The Southern Ocean is the ocean that encircles Antarctica, generally taken to be all the water south of 60°S. It covers about 21.96 million km² (8.48 million sq mi), making it the second-smallest of the five oceans after the Arctic, and its deepest known point is 7,434 m, in the part of the South Sandwich Trench that lies south of 60°S. Its limits are the newest of any ocean: the International Hydrographic Organization (IHO) voted for a 60°S limit in 2000, though that definition has never been formally published, and the National Geographic Society officially recognised it in June 2021. What makes it a distinct ocean is the Antarctic Circumpolar Current, the largest current on Earth, which flows unbroken around the continent and keeps Antarctica cold. Each year its sea ice grows from about 3 million km² in February to about 18 million km² in September.
Where Does the Southern Ocean Begin? From Land Limits in 1928 to the 60th Parallel
The Southern Ocean's border has been redrawn again and again. The IHO, whose predecessor the International Hydrographic Bureau held its first international conference in 1919, published the first edition of its Limits of Oceans and Seas in 1928. That edition drew the ocean's northern edge along land: from Cape Horn east to Cape Agulhas in South Africa, then along the southern coast of Australia to Tasmania and across to New Zealand. In effect, everything south of the southern continents counted.
The 1937 second edition pulled the line south, to 40°S between Cape Agulhas and Cape Leeuwin in Western Australia and to 55°S between the Auckland Islands and Cape Horn. Then in 1953 the IHO dropped the ocean altogether. Its third edition noted that most members saw "no real justification" for treating the waters as an ocean, because their northern limits shifted with the seasons, and so the Atlantic, Pacific and Indian oceans were extended all the way to Antarctica. National hydrographic offices were left to draw their own lines, and Britain used 55°S.
The name itself is older than any of these definitions. Balboa had used "Southern Ocean" for the Pacific in the 16th century, and 18th- and 19th-century gazetteers placed its northern edge anywhere from 40°S to 50°S. James Cook's voyages of the 1770s proved there was open water all the way around the southern latitudes.
The IHO Vote of 2000 and National Geographic's 2021 Map
Oceanographers kept treating the Southern Ocean as a single system because of the circulation around Antarctica, and in 2000 the IHO surveyed its members again. Of 68 member states, 28 replied, and all of them except Argentina agreed to define the ocean once more. The name "Southern Ocean" won 18 votes, beating "Antarctic Ocean". Half the votes backed a northern limit at 60°S, a latitude that crosses no land anywhere around the globe; the rest were split between other lines, mostly 50°S, with a few as far north as 35°S.
A draft fourth edition containing the 60°S definition was sent to member states in August 2002 but has never been formally published, held up by unrelated disputes, chiefly over the name of the Sea of Japan. Australia also lodged a reservation, because it regards the waters immediately south of its own coast as the Southern Ocean. Even so, the 60°S limit is widely used: the CIA World Factbook and Merriam-Webster use it, and in June 2021 the National Geographic Society officially recognised the Southern Ocean as the world's fifth ocean, after decades of showing the other three oceans running down to Antarctica on its maps.
The deepest point within the 60°S limit was only pinned down recently. In February 2019 the Five Deeps Expedition's sonar team found the deepest point south of 60°S, in the South Sandwich Trench at 60°28′S, at 7,434 m, and on 3 February its leader, Victor Vescovo, became the first person to reach the bottom of the Southern Ocean. He proposed calling the spot the Factorian Deep, after his submersible Limiting Factor.
The Antarctic Circumpolar Current: 137 Million Cubic Metres a Second
The reason oceanographers wanted the Southern Ocean back on the map is the Antarctic Circumpolar Current (ACC), also called the West Wind Drift. It flows from west to east all the way around Antarctica, clockwise as seen from the South Pole, carrying an estimated 137 million cubic metres of water a second (137 sverdrups), give or take 7, which makes it the largest ocean current on Earth. That is more than 200 times the combined flow of all the rivers draining into the Atlantic.
Because no land interrupts it, the current links the Atlantic, Pacific and Indian oceans and acts as a barrier, keeping warm subtropical water away from the Antarctic coast so that the continent can hold its enormous ice sheet. Along its northern side runs the Antarctic Convergence, where cold Antarctic surface water sinks beneath warmer subantarctic water and the mixing brings nutrients up. The current is split and steered by features on the seafloor, including the Scotia Arc east of Drake Passage and the Kerguelen Plateau in the Indian Ocean sector, and it drives two large gyres, the Ross Gyre and the Weddell Gyre.
Sailors have known it for centuries: it speeds any voyage from west to east, while sailing the other way, against the current and above all the prevailing westerlies, is hard going; the struggle that preceded the mutiny on the Bounty is one well-known case. Sailors call the latitudes from 40°S to 70°S the Roaring Forties, Furious Fifties and Shrieking Sixties for the winds that blow round the globe unimpeded by land. Read more in our guide to the Antarctic Circumpolar Current.
Drake Passage: 800 Kilometres from Cape Horn to Livingston Island
The ACC's tightest squeeze is the Drake Passage, the stretch of water between Cape Horn in Chile and the South Shetland Islands of Antarctica. About 800 km separate Cape Horn from Livingston Island, the shortest crossing between Antarctica and any other landmass. With nothing to slow the current or the wind, waves there can top 12 m, and the crossing has a reputation as one of the roughest sea journeys on Earth. The IHO places the boundary between the Atlantic and Pacific on the meridian through Cape Horn, 67°16′W, inside the passage.
The Spanish navigator Francisco de Hoces is thought to have been blown into these waters in 1525, which is why Spanish maps call it the Mar de Hoces. The English name comes from Sir Francis Drake, whose Golden Hind was driven south by a storm after leaving the Strait of Magellan in 1578. In 1616 the Dutch navigator Willem Schouten became the first to sail around Cape Horn. On 25 December 2019 a crew of six became the first to row across the passage.
The passage also shaped the planet's climate. When South America and Antarctica drifted apart, somewhere between about 49 and 17 million years ago by different estimates, the gap let the circumpolar current form and cut Antarctica off from warmer water, which helped its ice sheets grow. Our guide to the Drake Passage covers the route ships take across it.
Antarctic Sea Ice: From 3 Million to 18 Million Square Kilometres
Every year the Southern Ocean freezes and thaws on a scale no other part of Earth matches. Antarctic sea ice reaches its peak of roughly 18 million km² in September, at the end of the southern winter, an area about as large as Antarctica itself, then shrinks to around 3 million km² by February. Because so little survives the summer, most of it is first-year ice, generally less than a few metres thick. As it forms, the ice leaves salt behind in the water below, creating the very cold, dense Antarctic Bottom Water that spreads along the floor of the world's oceans.
Unlike the Arctic, Antarctic sea ice showed no clear long-term decline for decades; satellite records from 1979 to 2014 even showed a slight increase. That has changed since 2016. In early January 2023 the National Snow and Ice Data Center reported that the extent was the lowest in the 45-year satellite record, more than 500,000 km² below the previous record low for the date, set in 2018, with four of the five lowest years for late December all since 2016. In September 2023 the winter maximum reached only 16.96 million km², nearly a million km² below the previous lowest maximum, set in 1986.
Krill, Penguins and the Ross Sea Marine Protected Area
The Southern Ocean's food web rests on one small crustacean. Antarctic krill, each about 6 cm long and just over a gram, have a combined biomass of around 500 million tonnes, making them one of the most abundant animal species on Earth. Their swarms can stretch for kilometres, with up to 30,000 animals in a single cubic metre of water. Whales, seals, penguins, squid, fish and seabirds all depend on them, directly or one step removed.
Above them sit some of the world's most specialised animals. No other penguin raises its chick through the Antarctic winter the way the emperor does, and the Adélie nests farther south than any of its relatives. Of the six seal species around Antarctica, the crabeater seal numbers around 15 million, which makes it one of the most numerous large animals on the planet, and the Weddell seal lives on the fast ice close to the continent, often right beside research bases. The southern elephant seal, the largest, can weigh up to 4,000 kg.
Heavy hunting of fur seals in the 18th and 19th centuries showed how fragile this system is, and concern over a growing krill catch led to the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR), part of the Antarctic Treaty System, which came into force on 7 April 1982. On 28 October 2016, meeting in Hobart, CCAMLR declared the Ross Sea marine protected area, agreed by 24 countries and the European Union. It covers more than 1.5 million km², and at the time it was the largest protected area in the world, though it carries a 35-year sunset clause. The penguins that feed in these waters are mapped colony by colony in our guide to Antarctica's penguin colonies.