Southern Ocean, also called Antarctic Ocean, the southern portions of the Pacific, Atlantic, and Indian oceans and their tributary seas surrounding Antarctica. Unbroken by any other continental landmass, the Southern Ocean’s narrowest constriction is the Drake Passage, 600 miles (about 1,000 km) wide, between South America and the tip of the Antarctic Peninsula.

The Southern Ocean. Encyclopædia Britannica, Inc.

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The structure of the ocean floor includes a continental shelf usually less than 160 miles (about 260 km) wide that attains its maximum width of more than 1,600 miles (2,600 km) in the vicinity of the Weddell and Ross seas. There are oceanic basins farther north that are as much as 14,800 feet (4,500 metres) deep, defined by oceanic rises and often marked by ranges of abyssal hills. There are also narrow oceanic trenches with high relief, such as the South Sandwich Trench on the eastern side of the South Sandwich Islands. Other relief features include oceanic plateaus that rise from the oceanic basins to depths of less than 6,650 feet (2,000 metres) below sea level and form rather flat regions, which are often covered by relatively thick sedimentary deposits. The most extensive such plateau is the Campbell, or New Zealand, Plateau, which rises southeast of New Zealand and extends southward beyond the Campbell Islands.

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The flow of currents in the Southern Ocean is complex. Water cooled by cold air, outgoing radiation, and katabatic winds off of the Antarctic continent sinks and flows northward along the ocean bottom and is replaced at the surface by an equal volume of warmer water flowing south from the Indian, Pacific, and Atlantic oceans. The meeting point of the two is the Antarctic Convergence, where conditions favour the development of phytoplankton, consisting of diatoms and other single-celled plants. The ocean’s most important organism in the higher food chain is the small, shrimplike krill. Animals on the sea bottom of the near-shore zone include the sessile hydrozoans, corals, sponges, and bryozoans, as well as the foraging, crablike sea spiders and isopods, the annelid worm polychaete, echinoids, starfish, and a variety of crustaceans and mollusks. At the sea bottom there are also eelpouts, sea snails, rat-tailed fishes, and codlike fishes. Rare nonbony types of fish include hagfish and skates. Many species of deep-sea fish are known south of the Antarctic Convergence, but only three, a barracuda and two lantern fishes, seem to be confined to this zone.