On the Conditional Frazil Ice Instability in Seawater

AbstractIt has been suggested that the presence of frazil ice can lead to a conditional instability in seawater. Any frazil forming in the water column reduces the bulk density of a parcel of frazil–seawater mixture, causing it to rise. As a result of the pressure decrease in the freezing point, this causes more frazil to form, causing the parcel to accelerate, and so on. This study uses linear stability analysis and a nonhydrostatic ocean model to study this instability. The authors find that frazil ice growth caused by the rising of supercooled water is indeed able to generate a buoyancy-driven instability. Even in a gravitationally stable water column, the frazil ice mechanism can still generate convection. The instability does not operate in the presence of strong density stratification, high thermal driving (warm water), a small initial perturbation, high background mixing, or the prevalence of large frazil ice crystals. In an unstable water column, the instability is not necessarily expressed in frazil ice at all times; an initial frazil perturbation may melt and refreeze. Given a large enough initial perturbation, this instability can allow significant ice growth. A model shows frazil ice growth in an Ice Shelf Water plume several kilometers from an ice shelf, under similar conditions to observations of frazil ice growth under sea ice. The presence of this instability could be a factor affecting the growth of sea ice near ice shelves, with implications for Antarctic Bottom Water formation.

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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

<p>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;<br>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.</p>

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Low salinity frazil ice generation at the base of a small Antarctic ice shelf

Antarctic Science ◽

10.1017/s0954102093000409 ◽

1993 ◽

Vol 5 (3) ◽

pp. 309-322 ◽

Cited By ~ 24

Author(s):

J.-L. Tison ◽

D. Ronveaux ◽

R. D. Lorrain

Keyword(s):

Interstitial Water ◽

Sea Water ◽

Freezing Point ◽

Free Water ◽

Ice Crystals ◽

Ice Shelf ◽

Apparent Contradiction ◽

Ice Shelves ◽

Frazil Ice ◽

Low Salinity

Chemical, isotopic and crystallographic characteristics of marine ice formed at the base of the Hells Gate Ice Shelf, Terra Nova Bay, allow a better understanding of the dynamics of marine ice accretion under small ice shelves. The observed properties of the different types of frazil ice found in the area immediately behind the ice shelf front, result from a progressive evolution of the individual frazil ice crystals initially accreted at the base of the ice-shelf. Basal melting caused by the descending plumes of water masses at a temperature above their local freezing point, initiates partial melting of the frazil ice crystals. This dilutes the interstitial water and initiates chemical sorting effects as diffusion proceeds from the normal sea water in the free water column to the diluted interstitial water in the loose frazil layer. Different environmental conditions will result in contrasting properties. Where the subglacial interface is sculptured with domes or inverted channels, it will favour the accumulation of thick units of frazil ice, in a calm environment, that will be further protected from convection mixing over long time periods. This will result in the formation of orbicular frazil showing c-axes at random, strong dilution and important sorting effects. On the contrary, where no channel or dome exist, or where those are already filled with frazil, rectangular or wave-like banded frazil will form with properties showing interfacial streaming effects induced by water currents. Strong c-axes concentration at a single maximum, less dilution and weaker chemical sorting effects are then observed. These findings provide a tentative explanation for the apparent contradiction between the very low salinity levels detected in marine ice at the base of ice shelves and the comparatively minor salinity fluctuations in sea water profiles near ice shelves.

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Remote Sensing of Sea-Ice Growth and Melt-Pool Evolution, Milne Ice Shelf, Ellesmere Island, Canada

Annals of Glaciology ◽

10.1017/s0260305500000525 ◽

1987 ◽

Vol 9 ◽

pp. 145-150 ◽

Cited By ~ 2

Author(s):

Martin O. Jeffries ◽

William M. Sackinger ◽

Harold V. Serson

Keyword(s):

Sea Ice ◽

Drainage System ◽

Outer Part ◽

The Arctic ◽

Ice Shelf ◽

Ice Shelves ◽

Ice Growth ◽

Radar Images ◽

Melt Pool ◽

Arctic Ice

Periodically since 1950, air photographs and SLAR images have been taken of the Arctic ice shelves. The study of air photographs and SLAR images of the outer part of Milne Ice Shelf had three aims: (1) to map losses and ice re-growth at the shelf front, (2) to map the evolution of melt pools on shelf ice and multi-year land-fast sea ice, and (3) to assess the usefulness of air photographs and SLAR for these purposes. For mapping of ice calvings and subsequent sea-ice growth, both air photographs and radar images have been used sucessfully. However, air photographs are better than radar for mapping ice-surface features. The ridge-and-trough systems that characterize the surface of the ice shelf and old sea ice are clearly visible on each type of imagery but, because of their larger scale, air photographs proved to be most useful for a study of melt-pool evolution. The orientation of the melt pools is parallel to the prevailing winds which drive water along the troughs. The drainage system evolves by a process of elongation and coalesence.

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Intercomparison of Antarctic ice-shelf, ocean, and sea-ice interactions simulated by MetROMS-iceshelf and FESOM 1.4

Geoscientific Model Development ◽

10.5194/gmd-11-1257-2018 ◽

2018 ◽

Vol 11 (4) ◽

pp. 1257-1292 ◽

Cited By ~ 9

Author(s):

Kaitlin A. Naughten ◽

Katrin J. Meissner ◽

Benjamin K. Galton-Fenzi ◽

Matthew H. England ◽

Ralph Timmermann ◽

...

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Water Mass ◽

Horizontal Resolution ◽

Ocean Model ◽

Small Scale ◽

Ice Shelf ◽

Vertical Coordinate ◽

Ice Shelves ◽

Basal Melting

Abstract. An increasing number of Southern Ocean models now include Antarctic ice-shelf cavities, and simulate thermodynamics at the ice-shelf/ocean interface. This adds another level of complexity to Southern Ocean simulations, as ice shelves interact directly with the ocean and indirectly with sea ice. Here, we present the first model intercomparison and evaluation of present-day ocean/sea-ice/ice-shelf interactions, as simulated by two models: a circumpolar Antarctic configuration of MetROMS (ROMS: Regional Ocean Modelling System coupled to CICE: Community Ice CodE) and the global model FESOM (Finite Element Sea-ice Ocean Model), where the latter is run at two different levels of horizontal resolution. From a circumpolar Antarctic perspective, we compare and evaluate simulated ice-shelf basal melting and sub-ice-shelf circulation, as well as sea-ice properties and Southern Ocean water mass characteristics as they influence the sub-ice-shelf processes. Despite their differing numerical methods, the two models produce broadly similar results and share similar biases in many cases. Both models reproduce many key features of observations but struggle to reproduce others, such as the high melt rates observed in the small warm-cavity ice shelves of the Amundsen and Bellingshausen seas. Several differences in model design show a particular influence on the simulations. For example, FESOM's greater topographic smoothing can alter the geometry of some ice-shelf cavities enough to affect their melt rates; this improves at higher resolution, since less smoothing is required. In the interior Southern Ocean, the vertical coordinate system affects the degree of water mass erosion due to spurious diapycnal mixing, with MetROMS' terrain-following coordinate leading to more erosion than FESOM's z coordinate. Finally, increased horizontal resolution in FESOM leads to higher basal melt rates for small ice shelves, through a combination of stronger circulation and small-scale intrusions of warm water from offshore.

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Observations of turbulence beneath sea ice in southern McMurdo Sound, Antarctica

Ocean Science Discussions ◽

10.5194/osd-6-1407-2009 ◽

2009 ◽

Vol 6 (2) ◽

pp. 1407-1436

Author(s):

C. L. Stevens ◽

N. J. Robinson ◽

M. J. M. Williams ◽

T. G. Haskell

Keyword(s):

Sea Ice ◽

Water Column ◽

Unit Area ◽

Supercooled Water ◽

Turbulent Energy ◽

Ice Crystals ◽

Small Scale ◽

Ice Shelf ◽

Frazil Ice ◽

Vertical Diffusivity

Abstract. The first turbulence profiler observations beneath land fast sea ice which is directly adjacent to an Antarctic ice shelf are described. The stratification in the 325 m deep water column consisted of a layer of supercooled water in the upper 40 m lying above a quasi-linearly stratified water column with a sharp step in density at mid-depth. Turbulent energy dissipation rates were on average 3×10−8 m2 s−3 with peak bin-averaged values reaching 4×10−7 m2 s−3. The local dissipation rate per unit area was estimated to be 10 mWm−2 on average with a peak of 50 mWm−2. These values are consistent with a moderate baroclinic response to the tides. The small-scale turbulent energetics lie on the boundary between isotropy and buoyancy-affected. This will likely influence the formation and aggregation of frazil ice crystals within the supercooled layer. An estimate of the vertical diffusivity of mass Kρ yields a coefficient of around 10−3 m2 s−1. Combining this estimate of Kρ with available observations of average and maximum currents suggests the layer of supercooled water can persist for a distance of ~20 km from the front of the McMurdo Ice Shelf.

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Observations of turbulence beneath sea ice in southern McMurdo Sound, Antarctica

Ocean Science ◽

10.5194/os-5-435-2009 ◽

2009 ◽

Vol 5 (4) ◽

pp. 435-445 ◽

Cited By ~ 21

Author(s):

C. L. Stevens ◽

N. J. Robinson ◽

M. J. M. Williams ◽

T. G. Haskell

Keyword(s):

Sea Ice ◽

Water Column ◽

Supercooled Water ◽

Turbulent Energy ◽

Small Scale ◽

Initial Growth ◽

Ice Shelf ◽

Mcmurdo Sound ◽

Frazil Ice ◽

Vertical Diffusivity

Abstract. The first turbulence profiler observations beneath land fast sea ice which is directly adjacent to an Antarctic ice shelf are described. The stratification in the 325 m deep water column consisted of a layer of supercooled water in the upper 40 m lying above a quasi-linearly stratified water column with a sharp step in density at mid-depth. Turbulent energy dissipation rates were on average 3×10−8 m2 s−3 with peak bin-averaged values reaching 4×10−7 m2 s−3. The local dissipation rate per unit area was estimated to be 10 m Wm−2 on average with a peak of 50 m Wm−2. These values are consistent with a moderate baroclinic response to the tides. The small-scale turbulent energetics lie on the boundary between isotropy and buoyancy-affected. This will likely influence the formation and aggregation of frazil ice crystals within the supercooled layer. The data suggest that the large crystals observed in McMurdo Sound will transition from initial growth at scales smaller than the Kolmogorov lengthscale to sizes substantially (1–2 orders of magnitude) greater than the Kolmogorov scale. An estimate of the experiment-averaged vertical diffusivity of mass Kρ yields a coefficient of around 2×10−4 m2s−1 although this increased by a factor of 2 near the surface. Combining this estimate of Kρ with available observations of average and maximum currents suggests the layer of supercooled water can persist for a distance of ~250 km from the front of the McMurdo Ice Shelf.

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Modelling the Antarctic marine cryosphere at the Last Glacial Maximum

Annals of Glaciology ◽

10.3189/2015aog69a792 ◽

2015 ◽

Vol 56 (69) ◽

pp. 425-435 ◽

Cited By ~ 12

Author(s):

Kazuya Kusahara ◽

Tatsuru Sato ◽

Akira Oka ◽

Takashi Obase ◽

Ralf Greve ◽

...

Keyword(s):

Sea Ice ◽

Last Glacial Maximum ◽

Ocean Model ◽

Glacial Maximum ◽

Ice Shelf ◽

Atlantic Sector ◽

Ice Shelves ◽

Last Glacial ◽

The Last Glacial Maximum ◽

The Last Glacial

AbstractWe estimate the sea-ice extent and basal melt of Antarctic ice shelves at the Last Glacial Maximum (LGM) using a coupled ice-shelf-sea-ice-ocean model. The shape of Antarctic ice shelves, ocean conditions and atmospheric surface conditions at the LGM are different from those in the present day; these are derived from an ice-shelf-ice-sheet model, a sea-ice-ocean model and a climate model for glacial simulations, respectively. The winter sea ice in the LGM is shown to extend up to ∼7° of latitude further equatorward than in the present day. For the LGM summer, the model shows extensive sea-ice cover in the Atlantic sector and little sea ice in the other sectors. These modelled sea-ice features are consistent with those reconstructed from sea-floor sedimentary records. Total basal melt of Antarctic ice shelves in the LGM was ∼2147 Gt a–1, which is much larger than the present-day value. More warm waters originating from Circumpolar Deep Water could be easily transported into ice-shelf cavities during the LGM because the full glacial grounding line extended to shelf break regions and ice shelves overhung continental slopes. This increased transport of warm water masses underneath an ice shelf and into their basal cavities led to the high basal melt of ice shelves in the LGM.

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A Model of Sea Ice Formation in Leads and Polynyas

Journal of Physical Oceanography ◽

10.1175/jpo-d-16-0224.1 ◽

2017 ◽

Vol 47 (7) ◽

pp. 1701-1718 ◽

Cited By ~ 6

Author(s):

Harold D. B. S. Heorton ◽

Nikhil Radia ◽

Daniel L. Feltham

Keyword(s):

Sea Ice ◽

Surface Wind ◽

Surface Wind Speed ◽

Ice Formation ◽

Ice Shelves ◽

Ice Growth ◽

Frazil Ice ◽

Sensitivity Studies ◽

The Vertical Distribution ◽

The Laptev Sea

AbstractCracks in the sea ice cover break the barrier between the ocean and atmosphere, exposing the ocean to the cold atmosphere during the winter. These cracks are known as leads within the continuous sea ice pack and polynyas near land or ice shelves. Sea ice formation starts with frazil ice crystals in supercooled waters, which grow and precipitate to the ocean surface to form grease ice, eventually consolidating into a layer of solid sea ice that grows downward. In this study, a numerical model is formulated to simulate the formation of sea ice in a lead or polynya from frazil ice to a layer of new sea ice. The simulations show the refreezing of a lead within 48 h of its opening. Grease ice covers the lead typically within 3–10 h and consolidates into sea ice within 15–30 h. This study uses its model to simulate an observed polynya event in the Laptev Sea showing the vertical distribution of frazil ice and water supercooling. Sensitivity studies are used to investigate the dependence of ice growth on the ambient environment with the surface wind speed shown to be of greatest importance to lead exposure time and total ice growth. The size and distribution of frazil crystals and the time taken for the lead to freeze over is shown to be highly dependent upon the ambient forcing and lead geometry.

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Intercomparison of Antarctic ice shelf, ocean, and sea ice interactions simulated by two models

10.5194/gmd-2017-268 ◽

2017 ◽

Author(s):

Kaitlin A. Naughten ◽

Katrin J. Meissner ◽

Benjamin K. Galton-Fenzi ◽

Matthew H. England ◽

Ralph Timmermann ◽

...

Keyword(s):

Southern Ocean ◽

Sea Ice ◽

Water Mass ◽

Horizontal Resolution ◽

Ocean Model ◽

Small Scale ◽

Ice Shelf ◽

Vertical Coordinate ◽

Ice Shelves ◽

Basal Melting

Abstract. An increasing number of Southern Ocean models now include Antarctic ice shelf cavities, and simulate thermodynamics at the ice-shelf/ocean interface. This adds another level of complexity to Southern Ocean simulations, as ice shelves interact directly with the ocean and indirectly with sea ice. Here we present the first published model intercomparison and evaluation of present-day ocean/sea-ice/ice-shelf interactions, as simulated by two models: a circumpolar Antarctic configuration of MetROMS (ROMS: Regional Ocean Modelling System coupled to CICE: Community Ice CodE) and the global model FESOM (Finite Element Sea-ice/ice-shelf Ocean Model), where the latter is run at two different levels of horizontal resolution. From a circumpolar Antarctic perspective, we compare and evaluate simulated ice shelf basal melting and sub-ice shelf circulation, as well as sea ice properties and Southern Ocean water mass characteristics as they influence the sub-ice shelf processes. Despite their differing numerical methods, the two models produce broadly similar results, and share similar biases in many cases. Both models reproduce many key features of observations, but struggle to reproduce others, such as the high melt rates observed in the small warm-cavity ice shelves of the Amundsen and Bellingshausen Seas. Several differences in model design show a particular influence on the simulations. For example, FESOM's greater topographic smoothing can alter the geometry of some ice shelf cavities enough to affect their melt rates; this improves at higher resolution, since less smoothing is required. In the interior Southern Ocean, the vertical coordinate system affects the degree of water mass erosion due to spurious diapycnal mixing, with MetROMS' terrain-following coordinates leading to more erosion than FESOM's z-coordinates. Finally, increased horizontal resolution in FESOM leads to higher basal melt rates for small ice shelves, through a combination of stronger circulation and small-scale intrusions of warm water from offshore.

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