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The Southern Ocean (also known as the Great Southern Ocean, Antarctic Ocean, South Polar Ocean and Austral Ocean) comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. As such, it is regarded as the fourth-largest of the five principal oceanic divisions (after the Pacific, Atlantic, and Indian Oceans, but larger than the Arctic Ocean). This ocean zone is where cold, northward flowing waters from the Antarctic mix with warmer subantarctic waters.
Geographers disagree on the Southern Ocean's northern boundary or even its existence, with many considering the waters part of the Pacific, Atlantic, and Indian Oceans instead. Others regard the Antarctic Convergence, an ocean zone which fluctuates seasonally, as separating the Southern Ocean from other oceans, rather than the 60th parallel. Australian authorities regard the Southern Ocean as lying immediately south of Australia.
The International Hydrographic Organization (IHO) has not yet ratified its 2000 definition of the ocean as being south of 60°S. Its latest published definition of oceans dates from 1953; this does not include the Southern Ocean. However, the more recent definition is used by the IHO and others.
The Southern Ocean includes the Antarctic Circumpolar Current (which circulates around Antarctica) and parts of the Drake Passage, a part of the Scotia Sea, the Weddell Sea, the King Haakon VII Sea, the Lazarev Sea, the Riiser-Larsen Sea, the Cosmonaut Sea, the Cooperation Sea, the Davis Sea, the Mawson Sea, the D'Urville Sea, the Somov Sea, the Ross Sea, the Amundsen Sea, and the Bellingshausen Sea (in clockwise order).
The Southern Ocean differs from the other oceans in that its largest boundary, the northern boundary, does not abut a landmass, but merges into the Atlantic, Indian and Pacific Oceans. This calls into question why geographers should consider the Southern Ocean a separate ocean, as opposed to a southward extension of the other three oceans. One reason stems from the fact that much of the water of the Southern Ocean differs from the water in the other oceans. Because of the Antarctic Circumpolar Current, that water gets transported around the Southern Ocean fairly rapidly, so that the water in the Southern Ocean south of, for example, New Zealand, resembles the water in the Southern Ocean south of South America more closely than it resembles the water in the Pacific Ocean.
Several processes operate along the coast of Antarctica to produce, in the Southern Ocean, types of water masses not produced elsewhere in the oceans of the Southern Hemisphere. One of these is the Antarctic Bottom Water, a very cold, highly saline, dense water that forms under sea ice.
The Southern Ocean, geologically the youngest of the oceans, was formed when Antarctica and South America moved apart, opening the Drake Passage, roughly 30 million years ago. The separation of the continents allowed the formation of the Antarctic Circumpolar Current.
Different organizations and nations have differing points of view over the extents and existence of the Southern Ocean, though many mariners have long regarded the term as traditional.
The 1937 second edition of the International Hydrographic Organization's (IHO) Limits of Oceans and Seas publication included a definition of an ocean around Antarctica. However, this ocean did not appear in the 1953 third edition because "the northern limits ... are difficult to lay down owing to their seasonal change" - instead the Atlantic, Indian and Pacific Oceans were extended southward.
The IHO readdressed the question of the ocean's existence in a survey in 2000. Of its 68 member nations, 28 responded, and all responding members except Argentina agreed to define a new ocean, reflecting the importance placed by oceanographers on ocean currents. The proposal for the name Southern Ocean won 18 votes, beating the alternative Antarctic Ocean. Half of the votes supported a definition of the ocean's northern limit at 60°S (with no land interruptions at this latitude), with the other 14 votes cast for other definitions, mostly 50°S, but a few for as far north as 35°S. However, the 4th edition of Limits of Oceans and Seas was never ratified or published due to a reservation lodged by Australia,[improper synthesis?] and so the 3rd edition (which does not delineate a separate Southern Ocean) has not been superseded. If and when the 4th edition is published, it will restore the Southern Ocean as originally outlined in the 2nd edition and subsequently omitted from the 3rd.
Despite this, the 4th edition definition has de facto usage by many organisations, scientists and nations - even by the IHO. Some nations' hydrographic offices have defined their own boundaries; the United Kingdom used the 55°S parallel for example.
Other sources such as the National Geographic Society continue to show the Atlantic, Pacific and Indian Oceans as extending to Antarctica, although articles on the National Geographic web site have begun to reference the Southern Ocean. 
In Australia, cartographical authorities defined the Southern Ocean as including the entire body of water between Antarctica and the south coasts of Australia and New Zealand, although maps published by New Zealand's authorities such as Land Information New Zealand do not necessarily follow suit. Coastal maps of Tasmania and South Australia label the sea areas as Southern Ocean, while Cape Leeuwin in Western Australia is described as the point where the Indian and Southern Oceans meet.
The Southern Ocean lies in the Southern Hemisphere. It has typical depths of between 4,000 and 5,000 m (13,000 to 16,000 ft) over most of its extent with only limited areas of shallow water. The Antarctic continental shelf appears generally narrow and unusually deep, its edge lying at depths up to 800 m (2,600 ft), compared to a global mean of 133 m (436 ft).
Equinox to equinox in line with the sun's seasonal influence, the Antarctic ice pack fluctuates from an average minimum of 2.6 million square kilometres (1.0×106 sq mi) in March to about 18.8 million square kilometres (7.3×106 sq mi) in September, more than a sevenfold increase in area.
The Antarctic Circumpolar Current moves perpetually eastward — chasing and joining itself, and at 21,000 km (13,000 mi) in length — it comprises the world's longest ocean current, transporting 130 million cubic metres per second (4.6×109 cu ft/s) of water – 100 times the flow of all the world's rivers.
The Southern Ocean's greatest depth of 7,236 m (23,740 ft) occurs at the southern end of the South Sandwich Trench, at 60°00'S, 024°W.
Sea temperatures vary from about −2 to 10 °C (28 to 50 °F). Cyclonic storms travel eastward around the continent and frequently become intense because of the temperature contrast between ice and open ocean. The ocean-area from about latitude 40 south to the Antarctic Circle has the strongest average winds found anywhere on Earth. In winter the ocean freezes outward to 65 degrees south latitude in the Pacific sector and 55 degrees south latitude in the Atlantic sector, lowering surface temperatures well below 0 degrees Celsius. At some coastal points, however, persistent intense drainage winds from the interior keep the shoreline ice-free throughout the winter.
Icebergs can occur at any time of year throughout the ocean. Some may have drafts up to several hundred meters; smaller icebergs, iceberg fragments and sea-ice (generally 0.5 to 1 m thick) also pose problems for ships. The deep continental shelf has a floor of glacial deposits varying widely over short distances.
Sailors know latitudes from 40 to 70 degrees south as the "Roaring Forties", "Furious Fifties" and "Shrieking Sixties" due to high winds and large waves that form as winds blow around the entire globe unimpeded by any land-mass. Ship ice, especially in May to October, makes the area even more dangerous. The remoteness of the region makes sources of search and rescue scarce.
Increased solar ultraviolet radiation resulting from the Antarctic ozone hole has reduced marine primary productivity (phytoplankton) by as much as 15% and has started damaging the DNA of some fish. Illegal, unreported, and unregulated fishing, especially the landing of an estimated five to six times more Patagonian toothfish than the regulated fishery, likely affects the sustainability of the stock. Long-line fishing for toothfish causes a high incidence of seabird mortality.
All international agreements regarding the world's oceans apply to the Southern Ocean. In addition, it is subject to these agreements specific to the region:
Many nations prohibit the exploration for and the exploitation of mineral resources south of the fluctuating polar front, which lies in the middle of the Antarctic Circumpolar Current and serves as the dividing line between the very cold polar surface waters to the south and the warmer waters to the north. The Antarctic Treaty covers the portion of the globe south of sixty degrees south, it prohibits new claims to Antarctica and to all islands in the Southern Ocean.
Between 1 July 1998 and 30 June 1999, fisheries landed 119,898 tonnes, of which 85% consisted of krill and 14% of Patagonian toothfish. International agreements came into force in late 1999 to reduce illegal, unreported, and unregulated fishing, which in the 1998–99 season landed five to six times more Patagonian toothfish than the regulated fishery.
Few ports or harbors exist on the southern (Antarctic) coast of the Southern Ocean, since ice conditions limit use of most shores to short periods in midsummer; even then some require icebreaker escort for access. Most Antarctic ports are operated by government research stations and, except in an emergency, remain closed to commercial or private vessels; vessels in any port south of 60 degrees south are subject to inspection by Antarctic Treaty observers.
The Southern Ocean's southernmost port operates at McMurdo Station at Winter Quarters Bay forms a small harbor, on the southern tip of Ross Island where a floating Ice pier makes port operations possible in summer. Operation Deep Freeze personnel constructed the first ice pier at McMurdo in 1973..
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