The sub-Antarctic zone is a region in the Southern Hemisphere, located immediately north of the

Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...

region. This translates roughly to a latitude of between 46° and 60° south of the

Equator The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can als ...

. The subantarctic region includes many islands in the southern parts of the Atlantic, Indian, and Pacific oceans, especially those situated north of the Antarctic Convergence. Sub-Antarctic glaciers are, by definition, located on islands within the sub-Antarctic region. All glaciers located on the continent of Antarctica are by definition considered to be Antarctic glaciers.

Geography

The sub-Antarctic region comprises two geographic zones and three distinct fronts. The northernmost boundary of the subantarctic region is the rather ill-defined Subtropical Front (STF), also referred to as the Subtropical Convergence. To the south of the STF is a geographic zone, the Subantarctic Zone (SAZ). South of the SAZ is the Subantarctic Front (SAF). South of the SAF is another marine zone, called the Polar Frontal Zone (PFZ). The SAZ and the PFZ together form the subantarctic region. The southernmost boundary of the PFZ (and hence, the southern border of the subantarctic region) is the Antarctic Convergence, located approximately 200 kilometers south of the Antarctic Polar Front (APF).

Influence of the Antarctic Circumpolar Current and thermohaline circulation

The sub-Antarctic Front, found between 48°S and 58°S in the Indian and Pacific Ocean and between 42°S and 48°S in the Atlantic Ocean, defines the northern boundary of the

Antarctic Circumpolar Current The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise (as seen from the South Pole) from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feat ...

(or ACC). The ACC is the most important ocean current in the Southern Ocean, and the only current that flows completely around the Earth. Flowing eastward through the southern portions of the Atlantic, Indian, and Pacific Oceans, the ACC links these three otherwise separate oceanic basins. Extending from the sea surface to depths of 2000–4000 meters, and with a width of as great as 2000 kilometers, the ACC transports more water than any other ocean current. The ACC carries up to 150 Sverdrups (150 million cubic meters per second), equivalent to 150 times the volume of water flowing in all the world's rivers. The ACC and the global thermohaline circulation strongly influence regional and global climate as well as underwater biodiversity. Another factor that contributes to the climate of the subantarctic region, though to a much lesser extent than the thermohaline circulation, is the formation of Antarctic Bottom Water (ABW) by halothermal dynamics. The halothermal circulation is that portion of the global ocean circulation that is driven by global density gradients created by surface heat and

evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

Definition of subantarctic: political versus scientific

Several distinct water masses converge in the immediate vicinity of the APF or Antarctic Convergence (in particular the Subantarctic Surface Water (Subantarctic Mode Water or SAMW),

Antarctic Surface Water The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...

, and the

Antarctic Intermediate Water Antarctic Intermediate Water (AAIW) is a cold, relatively low salinity water mass found mostly at intermediate depths in the Southern Ocean. The AAIW is formed at the ocean surface in the Antarctic Convergence zone or more commonly called the Ant ...

). This convergence creates a unique environment, noted for its very high marine productivity, especially for

antarctic krill Antarctic krill (''Euphausia superba'') is a species of krill found in the Antarctic waters of the Southern Ocean. It is a small, swimming crustacean that lives in large schools, called swarms, sometimes reaching densities of 10,000–30,000 ind ...

. Because of this, all lands and waters situated south of the Antarctic Convergence are considered to belong to the Antarctic from a climatological,

biological Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary in ...

and hydrological standpoint. However, the text of the Antarctic Treaty, article VI ("Area covered by Treaty") states: "The provisions of the present Treaty shall apply to the area south of 60° South latitude". Therefore, Antarctica is defined from a political standpoint as all land and ice shelves south of 60°S latitude.

Subantarctic islands

The Tristan da Cunha Group,

Gough Island upright=1.3, Map of Gough island Gough Island ( ), also known historically as Gonçalo Álvares, is a rugged volcanic island in the South Atlantic Ocean. It is a dependency of Tristan da Cunha and part of the British overseas territory of Sain ...

, Amsterdam Island, and Saint Paul Island are all isolated volcanic islands situated at between 37°– 40° south of the Equator, just south of the

southern Southern may refer to: Businesses * China Southern Airlines, airline based in Guangzhou, China * Southern Airways, defunct US airline * Southern Air, air cargo transportation company based in Norwalk, Connecticut, US * Southern Airways Express, M ...

horse latitudes. Because they are located far to the north of the Antarctic Convergence and have a relatively temperate climate, they are not typically considered to be subantarctic islands. At between about 46°– 50° south of the Equator, in the region often referred to as the '' Roaring Forties'', are the

Crozet Islands The Crozet Islands (french: Îles Crozet; or, officially, ''Archipel Crozet'') are a sub-Antarctic archipelago of small islands in the southern Indian Ocean. They form one of the five administrative districts of the French Southern and Antarcti ...

, Prince Edward Islands,

Bounty Islands The Bounty Islands ( mi, Moutere Hauriri; "Island of angry wind") are a small group of 13 uninhabited granite islets and numerous rocks, with a combined area of , in the South Pacific Ocean. Territorially part of New Zealand, they lie about e ...

, Snares Islands,

Kerguelen Islands The Kerguelen Islands ( or ; in French commonly ' but officially ', ), also known as the Desolation Islands (' in French), are a group of islands in the sub-Antarctic constituting one of the two exposed parts of the Kerguelen Plateau, a large ...

, Antipodes Islands, and Auckland Islands. The geography of these islands is characterized by tundra, with some trees on Snares and Auckland Islands. These islands are all located near the Antarctic Convergence (with Kerguelen south of the Convergence) and are properly considered to be subantarctic islands. At between 51°– 56° south of the Equator, the Falkland Islands, Isla de los Estados, Ildefonso Islands, Diego Ramírez Islands, and other islands associated with Tierra del Fuego and Cape Horn, lie north of the Antarctic Convergence in the region often referred to as the ''

Furious Fifties The Roaring Forties are strong westerly winds found in the Southern Hemisphere, generally between the latitudes of 40°S and 50°S. The strong west-to-east air currents are caused by the combination of air being displaced from the Equator ...

''. Unlike other subantarctic islands, these islands have trees, temperate grasslands (mostly '' tussac grass''), and even arable land. They also lack tundra and

permanent snow The climatic snow line is the boundary between a snow-covered and snow-free surface. The actual snow line may adjust seasonally, and be either significantly higher in elevation, or lower. The permanent snow line is the level above which snow wil ...

and ice at their lowest elevations. Despite their more southerly location, it is debatable whether these islands should be considered as such because their climate and geography differs significantly from other subantarctic islands. At between 52°– 57° south of the Equator, the Campbell Island Group, Heard Island and McDonald Islands, Bouvet Island, the South Georgia Group, Macquarie Island, and the

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are also located in the Furious Fifties. The geography of these islands is characterized by tundra,

permafrost Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...

, and volcanoes. These islands are situated close to or south of the Antarctic Convergence, but north of 60° S latitude (the continental limit according to the Antarctic Treaty). Therefore, although some are located south of the Antarctic Convergence, they should still be considered as subantarctic islands by virtue of their location north of 60° S. At between 60°– 69° south of the Equator, the

South Orkney Islands The South Orkney Islands are a group of islands in the Southern Ocean, about north-east of the tip of the Antarctic PeninsulaSouth Shetland Islands,

Balleny Islands The Balleny Islands () are a series of uninhabited islands in the Southern Ocean extending from 66°15' to 67°35'S and 162°30' to 165°00'E. The group extends for about in a northwest-southeast direction. The islands are heavily glaciated an ...

, Scott Island, and

Peter I Island Peter I Island ( no, Peter I Øy) is an uninhabited volcanic island in the Bellingshausen Sea, from continental Antarctica. It is claimed as a dependency of Norway and, along with Bouvet Island and Queen Maud Land, composes one of the three No ...

are all properly considered to be Antarctic islands for the following three reasons: #they are all located south of the Antarctic Convergence #they are all located within the Southern (or Antarctic) Ocean #they are all located south of the 60th parallel south (in the region often referred to as the ''

Shrieking Sixties The Roaring Forties are strong westerly winds found in the Southern Hemisphere, generally between the latitudes of 40°S and 50°S. The strong west-to-east air currents are caused by the combination of air being displaced from the Equator ...

'') In light of the above considerations, the following should be considered to be subantarctic islands: } or officially ''Archipel Crozet'') , , Indian Ocean , France , - , Heard Island and McDonald Islands (HIMI) , , Indian Ocean ,

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

, - ,

, , Indian Ocean , France , - , Macquarie Island , , Pacific Ocean , Australia , - , Prince Edward Islands , , Indian Ocean , South Africa , - , South Georgia Group , , Atlantic Ocean , United Kingdom , - ,

, , Atlantic Ocean , United Kingdom , - , Snares Islands , , Pacific Ocean , New Zealand

Subantarctic glaciers

This is a list of glaciers in the subantarctic. This list includes one snow field ( Murray Snowfield). Snow fields are not glaciers in the strict sense of the word, but they are commonly found at the accumulation zone or head of a glacier. For the purposes of this list, Antarctica is defined as any latitude further south than 60° (the continental limit according to the Antarctic Treaty). Thatcher-Peninsula

Climate

Impact of climate change on SAMW

Together, the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) act as a carbon sink, absorbing atmospheric carbon dioxide and storing it in solution. If the SAMW temperature increases as a result of climate change, the SAMW will have less capacity to store dissolved carbon dioxide. Research using a computerized climate system model suggests that if atmospheric carbon dioxide concentration were to increase to 860 ppm by the year 2100 (roughly double today's concentration), the SAMW will decrease in density and salinity. The resulting reductions in the subduction and transport capacity of SAMW and AAIW water masses could potentially decrease the absorption and storage of CO₂ in the Southern Ocean.

Flora and fauna

*''Main: Flora of subantarctic islands, and Fauna of subantarctic islands''. The Antarctic realm and Antarctic Floristic Kingdom include most of the subantarctic islands native

biota Biota may refer to: * Biota (ecology), the plant and animal life of a region * Biota (plant), common name for a coniferous tree, ''Platycladus orientalis'' * Biota, Cinco Villas, a municipality in Aragon, Spain * Biota (band), a band from Color ...

, with many endemic

genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...

and species of flora and fauna.

Subantarctic island example

The physical landscape and biota communities of Heard Island and McDonald Islands are constantly changing due to volcanism, strong winds and waves, and climate change. Volcanic activity has been observed in this area since the mid-1980s, with fresh lava flows on the southwest flanks of Heard Island.

Satellite imagery Satellite images (also Earth observation imagery, spaceborne photography, or simply satellite photo) are images of Earth collected by imaging satellites operated by governments and businesses around the world. Satellite imaging companies sell ima ...

shows that McDonald Island increased in size from about 1 to 2.5 square kilometers between 1994 and 2004, as a result of volcanic activity. In addition to new land being produced by volcanism, global warming of the climate is causing the retreat of glaciers on the islands (''see section below ''). These combined processes produce new ice-free terrestrial and

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...

ecoregions, such as

moraine A moraine is any accumulation of unconsolidated debris (regolith and rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice shee ...

s and lagoons, which are now available for colonization by plants and animals. Heard Island has vast colonies of

penguin Penguins (order (biology), order List of Sphenisciformes by population, Sphenisciformes , family (biology), family Spheniscidae ) are a group of Water bird, aquatic flightless birds. They live almost exclusively in the Southern Hemisphere: on ...

s and petrels, and large harems of land-based marine predators such as elephant seals and fur seals. Due to the very high numbers of seabirds and marine mammals on Heard Island, the area is considered a "biological hot spot". The marine environment surrounding the islands features diverse and distinctive benthic habitats that support a range of species including corals, sponges,

barnacle A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in eros ...

s and echinoderms. This marine environment also serves as a nursery area for a range of fishes, including some species of commercial interest.

Retreat of subantarctic glaciers

Glaciers are currently retreating at significant rates throughout the southern hemisphere. With respect to glaciers of the

Andes mountains The Andes, Andes Mountains or Andean Mountains (; ) are the List of mountain ranges#Mountain ranges by length, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range i ...

in South America, abundant evidence has been collected from ongoing research at Nevado del Ruiz in

Colombia Colombia (, ; ), officially the Republic of Colombia, is a country in South America with insular regions in North America—near Nicaragua's Caribbean coast—as well as in the Pacific Ocean. The Colombian mainland is bordered by the Car ...

Quelccaya Ice Cap The Quelccaya Ice Cap (also known as Quenamari Ice Cap) is the second largest glaciated area in the tropics, after Coropuna. Located in the Cordillera Oriental section of the Andes mountains in Peru, the cap covers an area of with ice up t ...

and

Qori Kalis Glacier The Qori Kalis Glacier is a tropical glacier located in the Cordillera Oriental section of the Andes mountains of Peru. It is one of the few tropical glaciers left in the world, and is the main outlet of the Quelccaya Ice Cap. This glacier has l ...

in Peru, Zongo, Chacaltaya and Charquini glaciers in

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, the Aconcagua River Basin in the central Chilean Andes, and the Northern Patagonian and Southern Patagonian ice fields. Retreat of glaciers in New Zealand and Antarctica is also well documented. Many subantarctic glaciers are also in retreat. Mass balance is significantly negative on many glaciers on Kergeulen Island, Heard Island, South Georgia and Bouvet Island.

Glaciers of Heard Island

Heard Island is a heavily glacierized, subantarctic volcanic island located in the Southern Ocean, roughly 4000 kilometers southwest of

. 80% of the island is covered in ice, with glaciers descending from 2400 meters to sea level. Due to the steep topography of Heard Island, most of its glaciers are relatively thin (averaging only about 55 meters in depth). The presence of glaciers on Heard Island provides an excellent opportunity to measure the rate of glacial retreat as an indicator of climate change. Available records show no apparent change in glacier mass balance between 1874 and 1929. Between 1949 and 1954, marked changes were observed to have occurred in the ice formations above on the southwestern slopes of

Big Ben Big Ben is the nickname for the Great Bell of the Great Clock of Westminster, at the north end of the Palace of Westminster in London, England, and the name is frequently extended to refer also to the clock and the clock tower. The officia ...

, possibly as a result of volcanic activity. By 1963, major recession was obvious below on almost all glaciers, and minor recession was evident as high as . Retreat of glacier fronts across Heard Island is evident when comparing

aerial photograph Aerial photography (or airborne imagery) is the taking of photographs from an aircraft or other airborne platforms. When taking motion pictures, it is also known as aerial videography. Platforms for aerial photography include fixed-wing ai ...

s taken in December 1947 with those taken on a return visit in early 1980. Retreat of Heard Island glaciers is most dramatic on the eastern section of the island, where the termini of former tidewater glaciers are now located inland. Glaciers on the northern and western coasts have narrowed significantly, while the area of glaciers and

ice cap In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets. Description Ice caps are not constrained by topographical features ...

s on Laurens Peninsula have shrunk by 30% - 65%. During the time period between 1947 and 1988, the total area of Heard Island's glaciers decreased by 11%, from 288 km² (roughly 79% of the total area of Heard Island) to only 257 km². A visit to the island in the spring of 2000 found that the

Stephenson Stephenson is a medieval patronymic surname meaning "son of Stephen". The earliest public record is found in the county of Huntingdonshire in 1279. There are variant spellings including Stevenson. People with the surname include: *Ashley Stephen ...

, Brown and Baudissin glaciers, among others, had retreated even further. The terminus of Brown Glacier has retreated approximately 1.1 kilometres since 1950. The total ice-covered area of Brown Glacier is estimated to have decreased by roughly 29% between 1947 and 2004. This degree of loss of glacier mass is consistent with the measured increase in temperature of +0.9 °C over that time span. The coastal ice cliffs of Brown Glacier and Stephenson Glacier, which in 1954 were over high, had disappeared by 1963 when the glaciers terminated as much as inland. Baudissin Glacier on the north coast has lost at least , and Vahsel Glacier on the west coast has lost at least . Winston Glacier, which retreated approximately between 1947 and 1963, appears to be a very sensitive indicator of glacier change on the island. The young moraines flanking

Winston Lagoon Winston Lagoon () is a lagoon indenting the southeast coast of Heard Island in the southern Indian Ocean, about 1 nautical mile (1.9 km) northeast of Cape Lockyer. The feature is roughly portrayed on an American sealer chart of the 1860 period ...

show that Winston Glacier has lost at least of ice within a recent time period. The glaciers of Laurens Peninsula, whose maximum elevation is only 500 m above sea level, are smaller and shorter than most of the other Heard Island glaciers, and therefore much more sensitive to temperature effects. Accordingly, their total area has decreased by over 30 percent.

Jacka Glacier Jacka Glacier () is a long glacier which flows northeast from Hayter Peak and terminates in icefalls opposite Vanhoffen Bluff on the north side of Heard Island in the southern Indian Ocean. The glacier appears to be roughly charted on an 1860 s ...

on the east coast of Laurens Peninsula has also demonstrated marked recession since 1955. In the early 1950s, Jacka Glacier had receded only slightly from its position in the late 1920s, but by 1997 it had receded about 700 m back from the coastline. Possible causes of glacier recession on Heard Island include: #Volcanic activity #Southward movement of the Antarctic Convergence: such a movement conceivably might cause glacier retreat through a rise in sea and air temperatures #Climatic change The Australian Antarctic Division conducted an expedition to Heard Island during the austral summer of 2003–04. A small team of scientists spent two months on the island, conducting studies on avian and terrestrial biology and glaciology. Glaciologists conducted further research on the Brown Glacier, in an effort to determine whether glacial retreat is rapid or punctuated. Using a portable echo sounder, the team took measurements of the volume of the glacier. Monitoring of climatic conditions continued, with an emphasis on the impact of Foehn winds on glacier mass balance. Based on the findings of that expedition, the rate of loss of glacier ice on Heard Island appears to be accelerating. Between 2000 and 2003, repeat

GPS The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...

surface surveys revealed that the rate of loss of ice in both the ablation zone and the accumulation zone of Brown Glacier was more than double average rate measured from 1947 to 2003. The increase in the rate of ice loss suggests that the glaciers of Heard Island are reacting to ongoing climate change, rather than approaching dynamic equilibrium. The retreat of Heard Island's glaciers is expected to continue for the foreseeable future.

References

External links

U.S. Geological Survey, Atlas of Antarctic Research
{{Portal bar, Geography * Geography of the Southern Ocean * Antarctic realm