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Subantarctic

 

Subantarctic

The Antarctica region and its boundary, the Antarctic Convergence.

The Subantarctic is a region in the southern hemisphere, located immediately north of the Antarctic region. This translates roughly to a latitude of between 46°60° south of the Equator. The subantarctic region includes many islands in the southern parts of the Indian Ocean, Atlantic Ocean and Pacific Ocean, especially those situated north of the Antarctic Convergence. Subantarctic glaciers are, by definition, located on islands within the subantarctic region. All glaciers located on the continent of Antarctica are by definition considered to be Antarctic glaciers.

Contents

  • Geography 1
    • Influence of the Antarctic Circumpolar Current and thermohaline circulation 1.1
    • Definition of subantarctic: political versus scientific 1.2
    • Subantarctic islands 1.3
    • Subantarctic glaciers 1.4
  • Climate 2
    • Impact of climate change on SAMW 2.1
  • Flora and fauna 3
    • Subantarctic island example 3.1
  • Retreat of subantarctic glaciers 4
    • Glaciers of Heard Island 4.1
  • See also 5
  • References 6
  • Further reading 7
  • External links 8

Geography

The subantarctic 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).[1]

Influence of the Antarctic Circumpolar Current and thermohaline circulation

Diagram of the major ocean currents, showing the Antarctic Circumpolar Current (ACC). In addition to the global thermohaline circulation, the ACC strongly influences regional and global climate.
Global thermohaline circulation strongly influences regional and global climate. Blue paths represent deep-water currents, while red paths represent surface currents.

The Subantarctic 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 (or ACC).[1] 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.[2] 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.[3] The ACC and the global thermohaline circulation strongly influence regional and global climate as well as underwater biodiversity.[4]

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.

Definition of subantarctic: political versus scientific

Diagram showing different water masses in the Southern Ocean.

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, and the Antarctic Intermediate Water). This convergence creates a unique environment, noted for its very high marine productivity, especially for Antarctic krill. Because of this, all lands and waters situated south of the Antarctic Convergence are considered to belong to the Antarctic from a climatological, biological 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".[5] Therefore, Antarctica is defined from a political standpoint as all land and ice shelves south of 60°S latitude.

Subantarctic islands

Antarctica and surrounding islands
Trees growing along the north shore of the Beagle Channel, 55°S.

Tristan da Cunha group, Île Amsterdam, Île Saint-Paul, and Gough Island are all isolated volcanic islands situated at between 37° - 40° south of the equator, just south of the southern 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 46°50° south of the equator, in the region often referred to as the Roaring Forties, are the Crozet Islands, Prince Edward Islands, Bounty Islands, Snares Islands, Kerguelen Islands, Antipodes Islands, and Auckland Islands. These islands, all located approximately on the Antarctic Convergence (roughly the northern boundary of the subantarctic region), 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. Unlike other subantarctic islands, these islands have trees, temperate grasslands (mostly tussac grass), and even arable land. They also lack tundra and permanent snow 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, South Sandwich Islands are also located in the Furious Fifties. The geography of these islands is characterized by tundra, permafrost, and volcanoes. These islands are situated south of the Antarctic Convergence, but north of 60°S latitude (the continental limit according to the Antarctic Treaty).[5] Therefore, despite their being located significantly south of the Antarctic Convergence, they should still be considered to be subantarctic islands by virtue of their location north of the 60° latitude.

At between 60° – 69° south of the equator, the South Orkney Islands, South Shetland Islands, Balleny Islands, Scott Island, and Peter I Island are all properly considered to be Antarctic islands for the following three reasons:

  1. they are all located south of the Antarctic Convergence
  2. they are all located within the Southern (or Antarctic) Ocean
  3. they are all located south of the 60° latitude (in the region often referred to as the Shrieking Sixties)

In light of the above considerations, the following should be considered to be subantarctic islands:

Name of island group Coordinates[6][7] Ocean[6] Claimed by
Antipodes Islands Pacific Ocean New Zealand
Auckland Islands Pacific Ocean New Zealand
Bounty Islands Pacific Ocean New Zealand
Bouvet Island (Bouvetøya) Atlantic Ocean Norway
Campbell Island group Pacific Ocean New Zealand
Crozet Islands (French: Îles Crozet or officially Archipel Crozet) Indian Ocean France
Heard Island and McDonald Islands (HIMI) Indian Ocean Australia
Kerguelen Islands Indian Ocean France
Macquarie Island Pacific Ocean Australia
Prince Edward Islands Indian Ocean South Africa
South Georgia Group Atlantic Ocean United Kingdom
South Sandwich Islands 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.[8] For the purposes of this list, Antarctica is defined as any latitude further south than 60° (the continental limit according to the Antarctic Treaty).[5]

Satellite image of the southern tip of Heard Island. Cape Arkona is seen on the left side of the image, with Lied Glacier just above and Gotley Glacier just below. Big Ben Volcano and Mawson Peak are seen at the lower right side of the image.
Satellite image of central Harker Glacier, Cumberland Bay, Thatcher Peninsula, Allardyce Range, Mount Paget.
South Georgia, circa 1882.
South Georgia, circa 1882.
South Georgia.
Name of Glacier Coordinates[6][7] Length or (Width)[6] Location
Abbotsmith Glacier 4.8 km Heard Island
Allison Glacier Heard Island
Austin Glacier South Georgia Group
Bary Glacier South Georgia Group
Baudissin Glacier (2.8 km) Heard Island
Bertrab Glacier "small" South Georgia Group
Bogen Glacier "small" South Georgia Group
Briggs Glacier South Georgia Group
Brøgger Glacier 13 km South Georgia Group
Brown Glacier Heard Island
Brunonia Glacier South Georgia Group
Buxton Glacier South Georgia Group
Challenger Glacier Heard Island
Christensen Glacier South Georgia Group
Christensen Glacier Bouvet Island
Christophersen Glacier South Georgia Group
Clayton Glacier South Georgia Group
Compton Glacier Heard Island
Cook Glacier South Georgia Group
Crean Glacier 6 km South Georgia Group
Deacock Glacier Heard Island
Dead End Glacier South Georgia Group
Downes Glacier Heard Island
Ealey Glacier Heard Island
Eclipse Glacier South Georgia Group
Esmark Glacier South Georgia Group
Fiftyone Glacier Heard Island
Fortuna Glacier South Georgia Group
Geikie Glacier South Georgia Group
Gotley Glacier 13.2 km Heard Island
Graae Glacier 3.2 km South Georgia Group
Grace Glacier South Georgia Group
Hamberg Glacier South Georgia Group
Harker Glacier South Georgia Group
Harmer Glacier South Georgia Group
Heaney Glacier South Georgia Group
Helland Glacier South Georgia Group
Henningsen Glacier South Georgia Group
Herz Glacier South Georgia Group
Hindle Glacier 10 km South Georgia Group
Hodges Glacier South Georgia Group
Horntvedt Glacier Bouvet Island
Jacka Glacier 1.3 km Heard Island
Jenkins Glacier South Georgia Group
Jewell Glacier South Georgia Group
Keilhau Glacier 8 km South Georgia Group
Kjerulf Glacier South Georgia Group
König Glacier South Georgia Group
Lancing Glacier South Georgia Group
Lewald Glacier South Georgia Group
Lied Glacier Heard Island
Lucas Glacier South Georgia Group
Lyell Glacier South Georgia Group
Mary Powell Glacier Heard Island
Morris Glacier South Georgia Group
Murray Snowfield South Georgia Group
Nachtigal Glacier South Georgia Group
Neumayer Glacier 13 km South Georgia Group
Nordenskjöld Glacier "large" South Georgia Group
Novosilski Glacier 13 km South Georgia Group
Paget Glacier 6 km South Georgia Group
Peters Glacier South Georgia Group
Philippi Glacier South Georgia Group
Posadowsky Glacier Bouvet Island
Price Glacier South Georgia Group
Purvis Glacier South Georgia Group
Quensel Glacier "small" South Georgia Group
Risting Glacier South Georgia Group
Ross Glacier 10 km South Georgia Group
Ryan Glacier South Georgia Group
Salomon Glacier South Georgia Group
Schmidt Glacier Heard Island
Schrader Glacier South Georgia Group
Spenceley Glacier South Georgia Group
Stephenson Glacier Heard Island
Storey Glacier South Georgia Group
Twitcher Glacier 6 km South Georgia Group
Tyrrell Glacier South Georgia Group
Vahsel Glacier Heard Island
Webb Glacier 3.2 km South Georgia Group
Weddell Glacier 3.2 km South Georgia Group
Wheeler Glacier 3.2 km South Georgia Group
Winston Glacier Heard Island

Climate

Impact of climate change on SAMW

Air-sea exchange of CO2

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.[9]

Flora and fauna

The Antarctic ecozone and Antarctic Floristic Kingdom include most of the subantarctic islands native biota, with many endemic genera 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 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.[10]

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 ecoregions, such as moraines and lagoons, which are now available for colonization by plants and animals.[10]

Heard Island has vast colonies of penguins 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".[10] The marine environment surrounding the islands features diverse and distinctive benthic habitats that support a range of species including corals, sponges, barnacles and echinoderms. This marine environment also serves as a nursery area for a range of fishes, including some species of commercial interest.[10]

Retreat of subantarctic glaciers

Retreat of San Rafael Glacier from 1990 to 2000. San Quintín Glacier is shown in the background

Glaciers are currently retreating at significant rates throughout the southern hemisphere. With respect to glaciers of the Andes mountains in South America, abundant evidence has been collected from ongoing research at Nevado del Ruiz in Colombia,[11][12] Quelccaya Ice Cap and Qori Kalis Glacier in Peru,[13][14] Zongo, Chacaltaya and Charquini glaciers in Bolivia,[15] the Aconcagua River Basin in the central Chilean Andes,[16] and the Northern Patagonian and Southern Patagonian ice fields.[17][18][19] Retreat of glaciers in New Zealand[20] and Antarctica is also well documented.

Bearing this in mind, it should come as no surprise that many subantarctic glaciers are also in retreat.

  • U.S. Geological Survey, Atlas of Antarctic Research

External links

  • U. Radok and D. Watts (1975). "A synoptic background to glacier variations of Heard Island". Snow and Ice (Proceedings of the Moscow Symposium, August 1971) (PDF) (104 ed.). Wallingford, Oxfordshire, UK: International Association of Hydrological Sciences. pp. 42–56. Retrieved 7 June 2010. 
  • Truffer, M., Thost, D. and Ruddell, A. (2001). "The Brown Glacier, Heard Island: its morphology, dynamics, mass balance and climate setting". Antarctic CRC Research Report No. 24. Hobart, Tasmania: Cooperative Research Centre for the Antarctic and Southern Ocean Environment, University of Tasmania. pp. 1–27. 
  • Kevin Kiernan and Anne McConnell (2002). "Glacier retreat and melt-lake expansion at Stephenson Glacier, Heard Island World Heritage Area" (PDF). Polar Record 38 (207): 297–308.  
  • Paul Carroll (1 March 2004). "The South Atlantic and Subantarctic Islands". Derby, United Kingdom: Paul Carroll. Archived from the original on 16 May 2006. Retrieved 14 June 2010. 

Further reading

  1. ^ a b Ryan Smith, Melicie Desflots, Sean White, Arthur J. Mariano, Edward H. Ryan (2008). "Surface Currents in the Southern Ocean:The Antarctic CP Current". The Cooperative Institute for Marine and Atmospheric Studies (CIMAS). Retrieved 1 June 2010. 
  2. ^ Klinck, J. M., W. D. Nowland Jr. (2001). "Antarctic Circumpolar Current". Encyclopedia of Ocean Science (1st ed.). New York: Academic Press. pp. 151–159. 
  3. ^ Joanna Gyory, Arthur J. Mariano, Edward H. Ryan. "The Gulf Stream". The Cooperative Institute for Marine and Atmospheric Studies (CIMAS). Retrieved 1 June 2010. 
  4. ^ Ray Lilley (19 May 2008). "Millions of tiny starfish inhabit undersea volcano". Associated Press. Retrieved 1 June 2010. 
  5. ^ a b c Office of Polar Programs (OPP) (26 April 2010). "The Antarctic Treaty". The  
  6. ^ a b c d , "Antarctic Names".  
  7. ^ a b "Antarctic Gazetteer".  
  8. ^ Dr. Sue Ferguson,  
  9. ^ Stephanie M. Downes, Nathaniel L. Bindoff, Stephen R. Rintoul (2009). "Impacts of Climate Change on the Subduction of Mode and Intermediate Water Masses in the Southern Ocean". Journal of Climate 22 (12): 3289–3302.  
  10. ^ a b c d e f Big brother' monitors glacial retreat in the sub-Antarctic"'". Kingston, Tasmania, Australia: Australian Antarctic Division. 8 October 2008. Retrieved 19 June 2013. 
  11. ^ Jon J. Major and Christopher G. Newhall (1989). "Snow and ice perturbation during historical volcanic eruptions and the formation of lahars and floods". Bulletin of Volcanology 52 (1): 1–27.  
  12. ^ Cristian Huggel; Ceballos, Jorge Luis; Pulgarín, Bernardo; Ramírez, Jair; Thouret, Jean-Claude (2007). "Review and reassessment of hazards owing to volcano–glacier interactions in Colombia" (PDF). Annals of Glaciology 45: 128–136.  
  13. ^ Richard S. Williams, Jr., and Jane G. Ferrigno (9 February 1999). "Peruvian Cordilleras".  
  14. ^ L.G. Thompson, E. Mosley-Thompson; et al. (1 June 2010). "Peru - Quelccaya (1974 - 1983)".  
  15. ^ Bernard Francou (Institut de Recherche pour le Développement) (17 January 2001). "Small Glaciers Of The Andes May Vanish In 10-15 Years". UniSci, International Science News. Retrieved 1 June 2010. 
  16. ^ Francisca Bown, Andres Rivera, Cesar Acuna (2008). "Recent glacier variations at the Aconcagua Basin, central Chilean Andes" (PDF). Annals of Glaciology 48 (2): 43–48.  
  17. ^ Jonathan Amos (27 April 2004). "Patagonian ice in rapid retreat". BBC News. Retrieved 1 June 2010. 
  18. ^ Mariano H. Masiokas, Andrés Rivera, Lydia E. Espizua,  
  19. ^  
  20. ^ "Glaciers of New Zealand". Satellite Image Atlas of Glaciers of the World. U.S. Geological Survey. Retrieved 1 June 2010. 
  21. ^ a b c d e f g Ian F. Allison and Peter L. Keage (1986). "Recent changes in the glaciers of Heard Island". Polar Record 23 (144): 255–272.  
  22. ^ a b c d e f Andrew Ruddell (2010-05-25). "Our subantarctic glaciers: why are they retreating?". Glaciology Program, Antarctic CRC and AAD. Retrieved 19 June 2013. 
  23. ^ a b c d e G.M. Budd, P.J. Stephenson (1970). "Recent glacier retreat on Heard Island" (PDF). International Association for Scientific Hydrology 86: 449–458. Retrieved 7 June 2010. 
  24. ^ a b c d e Douglas E. Thost, Martin Truffer (February 2008). "Glacier Recession on Heard Island, Southern Indian Ocean". Arctic, Antarctic, and Alpine Research 40 (1): 199–214.  
  25. ^ Quilty, P.G. and Wheller, G. (2000). "Heard Island and the McDonald Islands: A window into the Kerguelen Plateau (Heard Island Papers)". Pap. Proc. R. Soc. Tasm. 133 (2): 1–12. 
  26. ^ Budd, G.M. (2000). Changes in Heard Island glaciers, king penguins and fur seals since 1947 (Heard Island Papers). Pap. Proc. R. Soc. Tasm. 133 (2). pp. 47–60. 
  27. ^ Heard Island and McDonald Islands (HIMI). "Australian Research Expeditions". Kingston, Tasmania, Australia:  

References

See also

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.[27] 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 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.[24] The retreat of Heard Island's glaciers is expected to continue for the foreseeable future.[21]

  1. Volcanic activity
  2. Southward movement of the Antarctic Convergence: such a movement conceivably might cause glacier retreat through a rise in sea and air temperatures
  3. Climatic change

Possible causes of glacier recession on Heard Island include:

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 on the east coast of Laurens Peninsula has also demonstrated marked recession since 1955.[23] 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.[21][22][25][26]

The coastal ice cliffs of Brown Glacier and Stephenson Glacier, which in 1954 were over 50 feet high, had disappeared by 1963 when the glaciers terminated as much as 100 yards inland.[23] Baudissin Glacier on the north coast, and Vahsel Glacier on the west coast have lost at least 100 and 200 vertical feet of ice, respectively.[23] Winston Glacier, which retreated approximately one mile between 1947 and 1963, appears to be a very sensitive indicator of glacier change on the island. The young moraines flanking Winston Lagoon show that Winston Glacier has lost at least 300 vertical feet of ice within a recent time period.[23]

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.[22] A visit to the island in the spring of 2000 found that the Stephenson, Brown and Baudissin glaciers, among others, had retreated even further.[22][24] The terminus of Brown Glacier has retreated approximately 1.1 kilometres since 1950.[10] The total ice-covered area of Brown Glacier is estimated to have decreased by roughly 29% between 1947 and 2004.[24] This degree of loss of glacier mass is consistent with the measured increase in temperature of +0.9 °C over that time span.[24]

Retreat of glacier fronts across Heard Island is evident when comparing aerial photographs taken in December 1947 with those taken on a return visit in early 1980.[21][24] 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.[21] Glaciers on the northern and western coasts have narrowed significantly, while the area of glaciers and ice caps on Laurens Peninsula have shrunk by 30% - 65%.[21][22]

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 5000 feet on the southwestern slopes of Big Ben, possibly as a result of volcanic activity. By 1963, major recession was obvious below 2000 feet on almost all glaciers, and minor recession was evident as high as 5000 feet.[23]

Heard Island is a heavily glacierized, subantarctic volcanic island located in the Southern Ocean, roughly 4000 kilometers southwest of Australia. 80% of the island is covered in ice, with glaciers descending from 2400 meters to sea level.[21] Due to the steep topography of Heard Island, most of its glaciers are relatively thin (averaging only about 55 meters in depth).[22] The presence of glaciers on Heard Island provides an excellent opportunity to measure the rate of glacial retreat as an indicator of climate change.[10]

Glaciers of Heard Island

[22][21]

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