The role of tides in ocean--ice-shelf interactions in the southwestern Weddell Sea

A novel regional ocean-sea-ice model configuration is designed to investigate the mechanisms of ocean&#8211;ice-shelf-melt interactions in the Weddell Sea. It features explicit resolution of the cavities of eastern Weddell, Larsen and Filchner-Ronne ice-shelves (FRIS, at 1.5-2.5 km horizontal resolution), as well as of the adjacent continental shelves (~2.5 km) and deep open-ocean gyre (at 2.5-4 km), in presence of interannually-varying atmospheric and ocean boundary forcing as well as explicit ocean tides. Simulated circulation, water mass and ice-shelf melt properties compare overall well with available open-ocean and cavity observations, and simulated Weddell ice-shelf melting reveals large variability on tidal, seasonal and year-to-year timescales.&#160;The presence of ocean tides, investigated explicitly, is revealed to result in a systematic time-average enhancement of both the production of ice-shelf meltwater as well as its refreezing on ascending branches of especially the FRIS cavity circulation. This tide-driven enhancement of the melt-induced FRIS cavity circulation acts to increase net ice-shelf melting (by 50%, ~50 Gt/yr) and the meltwater export by the FRIS outflow, and modulates their seasonal and lower frequency variability.&#160;The tidal impact on ice-shelf melting is consistent with being primarily driven mechanically through enhanced kinetic energy of the time-varying flow in contact with the ice drafts. The dynamically-driven tide-induced melting is thereby to almost 90% compensated by cooling through meltwater produced, but not quickly exported from regions of melting in the Weddell cold-cavity regime. Ocean boundary layer thermal adjustment underneath ice drafts, minimizing departures from the in-situ freezing point, thus substantially dampens the impact of tides on Weddell ocean&#8211;ice-shelf interactions.&#160;Simulations furthermore suggest attendant changes on the open-ocean continental shelves, characterized by overall freshening and modest year-round sea-ice thickening, as well as a marked freshening of newly-formed bottom waters in the southwestern Weddell Sea.

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Can we reconstruct the formation of large open ocean polynyas in the Southern Ocean using ice core records?

10.5194/cp-2020-91 ◽

2020 ◽

Author(s):

Hugues Goosse ◽

Quentin Dalaiden ◽

Marie G. P. Cavitte ◽

Liping Zhang

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Climate Models ◽

Ice Core ◽

Ice Cores ◽

Open Ocean ◽

Weddell Sea ◽

High Accumulation ◽

Surface Mass ◽

The Impact

Abstract. Large open-ocean polynyas, defined as ice-free areas within the sea ice pack, have been observed only rarely over the past decades in the Southern Ocean. In addition to smaller recent events, an impressive sequence occurred in the Weddell Sea in 1974, 1975 and 1976 with openings of more than 300,000 km2 that lasted the full winter. Those big events have a huge impact on the sea ice cover, deep-water formation and more generally on the Southern Ocean and the Antarctic climate. However, we have no estimate of the frequency of the occurrence of such large open-ocean polynyas before the 1970s. Our goal here is to test if polynya activity could be reconstructed using continental records, and specifically, observations derived from ice cores. The fingerprint of big open-ocean polynyas is first described in reconstructions based on data from weather stations, in ice cores for the 1970s and in climate models. It shows a clear signal, characterized by a surface air warming and increased precipitation in coastal regions adjacent to the eastern part of the Weddell Sea where several high-resolution ice cores have been collected. The signal of isotopic composition of precipitation is more ambiguous and we thus base our reconstructions on surface mass balance records only. A first reconstruction is obtained by performing a simple average of standardized records. Given the similarity between the observed signal and the one simulated in models, we also use data assimilation to reconstruct past polynya activity. The impact of open ocean polynyas on the continent is not large enough compared to the changes due, for instance, to atmospheric variability to detect without ambiguity the polynya signal and additional observations would be required to discriminate clearly the years with and without open ocean polynya. It is thus reasonable to consider that, in these preliminary reconstructions, some high accumulation events may be wrongly interpreted as the consequence of polynya formation while some years with polynya formation may be missed. Nevertheless, our reconstructions suggest that big open ocean polynyas, such as the ones that were observed in the 1970s, are rare events, occurring at most a few times per century. Century-scale changes in polynya activity are also likely but our reconstructions are unable to assess precisely this aspect at this stage.

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Can we reconstruct the formation of large open-ocean polynyas in the Southern Ocean using ice core records?

Climate of the Past ◽

10.5194/cp-17-111-2021 ◽

2021 ◽

Vol 17 (1) ◽

pp. 111-131

Author(s):

Hugues Goosse ◽

Quentin Dalaiden ◽

Marie G. P. Cavitte ◽

Liping Zhang

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Climate Models ◽

Ice Core ◽

Ice Cores ◽

Snow Accumulation ◽

Open Ocean ◽

Weddell Sea ◽

Surface Mass ◽

The Impact

Abstract. Large open-ocean polynyas, defined as ice-free areas within the sea ice pack, have only rarely been observed in the Southern Ocean over the past decades. In addition to smaller recent events, an impressive sequence occurred in the Weddell Sea in 1974, 1975 and 1976 with openings of more than 300 000 km2 that lasted the full winter. These big events have a huge impact on the sea ice cover, deep-water formation, and, more generally, on the Southern Ocean and the Antarctic climate. However, we have no estimate of the frequency of the occurrence of such large open-ocean polynyas before the 1970s. Our goal here is to test if polynya activity could be reconstructed using continental records and, specifically, observations derived from ice cores. The fingerprint of big open-ocean polynyas is first described in reconstructions based on data from weather stations, in ice cores for the 1970s and in climate models. It shows a signal characterized by a surface air warming and increased precipitation in coastal regions adjacent to the eastern part of the Weddell Sea, where several high-resolution ice cores have been collected. The signal of the isotopic composition of precipitation is more ambiguous; thus, we base our reconstructions on surface mass balance records alone. A first reconstruction is obtained by performing a simple average of standardized records. Given the similarity between the observed signal and the one simulated in models, we also use data assimilation to reconstruct past polynya activity. The impact of open-ocean polynyas on the continent is not large enough, compared with the changes due to factors such as atmospheric variability, to detect the polynya signal without ambiguity, and additional observations would be required to clearly discriminate the years with and without open-ocean polynya. Thus, it is reasonable to consider that, in these preliminary reconstructions, some high snow accumulation events may be wrongly interpreted as the consequence of polynya formation and some years with polynya formation may be missed. Nevertheless, our reconstructions suggest that big open-ocean polynyas, such as those observed in the 1970s, are rare events, occurring at most a few times per century. Century-scale changes in polynya activity are also likely, but our reconstructions are unable to precisely assess this aspect at this stage.

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Attenuated carbon sequestration by Weddell Sea dense waters over the 21st century – an assessment with FESOM-REcoM

10.5194/egusphere-egu21-6248 ◽

2021 ◽

Author(s):

Cara Nissen ◽

Ralph Timmermann ◽

Mario Hoppema ◽

Judith Hauck

Keyword(s):

Carbon Sequestration ◽

Sea Ice ◽

Continental Shelf ◽

Bottom Water ◽

Water Mass ◽

Weddell Sea ◽

Ice Shelf ◽

Deep Waters ◽

Water Formation ◽

Water Mass Transformation

Deep and bottom water formation regions have long been recognized to be efficient vectors for carbon transfer to depth, leading to carbon sequestration on time scales of centuries or more. Precursors of Antarctic Bottom Water (AABW) are formed on the Weddell Sea continental shelf as a consequence of buoyancy loss of surface waters at the ice-ocean or atmosphere-ocean interface, which suggests that any change in water mass transformation rates in this area affects global carbon cycling and hence climate. Many of the models previously used to assess AABW formation in present and future climates contained only crude representations of ocean-ice shelf interaction. Numerical simulations often featured spurious deep convection in the open ocean, and changes in carbon sequestration have not yet been assessed at all. Here, we present results from the global model FESOM-REcoM, which was run on a mesh with elevated grid resolution in the Weddell Sea and which includes an explicit representation of sea ice and ice shelves. Forcing this model with ssp585 scenario output from the AWI Climate Model, we assess changes over the 21st century in the formation and northward export of dense waters and the associated carbon fluxes within and out of the Weddell Sea. We find that the northward transport of dense deep waters (&#963;2>37.2 kg m-3 below 2000 m) across the SR4 transect, which connects the tip of the Antarctic Peninsula with the eastern Weddell Sea, declines from 4 Sv to 2.9 Sv by the year 2100. Concurrently, despite the simulated continuous increase in surface ocean CO2 uptake in the Weddell Sea over the 21st century, the carbon transported northward with dense deep waters declines from 3.5 Pg C yr-1 to 2.5 Pg C yr-1, demonstrating the dominant role of dense water formation rates for carbon sequestration. Using the water mass transformation framework, we find that south of SR4, the formation of downwelling dense waters declines from 3.5 Sv in the 1990s to 1.6 Sv in the 2090s, a direct result of the 18% lower sea-ice formation in the area, the increased presence of modified Warm Deep Water on the continental shelf, and 50% higher ice shelf basal melt rates. Given that the reduced formation of downwelling water masses additionally occurs at lighter densities in FESOM-REcoM in the 2090s, this will directly impact the depth at which any additional oceanic carbon uptake is stored, with consequences for long-term carbon sequestration.

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Validation of atmospheric reanalyses over the Weddell Sea, Antarctica, using observations from drifting buoys

10.5194/egusphere-egu21-10127 ◽

2021 ◽

Author(s):

John King ◽

Gareth Marshall ◽

Steve Colwell ◽

Clare Allen-Sader ◽

Tony Phillips

Keyword(s):

Sea Ice ◽

Austral Summer ◽

Longwave Radiation ◽

Weddell Sea ◽

Balance Model ◽

Atmospheric Conditions ◽

Snow Layer ◽

Near Surface ◽

The Impact ◽

Drifting Buoys

&#160;Global atmospheric reanalyses are frequently used to drive ocean-ice models but few data are available to assess the quality of these products in the Antarctic sea ice zone. We utilise measurements from three drifting buoys that were deployed on sea ice in the southern Weddell Sea in the austral summer of 2016 to validate the representation of near-surface atmospheric conditions in the ERA-Interim and ERA5 reanalyses produced by the European Centre for Medium Range Weather Forecasts (ECMWF). The buoys carried sensors to measure atmospheric pressure, air temperature and humidity, wind speed and direction, and downwelling shortwave and longwave radiation. One buoy remained in coastal fast ice for most of 2016 while the other two drifted northward through the austral winter and exited the pack ice during the following austral summer. Comparison of buoy measurements with reanalysis data indicates that both reanalyses represent the surface pressure field in this region accurately. Reanalysis temperatures are, however, biased warm by around 2 &#176;C in both products, with the largest biases seen at the lowest temperatures. We suggest that this bias is a result of the simplified representation of sea ice in the reanalyses, in particular the lack of an insulating snow layer on top of the ice. We use a simple surface energy balance model to investigate the impact of the reanalysis biases on sea ice thermodynamics.

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Ice-Ocean Interaction On Ronne Ice Shelf, Antarctica

Annals of Glaciology ◽

10.1017/s026030550000906x ◽

1990 ◽

Vol 14 ◽

pp. 341

Author(s):

A. Jenkins ◽

C.S.M. Doake

Keyword(s):

Cold Water ◽

Sea Water ◽

Freezing Point ◽

Accumulation Rate ◽

Weddell Sea ◽

Shear Stresses ◽

Strain Rates ◽

Horizontal Shear ◽

Ice Shelf ◽

Ice Stream

A detailed glaciological study of Ronne Ice Shelf has been undertaken along a flowline extending from Rutford Ice Stream grounding line to the ice front. Measurements of velocity, surface elevation, ice thickness, surface temperature and accumulation rate have been made at a total of 28 sites; at 17 of these ice deformation rates are also known. Although no direct measurements of basal conditions have been made, these can be deduced from observations made at the surface. Assuming the ice shelf to be in a steady state, the basal mass balance can be calculated at points where strain-rates are known. Information on the spatial distribution of basal saline ice layers can also be obtained from radio-echo sounding data. The derived pattern of basal melting and freezing influences both the ice shelf and the underlying ocean. Vertical heat advection modifies the temperature distribution within the ice shelf, which determines its dynamic response to driving and restraining forces through the temperature-dependent ice-flow law. Using measured strain-rates and calculated temperature profiles, the restraint generated by horizontal shear stresses can be derived for points on the flowline. It is the cumulative effect of these forces which controls the discharge of grounded ice from Rutford Ice Stream. Cooling of sea-water to its pressure melting point by melting of ice at depth has two important results. The outflow of cold, dense Ice Shelf Water, produced by this mechanism, is a major source of Antarctic Bottom Water, formed as it mixes at depth with the warmer waters of the Weddell Sea (Foldvik and Gammelsrod, 1988). If the cold water is forced up to shallower depths, frazil ice will be produced as the pressure freezing point rises, resulting in basal accretion if this occurs beneath the ice shelf.

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Variability Of Thermohaline Circulation Under An Ice Shelf

Annals of Glaciology ◽

10.1017/s0260305500009009 ◽

1990 ◽

Vol 14 ◽

pp. 338

Author(s):

H.H. Hellmer

Keyword(s):

Sea Ice ◽

Boundary Conditions ◽

Thermohaline Circulation ◽

Circulation Model ◽

Melting Zone ◽

Weddell Sea ◽

Shelf Water ◽

Ice Shelf ◽

Grounding Line ◽

Ice Shelf Water

The production of Antarctic Bottom Water is mainly influenced by Ice Shelf Water, which is formed through the modification of shelf water masses under huge ice shelves. To simulate this modification a two-dimensional thermohaline circulation model has been developed for a section perpendicular to the ice-shelf edge. Hydrographic data from the Filchner Depression enter into the model as boundary conditions. In the outflow region they also serve as a verification of model results. The standard solution reveals two circulation cells. The dominant one transports shelf water near the bottom toward the grounding line, where it begins to ascend along the inclined ice shelf. The contact with the ice shelf causes melting with a maximum rate of 1.5 m a−1 at the grounding line. Freezing and therefore the accumulation of “sea ice” at the bottom of the ice shelf occurs at the end of the melting zone at a rate on the order of 0.1 ma−1. Both rates are comparable with values estimated or predicted by models concerning ice-shelf dynamics. As one example of model sensitivity to changing boundary conditions, a higher sea-ice production in the southern Weddell Sea, as might be expected for a general climatic cooling event, is assumed. The resultant decrease/ increase in temperature/salinity of the inflow (Western Shelf Water) reduces the circulation under the ice shelf and therefore the outflow of Ice Shelf Water by 40%. The maximum melting and freezing rate decreases by 0.1 ma−1 and 0.01 m a−1, respectively. and the freezing zone shifts toward the grounding line by 100 km. In general the intensity of the circulation cells, the characteristics of Ice Shelf Water, the distribution of melting and freezing zones and the melting and freezing rates differ from the standard results with changing boundary conditions. These are the temperature and salinity of the inflow, the surface temperature at the top, and the extension and morphology of the ice shelf.

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Pack-Ice Motion in the Weddell Sea in Relation to Weather Systems and Determination of a Weddell Sea Sea-Ice Budget

Annals of Glaciology ◽

10.1017/s0260305500007047 ◽

1989 ◽

Vol 12 ◽

pp. 104-112 ◽

Cited By ~ 24

Author(s):

D.W.S. Limbert ◽

S.J. Morrison ◽

C.B. Sear ◽

P. Wadhams ◽

M.A. Rowe

Keyword(s):

Sea Ice ◽

Surface Pressure ◽

Ice Cover ◽

Weddell Sea ◽

Air Pressure ◽

Ice Formation ◽

Pack Ice ◽

The Impact

As part of the Winter Weddell Sea Project 1986 (WWSP 86), a buoy, transmitting via TIROS-N satellites to Service Argos, was inserted into an ice floe in the southern Weddell Sea. Operational U.K. Meteorological Office numerical surface-pressure analyses, which utilized the buoy’s measured values of air pressure and temperature, are used to assess the impact of weather systems on pack-ice movement. The motion of the buoy is shown to be related closely to the position of the circumpolar trough and to the tracks of depressions crossing the area. The tracks of this and other buoys deployed during WWSP 86 are analysed, together with the known drifts of some ice-bound vessels, to establish the overall movement of sea ice in the central and western Weddell Sea. Using these data, the area of ice transported northward out of the Weddell Sea is determined. Roughly 60% of the winter sea-ice cover is discharged out of the area, and is replaced by new ice formation in coastal polynyas and by influx of new ice from the east. In summer, a further 30% is discharged northward out of the region, leaving 40% cover and by implication a 30% loss by melting.

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Evolution of supercooling under coastal Antarctic sea ice during winter

Antarctic Science ◽

10.1017/s0954102011000265 ◽

2011 ◽

Vol 23 (4) ◽

pp. 399-409 ◽

Cited By ~ 30

Author(s):

Gregory H. Leonard ◽

Patricia J. Langhorne ◽

Michael J.M. Williams ◽

Ross Vennell ◽

Craig R. Purdie ◽

...

Keyword(s):

Sea Ice ◽

Freezing Point ◽

Circulation Pattern ◽

Late Winter ◽

Ice Shelf ◽

Mcmurdo Sound ◽

The North ◽

Antarctic Sea Ice ◽

Ice Production

AbstractHere we describe the evolution through winter of a layer of in situ supercooled water beneath the sea ice at a site close to the McMurdo Ice Shelf. From early winter (May), the temperature of the upper water column was below its surface freezing point, implying contact with an ice shelf at depth. By late winter the supercooled layer was c. 40 m deep with a maximum supercooling of c. 25 mK located 1–2 m below the sea ice-water interface. Transitory in situ supercooling events were also observed, one lasting c. 17 hours and reaching a depth of 70 m. In spite of these very low temperatures the isotopic composition of the water was relatively heavy, suggesting little glacial melt. Further, the water's temperature-salinity signature indicates contributions to water mass properties from High Salinity Shelf Water produced in areas of high sea ice production to the north of McMurdo Sound. Our measurements imply the existence of a heat sink beneath the supercooled layer that extracts heat from the ocean to thicken and cool this layer and contributes to the thickness of the sea ice cover. This sink is linked to the circulation pattern of the McMurdo Sound.

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The sensitivity of sea ice growth to the choice of ice shelf-ocean coupling algorithms.

10.5194/egusphere-egu2020-12502 ◽

2020 ◽

Author(s):

Stefan Jendersie ◽

Alena Malyarenko

Keyword(s):

Sea Ice ◽

Ocean Model ◽

Data Sets ◽

Ice Shelf ◽

Ice Shelves ◽

Ice Growth ◽

Continental Scale ◽

Complex Models ◽

Coupling Algorithms ◽

The Impact

To quantify Antarctic ice mass loss and the subsequent sea level rise the geophysical modelling community is pushing towards frameworks that fully couple increasingly complex models of atmosphere, ocean, sea ice and ice sheets & shelves. &#160;One particular hurdle remains the accurate representation of the vertical ocean-ice interaction at the base of ice shelves. &#160;Parameterizations that are tuned to particular data sets naturally perform best in comparable ice shelf cavity environments. This poses the challenge in continental scale ocean-ice shelf models to chose one melt parameterizaton that performs sufficiently well in diverse cavity environment. &#160;Thus adding uncertainty in ice shelf induced ocean freshening crucially affects modelled sea ice growth. &#160;The impact magnitude of ice shelf supplied melt water on growth rates, thickness and extent of sea ice in the open ocean is currently debated in the literature. &#160; We reviewed and compared 16 commonly utilized melting/freezing parameterizations in coupled ocean-ice shelf models. &#160;Melt rates differ hugely, in identical idealized conditions between 0.1m/yr to 3m/yr. &#160;In this talk we present results of a realistic circum-Antarctic ice shelf and sea ice coupled ocean model (CICE, ROMS), where we look at the effects of the chosen ice shelf melt parameterization on modeled sea surface conditions and sea ice growth, regionally and circum Antarctic.

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The Weddell Sea Expedition 2019

10.5194/egusphere-egu2020-21896 ◽

2020 ◽

Author(s):

John Shears ◽

Julian Dowdeswell ◽

Freddie Ligthelm ◽

Paul Wachter

Keyword(s):

Sea Ice ◽

Autonomous Underwater Vehicles ◽

Remote Sensing Data ◽

Open Water ◽

Weddell Sea ◽

Scientific Programme ◽

Ice Shelf ◽

Remotely Operated Vehicle ◽

Radar Images ◽

Ice Conditions

The Weddell Sea Expedition 2019 (WSE) was conceived with dual aims: (i) to undertake a comprehensive international inter-disciplinary programme of science centred in the waters around Larsen C Ice Shelf, western Weddell Sea; and (ii) to search for, survey and image the wreck of Sir Ernest Shackleton&#8217;s Endurance, which sank in the Weddell Sea in 1915.&#160;The 6-week long expedition, funded by the Flotilla Foundation, required the use of a substantial ice-strengthened vessel given the very difficult sea-ice conditions encountered in the Weddell Sea, and especially in its central and western parts. The South African ship SA Agulhas II was chartered for its Polar Class 5 icebreaking capability and design as a scientific research vessel. The expedition was equipped with state-of-the-art Autonomous Underwater Vehicles (AUVs) and a Remotely Operated Vehicle (ROV) which were capable of deployment to waters more than 3,000 m deep, thus making the Larsen C continental shelf and slope, and the Endurance wreck site, accessible. During the expedition, a suite of passive and active remote-sensing data, including TerraSAR-X radar images delivered in near real-time, was provided to the ice-pilot onboard the SA Agulhas II. These data were instrumental for safe vessel navigation in sea ice and the detection and tracking of icebergs and ice floes of scientific interest.The scientific programme undertaken by the WSE was very successful and produced many new geological, geophysical, marine biological and oceanographic observations from a part of the Weddell Sea that has been little studied previously, particularly the area east of Larsen C Ice Shelf. The expedition also reached the sinking location of Shackleton&#8217;s Endurance, where the presence of open-water sea ice leads allowed the deployment of an AUV to the ocean floor to try and locate and survey the wreck. Unfortunately, SA Agulhas II later lost communication with the AUV, and deteriorating weather and sea ice conditions meant that the search had to be called off.

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