Subantarctic

The Antarctic Convergence, approximately 200 km south of the Antarctic Polar Front, is the border between two distinct regions: the subantarctic and the antarctic.

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.

Geography

The subantarctic region is comprised of 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

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). 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.

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 this point 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 60º South latitude". Therefore, Antarctica is defined from a political standpoint as all land and ice shelves south of 60°S latitude.

Subantarctic islands

See also: List of Antarctic and sub-Antarctic 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, Campbell Island group, Heard Island and McDonald Islands, Bouvet Island, South Georgia Group, Macquarie Island, and 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 the 60° latitude (the continental limit according to the Antarctic Treaty). Therefore despite their being located 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	Ocean	Claimed by
Antipodes Islands	49°40′S 178°46′E	Pacific Ocean	New Zealand
Auckland Islands	50°42′S 166°05′E	Pacific Ocean	New Zealand
Bounty Islands	47°45′S 179°03′E	Pacific Ocean	New Zealand
Bouvet Island (Bouvetøya)	54°26′S 03°24′E	Atlantic Ocean	Norway
Campbell Island group	52°32′S 169°08′E	Pacific Ocean	New Zealand
Crozet Islands (French: Îles Crozet or officially Archipel Crozet)	46°25′S 51°59′E	Indian Ocean	France
Heard Island and McDonald Islands (HIMI)	53°04′S 73°00′E	Indian Ocean	Australia
Kerguelen Islands	49°15′S 69°35′E	Indian Ocean	France
Macquarie Island	54°38′S 158°52′E	Pacific Ocean	Australia
Prince Edward Islands	46°46′S 37°51′E	Indian Ocean	South Africa
South Georgia Group	54°30′S 37°00′W	Atlantic Ocean	United Kingdom
South Sandwich Islands	57°30′S 27°00′W	Atlantic Ocean	United Kingdom
Snares Islands	48°01′S 166°32′E	Pacific Ocean	New Zealand

Subantarctic glaciers

See also: List of glaciers in the Antarctic and List of Antarctic ice streams

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 60º (the continental limit according to the Antarctic Treaty).

Gotley Glacier and Lied Glacier, Heard Island.

Harker Glacier is one of several tidewater glaciers in Cumberland Bay, South Georgia.

Neumayer Glacier, Cumberland West Bay, South Georgia, circa 1882.

Neumayer Glacier, Cumberland West Bay, South Georgia, circa 1882.

Name of Glacier	Coordinates	Length	Location
Abbotsmith Glacier	53°6′S 73°24′E	4.8 km	Heard Island
Austin Glacier	54°4′S 37°12′W		South Georgia Group
Bary Glacier	54°26′S 36°47′W		South Georgia Group
Baudissin Glacier	53°2′S 73°26′E		Heard Island
Bertrab Glacier	54°37′S 35°57′W		South Georgia Group
Bogen Glacier	54°48′S 35°56′W		South Georgia Group
Briggs Glacier	54°1′S 37°8′W		South Georgia Group
Brøgger Glacier	54°32′S 36°26′W	13 km	South Georgia Group
Brown Glacier	53°4′S 73°39′E		Heard Island
Brunonia Glacier	54°3′S 37°29′W		South Georgia Group
Buxton Glacier	54°26′S 36°12′W		South Georgia Group
Challenger Glacier	53°2′S 73°28′E		Heard Island
Christensen Glacier	54°2′S 36°52′W		South Georgia Group
Christensen Glacier	54°28′S 3°24′E		Bouvet Island
Christophersen Glacier	54°25′S 36°47′W		South Georgia Group
Clayton Glacier	54°4′S 37°26′W		South Georgia Group
Compton Glacier	53°3′S 73°37′E		Heard Island
Cook Glacier	54°27′S 36°11′W		South Georgia Group
Crean Glacier	54°8′S 37°1′W	6 km	South Georgia Group
Deacock Glacier	53°11′S 73°31′E		Heard Island
Dead End Glacier	54°47′S 35°56′W		South Georgia Group
Downes Glacier	53°2′S 73°31′E		Heard Island
Ealey Glacier			Heard Island
Eclipse Glacier	54°23′S 36°5′W		South Georgia Group
Esmark Glacier	54°13′S 37°13′W		South Georgia Group
Fiftyone Glacier	53°11′S 73°34′E		Heard Island
Fortuna Glacier	54°6′S 36°51′W		South Georgia Group
Geikie Glacier	54°17′S 36°41′W		South Georgia Group
Gotley Glacier	53°1′S 73°27′E	13.2 km	Heard Island
Graae Glacier	54°48′S 36°1′W	3.2 km	South Georgia Group
Grace Glacier	54°4′S 37°23′W		South Georgia Group
Hamberg Glacier	54°21′S 36°31′W		South Georgia Group
Harker Glacier	54°22′S 36°32′W		South Georgia Group
Harmer Glacier	54°46′S 36°15′W		South Georgia Group
Heaney Glacier	54°25′S 36°12′W		South Georgia Group
Helland Glacier	54°29′S 36°37′W		South Georgia Group
Henningsen Glacier	54°27′S 36°42′W		South Georgia Group
Herz Glacier	54°41′S 35°58′W		South Georgia Group
Hindle Glacier	54°34′S 36°5′W	10 km	South Georgia Group
Hodges Glacier	54°16′S 36°32′W		South Georgia Group
Horntvedt Glacier	54°25′S 3°21′E		Bouvet Island
Jacka Glacier	53°00′S 73°20′E	1.3 km	Heard Island
Jenkins Glacier	54°46′S 36°7′W		South Georgia Group
Jewell Glacier	54°16′S 37°8′W		South Georgia Group
Keilhau Glacier	54°16′S 37°4′W	8 km	South Georgia Group
Kjerulf Glacier	54°21′S 36°51′W		South Georgia Group
König Glacier	54°1′S 36°48′W		South Georgia Group
Lancing Glacier	54°2′S 36°56′W		South Georgia Group
Lewald Glacier	54°45′S 35°52′W		South Georgia Group
Lied Glacier	53°9′S 73°26′E		Heard Island
Lucas Glacier	54°4′S 37°18′W		South Georgia Group
Lyell Glacier	54°17′S 36°37′W		South Georgia Group
Mary Powell Glacier			Heard Island
Morris Glacier	54°5′S 37°14′W		South Georgia Group
Murray Snowfield	54°9′S 37°9′W		South Georgia Group
Nachtigal Glacier	54°29′S 36°9′W		South Georgia Group
Neumayer Glacier	54°15′S 36°41′W	13 km	South Georgia Group
Nordenskjöld Glacier	54°22′S 36°22′W	"large"	South Georgia Group
Novosilski Glacier	54°4′S 36°18′W	13 km	South Georgia Group
Paget Glacier	54°24′S 36°28′W	6 km	South Georgia Group
Peters Glacier	54°8′S 37°33′W		South Georgia Group
Philippi Glacier	54°49′S 36°3′W		South Georgia Group
Posadowsky Glacier	54°25′S 32°2′E		Bouvet Island
Pr Ice Glacier	54°7′S 37°29′W		South Georgia Group
Purvis Glacier	54°6′S 37°1′W		South Georgia Group
Quensel Glacier	54°46′S 35°5′W	"small"	South Georgia Group
Risting Glacier	54°46′S 36°6′W		South Georgia Group
Ross Glacier	54°33′S 36°6′W	10 km	South Georgia Group
Ryan Glacier	54°3′S 37°36′W		South Georgia Group
Salomon Glacier	54°47′S 35°54′W		South Georgia Group
Schmidt Glacier	53°3′S 73°24′E		Heard Island
Schrader Glacier	54°7′S 37°39′W		South Georgia Group
Spenceley Glacier	54°35′S 36°19′W		South Georgia Group
Stephenson Glacier	53°6′S 73°42′E		Heard Island
Storey Glacier	54°47′S 36°1′W		South Georgia Group
Twitcher Glacier	54°43′S 35°56′W	6 km	South Georgia Group
Tyrrell Glacier	54°22′S 36°31′W		South Georgia Group
Vahsel Glacier	53°4′S 73°23′E		Heard Island
Webb Glacier	54°32′S 36°1′W	3.2 km	South Georgia Group
Weddell Glacier	54°35′S 36°00′W	3.2 km	South Georgia Group
Wheeler Glacier	54°36′S 36°22′W	3.2 km	South Georgia Group
Winston Glacier	53°9′S 73°38′E		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.

Retreat of subantarctic glaciers

See also: Retreat of glaciers since 1850 and Glacier mass balance

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

Glaciers appear to be 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, Quelccaya Ice Cap and Qori Kalis Glacier in Peru, Zongo, Chacaltaya and Charquini glaciers in Bolivia, 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.

Bearing this in mind, it should come as no surprise that 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. In 1947, Heard Island's glaciers covered 288 km2 or 79 percent of the island. By 1988 this had decreased by 11 percent to 257 km2. By 2000, the Stephenson, Brown and Baudissin glaciers had retreated even further.

The island's steep topography means that glaciers are relatively thin - about 55m deep on average. The largest glacier, Gotley, originating on the island's highest point, Mawson Peak (2745 m) is 27 km2 in area and 13.2 km long. Smaller glaciers such as Nares, Mary Powell, Brown, Deacock and those on the Laurens Peninsula have significantly receded, but the larger glaciers such as Gotley, Abbotsmith, Downes and Ealey show little or no change since 1947.

Heard Island is cold and steep with a high snowfall. Many larger glaciers are unable to lose much ice in the narrow coastal melt zone. If this zone were wider some of these glaciers would be several kilometers longer, but instead the sea's action removes ice from glacier tongues. For such glaciers, up to 80 percent of their volume loss is through calving. Loss through melting is unlikely to be considerable even if conditions were significantly warmer.

The smaller, shorter glaciers are much more sensitive to temperature effects. Laurens Peninsula glaciers, whose maximum elevation is only 500m above sea level, are particularly vulnerable. With a warming of 1°C the accumulation zone above the end-of-summer snowline elevation of about 300m would recede to about 450m elevation due to rain and increased melt. With an accumulation zone reduced to a range of only 50m elevation, the glacier retreats until the elevation range of the melt zone is similar in size. This illustrates much of what has been happening for Laurens Peninsula glaciers since 1947, during which time their total area has decreased by over 30 percent. Jacka Glacier in the early 1950s had receded only slightly from its position in the late 1920s, but by 1997 it had receded about 700m back from the coastline.

See also

• Extreme points of the Antarctic
• List of glaciers

External links

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