still have produced a record minimum.
Citation: Zhang, J., R. Lindsay, A. Schweiger, and M. Steele (2013), The impact of an intense summer cyclone on 2012 Arctic sea ice retreat, Geophysical Research Letters, 40, 720-726, doi:10.1002/grl.50190.
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The impact of winds and sea surface temperatures on the Barents Sea ice extent, a statistical approach
Abstract: An analysis was made of the processes controlling the incidence of sea ice in the Barents Sea for the period 1979-2010. The influence of atmospheric circulation and ocean temperature on the interannual variability of sea-ice extent (SIE) in the Barents Sea was investigated using sea-ice concentrations obtained from passive microwave satellite imagery, sea surface temperatures (SSTs), and NCEP-NCAR sea level pressure (SLP) data. Data from April and September were analysed, the months when SIE in the Barents Sea is respectively at its maximum and minimum. The strongest negative correlations (-0.65 to -0.77) were found between the SIE in the Barents Sea and SST in the regions most influenced by Atlantic Water. The patterns of distribution of correlation coefficients between interannual variability of SIE in the Barents Sea and SLP over the Nordic Seas and Siberia showed two well-defined SLP zones: one with a high positive correlation (0.60 to 0.65) over the Norwegian and Greenland seas, and a zone with high negative correlation (-0.60 to -0.63) in the area of western Siberia. We conclude that winds induced by changes in SLP gradient between these zones affect both the redistribution of sea ice and the advection of heat into the Barents Sea.
Citation: Pavlova, O., V. Pavlov, and S. Gerland (2014), The impact of winds and sea surface temperatures on the Barents Sea ice extent, a statistical approach, Journal of Marine Systems, 130, 248-255, doi:10.1016/j.jmarsys.2013.02.011.
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Mixing/Turbulence
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Observations of turbulent mixing and hydrography in the marginal ice zone of the Barents Sea
Abstract: Measurements of hydrography, currents, microstructure shear, and temperature were made at ice drift stations in the marginal ice zone (MIZ) of the northern Barents Sea. Highly variable mixing regimes were observed within and below the pycnocline. Elevated turbulent dissipation (5-15 × 10-7 W kg-1) was associated with strong vertical shear between the surface layer and the subsurface currents, as well as strong tidal flow over shallow topography. Dissipation in the pycnocline was enhanced at stations with strong wind forcing. During drifts under relatively calm wind and away from strong fronts and abrupt topography, station-mean dissipation values were up to a factor 50 lower and double diffusion contributed significantly to the vertical heat flux where hydrography favored diffusive layering. Independent measures of turbulent length scale from density overturns compared well with those inferred from the dissipation measurements. The variability of dissipation was better captured using a scaling by shear suggested for shelves rather than shear variance models appropriate to the deep open ocean. Sufficiently resolved patches of enhanced temperature microstructure used in combination with dissipation measurements suggest mixing efficiency of Rf ~ 0.2 for patches stable to double diffusion, comparable to the conventional upper bound of Rf ~ 0.17. Mixing efficiency for double-diffusive convection favorable cases is found to be significantly larger, Rf ~ 0.36. Water mass modification and fluxes of nutrients and dissolved carbon were found to have large local variability in accordance with the observed variability of vertical mixing in the MIZ.
Citation: Sundfjord, A., I. Fer, Y. Kasajima, and H. Svendsen (2007), Observations of turbulent mixing and hydrography in the marginal ice zone of the Barents Sea, Journal of Geophysical Research, 112, C05008, doi:10.1029/2006JC003524.
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Early spring oceanic heat fluxes and mixing observed from drift stations north of Svalbard
Abstract: From several drifting ice stations north of Svalbard, Norway, observations were made in early spring of the ocean turbulent characteristics in the upper 150 m using a microstructure profiles and close to the under-ice surface using eddy correlation instrumentation. The dataset is used to obtain average heat fluxes at the ice-water interface, in the mixed layer, across the main pycnocline, as well over different water masses in the region. The results are contrasted with proximity to the branches of the warm and saline Atlantic water current, the West Spitsbergen Current (WSC), which is the main oceanis heat and salinity source both to the region and to the Arctic Ocean. Hydrographic properties show that the surface water mass modification is typically due to atmospheric cooling with relatively less infleunce of ice melting. Surface heat fluxes of O(100) W m-2 are found within the branches of the WSC and over shelf areas with elevated levels of mixing due to strong tides. Away from the shelves and WSC, however, ocean-to-ice turbulent heat fluxes are typical of the central Arctic. Deeper in the water column, entrainment from below together with equally important horizontal advection and diffusion increase the heat content of the mixed layer and contribute to the heat flux maximum in the upper layers. The results in this study emphasize the importance of mixing along the boundaries, over shelves, and topography for the cooling of the Atlantic water layer in the Arctic in general, and for the regional heat budget, hence the ice cover and cooling of the WSC north of Svalbard, in particular.
Citation: Sirevaag, A., and I. Fer (2009), Early spring oceanic heat fluxes and mixing observed from drift stations north of Svalbard, Journal of Physical Oceanography, 39, 3049-3069, doi:10.1175/2009JPO4172.1.
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Mixing, heat fluxes and heat content evolution of the Arctic Ocean mixed layer
Abstract: A comprehensive measurement program was conducted during 16 days of a 3 week long ice pack drift, from 15 August to 1 September 2008 in the central Amundsen Basin, Arctic Ocean. The data, sampled as part of the Arctic Summer Cloud Ocean Study (ASCOS), included upper ocean stratification, mixing and heat transfer as well as transmittance of solar radiation through the ice. The observations give insight into the evolution of the upper layers of the Arctic Ocean in the transition period from melting to freezing. The ocean mixed layer was found to be heated from above and, for summer conditions, the net heat flux through the ice accounted for 22% of the observed change in mixed layer heat content. Heat was mixed downward within the mixed layer and a small, downward heat flux across the base of the mixed layer accounted for the accumulated heat in the upper cold halocline during the melt season. On average, the ocean mixed layer was cooled by an ocean heat flux at the ice/ocean interface (1.2 W m-2) and heated by solar radiation through the ice (-2.6 W m-2). An abrupt change in surface conditions halfways into the drift due to freezing and snowfall showed distinct signatures in the data set and allowed for inferences and comparisons to be made for cases of contrasting forcing conditions. Transmittance of solar radiations was reduced by 59% in the latter period. From hydrographic observations obtained earlier in the melting season, in the same region, we infer a total fresh water equivalent of 3.3 m accumulated in the upper ocean, which together with the observed saltier winter mixed layer indicates a transition towards a more seasonal ice cover in the Arctic.
Citation: Sirevaag, A., S. de la Rosa, I. Fer, M. Nicolaus, M. Tjernsttrom, and M.G. McPhee (2011), Mixing, heat fluxes and heat content evolution of the Arctic Ocean mixed layer, Ocean Science, 7, 335-349, doi:10.5194/os-7-335-2011.
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Influences of the ocean surface mixed layer and thermohaline stratification on Arctic Sea ice in the central Canada Basin
Abstract: Variations in the Arctic central Canada Basin mixed layer properties are documented based on a subset of nearly 6500 temperature and salinity profiles acquired by Ice-Tethered Profilers during the period summer 2004 to summer 2009 and analyzed in conjunction with sea ice observations from ice mass balance buoys and atmosphere‐ocean heat flux estimates. The July–August mean mixed layer depth based on the Ice‐Tethered Profiler data averaged 16 m (an overestimate due to the Ice‐Tethered Profiler sampling characteristics and present analysis procedures), while the average winter mixed layer depth was only 24 m, with individual observations rarely exceeding 40 m. Guidance interpreting the observations is provided by a 1‐D ocean mixed layer model. The analysis focuses attention on the very strong density stratification at the base of the mixed layer in the Canada Basin that greatly impedes surface layer deepening and thus limits the flux of deep ocean heat to the surface that could influence sea ice growth/ decay. The observations additionally suggest that efficient lateral mixed layer restratification processes are active in the Arctic, also impeding mixed layer deepening.
Citation: Toole, J. M., M. -L. Timmermans, D. K. Perovich, R. A. Krishfield, A. Proshutinsky, and J. A. Richter-Menge (2010), Influences of the ocean surface mixed layer and thermohaline stratification on Arctic Sea ice in the central Canada Basin, Journal of Geophysical Research, 115, C10018, doi:10.1029/2009JC005660.
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Biological/Ecological Importance
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Massive Phytoplankton Blooms Under Arctic Sea Ice
Abstract: Science Brevia do not contain abstracts.
Citation: Arrigo, K. R. et al. (2012), Massive Phytoplankton Blooms Under Arctic Sea Ice, Science, 336, doi: 10.1126/science.1215065.
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Floating Ice-Algal Aggregates below Melting Arctic Sea Ice
Abstract: During two consecutive cruises to the Eastern Central Arctic in late summer 2012, we observed floating algal aggregates in the melt-water layer below and between melting ice floes of first-year pack ice. The macroscopic (1-15 cm in diameter) aggregates had a mucous consistency and were dominated by typical ice-associated pinnate diatoms embedded within the mucous matrix. Aggregates maintained buoyancy and accumulated just above a strong pycnocline that separated meltwater and seawater layers. We were able, for the first time, to obtain quantitative abundance and biomass estimates of these aggregates. Although their biomass and production on a square metre basis was small compared to ice-algal blooms, the floating ice-algal aggregates supported high levels of biological activity on the scale of the individual aggregate. In addition they constituted a food source for the ice-associated fauna as revealed by pigments indicative of zooplankton grazing, high abundance of naked ciliates, and ice amphipods associated with them. During the Arctic melt season, these floating aggregates likely play an important ecological role in an otherwise impoverished near-surface sea ice environment. Our findings provide important observations and measurements of a unique aggregate-based habitat during the 2012 record sea ice minimum year.
Citation: Assmy P., J. K. Ehn, M. Fernández-Méndez, H. Hop, C. Katlein, et al. (2013), Floating Ice-Algal Aggregates below Melting Arctic Sea Ice, PLoS ONE, 8, 10, doi:10.1371/journal.pone.0076599
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Diatoms and biomarkers evidence for major changes in sea ice conditions prior the instrumental period in Antarctic Peninsula
Abstract: The Antarctic Peninsula (AP) has been identified as one of the most rapidly warming region on Earth. Satellite monitoring currently allows for a detailed understanding of the relationship between sea ice extent and duration and atmospheric and oceanic circulations in this region. However, our knowledge on ocean-ice-atmosphere interactions is still relatively poor for the period extending beyond the last 30 years. Here, we describe environmental conditions in Northwestern and Northeastern Antarctic Peninsula areas over the last century using diatom census counts and diatom specific biomarkers (HBIs) in two marine sediment multicores (MTC-38C and -18A, respectively). Diatom census counts and HBIs show abrupt changes between 1935 and 1950, marked by ocean warming and sea ice retreat in both sides of the AP. Since 1950, inferred environmental conditions do not provide evidence for any trend related to the recent warming but demonstrate a pronounced variability on pluri-annual to decadal time scale. We propose that multi-decadal sea ice variations over the last century are forced by the recent warming, while the annual-to-decadal variability is mainly governed by synoptic and regional wind fields in relation with the position and intensity of the atmospheric low-pressure trough around the AP. However, the positive shift of the SAM since the last two decades cannot explain the regional trend observed in this study, probably due to the effect of local processes on the response of our biological proxies.
Citation: Barbara, L., X. Crosta, S. Schmidt, and G. Massé (2010), Diatoms and biomarkers evidence for major changes in sea ice conditions prior the instrumental period in Antarctic Peninsula, Quaternary Science Reviews, 79, 99-110.
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Consequences of changing sea-ice cover for primary and secondary producers in the European Arctic shelf seas: Timing, quantity, and quality
Abstract: The Arctic ice cover poses severe limitations on the productive period of marine autotrophs that form the base of the marine food web. Sea-ice algae begin to grow in early spring within and underneath the ice, producing a substantial biomass despite very low light intensities. Pelagic algal blooms, in contrast, normally occur after ice breakup, at high latitudes as late as July–September. The timing of these blooms is crucial for the quantity and quality of primary and secondary production, and therefore for the transfer of energy and matter to higher trophic levels. Recent findings from Rijpfjorden, north-eastern Svalbard indicate that ice algae, rather than pelagic algae, trigger the reproduction of Arctic zooplankton around Svalbard. The key herbivore in Arctic shelf seas, the copepod Calanus glacialis, timed its seasonal migration, foraging, and reproduction to the ice algal bloom, which preceded the pelagic algal bloom by two months. The growth of this secondary producer's offspring, however, was dependent on the later bloom of phytoplankton and higher sea-water temperatures. In 2007, reproduction and growth of C. glacialis and the primary production regime matched perfectly. The persistent ice cover in summer 2008, however, led to a mismatch between the pelagic algal bloom and the growth of the new copepod generation, resulting in a fivefold lower biomass of C. glacialis in August 2008 compared to 2007. The initiation of the ice algal bloom is mainly determined by the solar angle, whereas the pelagic algal bloom requires more light and is therefore governed to a larger degree by ice thinning and the unpredictable ice breakup. We conclude that both a too early as well as a too late ice breakup can cause a mismatch between primary and secondary producers, with negative consequences for the entire lipid-based Arctic marine food web.
Citation: Leu, E., J. E. Søreide, D. O. Hessen, S. Falk-Petersen, and J. Berge (2011), Consequences of changing sea-ice cover for primary and secondary producers in the European Arctic shelf seas: Timing, quantity, and quality, Progress in Oceanography, 90, 18–32, doi:10.1016/j.pocean.2011.02.004.
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Timing of blooms, algal food quality and Calanus glacialis reproduction and growth in a changing Arctic
Abstract: The Arctic bloom consists of two distinct categories of primary producers, ice algae growing within and on the underside of the sea ice, and phytoplankton growing in open waters. Long chain omega-3 fatty acids, a subgroup of polyunsaturated fatty acids (PUFAs) produced exclusively by these algae, are essential to all marine organisms for successful reproduction, growth, and development. During an extensive field study in the Arctic shelf seas, we followed the seasonal biomass development of ice algae and phytoplankton and their food quality in terms of their relative PUFA content. The first PUFA-peak occurred in late April during solid ice cover at the onset of the ice algal bloom, and the second PUFA-peak occurred in early July just after the ice break-up at the onset of the phytoplankton bloom. The reproduction and growth of the key Arctic grazer Calanus glacialis perfectly coincided with these two bloom events. Females of C. glacialis utilized the high-quality ice algal bloom to fuel early maturation and reproduction, whereas the resulting offspring had access to ample high-quality food during the phytoplankton bloom 2 months later. Reduction in sea ice thickness and coverage area will alter the current primary production regime due to earlier ice break-up and onset of the phytoplankton bloom. A potential mismatch between the two primary production peaks of high-quality food and the reproductive cycle of key Arctic grazers may have negative consequences for the entire lipid-driven Arctic marine ecosystem.
Citation: Søreide, J. E., E. Leu, J. Berge, M. Graeve, and S. Falk-Petersen (2010), Timing of blooms, algal food quality and Calanus glacialis reproduction and growth in a changing Arctic, Global Change Biology, 16, 3154–3163, doi: 10.1111/j.1365-2486.2010.02175.x.
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Journals
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General
Antarctic, Arctic, and Alpine Research
Bulletin of the American Meteorological Society
Earth and Planetary Science Letters
IEEE Transactions on Geoscience and Remote Sensing
Journal of Geophysical Research
Proceedings of the National Academy of Sciences the United States of America
Progress in Physical Geography
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Ice
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Oceans