The Arctic OceanWord文件下载.docx

1、 the Arctic Basin marine region is characterized by the year-round presence of sea ice, while the other Arctic subregions have ice-free periods ranging from less than a month to up to four months. The Arctic coast （including islands） encompasses 68% of Canadas coastline, stretching 165,000 kilometer

2、s （km） from James Bay and Baffin Island to the Yukon. The region exhibits a wide range of coastal landforms and reliefs fashioned by the processes of volcanism, glaciation, faulting and folding. The tidal range is generally less than 0.5 meter （m） in the northern and western sectors but increases to

3、 1.0-5.0 m in the east and south.Arctic Sea Ice （Source: Biodiversity Institute of Ontario） The Arctic Ocean waters are so cold that they develop a thick layer of sea ice on the surface during most of the year. The cold waters of the Arctic would seem to be inhospitable for life, but a rich ecosyste

4、m is present.Sea Ice FormationSalt lowers the freezing temperature of water, so that seawater freezes at about -2 C, whereas pure water freezes at 0C. When the air temperature drops in the Arctic, a thick layer of seawater becomes ready to freeze all at once. The first signs of freezing are changes

5、in color and texture as millions of tiny floating ice needles form, creating a dense “soup” called frazil ice. As the temperature falls further, the frazil ice thickens and traps pockets of salty seawater, or brine, within its layers creating a slushy mixture called grease ice. Eventually the grease

6、 ice accumulates, reaching a thickness of up to 1 m. In response to gravity, the pockets of brine move downward, and within a few weeks accumulate in the lower layers. As a result the ice at the surface contains little salt. Their removal from the upper layers increases its freezing point, aiding th

7、e change from grease ice to solid ice. Sea ice thus freezes solid from the top layer down. Eventually, all of the salt water is drained and the ice layer is frozen solid into pack ice. During the winter, the area of the Arctic Ocean covered with pack ice expands to form a continuous vast sheet. When

8、 melting occurs in the spring and summer, the pack ice retreats and breaks up into smaller pieces.Types of Sea IceThere are two forms of pack ice in the Arctic. The winter ice pack consists of annual ice, which melts in the spring and freezes again in the fall. Multiyear ice, which remains frozen th

9、roughout the summer, forms farther north in two regions: In the central part of the Arctic Ocean, there is the massive permanent polar pack, otherwise known as the Arctic or polar ice cap, that slowly rotates clockwise as a unit. The summer ice pack, which makes up the other region of multiyear ice,

10、 surrounds the permanent polar pack, and breaks up in the summer. These broken pieces of ice block many of the channels between the Arctic islands year-round.Multiyear IceMultiyear ice remains frozen year-round, but still goes through a cycle of changes. The surface layer of this ice melts in the su

11、mmer and the water then drains through pores in the ice to the bottom. This salt-free meltwater remains floating beneath the ice atop the denser salt water. In the winter, the meltwater refreezes as a layer on the bottom of the pack ice. One year at a time, the layers travel upward in a continuous c

12、ycle of loss and replacement. This cycle prevents the multiyear ice from thickening indefinitely. Multiyear ice forms the permanent polar pack in the center of the Arctic Ocean, as well as the summer ice pack that surrounds the permanent polar ice pack, and breaks up in the summer. Unlike annual ice

13、, the summer ice pack never completely melts.Annual IceAnnual ice typically reaches a thickness of about 2 meters over the winter. This type of pack ice begins to form by the middle of September, and by the end of October the ice field is almost solid along the coasts. As winter progresses, the annu

14、al ice extends offshore, connecting some islands with a solid sheet of sea ice. The thickness of the ice increases until May at which point it begins to thaw, with shallow blue pools forming on the ice surface. Rising summer temperatures cause the ice to melt further and eventually it breaks up. Cra

15、cks form that soon widen into channels or leads that separate the ice pack into chunks called floes. Floes sometimes collide with each other, and break into many pieces. Ice breakup occurs in early July in the low Arctic, and at the end of the month in the high Arctic. The ice density declines until

16、 late August, but by mid-September the freezing cycle begins again.Segments of the ice pack that are firmly frozen to the shore are known as landfast ice. Offshore winds push ice away from the coast creating a gap of open water between the landfast ice and the pack ice - a shore lead. The winds also

17、 push the pack ice further out to sea, causing the drifting floes to collide and ride up over each other, or hummock. The result is a distinct ridge of jumbled ice blocks.The loose, floating pieces of ice present in the summer scour the shorelines, preventing most plants and animals from settling in

18、 shallow water. This makes the intertidal zone, the area between the shoreline and the near shore bottom, much more barren than it is in other marine ecosystems.Where to Find Arctic Sea IceAbout 12-13 million square kilometers of the Arctic Basin are covered by ice in winter. Additional sea ice cove

19、rs much of Baffin Bay and Hudson Bay, as well as the Labrador Sea. Nearly all of the Arctic and sub-Arctic shores are ice-bound throughout the winter. By late summer, the amount of Arctic sea ice is reduced to about 9 million square kilometers. During this season, loose pack ice from the Arctic move

20、s southward along the east Greenland coast and from Baffin Bay down the Labrador coast into the north Atlantic Ocean. Although lanes of water open in the summer along the mainland, many of the channels in the Canadian archipelago remain ice-clogged year-round.PolynyasA polynya is a large area of ope

21、n water surrounded by sea ice that is found in the same region year after year. Although polynyas can be hundreds of kilometers wide, their surface area is far less than the area of sea ice which surrounds them. However, the open waters of polynyas are very important to life in the Arctic Ocean. Pol

22、ynyas teem with animal and plant life. It is only here, where the sea ice is absent, that the suns energy directly reaches the waters. Snow and ice ordinarily reflect much of the suns light energy, but the open waters of polynyas absorb it. Phytoplankton in the polynya use this energy to produce a n

23、utrient rich grazing area for zooplankton. Feeding on these small animals are whales and fishes that in turn feed seals, walruses and polar bears. Often seen as oases in the harsh climate of the Arctic marine environment, polynyas have intrigued scientists for more than a century.FormationPolynyas o

24、ccur in areas where swiftly moving sea currents prevent the freezing of surface water. Many polynyas are small, measuring only a few hundred square kilometers, and may freeze over at the height of winter. The larger polynyas, however, stay open all winter, although they may shrink in size in extreme

25、ly cold temperatures. A distinct characteristic of all polynyas is that they are recurrent. The largest Arctic polynya, North Water, occurs at the head of Baffin Bay, and is like an inland sea surrounded by ice. This polynya was discovered in 1616, and is usually about as large as Lake Superior!Comp

26、ared to the surrounding air, the waters of a polynya are so warm that steam billows up. The average temperature of the waters within a polynya is about 0C, and it never dips below -2C, the freezing point of seawater. Polynyas allow heat from warm ocean currents to escape; this is important in the re

27、gulation of the Earths temperature.In calm weather, a layer of new ice can form across a polynya in minutes. Inevitably, a strong wind or wave smashes this new ice layer, driving it out to sea and once again opening the surface of the polynya. This process occurs repeatedly, making polynyas an impor

28、tant source of new Arctic sea ice.LeadsA lead is a fracture or large crack in the sea ice that results in a passageway of open water. Leads are less important to sea life than the open areas of polynyas, but they are still important to many organisms. Leads provide important arteries for the navigat

29、ion of marine mammals and birds.Leads can occur almost anywhere that sea ice is packed relatively loosely. They may appear suddenly, and tend to open and close over a period of just a few hours following changes in winds or tides. There are places, however, where wide, predictable leads form between

30、 the landfast ice and the free-floating pack ice. These long-lasting leads are created by winds blowing offshore that push ice away from the coast, causing the pack ice to aggregate. These distinct leads provide open water for new ice formation.Leads can often be detected from a great distance because they make the clouds above them appear dark. This occurs because the open water in leads absorbs sunlight, which is normally reflected off ice or snow. This causes a dark streak to appear on the underside