Southern Ocean
The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of, it is the second-smallest of the five principal oceanic divisions, smaller than the Pacific, Atlantic and Indian oceans, and larger than the Arctic Ocean.
The maximum depth of the Southern Ocean, using the definition that it lies south of 60th parallel, was surveyed by the Five Deeps Expedition in early February 2019. The expedition's multibeam sonar team identified the deepest point at 60° 28' 46"S, 025° 32' 32"W, with a depth of. The expedition leader and chief submersible pilot, Victor Vescovo, has proposed naming this deepest point the "Factorian Deep", based on the name of the crewed submersible DSV Limiting Factor, in which he successfully visited the bottom for the first time on February 3, 2019.
By way of his voyages in the 1770s, James Cook proved that waters encompassed the southern latitudes of the globe. Yet, geographers have often disagreed on whether the Southern Ocean should be defined as a body of water bound by the seasonally fluctuating Antarctic Convergence — an oceanic zone where cold, northward flowing waters from the Antarctic mix with warmer Subantarctic waters — or not defined at all, with its waters instead treated as the southern limits of the Pacific, Atlantic, and Indian oceans. The International Hydrographic Organization finally settled the debate after the full importance of Southern Ocean overturning circulation had been ascertained, and the term Southern Ocean now defines the body of water which lies south of the northern limit of that circulation.
The Southern Ocean overturning circulation is important because it makes up the second half of the global thermohaline circulation, after the better known Atlantic meridional overturning circulation. Much like AMOC, it has also been substantially affected by climate change, in ways that have increased ocean stratification, and which may also result in the circulation substantially slowing or even passing a tipping point and collapsing outright. The latter would have adverse impacts on global weather and the function of marine ecosystems here, unfolding over centuries. The ongoing warming is already changing marine ecosystems here.
Definition and term use
Borders and names for oceans and seas were internationally agreed when the International Hydrographic Bureau, the precursor to the IHO, convened the First International Conference on 24 July 1919. The IHO then published these in its Limits of Oceans and Seas, the first edition being 1928. Since the first edition, the limits of the Southern Ocean have moved progressively southward; since 1953, it has been omitted from the official publication and left to local hydrographic offices to determine their own limits.The IHO included the ocean and its definition as the waters south of the 60th parallel south in its 2000 revisions, but this has not been formally adopted, due to continuing impasses about some of the content, such as the naming dispute over the Sea of Japan. The 2000 IHO definition was circulated as a draft edition in 2002, and is used by some within the IHO and other organizations, such as the CIA World Factbook and Merriam-Webster.
The Australian Government regards the Southern Ocean as lying immediately south of Australia.
The National Geographic Society recognized the ocean officially in June 2021. Prior to this, it depicted it in a typeface different from the other world oceans; instead, it showed the Pacific, Atlantic, and Indian Oceans extending to Antarctica on both its print and online maps. Map publishers using the term Southern Ocean on their maps include Hema Maps and GeoNova.
Pre-20th century
"Southern Ocean" is an obsolete name for the Pacific Ocean or South Pacific, coined by the Spanish explorer Vasco Núñez de Balboa, the first European to discover the Pacific, who approached it from the north in Panama. The "South Seas" is a less archaic synonym. A 1745 British Act of Parliament established a prize for discovering a Northwest Passage to "the Western and Southern Ocean of America".Authors using "Southern Ocean" to name the waters encircling the unknown southern polar regions used varying limits. James Cook's account of his second voyage implies New Caledonia borders it. Peacock's 1795 Geographical Dictionary said it lay "to the southward of America and Africa"; John Payne in 1796 used 40 degrees as the northern limit; the 1827 Edinburgh Gazetteer used 50 degrees. The Family Magazine in 1835 divided the "Great Southern Ocean" into the "Southern Ocean" and the "Antarctick Ocean" along the Antarctic Circle, with the northern limit of the Southern Ocean being lines joining Cape Horn, the Cape of Good Hope, Van Diemen's Land and the south of New Zealand.
The United Kingdom's South Australia Act 1834 described the waters forming the southern limit of the new province of South Australia as "the Southern Ocean". The Colony of Victoria's Legislative Council Act 1881 delimited part of the division of Bairnsdale as "along the New South Wales boundary to the Southern ocean".
1928 delineation
In the 1928 first edition of Limits of Oceans and Seas, the Southern Ocean was delineated by land-based limits: Antarctica to the south, and South America, Africa, Australia, and Broughton Island, New Zealand to the north.The detailed land-limits used were from Cape Horn in Chile eastward to Cape Agulhas in Africa, then further eastward to the southern coast of mainland Australia to Cape Leeuwin, Western Australia. From Cape Leeuwin, the limit then followed eastward along the coast of mainland Australia to Cape Otway, Victoria, then southward across Bass Strait to Cape Wickham, King Island, along the west coast of King Island, then the remainder of the way south across Bass Strait to Cape Grim, Tasmania.
The limit then followed the west coast of Tasmania southward to the South East Cape and then went eastward to Broughton Island, New Zealand, before returning to Cape Horn.
1937 delineation
The northern limits of the Southern Ocean were moved southward in the IHO's 1937 second edition of the Limits of Oceans and Seas. From this edition, much of the ocean's northern limit ceased to abut land masses.In the second edition, the Southern Ocean then extended from Antarctica northward to latitude 40°S between Cape Agulhas in Africa and Cape Leeuwin in Western Australia, and extended to latitude 55°S between Auckland Island of New Zealand and Cape Horn in South America.
As is discussed in more detail below, prior to the 2002 edition the limits of oceans explicitly excluded the seas lying within each of them. The Great Australian Bight was unnamed in the 1928 edition, and delineated as shown in the figure above in the 1937 edition. It therefore encompassed former Southern Ocean waters—as designated in 1928—but was technically not inside any of the three adjacent oceans by 1937.
In the 2002 draft edition, the IHO have designated "seas" as subdivisions within "oceans", so the Bight would have still been within the Southern Ocean in 1937 if the 2002 convention were in place then. To perform direct comparisons of current and former limits of oceans it is necessary to consider, or at least be aware of, how the 2002 change in IHO terminology for "seas" can affect the comparison.
1953 delineation
The Southern Ocean did not appear in the 1953 third edition of Limits of Oceans and Seas, a note in the publication read:Instead, in the IHO 1953 publication, the Atlantic, Indian and Pacific Oceans were extended southward, the Indian and Pacific Oceans now abutted at the meridian of South East Cape, and the southern limits of the Great Australian Bight and the Tasman Sea were moved northward.
2002 draft delineation
The IHO readdressed the question of the Southern Ocean in a survey in 2000. Of its 68 member nations, 28 responded, and all responding members except Argentina agreed to redefine the 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 the 60th parallel south—with no land interruptions at this latitude—with the other 14 votes cast for other definitions, mostly the 50th parallel south, but a few for as far north as the 35th parallel south. Notably, the Southern Ocean Observing System collates data from latitudes higher than 40 degrees south.A draft fourth edition of Limits of Oceans and Seas was circulated to IHO member states in August 2002. It has yet to be published due to 'areas of concern' by several countries relating to various naming issues around the world – primarily the Sea of Japan naming dispute – and there have been various changes: 60 seas were given new names, and even the name of the publication was changed. A reservation had also been lodged by Australia regarding the Southern Ocean limits. Effectively, the third edition—which did not delineate the Southern Ocean leaving delineation to local hydrographic offices—has yet to be superseded.
Despite this, the fourth edition definition has partial de facto usage by many nations, scientists, and organisations such as the U.S. and Merriam-Webster, scientists and nations – and even by some within the IHO. Some nations' hydrographic offices have defined their own boundaries; the United Kingdom used the 55th parallel south for example. Other organisations favour more northerly limits for the Southern Ocean. For example, Encyclopædia Britannica describes the Southern Ocean as extending as far north as South America, and confers great significance on the Antarctic Convergence, yet its description of the Indian Ocean contradicts this, describing the Indian Ocean as extending south to Antarctica.
Other sources, such as the National Geographic Society, show the Atlantic, Pacific, and Indian oceans as extending to Antarctica on its maps, although articles on the National Geographic web site have begun to reference the Southern Ocean.
A radical shift from past IHO practices was also seen in the 2002 draft edition when the IHO delineated "seas" as subdivisions within the boundaries of "oceans". While the IHO are often considered the authority for such conventions, the shift brought them into line with the practices of other publications which already adopted the principle that seas are contained within oceans. This difference in practice is markedly seen for the Pacific Ocean in the adjacent figure. Thus, for example, previously the Tasman Sea between Australia and New Zealand was not regarded by the IHO as part of the Pacific, but as of the 2002 draft edition it is.
The new delineation of seas as subdivisions of oceans has avoided the need to interrupt the northern boundary of the Southern Ocean where intersected by Drake Passage which includes all of the waters from South America to the Antarctic coast, nor interrupt it for the Scotia Sea, which also extends below the 60th parallel south. The new delineation of seas has also meant that the long-time named seas around Antarctica, excluded from the 1953 edition, are automatically part of the Southern Ocean.