The
Antarctic Circumpolar Current (
ACC) is an
ocean currentAn ocean current is a continuous, directed movement of ocean water generated by the forces acting upon the water, such as the wind, Coriolis force, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
that flows from west to east around
Antarctica| style="border-top:solid 1px #ccd2d9; padding:0.4em 1em 0.4em 0; vertical-align:top;" | 14,000,000 km
2 280,000 km
2 13,720,000 km
2 |-! style="border-top: solid 1px #ccd2d9; padding: 0.4em 1em 0.4em 0; vertical-align: top;...
. An alternative name for the ACC is the
West Wind Drift. The ACC is the dominant circulation feature of the
Southern OceanThe Southern Ocean, also known as the Great Southern Ocean, the Antarctic Ocean and the South Polar Ocean, comprises the southernmost waters of the World Ocean south of 60° S latitude. The International Hydrographic Organization has designated the Southern Ocean as an oceanic division encircling...
and, at approximately 125
SverdrupThe sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...
s, the largest ocean current
http://oceancurrents.rsmas.miami.edu/southern/antarctic-cp.html.
The
Antarctic Circumpolar Current (
ACC) is an
ocean currentAn ocean current is a continuous, directed movement of ocean water generated by the forces acting upon the water, such as the wind, Coriolis force, temperature and salinity differences and tides caused by the gravitational pull of the Moon and the Sun...
that flows from west to east around
Antarctica| style="border-top:solid 1px #ccd2d9; padding:0.4em 1em 0.4em 0; vertical-align:top;" | 14,000,000 km
2 280,000 km
2 13,720,000 km
2 |-! style="border-top: solid 1px #ccd2d9; padding: 0.4em 1em 0.4em 0; vertical-align: top;...
. An alternative name for the ACC is the
West Wind Drift. The ACC is the dominant circulation feature of the
Southern OceanThe Southern Ocean, also known as the Great Southern Ocean, the Antarctic Ocean and the South Polar Ocean, comprises the southernmost waters of the World Ocean south of 60° S latitude. The International Hydrographic Organization has designated the Southern Ocean as an oceanic division encircling...
and, at approximately 125
SverdrupThe sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...
s, the largest ocean current
http://oceancurrents.rsmas.miami.edu/southern/antarctic-cp.html. It keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge
ice sheetAn ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² . The only current ice sheets are in Antarctica and Greenland; during the last glacial period at Last Glacial Maximum the Laurentide ice sheet covered much of Canada and North America, the...
.
The ACC has been known to sailors for many years; circumstances preceding the
Mutiny on the BountyThe mutiny on the Bounty occurred aboard a British Royal Navy ship on 28 April 1789, and has been commemorated by several books, films, and popular songs, many of which take considerable liberties with the facts. The mutiny was led by Fletcher Christian against the commanding officer, William Bligh...
and
Jack LondonJack London was an American author who wrote The Call of the Wild, White Fang, and The Sea Wolf along with many other popular books...
's story "Make Westing" poignantly illustrated the difficulty it caused for mariners seeking to round
Cape HornCape Horn island is the southernmost headland of the Tierra del Fuego archipelago of southern Chile....
on the clipper ship route between New York and California.
The current creates two Antarctic gyres.
Structure
The ACC connects the
AtlanticThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about 106.4 million square kilometres , it covers approximately one-fifth of the Earth's surface and about one-quarter of its water surface area. The first part of its name refers to the Atlas of Greek...
,
PacificThe Pacific Ocean is the largest of the Earth's oceanic divisions. Its name is derived from the Latin name Tepre Pacificum, "peaceful sea", bestowed upon it by the Portuguese explorer Ferdinand Magellan. It extends from the Arctic in the north to Antarctica in the south, bounded by Asia and...
and
Indian OceanThe Indian Ocean is the third largest of the world's oceanic divisions, covering about 20% of the water on the Earth's surface. It is bounded on the north by South Asia ; on the west by Africa; on the east by Indochina, the Sunda Islands, and Australia; and on the south by the Southern Ocean...
basins, and as such serves as a principal pathway of exchange between these basins. The current is strongly constrained by
landformIn the earth sciences and geology sub-fields, a landform or physical feature comprises a geomorphological unit, and is largely defined by its surface form and location in the landscape, as part of the terrain, and as such, is typically an element of topography...
and bathymetric features. Starting at
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
, it flows through the
Drake PassageThe Drake Passage or Mar de Hoces -Sea of "Hoces"- is the body of water between the southern tip of South America at Cape Horn, Chile and the South Shetland Islands of Antarctica...
between South America and the
Antarctic PeninsulaThe Antarctic Peninsula is the northernmost part of the mainland of Antarctica. It extends from a line between Cape Adams and a point on the mainland south of Eklund Islands....
and then is split by the
Scotia ArcScotia Arc is the island arc system forming the north, east and south border of Scotia Sea. The arc comprises submarine ridges and the island groups of South Orkneys, South Sandwich Islands, Clerke Rocks, South Georgia, Shag Rocks, Isla de los Estados, and Burdwood Bank linking the mountains of...
to the east, with a shallow warm branch flowing to the north in the
Falkland CurrentThe Falkland Current is a cold water current that flows northward along the Atlantic coast of Patagonia as far north as the mouth of the Río de la Plata. This current results from the movement of water from the West Wind Drift as it rounds Cape Horn. It takes its name from the Falkland Islands....
and a deeper branch passing through the Arc more to the east before also turning to the north. Passing through the Indian Ocean, the current is split by the
Kerguelen PlateauThe Kerguelen Plateau is an underwater volcanic large igneous province in the southern Indian Ocean. It lies about 3,000 km to the southwest of Australia and is nearly three times the size of Japan...
in the Indian Ocean, and then moving northward again. Deflection is also seen as it passes over the
mid-ocean ridgeA mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading center, which is responsible for seafloor...
in the Southeast Pacific.
The current consists of a number of
frontFront may refer to:* The Front, 1976 film* "The Front" * Front , British men's magazine* Front , area where armies are engaged in conflict...
s. The northern boundary of the ACC is defined by the
Subtropical FrontSubtropical Front is a term used in Oceanography to describe a boundary between water systems based on temperature and salinity.- Northern Subtropical Front :A subtropical front in the Pacific Ocean lying between 25 and 30 degrees north latitude....
. This marks the boundary between warm, salty subtropical waters (generally with a
salinitySalinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
of greater than 34.9 parts per thousand) and fresher, cooler subpolar waters. Moving southward we find the Subantarctic Front, along which much of the ACC transport is carried, which is defined as the latitude at which a subsurface salinity minimum or a thick layer of unstratified
Subantarctic Mode WaterSubantarctic mode water is an important water mass in the earth's oceans. It is formed near the Subantarctic Front on the northern flank of the Antarctic Circumpolar Current...
first appears. Still further south lies the Polar Front, which is marked by a transition to very cold, relatively fresh, Antarctic Surface Water at the surface. Further south still is the Southern Boundary front, which is determined as the point where very dense
abyssal watersThe abyssal zone is the abyssopelagic layer of pelagic zone that contains the very deep benthic communities near the bottom of oceans. "Abyss" is from the Greek word meaning "bottomless sea". At depths of 4,000 to 6,000 meters , this zone remains in perpetual darkness and never receives daylight...
upwell to within a few hundred meters of the surface. The bulk of the transport is carried in the middle two fronts. The total transport of the ACC at Drake Passage is estimated to be around 135
SverdrupThe sverdrup, named in honour of the pioneering oceanographer Harald Sverdrup, is a unit of measure of volume transport. It is used almost exclusively in oceanography, to measure the transport of ocean currents. Its symbol is Sv. Note that the sverdrup is not an SI unit, and that its symbol...
s (135,000,000 m³/s), or about 135 times the transport of all the world's rivers combined. There is a relatively small addition of flow in the Indian Ocean, with the transport south of
TasmaniaTasmania is an Australian island and state. It is located south of the eastern side of the continent, from which it is separated by Bass Strait. The state includes the island of Tasmania – the 26th largest island in the world – and the surrounding islands. The state has a population of 500,000 ,...
reaching around 147 Sv, at which point the current is probably the largest on the planet.
Dynamics
The Circumpolar Current is driven by the strong westerly
windWind is the flow of air or other gases that compose an atmosphere . On Earth, wind consists of the bulk movement of air...
s which are found in the latitudes of the Southern Ocean.
In latitudes where there are continents, winds blowing on light surface water can simply pile up light water against these continents. But in the Southern Ocean, the momentum imparted to the surface waters cannot be balanced in this way. Different theories of the Circumpolar Current balance the momentum imparted by the winds in different ways. The increasing eastward momentum imparted by the winds causes water parcels to drift outwards from the axis of the Earth's rotation (in other words, northward) as a result of the Coriolis force. This northward transport is balanced by a southward, pressure-driven flow below the depths of the major ridge systems. Some theories connect these flows directly, implying that there is significant upwelling of dense deep waters within the Southern Ocean, transformation of these waters into light surface waters, and a transformation of waters in the opposite direction to the north. Such theories link the magnitude of the Circumpolar Current with the global
thermohaline circulationThe term thermohaline circulation refers to the part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....
, particularly the properties of the North Atlantic.
Alternatively, ocean eddies, the oceanic equivalent of atmospheric storms, or the large scale meanders of the Circumpolar Current may directly transport momentum downwards in the water column. This is because such flows can produce a net southward flow in the troughs and a net northward flow over the ridges without requiring any transformation of density. In practice both the thermohaline and the eddy/meander mechanisms are likely to be important.
The current flows at a rate of about four km per hour. Recent studies have indicated that the Antarctic Circumpolar Current varies with time. Evidence of this is the
Antarctic Circumpolar WaveThe Antarctic Circumpolar Wave is a coupled ocean/atmosphere wave that circles the Southern Ocean in approximately eight years. Since it is a wave-2 phenomenon at each fixed point in space a signal with a period of four years is seen...
, a periodic
oscillationOscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power...
that affects the climate of much of the southern hemisphere. There is also the
Antarctic oscillationThe Antarctic oscillation is a low-frequency mode of atmospheric variability of the southern hemisphere...
, which involves changes in the location and strength of Antarctic winds. Trends in the Antarctic Oscillation have been hypothesized to account for an increase in the transport of the Circumpolar Current over the past two decades.
Formation
The Antarctic Circumpolar Current formed during Late
EoceneThe Eocene epoch, lasting from 55.8 ± 0.2 to 33.9 ± 0.1 Ma , is a major division of the geologic timescale and the second epoch of the Palaeogene period in the Cenozoic era. The Eocene spans the time from the end of the Paleocene epoch to the beginning of the Oligocene epoch. The start of the...
through Early
OligoceneThe Oligocene is a geologic epoch of the Paleogene period and extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
, as
Antarctica| style="border-top:solid 1px #ccd2d9; padding:0.4em 1em 0.4em 0; vertical-align:top;" | 14,000,000 km
2 280,000 km
2 13,720,000 km
2 |-! style="border-top: solid 1px #ccd2d9; padding: 0.4em 1em 0.4em 0; vertical-align: top;...
and
South AmericaSouth America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere...
finally separated enough for the
Drake PassageThe Drake Passage or Mar de Hoces -Sea of "Hoces"- is the body of water between the southern tip of South America at Cape Horn, Chile and the South Shetland Islands of Antarctica...
to form some 30 to 34 million years ago. As Antarctica became isolated from warmer waters it cooled and glaciers began to form on the formerly forested continent.
Studies
An expedition in May 2008 by 19 scientists studied the geology and biology of eight Macquarie Ridge sea mounts, as well as the Antarctic Circumpolar Current to investigate the effects of climate change of the southern Ocean. The circumpolar current merges the waters of the Atlantic, Indian, and Pacific Oceans and carries up to 150 times the volume of water flowing in all of the world's rivers. After studying the circumpolar current it is clear that it strongly influences regional and global climate as well as underwater biodiversity.
External links
- http://www.literaturecollection.com/a/london/149/
- http://oceancurrents.rsmas.miami.edu/southern/antarctic-cp.html
- http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_04.htm
- http://earth.usc.edu/~stott/Catalina/Oceans.html Good graphics later in article.