Smallest of the world's oceans, centring approximately on the North Pole, the Arctic Ocean and its marginal seas (the Chukchi, East Siberian, Laptev, Kara, Barents, White, Greenland, and Beaufort; some oceanographers also include the Bering and Norwegian seas) are the least-known basins and bodies of water in the world ocean. This lack of knowledge about them results from their remoteness, hostile weather, and perennial or seasonal ice cover. This is changing, however, because the Arctic may exhibit a strong response to global change and may be capable of initiating dramatic climatic changes through alterations induced in the oceanic thermohaline circulation by its cold, southward-moving currents or through its effects on the global albedo resulting from changes in its total ice cover.
Although the Arctic Ocean is by far the smallest of the Earth's oceans, having only a little more than one-sixth the area of the next largest, the Indian Ocean, its area of 14,090,000 square km (5,440,000 square miles) is five times larger than that ofthe largest sea, the Mediterranean. The deepest sounding obtained in Arctic waters is 5,502 metres (18,050 feet), but the average depth is only 987 metres (3,240 feet).
Distinguished by several unique features, including a cover of perennial ice and almost complete encirclement by the landmasses of North America, Eurasia, and Greenland, the north polar region has been a subject of speculation since the earliest concepts of a spherical Earth. From astronomical observations, the Greeks theorized that north of the Arctic Circle there must be a midnight sun at midsummer and continual darkness at midwinter. The enlightened view was that both the northern and southern polar regions were uninhabitable frozen wastes, whereas the more popular belief was that there was a halcyon land beyond the north wind where the sun always shone and people called Hyperboreans led a peaceful life. Such speculations provided incentives for adventurous men to risk the hazards of severe climate and fear of the unknown to further geographic knowledge and national and personal prosperity
Several factors in the Arctic Ocean make its physical, chemical, and biological processes significantly different from those in the adjoining North Atlantic and Pacific Oceans. Most notable is the covering ice pack, which reduces the exchange of energy between ocean and atmosphere by about 100 times. In addition, sea ice greatly reduces the penetration of sunlight needed for the photo-synthetic processes of marine life and impedes the mixing effect of the winds. A further significant distinguishing feature is the high ratio of freely connected shallow seas to deep basins. Whereas the continental shelf on the North American side of the Arctic Ocean is of a normal width (approximately 40 miles), the Eurasian sector is hundreds of miles broad, with peninsulas and islands dividing it into five main marginal seas: the Chukchi, East Siberian, Laptev, Kara, and Barents. These marginal seas occupy 36 percent of the area of the Arctic Ocean, yet they contain only 2 percent of its water volume. With the exception of the Mackenzie River of Canada and the Colville River of Alaska, all major rivers discharge into these marginal shallow seas. The combination of large marginal seas, with a high ratio of exposed surface to total volume, plus large summer inputs of fresh water, greatly influences surface-water conditions in the Arctic Ocean.
As an approximation, the Arctic Ocean may be regarded as an estuary of the Atlantic Ocean. The major circulation into and from the Arctic Basin is through a single deep channel, the Fram Strait, which lies between the island of Spitsbergen and Greenland. A substantially smaller quantity (approximately one-quarter of the volume) of water is transported southward through the Barents and Kara seas and the Canadian Archipelago. The combined outflow to the Atlantic appears to be of major significance to the large-scale thermohaline circulation and mean temperature of the world ocean with a potentially profound impact on global climate variability Warm waters entering the Greenland/Iceland/Norwegian (GIN) Sea plunge downward when they meet the colder waters from more northerly produced freshwater, southward-drifting ice, and a colder atmosphere. This produces North Atlantic Deep Water (NADW), which circulates in the world ocean. An increase in this freshwater and ice export could shut down the thermocline convection in the GIN Sea; alternatively, a decrease in ice export might allow for convection and ventilation in the Arctic Ocean itself.
Low-salinity waters enter the Arctic Ocean from the Pacific through the shallow Bering Strait. Although the mean inflow seems to be driven by a slight difference in sea level between the North Pacific and Arctic oceans, a large source of variability is induced by the wind field, primarily large-scale atmospheric circulation over the North Pacific. The amount of freshwater entering the Arctic Ocean is about 2 percent of the total input. Precipitation is believed to be about 10 times greater than loss by evaporation, although both figures can be only roughly estimated. Through all these various routes and mechanisms, the exchange rate of the Arctic Ocean is estimated to be approximately 5.9 million cubic metres (210 million cubic feet) per second.
All waters of the Arctic Ocean are cold. Variations in density are thus mainly determined by changes in salinity. Arctic waters have a two-layer system: a thin and less dense surface layer is separated by a strong density gradient, referred to as a pycnocline, from the main body of water, which is of quite uniform density. This pycnocline restricts convective motion and the vertical transfer of heat and salt, and hence the surface layer acts as a cap over the larger masses of warmer water below.
Despite this overall similarity in gross oceanographic structure, the waters of the Arctic Ocean can be classified into three major masses and one lesser mass.
i. The water extending from the surface to a depth of about 200 metres (about 650 feet) is the most variable and heterogeneous of all that in the Arctic. This is because of the latent heat of freezing and thawing; brine addition from the process of ice freezing; freshwater addition by rivers, ice melting, and precipitation; and great variations in insolation (rate of delivery of solar energy) and energy flux as a result of sea ice cover. Water temperature may vary over a range of 7°F (4°C) and salinity from 28 to 34 grams of salt per kilogram of seawater (28 to 34 parts per thousand C°/gg}).
2. Warmer Atlantic water everywhere underlies Arctic surface water from a depth of about 200 to 914 metres (650 to 3,000 feet). As it cools it becomes so dense that it slips below the surface layer on entering the Arctic Basin. The temperature of this water is about 34 to 3 °F (1 to 3°C) as it enters the basin, but it is gradually cooled so that by the time it spreads to the Beaufort Sea it has a maximum temperature of 32.9 to 33.1T (0.5 to °.6°C). The salinity of the Atlantic layer varies between 34.5 and 35°/°°.
3. Bottom water extends beneath the Atlantic layer to the ocean floor. This is colder than the Atlantic water (below 32°F, or °°C) but has the same salinity
4. An inflow of Pacific water can be observed in the Amerasia Basin but not in the Eurasia Basin. This warmer and fresher water mixes with colder and more saline water in the Chukchi Sea, where its density enables it to flow as a wedge between the Arctic and Atlantic waters. The Pacific water, by the time it reaches the Canada Basin, has a temperature range of 31.1 to 30.8T (-0.5 to -°.7°C) and salinities between 31.5 and 33°/°°.
Arctic waters are driven by the wind and by density differences. The net effect of tides is unknown but could have some modifying effect on gross circulation. The motion of surface waters is best known from observations of ice drift. The most striking feature of the surface circulation pattern is the large clockwise gyre (circular motion)
that covers almost the entire Amerasia Basin. Fletcher's Ice Island (T-3) made two orbits in this gyre over a 20-year period, which is some indication of the current speed. The northern extremity of the gyre bifurcates and jets out of the Greenland-Spitsbergen passage as the East Greenland Current, attaining speeds of 15 to 40 centimeters (6 to 16 inches) per second. Circulation of the shallow Eurasian shelf seas seems to be a complex series of counterclockwise gyres, complicated by islands and other topographic relief.
Circulation of the deeper Atlantic water is less well known. On entering the Eurasia Basin, the plunging Greenland Sea water appears to flow eastward along the edge of the continental margin until it fans out and enters the Amerasia Basin along a broad front over the crest of the Lomonosov Ridge. There seems to be a general counterclockwise circulation in the Eurasia Basin and a smaller clockwise gyre in the Beaufort Sea. Speeds are slow—probably less than two inches per second.
The circulation of the bottom water is unknown but can be inferred to be similar to the Atlantic layer. Measured values of dissolved oxygen show that the bottom water is well ventilated, dissolved oxygen everywhere exceeding 70 percent of saturation.
The cover of sea ice suppresses wind stress and wind mixing, reflects a large proportion of incoming solar radiation, imposes an upper limit on the surface temperature, and impedes evaporation. Wind and water stresses keep the ice pack in almost continuous motion, causing the formation of cracks (leads), open ponds (polynya), and pressure ridges. Along these ridges the pack ice may be locally stacked high and project downward about 10-25 metres (33-80 feet) into the ocean. Besides its deterrence to the exchange of energy between the ocean and the atmosphere, the formation of sea ice generates vast quantities of cold water that help drive the circulation of the world ocean system.
Sea ice rarely forms in the open ocean below a latitude of 60° N but does occur in more southerly enclosed bays, rivers, and seas. Between about 60° and 75° N the occurrence of sea ice is seasonal, and there is usually a period of the year when the water is ice-free. Above a latitude of 75°N there is a more or less permanent ice cover. Even there, however, as much as 10 percent of the area consists of open water owing to the continual opening of leads and polynyas.
In the process of freezing, the salt in seawater is expelled as brine. The degree to which this rejection takes place increases as the rate of freezing decreases. Typically, newly formed sea ice has a salinity of 4 to 6 %0. Even after freezing the process of purification continues but at a much slower rate. By the time the ice is one year old, it is sufficiently salt-free to be melted for drinking. This year-old, or older, salt-free sea ice is referred to as multiyear sea ice or polar pack. It can be distinguished by its smoother, rounded surface and pale blue colour. Younger ice is more jagged and grayer in colour. Because the hardness and strength of ice increases as the salts are expelled, polar pack is a special threat to shipping. First-year ice has a characteristic thickness of up to 2 metres (6 feet), whereas multiyear ice averages about 4 metres (about 12 feet) in thickness.
There is no direct evidence as to the onset of the Arctic Ocean ice cover. The origin of the ice pack was influenced by a number of factors, such as the formation of terrestrial ice caps and the interaction of the Arctic and North Atlantic waters—with their different temperature and salinity structures—with atmospheric climate variables.
What can be inferred from available data is that there was not a continuous ice cover throughout the Pleistocene Epoch (about 2.6 million to 11,700 years ago). Rather, there was a continually warm ocean until approximately 2,000,000 years ago, followed by a permanent ice pack about 850,000 years ago.
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