Sea Ice Formation and Features
Sea ice that is not more than one winter old is known as first-yearice. Seaice that survives one or more summers is known as multiyear ice. Most Antarctic sea ice is first-year pack ice. Multiyear ice is common in the Arctic, where most of it occurs as pack ice in the Arctic Ocean.
浮冰是由许多我的各个部分ce known as cakes, if they are less than 20 metres (about 66 feet) across, and floes, which vary from small (20-100 metres [about 66-330 feet] across) to giant (greater than 10 km [about 6 miles] across). As the ice drifts, it often breaks apart, and open water appears within leads and fractures. Leads are typically linear features that are widespread in the pack ice at any time of year, extend for hundreds of kilometres, and vary from a few metres to hundreds of metres in width. In winter, leads freeze quickly Both new and young ice are often thickened mechanically by rafting and ridging, when they are compressed between thicker floes. A pressure ridge is composed of a sail above the waterline and a keel below. In the Arctic most keels are 10-25 metres (about 33-80 feet) deep and typically four times the sail height. Keel widths are typically 2-3 times the sail width. Antarctic pressure ridges are less massive than Arctic pressure ridges. Though they only make up about 25 percent of the total ice area in both polar regions, approximately 40-60 percent of the total ice mass is contained within pressure ridges.
Ice crystals growing on the ocean surface typically break down quickly into smaller pieces that form a soupy suspension known as frazil or grease ice. Under calm conditions the crystals freeze together to form a continuous sheet of new ice called nilas. It is up to 10 cm (about 4 inches) thick and looks dark gray. As the sheet ice thickens by freezing at the bottom, it becomes young ice that is gray to grayish white and up to 30 cm (about 1 foot) thick. If new and young ice are not deformed into rafts or ridges, they will continue to grow by a bottom-freezing process known as congelation.Congelation ice, with its

Pressure ridge in multiyear sea ice thrust up against the northernmost coast of Ellesmere Island, Queen Elizabeth Islands, Canada. M.O. Jeffries, University of Alaska Fairbanks distinctive columnar crystal texture due to the downward growth of the ice crystals into the water, is very common in Arctic pack ice and fast ice.
Under more turbulent conditions, when the water is disturbed by wind and waves, frazil crystals agglomerate into discs known as pancakes. As they grow from a few centimetres to a few metres across, theysolidifyand thicken mechanically by rafting on top of each other. Pancakes freeze together to form cakes and floes, which contain a large amount of ice with a granular texture. The "frazil-pancake cycle," though it occurs in both hemispheres, is particularly important in Antarctica, where it accounts for the rapid expansion of ice cover during the autumn and winter. Consequently, Antarctic ice floes generally contain a larger amount of granular ice and a smaller amount of columnar ice than Arctic ice floes.
Frazil, grease, and pancake ice formation also occur in polynyas, which are recurrent features that remain partially or totally ice-free in areas normally expected to be covered with sea ice. They are particularly common in Antarctica, wherekatabatic windsblowing off the continent force the ice at the coast away from shore, leaving the ocean surface ice-free and open to further ice growth. Ice formation and removal can be almost continuous in coastal polynyas. Consequently, they are sometimes referred to as "ice factories."
Antarctic ice floes also contain a significant amount of granular ice because the weight of snow is often sufficient to depress the ice surface below sea level, soaking the base of the snow with seawater and producing a slush. When the slush freezes, a layer of granular snow ice is added near the top of the floes.
Platelet ice is perhaps the most exotic form of sea ice besides marine ice. In Antarctica, where cold, relatively low-salinity seawater flows out from beneath ice shelves, platelet ice grows both in the water column and at the bottom of the sea ice on the ocean surface. Whereas platelet ice has been found frozen into pack ice floes, it is most common in fast ice such as the type found in McMurdo Sound. In the Arctic, platelet ice grows primarily in pools of low-salinity water. These pools form at the base of ice floes during the summer months from meltwater runoff.
PACk ICE DrifT AND ThickNESS
The large-scale drift of sea ice in the Arctic Ocean is dominated by the Beaufort Gyre (a roughly circular current flowing clockwise within the surface waters ofthe Beaufort Sea in the western or North American Arctic) and the Transpolar Drift (the major current flowing into the Atlantic Ocean from the eastern or Eurasian Arctic). The clockwise rotation of the Beaufort Gyre and the movement of the Transpolar Drift, the result of large-scale atmospheric circulation, are dominated by a high-pressure centre over the western Arctic Ocean. The pattern is not constant but varies in both strength and position about every decade or so, as the high-pressure centre weakens and moves closer to both Alaska and the Canadian Arctic. This decadal shift in the high-pressure centre is known as the Arctic Oscillation.
The Transpolar Drift exports large volumes of ice from the Arctic Ocean south through Fram Strait and along the east coast of Greenland into the North AtlanticOcean. Icedrift speeds, determined from buoys placed on the ice, average 10-15 km (about 6-9 miles) per day in the Fram Strait. Ice can drift in the Beaufort Gyre for as much as seven years at rates that vary between zero at the centre to an average of 4-5 km (about 2.5-3 miles) per day at the edge. Together, the Beaufort Gyre and Transpolar
Drift strongly influence the Arctic Ocean ice thickness distribution, which has been determined largely from submarine sonar measurements of the ice draft. Ice draft is a measurement of the ice thickness below the waterline and often serves as a close proxy for total ice thickness. The average draft increases from about one metre (about three feet) near the Eurasian coast to 6-8 metres (about 20-26 feet) along the coasts ofnorth Greenland and the Canadian Arctic islands, where the ice is heavily ridged.
In Antarctica the large-scale sea circulation is dominated by westward motion along the coast and eastward motion farther offshore in theWest Wind Drift(also known as the Antarctic Circumpolar Current). The average drift speed is 20 km (about 12 miles) per day in the westward flow and 15 km (about 9 miles) per day in the eastward flow. Where katabatic winds force the ice away from the coast and create polynyas, local sea ice motion is roughly perpendicular to the shore. There are gyres in the Ross Sea and Weddell Sea where the westward-moving iceis deflected to the north and meets the eastward-moving ice further offshore. Unlike the Beaufort Gyre in the Arctic Ocean, these gyres do not appear to recirculate ice. Ice thickness data from drilling on floes, visual estimates by observers on ships, and a few moored sonars indicate that Antarctic sea ice is thinner than Arctic sea ice. Typically, Antarctic first-year ice is less than one metre (about three feet) thick, while multiyear ice is less than two metres (about 6.5 feet) thick.
Sea Ice and its iNtEractioNs witH the Oceans, Atmosphere, and CliMatE
The growth and decay of sea ice influences local, regional, and global climate through interactions with the atmosphere and ocean. Whereas snow-covered sea ice is an
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- Frost smoke, open water, and new and young sea ice at a small lead surrounded by pack ice and icebergs in the Bellingshausen Sea, Antarctica. M.O. Jeffries, University of Alaska Fairbanks
effective insulator that restricts heat loss from the relativelywarm ocean冷的气氛,有显著的病重bulent heat and mass transfer from leads and polynyas to the ocean and atmosphere during the winter months. These losses are manifested as frost smoke from evaporation and condensation at the water surface, and they affect atmospheric processes hundreds of metres above and hundreds of kilometres downstream from leads and polynyas. Brine rejected from ice growing within leads and polynyas drives the deep mixing of the ocean. Rejected brine also affectsglobal ocean circulationand ventilation processes by increasing the salt concentration of the water it is released into. The conversion of both new and young ice into pressure ridges creates rough top and bottom surfaces that enhance the transfer of momentum from the atmosphere to theocean. Ridgesat the ice surface act as sails and catch the wind. The subsequent movement of the ice floes transfers energy to the underlying water via the keels on the underside of the ice.
Snow and icereduce the amount of solar radiation available for organisms residing in the ice and water. This decrease in the amount of available energy affects and often reduces the productivity of plants, animals, and microorganisms. Snow has a high albedo (it reflects a significant proportion of solar shortwave radiation back to the atmosphere), and thus the temperature at the surface remains cool. In the Arctic the surface albedo decreases in summer as thesnow meltscompletely, ponds of meltwater form on the ice surface that absorb a greater share ofincoming shortwave radiation, and the overall ice concentration (the ratio of ice area to open water area) decreases. The increase in shortwaveradiation absorptionby meltwater ponds and the open ocean accelerates the melting process and further reduces surface albedo. This ice-albedo positive feedback plays a key role in the interaction of sea ice with climate.
The Emerging Impacts of Recent Changes to Sea Ice
Submarine sonar data obtained since 1958 have revealed that the average ice draft in the Arctic Ocean in the 1990s decreased by over 1 metre (about 3 feet) and that ice volume was 40 percent lower than during the period 1958-76. The greatest ice draft reduction occurred in the central and eastern Arctic. Remote sensing also revealed a reduction of 3 percent per decade in Arctic sea ice extent from 1978, with particularly rapid losses occurring from the late 1980s. This included the eastern Arctic, where both the ice concentration and the duration of the ice-covered season also decreased. Computer simulations suggest that sea ice changes in this region were due to changes in atmospheric circulation, and thusice dynamics, rather than higher air temperatures. Yet it is not clear whether these changes are due to natural variability—i.e., the Arctic Oscillation—or whether they represent a regime shift that will persist and perhaps become even more severe in the future.
Since computer models of climate change predict that theconsequences of global warmingwill occur earlier and be most pronounced in the polar regions, particularly the Arctic, monitoring and understanding the behaviour of sea ice are important. Continued reductions of Arctic sea ice extent could have potentially severe ecological impacts. One such event may have arisen in western Hudson Bay, Canada, where a significant decline in the physical condition and reproductive success of polar bears occurred as the duration and extent of sea ice cover decreased during the 1980s and 1990s. On the other hand, a reduction in sea ice could be advantageous for oil and mineral exploration, production, and transport, and for navigation through the Northern Sea Route (Northeast Passage), a water route connecting the Atlantic and Pacific Oceans along the northern coast of Europe and Russia, and the Northwest Passage, a similar route along the northern coast of North America.
Whaling records suggest that Antarctic sea ice extent decreased by approximately 25 percent between the mid-1950s and early 1970s, whereasice core samplessuggest a 20 percent decrease in sea ice extent since 1950. Since then, remote sensing data have indicated an increase in Antarctic sea ice extent parallel to the decrease in Arctic sea ice extent through the 1980s and 1990s. Yet the increase in Antarctic sea ice extent has not been uniformly distributed. A reduction in sea ice extent west of the Antarctic Peninsula has been correlated with slight declines in Adélie penguin numbers and a significant rise in the Chinstrap penguin population. There is speculation that if ice extent continues to decrease in this region, krill numbers will diminish significantly as they lose their under-ice habitat and face growing competition from salps.
Continue reading here:The Arctic
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