Subantarctic

Lore & Background

The subantarctic region is defined by oceanographic fronts rather than landmasses. Its northern boundary is the ill-defined Subtropical Front, while its southern boundary is the Antarctic Convergence, located about 200 kilometers south of the Antarctic Polar Front. The region includes the Subantarctic Zone and the Polar Frontal Zone, which together host the Antarctic Circumpolar Current—the only current that flows completely around the Earth, transporting up to 150 Sverdrups of water. This current links the Atlantic, Indian, and Pacific oceans and strongly influences global climate and underwater biodiversity. Subantarctic islands are categorized by latitude and proximity to the Antarctic Convergence. Islands between 46°–50° south, such as the Crozet and Kerguelen groups, feature tundra and are properly considered subantarctic. Those between 51°–56° south, including the Falkland Islands and Tierra del Fuego-associated islands, have trees and temperate grasslands, leading to debate over their classification. Islands between 52°–57° south, like Heard Island and South Georgia, have tundra, permafrost, and volcanoes, and are considered subantarctic because they lie north of 60° south latitude, the political boundary of Antarctica under the Antarctic Treaty. The region's waters are a convergence zone for several water masses, including Subantarctic Mode Water and Antarctic Intermediate Water, which act as a carbon sink. Climate change may reduce the capacity of these waters to absorb atmospheric carbon dioxide, potentially decreasing the Southern Ocean's role in carbon storage.

Reader's Guide

The subantarctic region holds significance as a transitional zone between temperate and Antarctic climates, defined by oceanographic fronts rather than political borders. Its islands serve as critical habitats for seabirds, penguins, seals, and unique benthic marine life, with Heard Island described as a 'biological hot spot.' The Antarctic Circumpolar Current, the most powerful ocean current on Earth, flows through the region, linking the world's major ocean basins and driving thermohaline circulation that affects global climate. The region's waters are also a major carbon sink, with Subantarctic Mode Water and Antarctic Intermediate Water absorbing atmospheric carbon dioxide. However, climate change threatens this function, as warming may reduce the density and salinity of these water masses, decreasing their capacity to store CO2. The political definition of Antarctica under the Antarctic Treaty (south of 60° south latitude) creates a distinction between subantarctic and Antarctic islands, with some islands south of the Antarctic Convergence but north of 60° south still classified as subantarctic. Glaciers in the subantarctic are retreating at significant rates, and volcanic activity on islands like Heard and McDonald is creating new land and freshwater habitats, altering local ecosystems. The region's unique combination of oceanography, climate, and biodiversity makes it a key area for studying global environmental change.

Did You Know?

• The Antarctic Circumpolar Current transports up to 150 Sverdrups (150 million cubic meters per second), equivalent to 150 times the volume of all the world's rivers combined.
• The subantarctic region is bounded to the north by the Subtropical Front and to the south by the Antarctic Convergence, located about 200 kilometers south of the Antarctic Polar Front.
• Under the Antarctic Treaty, Antarctica is defined politically as all land and ice shelves south of 60° south latitude, so some islands south of the Antarctic Convergence are still considered subantarctic.

Geographic Scope and the Deepest Point

The Southern Ocean occupies the waters south of the 60th parallel south, encircling Antarctica. At roughly 21.96 million square kilometers, it ranks as the second-smallest of the five major oceanic divisions—dwarfed by the Pacific, Atlantic, and Indian Oceans yet exceeding the Arctic. In early February 2019, the Five Deeps Expedition used multibeam sonar to locate the ocean's deepest point at coordinates 60° 28' 46"S, 25° 32' 32"W, measuring 7,434 metres below the surface. Expedition leader Victor Vescovo, piloting the crewed submersible DSV Limiting Factor, reached that bottom on February 3, 2019, and proposed the name "Factorian Deep" for the site. This survey confirmed the ocean's true vertical extent under the 60th-parallel definition and marked the first human visit to that particular seafloor.

The Enduring Naming and Boundary Dispute

For centuries, the waters surrounding Antarctica lacked a stable identity. The phrase "Southern Ocean" originally referred to the Pacific as seen from Panama, a usage dating to Vasco Núñez de Balboa. By the 18th and 19th centuries, various authors assigned different northern boundaries—Cook implied New Caledonia bordered it, while later writers placed the limit at 40, 50, or even the Antarctic Circle. The IHO's 1928 first edition of Limits of Oceans and Seas drew land-based borders touching South America, Africa, Australia, and New Zealand. Yet by 1953 the ocean was dropped from the official publication entirely, leaving regional hydrographic offices to decide their own limits. The 2000 IHO revision reinstated the ocean south of the 60th parallel, but that definition never achieved formal adoption because of unrelated disputes, notably over the Sea of Japan. The Australian Government simply treats the Southern Ocean as the waters immediately south of its own coastline, while National Geographic did not officially recognize the name until June 2021.

Role in Global Climate Circulation

The Southern Ocean's overturning circulation forms the second major component of Earth's thermohaline conveyor, complementing the more widely discussed Atlantic meridional overturning circulation. This deep-water movement is critical to redistributing heat and nutrients across the globe. However, ongoing climate change is already disrupting the system. Warming has increased ocean stratification, and scientists warn the circulation could slow dramatically or even cross a tipping point toward outright collapse. Such a collapse would unfold over centuries but would carry severe consequences for global weather patterns and the marine ecosystems that depend on the ocean's biological productivity. The full importance of this overturning circulation was only ascertained in the late 20th century, and it was precisely this understanding that prompted the IHO to formally define the Southern Ocean in its 2000 revisions. The ocean is thus not merely a geographic label but a functional engine of the planetary climate system.

From Cook's Voyages to Modern Cartographic Acceptance

James Cook's 1770s expeditions demonstrated that open waters encircled the southern latitudes, yet the question of whether those waters constituted a distinct ocean persisted for over two centuries. Before the 20th century, the term was applied loosely: a 1745 British act referenced a "Southern Ocean of America" in the context of a Northwest Passage prize, and the 1834 South Australia Act used the phrase to describe the province's southern maritime boundary. The IHO's 1919 First International Conference set the stage for formal delineation, and the 1928 publication established land-anchored limits running from Cape Horn through Africa, Australia, and New Zealand back to its starting point. The 1937 edition shifted those northern boundaries southward, detaching much of the limit from coastlines. Despite these efforts, the ocean remained absent from official IHO publications from 1953 onward. It was not until National Geographic's June 2021 decision to print the name in the same typeface as the other four oceans that the Southern Ocean achieved broad mainstream cartographic legitimacy.