Southern Ocean is the body of water that surrounds Antarctica. The Southern Ocean covers about 8.5 million square miles (22 million square kilometers). It is the world’s fourth largest ocean, ranking behind the Pacific, Atlantic, and Indian oceans. The Southern Ocean has also been referred to as the Antarctic Ocean.
Scientists long disagreed about whether the waters surrounding Antarctica should be considered an ocean. Oceanographers referred to those waters as the Southern Ocean, but geographers regarded the waters as extensions of the Pacific, Atlantic, and Indian oceans. In 2000, the International Hydrographic Organization (IHO), a group of major maritime nations, proposed that the waters around Antarctica be recognized as the Southern Ocean.
Boundaries.
The southern boundary of the Southern Ocean is the coastline of Antarctica. The IHO set the northern boundary at 60° south latitude. That is also the northern boundary specified in the Antarctic Treaty, the most important international agreement on the use and protection of Antarctica. That treaty took effect in 1961.
There is a natural border between the surface waters of the Southern Ocean and those of the Pacific, Atlantic, and Indian oceans. However, that border is too indefinite to be useful as an official boundary. The border is an imaginary line where the cold waters of the Southern Ocean meet the warmer waters of the other three oceans. It is known as the Antarctic Convergence (AAC) or the polar front. The AAC would not be useful as an official boundary because it shifts back and forth with the seasons and from year to year. Its general location ranges from 48° to 60° south latitude.
Two large seas, the Weddell Sea and the Ross Sea, extend far into Antarctica. Permanent ice in those two seas reaches as far south as 80° to 85° south latitude.
A land mass known as the Antarctic Peninsula extends from Antarctica toward South America. Many islands are clustered around the peninsula.
People in Australia commonly use the term Southern Ocean to refer to all waters south of Australia. Those waters include part of the Indian Ocean.
The ocean floor.
The continental shelf surrounding Antarctica is narrow, and deep at its outer edge. The depth at the outer edge ranges from about 1,300 to 2,600 feet (400 to 790 meters). By contrast, the depth of the shelf around other continents is less than 600 feet (200 meters). Antarctica’s shelf edge is so deep because massive amounts of ice press the continent down.
The floor of the Southern Ocean includes five major basins (broad, deep regions): the Amundsen Abyssal Plain, the Australian-Antarctic Basin, the Bellingshausen Abyssal Plain, the Enderby Abyssal Plain, and the Weddell Abyssal Plain. Some areas of those basins reach depths of more than 16,400 feet (5,000 meters). The greatest depth in the Southern Ocean lies 23,737 feet (7,235 meters) below sea level at the southern end of the South Sandwich Trench.
Temperature.
The Southern Ocean is often cold. At a latitude of 70° south, the sun never rises during midwinter, which occurs in June and July. In midsummer, which occurs in December and January, the sun never sets.
The surface waters reach their lowest temperatures of 28 to 30 °F (–2 to –1 °C) in August and their highest temperatures of 30 to 43 °F (–1 to 6 °C) in February. The lowest temperatures generally occur near Antarctica, and the highest temperatures near 60° south latitude.
Surface seawater freezes during the winter. As the water freezes, salts come out of the ice. The surrounding waters become more salty and thus more dense. During the winter, the surface waters freeze as far north as 55° south latitude on the Atlantic side and 65° south latitude on the Pacific side. During the summer, the sea ice retreats to about 67° to 70° south latitude.
Winds.
The average wind speed between 40° and 67° south latitude is roughly 35 miles per hour (mph), or 55 kilometers per hour (kph)—higher than at any other place in the world. The winds there are prevailing westerlies—that is, they come from the west. They blow eastward in a circle around Antarctica.
Winds that blow away from Antarctica can reach tremendous speeds. Those winds, known as katabatic winds, originate high on the mass of ice that covers Antarctica. At that location, the air becomes extremely cold and dense. Due to the force of gravity, the heavy air then moves down the slopes at ever increasing speeds. The katabatic winds reach their maximum speeds, in excess of 100 mph (160 kph), as they move down valleys and out over the Southern Ocean. The winds blow across the ocean more than 100 miles (160 kilometers) from some parts of Antarctica. The term katabatic comes from a Greek word meaning to go down.
Currents.
The prevailing westerlies drive waters of the Southern Ocean eastward around Antarctica, creating the Antarctic Circumpolar Current. That current extends from north of the AAC to about 67° south latitude. The current transports about 4.6 billion cubic feet (130 million cubic meters) of water per second. That is about 100 times the flow of water from all the world’s rivers. The current reaches depths of 9,800 feet (3,000 meters).
At latitudes higher than 67° south latitude and near Antarctica, the prevailing winds blow from the east. Those winds cause a narrow current to flow to the west on the surface waters around much of Antarctica.
Water masses.
The Southern Ocean plays a key role in the global circulation of water masses, layers of water with different circulation patterns. The Antarctic Circumpolar Water is a major mass in the Southern Ocean. It occurs at depths of about 980 to 9,800 feet (300 to 3,000 meters). This water circulates from west to east around Antarctica and mixes northward into the other oceans.
At several places near Antarctica, and especially in the Weddell Sea, freezing at the ocean surface makes the water salty and dense. That water flows to the ocean floor and away from Antarctica, mixing with the Antarctic Circumpolar Water along the way. The resulting mass of water, known as the Antarctic Bottom Water, mixes and spreads into the basins of the other oceans. The Antarctic Bottom Water affects the temperature and saltiness of waters as far north as the equator.
The Antarctic Intermediate Water forms at the AAC when Antarctic Bottom Water mixes with cold, fresh water released by melting ice. The Antarctic Intermediate Water is therefore cooler and less salty than the Antarctic Bottom Water. The Antarctic Intermediate water spreads into the Pacific, Atlantic, and Indian oceans at depths of 1,300 to 3,900 feet (400 to 1,200 meters). Its effects can be detected as far north as the North Atlantic.
Commercial resources
of the Southern Ocean include fish and small shrimplike animals known as krill. Much of the krill caught is used as fish meal or animal feed. Many people eat krill that has been cut up into small pieces or made into a paste.
Whaling crews once hunted whales in the Southern Ocean. But in 1963, the International Whaling Commission (IWC) banned the killing of humpback whales in the Southern Hemisphere. The IWC banned the killing of blue whales in those waters in 1967. In 1994, the IWC declared that no commercial whaling would be allowed in Antarctic waters.
The floor of the Southern Ocean may contain much petroleum and natural gas. However, prospectors have not yet explored extensively for those resources. The severe conditions in the ocean would make the oil and gas expensive to bring to the surface.
Scientific research.
Many international scientific groups study the Southern Ocean. Major research topics include the ability of the ocean to dissolve carbon dioxide (CO2). That topic is related to the issue of global warming, an increase in the average temperature of Earth’s surface. Human activities are responsible for most of the warming that has occurred. The chief activity contributing to global warming has been the burning of fossil fuels—coal, oil, and natural gas. The burning of those fuels has increased the amount of CO2 in the atmosphere. CO2 is a greenhouse gas, one that contributes to global warming through a complex process involving sunlight, gases, and particles in the atmosphere. The Southern Ocean and the other oceans can dissolve some of the CO2 that enters the atmosphere, thereby reducing the amount of global warming that will occur.
Global warming may affect the Southern Ocean and other areas of high latitude more than it affects regions of low latitude. Satellites have already detected huge chunks of ice breaking free from Antarctica. Increases in the amount of ice in the Southern Ocean could lead to a rise in the sea level throughout the world.
Other researchers are studying how an increase in ultraviolet radiation might harm living things in the Southern Ocean. A layer of a gas called ozone in the upper atmosphere shields Earth from 95 to 99 percent of the sun’s ultraviolet rays. But since the late 1970’s, scientists have observed a thinning of the ozone layer over Antarctica and the Southern Ocean.
See also Antarctica; Global warming; Krill; Ocean; Ozone hole.
