Sea ice

Sea ice is formed from ocean water that freezes. Because the oceans are salty, this occurs at about minus 1.8 °C. Fast ice is sea ice that has frozen along coasts and extends out from land. Pack ice is floating consolidated sea ice that's either detached from land and freely floating, or has been blocked by land-attached ice while drifting past. An ice floe is a floating chunk of sea ice, that is less than 10 kilometers (six miles) in its greatest dimension. Wider chunks of ice are called ice fields. Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Since 1979, sea ice has decreased significantly in the Arctic and increased insignificantly in the Antarctic.

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Formation of sea ice

Only the top layer of water needs to cool to the freezing point. Convection of the surface layer involves the top 100–150 m, down to the pycnocline of increased density.

In calm water, the first sea ice to form on the surface is a skim of separate crystals which initially are in the form of tiny discs, floating flat on the surface and of diameter less than 2-3 mm. Each disc has its c-axis vertical and grows outwards laterally. At a certain point such a disc shape becomes unstable, and the growing isolated crystals take on a hexagonal, stellar form, with long fragile arms stretching out over the surface. These crystals also have their c-axis vertical. The dendritic arms are very fragile, and soon break off, leaving a mixture of discs and arm fragments. With any kind of turbulence in the water, these fragments break up further into random-shaped small crystals which form a suspension of increasing density in the surface water, an ice type called frazil or grease ice. In quiet conditions the frazil crystals soon freeze together to form a continuous thin sheet of young ice; in its early stages, when it is still transparent, it is called nilas. When only a few centimetres thick this is transparent (dark nilas) but as the ice grows thicker the nilas takes on a grey and finally a white appearance. Once nilas has formed, a quite different growth process occurs, in which water molecules freeze on to the bottom of the existing ice sheet, a process called congelation growth. This growth process yields first-year ice, which in a single season reaches a thickness of 1.5–2 m.

In rough water, fresh sea ice is formed by the cooling of the ocean as heat is lost into the atmosphere. The uppermost layer of the ocean is supercooled to slightly below the freezing point, at which time tiny ice platelets, known as frazil ice, form. As more frazil ice forms, the ice forms a mushy surface layer, known as grease ice. Frazil ice formation may also be started by snowfall, rather than supercooling.

Waves and wind then act to compress these ice particles into larger plates, of several metres in diameter, called pancake ice. These float on the ocean surface, and collide with one another, forming upturned edges. In time, the pancake ice plates may themselves be rafted over one another or frozen together into a more solid ice cover, known as consolidated ice pancake ice. Such ice has a very rough appearance on top and bottom.

The sea ice itself is largely fresh, since the ocean salt, by a process called brine rejection, is expelled from the forming and consolidating ice. The resulting highly saline (and hence dense) water is an important influence on the ocean overturning circulation.

Pack ice

Pack ice is formed from seawater in the Earth's polar regions, and coverage increases during winter. In spring and summer, when melting occurs, the margins of the sea ice retreat. The vast bulk of the world's sea ice forms in the Arctic ocean and the oceans around Antarctica. The Antarctic ice cover is highly seasonal, with very little ice in the austral summer, expanding to an area roughly equal to that of Antarctica in winter. Consequently, most Antarctic sea ice is first year ice, up to 1 meter thick. The situation in the Arctic is very different (a polar sea surrounded by land, as opposed to a polar continent surrounded by sea) and the seasonal variation much less, consequently much Arctic sea ice is multi-year ice, and thicker: up to 3–4 meters thick over large areas, with ridges up to 20 meters thick.

The amount of sea ice around both poles in winter is similar in scale. The amount melted each summer is affected by the different environments: the cold Antarctic pole is over land so sea ice is around edge, and the Antarctic sea ice is in the freely-circulating Southern Ocean.

In the spring, krill can scrape off the green lawn of ice algae from the underside of the pack ice. In this image most krill swim in an upside down position directly under the ice. Only one animal (in the middle) is hovering in the open water.

Drift ice

Climatic importance

Sea ice has an important effect on the heat balance of the polar oceans, since it acts to insulate the (relatively) warm ocean from the much colder air above, thus reducing heat loss from the oceans. Especially when covered with snow, sea ice has a high albedo — about 0.8 — and thus the ice also affects the absorption of sunlight at the surface. The sea ice cycle is also an important source of dense (saline) "bottom water". While freezing, water rejects its salt content (leaving pure ice) and the remaining surface, made dense by the extra salinity sinks, leading to the productions of dense water masses, such as Antarctic Bottom Water. This production of dense water is a factor in maintaining the thermohaline circulation, and the accurate representation of these processes is an additional difficulty to climate modelling.

In the Arctic, a key area where pancake ice forms the dominant ice type over an entire region is the so-called Odden ice tongue in the Greenland Sea. The Odden (the word is Norwegian for headland) grows eastward from the main East Greenland ice edge in the vicinity of 72–74°N during the winter because of the presence of very cold polar surface water in the Jan Mayen Current, which diverts some water eastward from the East Greenland Current at that latitude. Most of the old ice continues south, driven by the wind, so a cold open water surface is exposed on which new ice forms as frazil and pancake in the rough seas. The salt rejected back into the ocean from this ice formation causes the surface water to become more dense and sink, sometimes to great depths (2500 m or more), making this one of the few regions of the ocean where winter convection occurs, which helps drive the entire worldwide system of surface and deep currents known as the thermohaline circulation.

Extent and trends of polar ice packs

Reliable measurements of sea ice edge begin within the satellite era. From the late 1970s, the Scanning Multichannel Microwave Radiometer (SMMR) on Seasat (1978) and Nimbus 7 (1978-1987) satellites provided information that was independent of solar illumination or meteorological conditions. The frequency and accuracy of passive microwave measurements improved with the launch of the DMSP F8 Special Sensor Microwave/Imager SSMI in 1987.

The trends since 1979 have been a statistically significant Arctic decrease and an Antarctic increase that is probably not significant, depending exactly on which time period is used. The Arctic trends of −2.5% ± 0.9% per decade; or about 3% per decade (Cavalieri et al. 2003). Climate models simulate this trend and attribute it to anthropogenic forcing. The September ice extent trend for 1979-2004 is declining by 7.7% per decade (Stroeve et al. 2005). In September 2002, sea ice in the Arctic reached a record minimum (Serreze et al. 2003), 4% lower than any previous September since 1978, and 14% lower than the 1978-2000 mean. In the past, a low ice year would be followed by a rebound to near-normal conditions, but 2002 has been followed by two more low-ice years, both of which almost matched the 2002 record. The Antarctic increase is 0.8% per decade [http://nsidc.org/sotc/sea_ice.html although this depends on the period being considered. Vinnikov et al GRL, 2006 find the NH reduction to be statistically significant but the SH trend is not.

In a modelling study of the 52-year period from 1948 to 1999 Rothrock and Zhang (2005) find a statistically significant trend in Arctic ice volume of -3% per decade; splitting this into wind-forced and temperature forced components shows it to be essentially all caused by the temperature forcing.

In the overall mass balance, the volume of sea ice depends on the thickness of the ice as well as the areal extent. While the satellite era has enabled better measurement of trends in areal extent, accurate ice thickness measurements remain a challenge.

Summer melting

In the Arctic, the overlying snow layer typically begins to melt from late May to early June. Melting of the snowcover leads to the development of melt ponds (meltwater pools)on the surface of the ice. On first year ice, which has a smooth upper surface at the end of winter (except where ridged), the pools are initially very shallow, forming in minor depressions in the ice surface, or simply being retained within surviving snow pack as a layer of slush. As summer proceeds, however, this initial random structure becomes more fixed as the pools melt their way down into the ice through preferential absorption of solar radiation by the water, which reflects only 15–40% of the radiation falling on it compared to 40–70% for bare ice.

As the melt pools grow deeper and wider they may eventually drain off into the sea, over the side of floes, through existing cracks, or by melting a thaw hole right through the ice at its thinnest point or at the melt pool's deepest point. The downrush of water when a thaw hole opens may be quite violent, and on very level ice, such as fast ice, a single thaw hole may drain a large area of ice surface. From the air such thaw holes give the appearance of "giant spiders", with the "body" being the thaw hole and the "legs" channels of melt water draining laterally towards the hole.

The underside of the ice cover also responds to the surface melt. Directly underneath melt pools the ice is thinner and is absorbing more incoming radiation. This causes an enhanced rate of bottom melt so that the ice bottom develops a topography of depressions to mirror the melt pool distribution on the top side. In this way an initially smooth first-year ice sheet acquires by the end of summer an undulating topography both on its top and bottom sides. Some of the drained melt water may in fact gather in the underside depressions to form under-ice melt pools, which refreeze in autumn and partially smooth off the underside, leaving it with bulges but not depressions.

Extent of the Arctic ice-pack 2002-2004 (NSIDC)

A final and most important role of the melt water is that some of it works its way down through the ice fabric through minor pores, veins and channels, and in doing so drives out much of the remaining brine. This process, called flushing, is the most efficient and rapid form of brine drainage mechanism, and it operates to remove nearly all of the remaining brine from the first-year ice. The hydrostatic head of the surface meltwater provides the driving force, but an interconnecting network of pores is necessary for the flushing process to operate. Given that the strength properties of sea ice depend on the brine volume, this implies that the flushing mechanism creates a surviving ice sheet which during its second winter of existence has much greater strength than in its first winter.

References

Konstantin Y. Vinnikov, Donald J. Cavalieri, and Claire L. Parkinson, V33, L05704, doi:10.1029/2005GL025282, 2006 ^*

External links

Earth phenomena | Bodies of ice | Glaciology | Water ice | Landforms

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