Introduction — Borders of the Oceans
The World or Global Ocean is a single, continuous envelope of saline water surrounding Earth; for practical, navigational and political purposes it is conventionally divided on maps into principal oceanic areas. Most geographers and international bodies recognize five such divisions—Pacific, Atlantic, Indian, Southern (commonly termed Antarctic), and Arctic—whose relative extents, when ordered by surface area, run from the Pacific as the largest to the Arctic as the smallest.
Ocean boundaries designate the limits of Earth’s oceanic waters, but these limits and even the number of oceans are not fixed: delineations vary according to historical practice, hydrographic criteria, ecological considerations, or political convention. Each principal ocean contains numerous named subregions—seas, gulfs, bays, straits and the like—that are cataloged for descriptive and navigational use. From a geological standpoint, an ocean denotes not only the overlying body of water but also the underlying oceanic crust, linking the hydrosphere with lithospheric structure.
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Overview
The planet’s saline waters function as a single, continuous World (or Global) Ocean, with fluids and properties exchanged relatively freely across its extent—an integrative concept central to oceanography. For practical and cartographic purposes that continuous mass is partitioned into major oceanic basins, commonly listed by area as the Pacific, Atlantic, Indian, Southern (Antarctic), and Arctic Oceans. Within and between those basins exist smaller named features—seas, gulfs, bays, straits and the like—whose limits and names reflect regional geography.
Boundaries among oceanic divisions arise from a mixture of factors: continental outlines and coastlines, the arrangement of island groups and archipelagos, and hydrographic or navigational criteria. Thus basin delineation is not purely geometric but combines Earth-surface configuration with functional and hydrological considerations. Because different criteria can be emphasized, authoritative assignments of particular seas to a given ocean vary among sources; organizations such as the International Hydrographic Organization provide hierarchical lists and gazetteers, but no single global convention governs every name or border.
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From a geological standpoint an “ocean” denotes a basin floored by oceanic crust—thin, dense, basaltic lithosphere formed at spreading centers—overlain by water, as opposed to thicker, lower-density continental crust. That crustal perspective leads some geologists to distinguish three oceanic basins on Earth: the continuous World Ocean plus the Caspian and Black Seas, because those enclosed basins occupy oceanic-style crust unlike surrounding continents. The Caspian and Black Seas are products of tectonic events associated with the collision and reconfiguration of microcontinents (e.g., Cimmeria) and larger landmasses (e.g., Laurasia), processes that generated isolated basins with oceanic-crust characteristics.
Regional connectivity has shifted through geologic and recent history. The Mediterranean has at times behaved as a semi-independent basin when tectonic movements closed its gateway at Gibraltar, whereas the Black Sea is linked to the Mediterranean today via the Bosporus—a relatively recent (about 7,000 years ago) channel cut through continental rock rather than an extension of oceanic seafloor. Other inland water bodies commonly called “seas” (for example the Aral Sea) are entirely disconnected from the World Ocean; the Caspian, however, is treated in some geological frameworks as a bona fide oceanic basin despite its present isolation.
List of oceans
The global ocean is conventionally divided into principal basins delineated by continental margins and by oceanographic features such as currents, straits and gyres; these principal divisions are the Atlantic, Arctic, Indian, Pacific and the Southern Ocean (the latter recognized by the International Hydrographic Organization in 2000). Each basin contains numerous subordinate seas, gulfs and bays.
The Pacific is the largest ocean basin, extending from the Southern Ocean northward to the Arctic and occupying the maritime space between Asia–Australia to the west and the Americas to the east; it connects hydrologically with the Atlantic around Cape Horn. The Atlantic, the second largest, spans between the Americas and Europe–Africa, reaches northward into the Arctic, and joins the Indian Ocean at Cape Agulhas to the south of Africa. The Indian Ocean, third in size, reaches northward toward the Indian subcontinent, the Arabian Peninsula and Southeast Asia, lies between Africa and Australia, and communicates with the Pacific near Australia at South East Cape.
The Arctic is the smallest named ocean and overlies the North Pole; it borders northern North America, Scandinavia and Siberia, links with the Atlantic near Greenland and Iceland and with the Pacific via the Bering Strait, and is seasonally covered by variable sea ice. The Southern Ocean encircles Antarctica (commonly defined as waters south of 60° S), is dominated by the Antarctic Circumpolar Current, sustains distinct ecosystems and exerts outsized influence on global climate; it is partially covered by seasonally changing sea ice and is the second smallest of the five named oceans.
Ocean identities and boundaries thus reflect both continental geometry and dynamic oceanographic junctions. Key connectors—such as the Bering Strait, Cape Horn, Cape Agulhas and South East Cape—mark principal inter-basin passages, and the Arctic linkage to both Atlantic and Pacific basins produces a continuous hydrological pathway among them. Seasonal sea-ice dynamics in the high latitudes are central to regional oceanography, altering albedo, ecological conditions and climate feedbacks. Viewed from the Antarctic, the Atlantic, Indian and Pacific basins appear as three northward-extending lobes, a morphology that underscores their oriented relationship around Antarctica.
Arctic Ocean
The Arctic Ocean occupies the central polar realm between northern North America and northern Eurasia; it is variously described as a distinct oceanic basin, a large sea, or, in some classifications, an estuarial extension of the Atlantic. Institutional definitions, however, attempt to delimit the ocean proper separately from its adjoining marginal seas.
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The International Hydrographic Organization (IHO) provides a formal circumscription that defines the limits of the Arctic Ocean while expressly excluding the marginal seas it lists and maps independently (for example the Kara Sea and the East Siberian Sea). Under the IHO scheme the ocean’s northern-and-polar boundary is traced in contiguous segments around the polar basin. In the Atlantic–European sector the northern limit runs between Greenland and West Spitsbergen (the northern boundary of the Greenland Sea) and then eastward between West Spitsbergen and North East Land along latitude 80° N.
Proceeding eastward, the IHO delineates the northern margins of the Barents, Kara and Laptev seas as a sequence of headland-to-headland lines: from Cape Leigh Smith to Cape Kohlsaat (Barents Sea), from Cape Kohlsaat to Cape Molotov (Kara Sea), and from Cape Molotov to the northern extremity of Kotelni Island (Laptev Sea). The eastern Arctic boundary continues from Kotelni Island to the northern point of Wrangel Island (northern limit of the East Siberian Sea) and thence to Point Barrow, Alaska (northern limit of the Chukchi Sea).
From Point Barrow the IHO boundary follows a complex arc through the Canadian Arctic Archipelago, linking a series of islands and capes—including Prince Patrick, Borden, Ellef Ringnes, Meighen, Axel Heiberg and Ellesmere—ultimately reaching Cape Columbia. A closing trans‑Greenland line is then drawn from Cape Columbia (Ellesmere Island) to Cape Morris Jesup (Greenland), completing the IHO’s circumscription of the Arctic Ocean proper.
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Despite the IHO’s formal exclusion of separately defined marginal seas, those seas are often treated in practice as components of the Arctic Ocean. Alternative portrayals of the ocean’s extent exist—most notably the limits published by the CIA World Factbook—which differ from the IHO outline and are reflected in comparative maps showing contrasting delineations.
The International Hydrographic Organization (IHO) delineates the high Arctic maritime limits by a sequence of straight lines and polylines that join defined capes and the northern extremities of island groups, thereby partitioning adjacent seas and marking oceanic boundaries across the polar shelf.
A polar boundary links the northernmost point of Spitzbergen (Svalbard) to the northernmost point of Greenland, providing a west–east delimitation across the central high Arctic between Svalbard and Greenland. From the Svalbard sector a line runs from Cape Leigh Smith across Storøya (Bolshoy Ostrov), Kvitøya (Gilles) and Victoria Island to Cape Mary Harmsworth (southwestern Alexandra Land), then follows the northern coasts of the Franz-Josef Land archipelago eastward to Cape Kohlsaat (81°14′N, 65°10′E), tracing the northern maritime edge of these islands.
From Cape Kohlsaat the IHO prescribes a straight connection to Cape Molotov (81°16′N, 93°43′E), the northern extremity of Komsomolets Island in Severnaya Zemlya, defining a zonal sector between Franz-Josef Land and Severnaya Zemlya. That sector continues eastward by a line joining Cape Molotov to the northern extremity of Kotelni Island (76°10′N, 138°50′E), linking Severnaya Zemlya with the New Siberian Islands.
Further east, a polyline begins at the northernmost point of Wrangel Island (179°30′W) and proceeds eastward to the De Long Islands (explicitly including Henrietta and Jeannette Islands), then to Bennett Island and on to the northern extremity of Kotelni Island. This chain connects the eastern Russian Arctic island groups across the Siberian Arctic shelf.
Two trans-Arctic links connect the Alaskan and Canadian sectors to these Russian delineations. One line runs northeast from Point Barrow, Alaska (71°20′N, 156°20′W) to the northernmost point of Wrangel Island (179°30′W), forming a maritime link across the Chukchi/Beaufort sector. A separate line from Point Barrow to Lands End, Prince Patrick Island (76°16′N, 124°08′W) establishes a southwestern boundary segment between the Alaskan Arctic coast and the Canadian Arctic Archipelago (Prince Patrick Island).
Collectively, these IHO-defined segments provide a continuous framework for demarcating oceanic and sea boundaries in the high Arctic by connecting prominent headlands and island extremities in a zonal, west‑to‑east sequence.
Atlantic Ocean
The Atlantic is delimited in different ways by major reference authorities, reflecting alternative cartographic conventions: some sources (e.g., the CIA World Factbook) depict a broad, shaded polygon that incorporates adjacent marginal seas and coastal basins, whereas the International Hydrographic Organization (IHO) promulgates a formal line that deliberately excludes those peripheral waterbodies. Geographically the Atlantic functions as the principal oceanic divide between the Americas on one side and Europe and Africa on the other, and it serves as the principal transoceanic corridor linking these continental regions. For many practical purposes—oceanography, climatology and navigation—the ocean is treated in two hemispheric sectors separated by the Equator, commonly referred to as the Northern and Southern Atlantic. The IHO’s exclusion of marginal seas has concrete consequences: which gulfs, seas and coastal basins are counted within “the Atlantic” alters mapped area totals, statistical summaries, maritime-jurisdictional delineations, and the composition of oceanographic datasets and comparative charts. Consequently, any map, dataset or description that invokes “the Atlantic Ocean” should be read with attention to the boundary convention used (broadly inclusive shading versus the IHO’s restrictive outline), since that choice determines which adjacent waterbodies are incorporated into spatial analyses and navigational products.
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The International Hydrographic Organization’s current (third) edition of Limits of Oceans and Seas defines the North Atlantic Ocean as an oceanic domain whose perimeter is drawn independently of the marginal seas and gulfs that lie within it. Its western limit is traced by the established confines of adjacent marginal basins: the eastern edge of the Caribbean Sea; the southeastern limit of the Gulf of Mexico from Cuba’s north coast to Key West; the southwestern limit of the Bay of Fundy; and the southeastern and northeastern limits of the Gulf of St. Lawrence. To the north the boundary follows polar and subpolar passages and sea limits, notably the southern limit of Davis Strait between Labrador and Greenland and the southwestern limits of the Greenland and Norwegian Seas extending eastward toward the Shetland Islands. The eastern margin is delineated by the limits separating the Atlantic from European marginal waters and channels—principally the northwestern limit of the North Sea; the northern and western limits of the Scottish Seas; the southern limit of the Irish Sea; and the western limits of the Bristol Channel, the English Channel, the Bay of Biscay and the Mediterranean. The southern boundary is the Equator, taken as the arc stretching from the Brazilian coast to the southwestern limit of the Gulf of Guinea. Although the IHO’s delimitation administratively excludes marginal waterbodies it defines separately (for example the Caribbean Sea or the North Sea), these basins are frequently regarded in general usage as integral parts of the Atlantic.
Under the International Hydrographic Organization’s current (3rd edition) Limits of Oceans and Seas, the South Atlantic is defined as a discrete oceanic region distinct from marginal seas that the IHO delimits separately. Its northern extent coincides with the southern limit of the North Atlantic as set by the IHO, while the western boundary in the South American sector follows the limit of the Río de la Plata estuary. To the northeast the South Atlantic is bounded by the limit of the Gulf of Guinea adjoining West African marginal seas. On the southern and southeastern flanks the IHO prescribes lines from Tierra del Fuego and the southern tip of Africa to the Antarctic continent: the southwestern limit runs along the meridian of Cape Horn (67°16′W) from Tierra del Fuego southward, with an additional local boundary drawn from Cape Virgins (52°21′S, 68°21′W) to Cape Espíritu Santo at the eastern entrance to the Strait of Magellan; the southeastern limit follows the meridian of 20°E from Cape Agulhas south to Antarctica. The Antarctic continent itself constitutes the southern boundary under the 3rd‑edition delineation. The IHO framework explicitly excludes marginal waterbodies that it defines separately (for example the Bay of Biscay and the Gulf of Guinea), even though such areas are often treated informally as parts of the Atlantic. A proposed 2002 IHO revision would have moved the Atlantic’s southern limit north to 60°S and reinstated a Southern Ocean south of that parallel; this draft was not ratified (Australia entered a formal reservation in 2003). Were it adopted and published as a 4th edition, the change would reintroduce the Southern Ocean concept used in the IHO’s 2nd edition; until any ratification, many authoritative atlases (e.g., the U.S. National Geographic Society’s 10th‑edition World Atlas) continue to depict the Atlantic, Indian and Pacific Oceans as extending uninterrupted to Antarctica.
The Notes establish a series of precisely defined maritime limits that separate adjacent oceanic and shelf basins across the Atlantic, its marginal seas, and adjoining tropical waters. These delineations combine meridians, parallels, straight-line joins between coastal extremities, and bathymetric contours to assign narrow channels, island groups and coastal embayments to particular sea basins.
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Caribbean and Gulf of Mexico
A composite boundary encloses the Lesser Antilles and delineates the seaward margin of the Caribbean basin. From Point San Diego (Puerto Rico) the limit proceeds north on the 65°39′W meridian to the 100-fathom contour, then arcs eastward and southward to include the Antillean islands and adjacent shoals, terminating at the northeast and southeast points of Trinidad (Galera Point; Galeota Point) and thence to Baja Point on the Venezuelan coast (9°32′N, 61°0′W). A separate coastal demarcation links Cape Catoche Light (Yucatán) with Cape San Antonio (Cuba), continues across Cuba to the 83°W meridian and thence via the latitude of the Dry Tortugas (24°35′N), Rebecca Shoal and the Florida Keys to the eastern entrance of Florida Bay, thereby incorporating the narrow waters between the Dry Tortugas and the Florida mainland into the Gulf of Mexico.
Northeastern North America and Labrador–Greenland
Maritime separations along the North American northeast coast are defined by chains of points between Nova Scotia and Maine (from Cape St. Mary through Machias Seal Island to Little River Head, Maine) and by a more complex route connecting Cape Canso with points on Cape Breton, the islands of Saint-Pierre-et-Miquelon and Morgan Island, thus distinguishing Canadian, insular and French maritime sectors. A secondary northern sequence runs from Cape Bauld and the east extreme of Belle Isle through the Northeast Ledge and on to the east extreme of Cape St. Charles in Labrador, marking the interface with Arctic-influenced waters. A simple latitudinal divider — the parallel of 60°N between Greenland and Labrador — is also employed as a constant northern limit in this sector. In the wider North Atlantic, a transoceanic link connects Straumness (northwest Iceland) to Cape Nansen (Greenland coast, 68°15′N 29°30′W).
British Isles and adjacent Atlantic margins
Northern British boundaries trace a continuous archipelagic route from Dunnet Head in mainland Scotland through the Orkney and Shetland island groups (via Tor Ness, Kame of Hoy, Breck Ness, Costa Head, Westray, Papa Westray, North Ronaldsay and Horse Island), thereby allocating the inter-island and coastal passages. To the west, a delineation beginning at Bloody Foreland (Ireland) runs by Tory Island to Barra Head and through the Hebrides so that principal islands’ west coasts face the open Atlantic while their inter-island channels are assigned to the Inner Seas up to the Butt of Lewis and Cape Wrath. Cross-channel separation is effected by a straight line from St. David’s Head (Wales) to Carnsore Point (southeastern Ireland). Additional regional transects include a southwestern British Isles line between Hartland Point and St. Govan’s Head, and the western approaches bounded by Isle Vierge (Brittany) to Land’s End, which together distinguish the English Channel approaches, the Celtic Sea and the Atlantic approaches.
Northeastern Iberia–Brittany, Gibraltar and African Atlantic coasts
A north–south axial line from Cap Ortegal (northern Spain) to Penmarch Point (Brittany) partitions adjacent Spanish and French Atlantic waters. At the entrance to the western Mediterranean, the straight join between Cape Trafalgar (Iberia) and Cape Spartel (northwest Africa) functions as the principal threshold marking the western approaches to the Strait of Gibraltar. Along the Gulf of Guinea, a southeastern axis from Cape Palmas (Liberia) to Cape Lopez (Gabon) provides a defined division of the West African Atlantic frontage.
Southern South America
A bilateral maritime limit in the South Atlantic is defined by the line connecting Punta del Este, Uruguay (approximately 34°58.5′S, 54°57.5′W) to Cabo San Antonio, Argentina (approximately 36°18′S, 56°46′W), establishing the seaward separation between Uruguayan and Argentine coastal waters.
Collectively, these note-boundaries employ a mix of geodetic lines, coastal extremities and bathymetric references to produce legally and geographically coherent separations between contiguous sea areas and to allocate channels, shoals and island waters to specific basins.
Indian Ocean
The Indian Ocean occupies the maritime space south of Asia between Africa and Australia, but its cartographic portrayal varies. Some sources, such as the CIA World Factbook, present the basin inclusively, whereas the International Hydrographic Organization (IHO) applies a more prescriptive delimitation that excludes marginal seas it defines separately.
The IHO’s current authoritative framework is the 3rd edition of Limits of Oceans and Seas, which supplies the reference meridians, coastal points and continental termini that determine the ocean’s extent. Under that edition the ocean’s northern edge is not a single latitude but is defined by the southern limits of a series of named marginal waterbodies — notably the Arabian Sea, the Lakshadweep Sea, the Bay of Bengal, the East Indian Archipelago and the Great Australian Bight — all of which the IHO treats as distinct from the ocean proper. The western boundary runs southward along the 20° East meridian passing through Cape Agulhas, while the eastern boundary follows longitude 146°55′ East south from South East Cape, Tasmania. The southern limit of the IHO-defined Indian Ocean is the Antarctic coastline itself, so the ocean is described as extending to Antarctica rather than ending at a sub‑Antarctic parallel.
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In practice, however, many maps and much scientific and popular usage do not observe the IHO’s separations: basins named as marginal seas are frequently depicted as integral parts of the Indian Ocean, and some authorities continue to show the Atlantic, Indian and Pacific oceans as continuous to Antarctica. A proposed IHO revision of 2002 would have relocated the southern limit northward to 60°S and reintroduced a formally defined Southern Ocean; that proposal remains unratified, was the subject of an Australian reservation in 2003, and would require publication in a future (4th) edition to take effect.
These definitional differences are not merely semantic. They affect cartographic nomenclature, scientific communication and legal regimes that use ocean boundaries to delimit jurisdiction. In Australia, for example, historical legislative language — including the Imperial South Australia Colonisation Act of 1834, rolled into the federation-era constitutional framework — explicitly employed the term “Southern Ocean,” so shifts in ocean nomenclature can have direct constitutional and maritime-law consequences.
The IHO delineations recorded in these notes establish precise straight‑line connections and waypoint coordinates that partition adjoining marginal seas and oceanic approaches across the Indian Ocean region and southern Australia. In the central Indian Ocean the IHO draws a zonal link between the southern extremity of Addu Atoll (Maldives) and Ras Hafun on the Somali coast (lat. 10°26′N), thereby linking the southern Maldives to northeastern Africa. A principal southward axis is defined from Dondra Head at Sri Lanka’s southern tip to the most southerly point of Addu Atoll, forming the principal maritime corridor from southern Sri Lanka into the central Maldives. The IHO limits explicitly incorporate Adam’s Bridge (the shallow reef/land connection between India and Sri Lanka) and specify a further connecting line from the south point of Dondra Head to the north point of Poeloe Bras at 5°44′N, 95°04′E (5.733°N, 95.067°E), which fixes a precise northern seaward coordinate east of the Indian landmass.
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Within the Indonesian archipelago the IHO prescribes composite boundaries that separate inter‑island seas by named waypoints and straight lines. For the Java Sea the southern limit follows Java’s north and west coasts to Java Hoofd (6°46′S, 105°12′E) and thence to Vlakke Hoek, the southern extreme of Sumatra. The Bali Sea is delimited by a line from Tanjong Bantenan via the southern points of adjacent islands to the southwest extremity of Lombok (Tanjong Bt Gendang), then along Lombok’s south coast to Tanjong Ringgit and onward to Tanjong Mangkoen (the southwest point of Sumbawa at 9°01′S, 116°43′E). The Savu Sea boundary runs from Timor’s southwest point to Roti, across Roti to Poeloe Dana (10°49′S, 121°17′E), then to the southern and western points of Sumba. Finally, the western limit of the Timor Sea is drawn from Cape Londonderry (Australia) to the southwest point of Roti (10°56′S, 122°48′E), collectively defining the maritime separations among Java, Sumatra, Bali, Lombok, Sumbawa, Timor, Roti, Poeloe Dana and Sumba.
In the southern Australasian sector the IHO specifies a straight line joining West Cape Howe on mainland Australia (35°08′S, 117°37′E) to South West Cape, Tasmania, thereby establishing a formal trans‑Bass Strait/approach alignment between Australia’s southernmost mainland promontory and Tasmania’s southwestern extremity.
Pacific Ocean
The Pacific Ocean is the vast basin that separates Asia and Australia on its western margins from North and South America on its eastern margins, serving as the principal oceanic divide between Eurasia–Australia and the Americas. Cartographic treatments vary: many public maps (for example the CIA World Factbook) depict the Pacific as an uninterrupted expanses of water, while the International Hydrographic Organization applies a more constrained delineation that excludes adjacent marginal seas, coastal gulfs and bays and thus traces a distinct formal outline. Omitting marginal waterbodies means enclosed or semi-enclosed seas and inlets are not included within the ocean’s official limits; this convention reduces the reported ocean area, alters the limits of named seas, and has practical consequences for hydrographic and nautical practice. The Equator (0° latitude) provides a primary latitudinal subdivision into the Northern and Southern Pacific, a partition widely used in climatology, biogeography and regional oceanography. In sum, the Pacific’s western borders are formed by Asia and Australia, its eastern borders by the Americas, it spans both hemispheres, and its mapped extent depends on whether marginal waters are treated as part of the ocean.
North Pacific Ocean
The delimitation of the North Pacific Ocean follows the International Hydrographic Organization’s Limits of Oceans and Seas (3rd edition) and explicitly excludes the internal seas contained within its area. Its perimeter is therefore defined by adjacent sea-limits and coastal boundaries rather than by a continuous natural margin.
To the southwest the North Pacific is bounded along the northeastern archipelagic limit of the East Indian Archipelago, running from the Equator northward to Luzon. The western and northwestern margins are formed by the eastern limits of the Philippine Sea and the Japan Sea together with the southeastern limit of the Sea of Okhotsk. On the north, the ocean ends where the southern limits of the Bering Sea and the Gulf of Alaska meet. The eastern boundary follows the western limit of the coastal waters of southeastern Alaska and British Columbia and the southern limit of the Gulf of California, defining the seaward edge of the northwest American coast and the entrance to the Gulf. The Equator serves as the principal southern limit, with the notable exceptions of the Gilbert and Galápagos island groups, which lie north of the Equator and are therefore included within the North Pacific.
South Pacific Ocean (IHO 3rd Edition)
The International Hydrographic Organization’s third edition of Limits of Oceans and Seas provides the operative legal-geographical definition of the South Pacific Ocean and serves as the reference for the delimitation described here.
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Under that delimitation the western limit is a fixed meridian: a line due south from Southeast Cape, Tasmania (the island’s southernmost point) along 146°55′E until it meets the Antarctic coastline. The ocean’s northwestern and southwestern margins follow the contiguous limits of adjacent seas and archipelagic extents rather than simple great‑circle arcs; specifically, the boundary traces the defined limits of the Tasman Sea, the Coral Sea, the Solomon and Bismarck seas, and the northeastern extent of the East Indian Archipelago from New Guinea to the Equator. The Equator itself is taken as the northern limit, and islands of the Gilbert and Galápagos groups that lie north of 0° latitude are included within the South Pacific as defined by the IHO.
To the east the IHO specifies a two‑part delimitation linking Tierra del Fuego to Antarctica: the principal eastern meridian is that of Cape Horn (67°16′W) running south to the Antarctic continent, supplemented by a closing line drawn between Cape Virgins (given as 52°21′S, 68°21′W; decimal −52.350, −68.350) and Cape Espiritu Santo at the eastern entrance to the Strait of Magellan. This eastern formulation has not received official acceptance from Argentina and Chile, and thus lacks unanimous national endorsement. The southern extent, in the current (3rd) edition, is the Antarctic continent itself so that waters between the Antarctic shore and the other specified limits constitute the ocean’s southern reach.
The IHO list explicitly excludes from the South Pacific waters that the organization defines as separate marginal seas (for example, the Gulf of Alaska and the Coral Sea), a distinction that may differ from common cartographic practice in which such named seas are often treated as parts of the Pacific. A 2002 IHO draft proposed altering the southern boundary of the Pacific to 60°S and restoring a distinct Southern Ocean for waters south of that parallel; that proposal was not ratified (Australia lodged a reservation in 2003) and therefore remains unadopted pending any future fourth edition. Consequently, significant variation persists among atlases and national authorities: many sources continue to depict the principal oceans extending uninterrupted to Antarctica, and the use of the term “Southern Ocean” remains common in other conventions.
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Notes — IHO delimitations of regional sea boundaries
The IHO delineations reported here employ lines drawn between prominent terrestrial and insular features (capes, headlands, island extremities, reefs and lighthouses) and frequently follow island chains or parallels to define the limits of named seas. A consistent principle is the inclusion of archipelagos and the narrow channels between their constituent islands within the adjoining sea by routing boundary lines so that intervening waters are encompassed rather than excluded.
In the northwest Pacific the Philippine Sea is delimited by the submarine and island ridge linking the main Japanese islands with the Bonin (Ogasawara), Volcano and Mariana groups, with those island groups and the waters between them incorporated into the sea. Local straits are fixed by headland-to-headland lines (for example, the Tsugaru Strait), while larger enclosed basins such as the Sea of Okhotsk are bounded by a chain of points running from Hokkaidō through the Kuril islands to Kamchatka so as to embrace all narrow channels. The Bering Sea limit likewise runs from the Alaskan peninsula through the Aleutians to the Commander Islands and Kamchatka, and the Gulf of Alaska lower limit is the line from Cape Spencer to the Alaskan/Bering boundary point, arranged to include adjacent islands.
Along the North American northwest coast the Coastal Waters of British Columbia and Southeast Alaska are defined by a complex westward line from Vancouver Island through the Scott Islands and Haida Gwaii, then northward along the outer margins of the Alexander Archipelago so as to incorporate the narrow passages between those islands. Off Mexico a simple coastwise limit is established by the straight line joining a mainland point near Piastla with the southern tip of Baja California.
In the southwest Pacific the IHO ties subantarctic island groups to Tasmania and to New Zealand by sequences of straight lines linking island extremities and caps: for example, the southern enclosure joining Auckland Island to South East Cape (Tasmania), and the route from Stewart Island via The Snares to Auckland Island that defines southeastern approaches. Eastern approaches between New Zealand’s islands are fixed by lines between named capes and lights in Cook Strait and Foveaux Strait, while the northeastern limit of the North Island sector is set by a line through the Three Kings to North Cape. Offshore reefs and parallels are also used: a segment near Elizabeth Reef is fixed by the intersection of a 30°S parallel with a line joining reef extremities.
The Coral Sea limits are established by linking island groups and reef features from Vanuatu and New Caledonia through Middleton and Elizabeth Reefs to the 30°S parallel, and by connecting the Duff/Wilson Group through Vanuatu (Mera Lava, Aneityum) so that those islands and their intervening straits lie within the Coral Sea. Northern and coastwise boundaries in the Solomon–New Guinea transition follow island-to-island tracks from New Ireland and Buka through Bougainville and the principal Solomon Islands, and continue along the northern and northeastern fringes of New Ireland, New Hanover, the Admiralties and associated groups to Baudissin Point, New Guinea. The western New Guinea (Vogelkop) sector is delimited by lines traversing Waigeo, Batanta and adjacent islets and by coastal points that close the limit toward western New Guinea.
Together these provisions illustrate the IHO practice of using named geographic features and simple connecting lines—often along island chains, reef extents or selected parallels—to produce coherent maritime boundaries that deliberately encompass archipelagic waters and narrow channels within adjacent sea areas.
Southern (Antarctic) Ocean
The Southern Ocean refers to the body of ocean surrounding Antarctica; it is frequently treated not as an entirely separate basin but as the southern extension of the Atlantic, Pacific and Indian Oceans. Its delimitation has been contested, reflecting contrasting principles of coastline-based boundaries and oceanographic criteria such as current systems.
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Early international hydrographic practice used land-based northern limits. The International Hydrographic Organization (IHO) first edition of Limits of Oceans and Seas (1928) drew the oceanal boundary by connecting Antarctic shores to specified southern extremities of South America, Africa, Australia and New Zealand (including Cape Horn, Cape Agulhas, Cape Leeuwin, South East Cape and Broughton Island). The 1937 IHO revision moved those limits southward and expressed them quantitatively: broad sectors were bounded by parallels at 40°S between Cape Agulhas (20°E) and Cape Leeuwin (115°E), and at 55°S between Auckland Island (≈165–166°E) and Cape Horn (67°W).
The IHO’s 1953 edition abandoned a fixed northern limit, citing seasonal and dynamic variability; it left delimitation to national hydrographic authorities and, in practice, allowed the Atlantic, Indian and Pacific Oceans to be mapped as extending farther south. Oceanographic considerations—most importantly the Antarctic Circumpolar Current—later prompted renewed discussion. A 2000 IHO survey of member states found majority support for reinstating a Southern Ocean name (preferred over “Antarctic Ocean”) and substantial support for a 60°S parallel as the northern boundary, though proposals ranged from about 35°S to 60°S.
Attempts to publish a definitive fourth edition have been delayed by naming and boundary disputes. A 2002 draft proposing numerous changes, including extensive renaming and new limits, met formal objections from several states; Australia lodged a reservation specifically concerning Southern Ocean limits. Consequently, the IHO regards the 1953 edition as still in force, even as the 2002 draft definitions have gained de facto currency in many scientific and national contexts and have sometimes been used within IHO working groups.
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National practices remain heterogeneous. Some hydrographic offices adopt their own parallels (for example, the United Kingdom has used 55°S), major reference works vary between treating Antarctica as bordered by the Atlantic/Pacific/Indian Oceans or by a distinct Southern Ocean, and certain national agencies—most notably Australian cartographic authorities—define the Southern Ocean as the waters between Antarctica and the southern coasts of Australia and New Zealand, a delimitation similar to earlier IHO arrangements. Under the IHO second-edition scheme the Great Australian Bight was the principal named feature separating the Australian mainland from the Southern Ocean; contemporary Australian maps commonly label adjacent waters as “Southern Ocean” and describe Cape Leeuwin as the meeting point of the Indian and Southern Oceans.
The debate over the Southern Ocean therefore exemplifies two persistent geographical tensions: first, the choice between land-based, cap-to-cap delimitation and limits derived from oceanographic phenomena (chiefly the Antarctic Circumpolar Current); and second, the administrative reality that international standards, national cartographic practice and scientific convention may diverge, yielding multiple, coexisting definitions keyed to different latitudes (commonly 40°, 50°, 55° or 60°S, and sometimes as far north as 35°S), named coastal points and meridians.
Natural delimitation between the Pacific and South Atlantic: Shackleton Fracture Zone versus Scotia Arc
A recent thesis advances a cartographic proposal that the Shackleton Fracture Zone constitutes the most appropriate natural boundary separating the Pacific Ocean from the South Atlantic in the high southern latitudes. This fracture zone is characterized by a linear bathymetric and lithospheric discontinuity: an abrupt change in seafloor morphology and tectonic fabric that can function as a sharp transition or barrier between oceanic sectors. As such, it is presented as a clear, geomorphologically grounded delimiter that concentrates tectonic and bathymetric contrasts along a narrow locus.
By contrast, the Scotia Arc offers an alternative delimitation premised on topographic continuity. Composed of an arcuate chain of submarine ridges and island fragments linking southern South America to the Antarctic Peninsula, the Arc forms a semi-continuous morphological barrier whose ridges and island chains can be read as a boundary defined by persistent topographic features rather than by a single tectonic discontinuity.
Choosing between these candidates rests on different geographical criteria: the Shackleton Fracture Zone emphasizes tectonic and bathymetric discontinuity, whereas the Scotia Arc emphasizes surface topographic continuity and island-ridge systems. Each choice implies different positions for oceanographic fronts, distinct pathways for surface and deep-water exchange, divergent delimitations of marine biogeographic provinces, and varying patterns of seafloor connectivity. Consequently, adopting one boundary over the other would alter mapped ocean extents and influence scientific interpretations—and potentially cartographic or legal conventions—pertaining to circulation, ecology, and regional classification.
Current scholarly usage, as reflected in the thesis and related literature, favors the Shackleton Fracture Zone as the more frequently advocated contemporary boundary, while the Scotia Arc remains a historically and morphologically defensible alternative.