Brash ice
Sea Ice |
GLOSSARY
This glossary contains a selection of commonly used sea ice terms, as well as a few glaciological terms pertinent to the study of sea ice. The italicised descriptions are the standard definitions published by the World Meteorological Organisation (World Meteorological Organisation. 1970. WMO sea-ice nomenclature, terminology, codes and illustrated glossary. WMO/OMM/BMO 259, TP 145. Geneva, World Meteorological Organisation).
Sea Ice
Any form of ice found at sea which has originated from the freezing of sea water.
Sea ice can be broadly described as new ice , young ice, first-year ice, and old ice. These categories broadly reflect the age of the ice and include different forms and thicknesses of ice at various stages of development:
| New ice | <10 cm thick | |||||
| Frazil ice | ||||||
| Grease ice | ||||||
| Slush | ||||||
| Shuga | ||||||
| Nilas | ||||||
| Pancake ice | (maybe >10 cm thick) | |||||
| Young ice | 10-30 cm thick | |||||
| Grey ice | 10-15 cm thick | |||||
| Grey-white ice | 15-30cm thick | |||||
| First-year ice | >30 cm thick | |||||
| Thin first-year or white ice | 30-70 cm thick | |||||
| Medium first-year ice | 70-120 cm thick | |||||
| Thick first-year ice | >120 cm thick | |||||
| Old ice | ||||||
| Second-year ice | ||||||
| Multi-year ice | ||||||
Accumulations of floating ice made up of fragments not more than 2 m across; the wreckage of other forms of ice.
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Brash ice |
Brash ice refreezing |
Brash is common between colliding floes or in regions where pressure ridges have collapsed.
Sea ice which forms and remains fast along the coast, where it is attached to the shore, to an ice wall, to an ice front, between shoals or grounded icebergs. Vertical fluctuations may be observed during changes of sea-level. Fast ice may be formed in situ from sea water or by freezing of pack ice of any age to the shore, and it may extend a few metres or several hundred kilometres from the coast. Fast ice may be more than one year old and may then be prefixed with the appropriate age category (old, second-year, or multi-year).
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Fast ice |
Unloading ship onto ice |
Tide cracks may form where the fast ice is attached to the coast, due to the rise and fall of the ice in response to fluctuating sea level. These may be sites of high ice production and heat exchange, and may allow penguins and seals living near the coast gain access to the ocean even in the middle of winter.
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Tide crack |
Tide crack |
Sea ice of not more than one winter's growth, developing from young ice; thickness (typically) 30 cm - 2 m. May be subdivided into thin first-year ice/white ice, medium first-year ice and thick first-year ice.
Thin first-year ice is 30-70 cm thick, medium first-year ice is 70-120 cm thick, and thick first-year ice is over 120 cm thick. First-year ice may be thicker than 200 cm when it is in the form of ridges. It is most unlikely that ice of this thickness could be achieved by thermodynamic processes alone due to the oceanic heat flux at the bottom of the ice cover.
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This image shows thin first-year ice along the top third, medium to thick first-year ice along the right, and new ice forming in the open water. |
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Mostly level medium-sized first-year floes. |
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Messy ridged first-year ice. |
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Mostly level first-year floes. A large ridge and be seen in the central floe. |
A floe is any contiguous piece of sea ice. Floes may be described in terms of several size categories:
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Floes less than 20 m across are called cake ice.
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| Aerial shot showing a range of floe sizes. | ||
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Any break or rupture through very close pack ice, compact pack ice, consolidated pack ice, fast ice, or a single floe resulting from deformation processes. Fractures may contain brash ice and/or be covered with nilas and/or young ice. Length may vary from a few metres to many kilometres.
Fractures, by definition, are narrower than leads and may not aid navigation of surface vessels. |
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Fine spicules or plates of ice, suspended in water.
Frazil ice formation represents the first stage of sea ice growth. The frazil crystals are usually suspended in the top few centimetres of the surface layer of the ocean and give the water an oily appearance. In the open ocean the crystals may form, or be stirred to a depth of several metres by wave-induced turbulence. |
| Frazil ice gives the water an oily appearance. |
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A later stage of freezing than frazil ice when the crystals have coagulated to form a soupy layer on the surface. Grease ice reflects little light, giving the surface a matt appearance.
Grease ice behaves in a viscous fluid-like manner, and does not form distinct ice floes. |
Young ice 10-15 cm thick. Less elastic than nilas and breaks on swell. Usually rafts under pressure.
Young ice 15-30 cm thick. Under pressure more likely to ridge than to raft.
| Thin grey-white ice showing the effects of ridging and rafting. |
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A massive piece of ice of greatly varying shape, more than 5 m above sea-level, which has broken away from a glacier (or an ice shelf), and which may be afloat or aground. Icebergs may be described as tabular, dome-shaped, sloping, pinnacled, weathered or glacier bergs (an irregularly shaped iceberg).
Icebergs are not sea ice. They originate from the ice mass of the Antarctic continent that has accumulated over many thousands of years. When they melt they add fresh water to the ocean. |
Any fracture or passage-way through sea ice which is navigable by surface vessels.
A more general description of a lead is an area of open water or new ice between ice floes, although the term is generally applied to linear features. If the open area is very large it may be called a polynya.
A lead between the shore and the pack ice is called a coastal lead or shore lead, and a lead between the fast ice and the pack ice is called a flaw lead.
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Leads permit large transports of heat and mass between the ocean and the atmosphere, visualised in this image by the water vapour rising from the open water (sea smoke). |
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Deep within the pack leads provide vital access to the ocean for seals and penguins, and breathing holes for whales. In this scene a pod of minke whales were taking turns coming to the surface through this lead to breath. |
The marginal ice zone (MIZ) is not a very well defined term and it is useful for an author to be specific about its meaning when using it. Probably the most accepted definition is that of Wadhams (1986) who describes it as "that part of the ice cover which is close enough to the open ocean boundary to be affected by its presence". This definition is open to interpretation, but is generally applied to that region of the pack which is significantly affected by ocean swell. In the Antarctic this region may extend hundreds of kilometres from the ice edge, and in some regions right to the coast. The MIZ is an area of enhanced ice drift, deformation and divergence.
Old ice up to 3 m or more thick which has survived at least two summers' melt. Hummocks (hillocks of broken ice that have been forced up by pressure) even smoother than in second-year ice, and the ice is almost salt-free. Colour, where bare, is usually blue. Melt pattern consists of large interconnecting irregular puddles and a well-developed drainage system.
Multi-year ice is less common in the Antarctic than the Arctic, and is usually confined to the western Weddell Sea and isolated embayments at other locations around the coast. Ocean currents and the atmospheric circulation result in a net divergence of sea ice around the continent, causing most of the ice to melt in the summer as it drifts into warmer waters, or as the upper ocean heats up as the open water areas within the pack absorb solar radiation.
The Weddell Sea accounts for about 80% of the multi-year ice in the Antarctic. The clockwise circulating current known as the Weddell Gyre is responsible for trapping sea ice along the eastern side of the Antarctic Peninsula, allowing it to survive for more than one year. Eventually the current transports the ice further north where it dissipates into the ocean and melts.
Melt patterns on multi-year ice are a feature most commonly observed in the Arctic. The ablation season of Antarctic sea ice is rarely associated with the presence of melt water on the surface of the ice.
A general term for recently formed ice which includes frazil ice, grease ice, slush and shuga.
A thin elastic crust of ice, easily bending on waves and swell and under pressure, thrusting in a pattern of interlocking "fingers" (finger rafting). Has a matt surface and is up to 10 cm in thickness. May be subdivided into dark nilas and light nilas.
Dark nilas is < 5 cm thick and very dark in colour. Light nilas is 5-10 cm thick and reflects proportionately more light than dark nilas, depending on its thickness.
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Vast sheets of nilas |
Term used in a wide sense to include any area of sea ice, other than fast ice, no matter what form it takes or how it is disposed.
The pack can be described as very open (with an ice concentration of 1/10 to 3/10), open (4/10 to 6/10, with many leads and polynyas and the floes generally not in contact with one another), close (7/10 to 8/10, composed of floes mostly in contact), very close (9/10 to less than 10/10), and compact (10/10, with no water visible, called consolidated pack ice if the floes are frozen together).
Predominantly circular pieces of ice from 30 cm - 3 m in diameter, and up to 10 cm in thickness (unrafted), with raised rims due to the pieces striking against one another. It may be formed on a slight swell from grease ice, shuga or slush or as the result of the breaking of ice rind, nilas or, under severe conditions of swell or waves, of grey ice.
A common process of sea ice development in the Antarctic is "the pancake cycle". The pancakes start with a diameter of tens of centimetres, but through wind and wave action they aggregate with loose frazil crystals to increase in diameter, and raft with other pancakes to increase in thickness. In this manner the pancakes can rapidly grow to a few metres in diameter and up to 1 m thick. Eventually the pancakes can freeze together into larger floes or a consolidated ice cover.
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Pancake ice can rapidly cover vast areas within the pack. The characteristic upturned edges of the pancakes result from continuous collisions. |
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Small pancakes welding together to form larger floes. |
Any non-linear shaped opening enclosed in ice. Polynyas may contain brash ice and/or be covered with new ice, nilas or young ice; submariners refer to these as skylights. Sometimes the polynya is limited on one side by the coast and is called a shore polynya or by fast ice and is called a flaw polynya. If it recurs in the same position every year, it is called a recurring polynya.
Polynyas range in size from relatively small to enormous. The largest polynya observed in the Antarctic was the Weddell Polynya of 1975-77, covering an area of 2x105 km2. The two main categories of polynya are sensible heat and latent heat, depending on the mechanism responsible for maintaining their presence. Latent heat polynyas are maintained by persistent katabatic winds that drain off the continent. Newly formed ice is advected away by the wind, leaving the surface ice-free and open to more ice formation. In this manner latent heat polynyas can be major sources of new ice production. Coastal polynyas are primarily of this type. Sensible heat polynyas are maintained by upwelling warm water that supplies a sufficiently large oceanic heat flux to the base of the ice to reduce its thickness, or melt it completely. These polynyas are not responsible for large quantities of new ice production. A polynya may also form by a combination of the sensible and latent heat processes.
Pressure process whereby one piece of ice overrides another. Most common in new and young ice (cf. finger rafting).
Finger rafting is a type of rafting whereby interlocking thrusts are formed, each floe thrusting "fingers" alternately over and under each other. Common in nilas and grey ice.
Rafting plays an important role in increasing ice thickness within the Antarctic pack. It is the dominant dynamic mechanism whereby floes reach between about 0.4 and 0.6 m thick in the early stages of ice development. Beyond this thickness, converging floes are more likely to form ridges than to raft.
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Thin grey-white ice showing |
Nilas |
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Nilas |
Nilas exhibiting finger rafting. |
The pressure process by which sea ice is forced into ridges.
A ridge is a line or wall of broken ice forced up by pressure. May be fresh or weathered. The submerged volume of broken ice under a ridge, forced downwards by pressure, is termed an ice keel.
In the Antarctic, ridges are commonly point features rather than the long linear features observed in the Arctic.
A considerable percentage of ice mass is contained within ridged areas of the Antarctic pack. Data from eight voyages into the East Antarctic pack show that by incorporating the ridged ice, the mean thickness increases, on average, by 1.7 times the observed mean undeformed ice thickness.
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Ice being pushed up to form ridges with compression of the pack. Flooding of the surface snow cover can be seen in the centre. |
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The RSV Aurora Australia stopped in heavily ridged ice |
Old ice which has survived only one summer's melt. Because it is thicker and less dense than first-year ice, it stands higher out of the water. In contrast to multi-year ice, summer melting produces a regular pattern of puddles. Bare patches and puddles are usually greenish-blue.
The regular pattern of puddles produced during the melt season is only a feature of Arctic se ice. Melt water does not usually accumulate on the surface of Antarctic sea ice.
Second-year ice is the most common form of old ice present in Antarctica.
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An accumulation of spongy white lumps, a few centimetres across; they are formed from grease ice or slush and sometimes from anchor ice rising to the surface. |
| With the interaction of surface winds and waves, shuga may line up along the wind direction and form characteristic ice bands. |
Snow which is saturated and mixed with water on land or ice surfaces, or as a viscous floating mass in water after heavy snowfall.
Snow ice forms by refreezing flooded snow, creating an ice layer that bonds firmly to the top surface of a floe. Ice formed by this process and makes a significant contribution to the total mass of Antarctic sea ice. The snow cover of sea ice can become flooded by sea water via a number of mechanisms, in particular when the mass of snow becomes great enough to depress the ice/snow interface below sea level. The snow cover is porous and sea water can easily infiltrate from the sides of floes to form a slush layer at the ice/snow boundary. The snow may also become flooded by water rising up brine channels within the sea ice. With sufficiently cold temperatures this slush layer freezes to form snow ice.
Ice in the transition stage between nilas and first-year ice, 10-30 cm in thickness. May be subdivided into grey ice and grey-white ice.
Glosary compliments of ASPeCT ( Antarctic Sea-Ice Processes and Climate
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For more infomation also contact:
Scientific Committee on Antarctic Research
