Antarctic Circumpolar Current

Last updated January 14, 2024

Animation of the thermohaline circulation. The later part of this animation shows the Antarctic Circumpolar Current.

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 feature of the Southern Ocean and has a mean transport estimated at 100–150 Sverdrups (Sv, million m³/s),^[1] or possibly even higher,^[2] making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.

Associated with the Circumpolar Current is the Antarctic Convergence, where the cold Antarctic waters meet the warmer waters of the subantarctic, creating a zone of upwelling nutrients. These nurture high levels of phytoplankton with associated copepods and krill, and resultant food chains supporting fish, whales, seals, penguins, albatrosses, and a wealth of other species.

The ACC has been known to sailors for centuries; it greatly speeds up any travel from west to east, but makes sailing extremely difficult from east to west, although this is mostly due to the prevailing westerly winds. Jack London's story "Make Westing" and the circumstances preceding the mutiny on the Bounty poignantly illustrate the difficulty it caused for mariners seeking to round Cape Horn westbound on the clipper ship route from New York to California.^[3] The eastbound clipper route, which is the fastest sailing route around the world, follows the ACC around three continental capes – Cape Agulhas (Africa), South East Cape (Australia), and Cape Horn (South America).

The current creates the Ross and Weddell gyres.

Structure

The ACC connects the Atlantic, Pacific, and Indian Oceans, and serves as a principal pathway of exchange among them. The current is strongly constrained by landform and bathymetric features. To trace it starting arbitrarily at South America, it flows through the Drake Passage between South America and the Antarctic Peninsula and then is split by the Scotia Arc to the east, with a shallow warm branch flowing to the north in the Falkland Current 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 first retroflects the Agulhas Current to form the Agulhas Return Current before it is split by the Kerguelen Plateau, and then moving northward again. Deflection is also seen as it passes over the mid-ocean ridge in the Southeast Pacific.

Fronts

The current is accompanied by three fronts: the Subantarctic front (SAF), the Polar front (PF), and the Southern ACC front (SACC).^[4] Furthermore, the waters of the Southern Ocean are separated from the warmer and saltier subtropical waters by the subtropical front (STF).^[5]

The northern boundary of the ACC is defined by the northern edge of the SAF, this being the most northerly water to pass through Drake Passage and therefore be circumpolar. Much of the ACC transport is carried in this front, which is defined as the latitude at which a subsurface salinity minimum or a thick layer of unstratified Subantarctic mode water first appears, allowed by temperature dominating density stratification. Still further south lies the PF, which is marked by a transition to very cold, relatively fresh, Antarctic Surface Water at the surface. Here a temperature minimum is allowed by salinity dominating density stratification, due to the lower temperatures. Farther south still is the SACC, which is determined as the southernmost extent of Circumpolar Deep Water (temperature of about 2 °C at 400 m). This water mass flows along the shelfbreak of the western Antarctic Peninsula and thus marks the most southerly water flowing through Drake Passage and therefore circumpolar. 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 Sv, 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 Tasmania 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 winds 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 offset in this way. There are different theories on how the Circumpolar Current balances the momentum imparted by the winds. The increasing eastward momentum imparted by the winds causes water parcels to drift outward from the axis of the Earth's rotation (in other words, northward) as a result of the Coriolis force. This northward Ekman 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 circulation, 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 downward 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 4 km/h (2.5 mph) over the Macquarie Ridge south of New Zealand.^[6] The ACC varies with time. Evidence of this is the Antarctic Circumpolar Wave, a periodic oscillation that affects the climate of much of the southern hemisphere.^[7] There is also the Antarctic oscillation, 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

Published estimates of the onset of the Antarctic Circumpolar Current vary, but it is commonly considered to have started at the Eocene/Oligocene boundary. The isolation of Antarctica and formation of the ACC occurred with the openings of the Tasmanian Passage and the Drake Passage. The Tasmanian Seaway separates East Antarctica and Australia, and is reported to have opened to water circulation 33.5 Ma.^[8] The timing of the opening of the Drake Passage, between South America and the Antarctic Peninsula, is more disputed; tectonic and sediment evidence show that it could have been open as early as pre-34 Ma,^[9] estimates of the opening of the Drake passage are between 20 and 40 Ma.^[10] The isolation of Antarctica by the current is credited by many researchers with causing the glaciation of Antarctica and global cooling in the Eocene epoch. Oceanic models have shown that the opening of these two passages limited polar heat convergence and caused a cooling of sea surface temperatures by several degrees; other models have shown that CO₂ levels also played a significant role in the glaciation of Antarctica.^[10]^[11]

Phytoplankton

Antarctic sea ice cycles seasonally, in February–March the amount of sea ice is lowest, and in August–September the sea ice is at its greatest extent.^[12] Ice levels have been monitored by satellite since 1973. Upwelling of deep water under the sea ice brings substantial amounts of nutrients. As the ice melts, the melt water provides stability and the critical depth is well below the mixing depth, which allows for a positive net primary production.^[13] As the sea ice recedes epontic algae dominate the first phase of the bloom, and a strong bloom dominate by diatoms follows the ice melt south.^[13]

Another phytoplankton bloom occurs more to the north near the Antarctic convergence, here nutrients are present from thermohaline circulation. Phytoplankton blooms are dominated by diatoms and grazed by copepods in the open ocean, and by krill closer to the continent. Diatom production continues through the summer, and populations of krill are sustained, bringing large numbers of cetaceans, cephalopods, seals, birds, and fish to the area.^[13]

Phytoplankton blooms are believed to be limited by irradiance in the austral (southern hemisphere) spring, and by biologically available iron in the summer.^[14] Much of the biology in the area occurs along the major fronts of the current, the Subtropical, Subantarctic, and the Antarctic Polar fronts, these are areas associated with well defined temperature changes.^[15] Size and distribution of phytoplankton are also related to fronts. Microphytoplankton (>20 μm) are found at fronts and at sea ice boundaries, while nanophytoplankton (<20 μm) are found between fronts.^[16]

Studies of phytoplankton stocks in the southern sea have shown that the Antarctic Circumpolar Current is dominated by diatoms, while the Weddell Sea has abundant coccolithophorids and silicoflagellates. Surveys of the SW Indian Ocean have shown phytoplankton group variation based on their location relative to the Polar Front, with diatoms dominating South of the front, and dinoflagellates and flagellates in higher populations North of the front.^[16]

Some research has been conducted on Antarctic phytoplankton as a carbon sink. Areas of open water left from ice melt are good areas for phytoplankton blooms. The phytoplankton takes carbon from the atmosphere during photosynthesis. As the blooms die and sink, the carbon can be stored in sediments for thousands of years. This natural carbon sink is estimated to remove 3.5 million tonnes from the ocean each year. 3.5 million tonnes of carbon taken from the ocean and atmosphere is equivalent to 12.8 million tonnes of carbon dioxide.^[17]

Studies

An expedition in May 2008 by 19 scientists^[18] 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. The study found that any damage on the cold-water corals nourished by the current will have a long-lasting effect.^[6] After studying the circumpolar current it is clear that it strongly influences regional and global climate as well as underwater biodiversity.^[19] The subject has been characterized recently as "the spectral peak of the global extra-tropical circulation at ≈ 10^4 kilometers".^[20]

The current helps preserve wooden shipwrecks by preventing wood-boring "ship worms" from reaching targets such as Ernest Shackleton's ship, the Endurance.^[21]

Related Research Articles

The 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 epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich from his studies of marine beds in Belgium and Germany. The name comes from the Ancient Greek ὀλίγος and καινός, and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period.

Antarctic krill 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 individual animals per cubic metre. It feeds directly on minute phytoplankton, thereby using the primary production energy that the phytoplankton originally derived from the sun in order to sustain their pelagic life cycle. It grows to a length of 6 centimetres (2.4 in), weighs up to 2 grams (0.071 oz), and can live for up to six years. It is a key species in the Antarctic ecosystem and in terms of biomass, is one of the most abundant animal species on the planet – approximately 500 million metric tons.

The Drake Passage is the body of water between South America's Cape Horn, Chile, Argentina and the South Shetland Islands of Antarctica. It connects the southwestern part of the Atlantic Ocean with the southeastern part of the Pacific Ocean and extends into the Southern Ocean. The passage is named after the 16th-century English explorer and privateer Sir Francis Drake.

A polynya is an area of open water surrounded by sea ice. It is now used as a geographical term for an area of unfrozen seawater within otherwise contiguous pack ice or fast ice. It is a loanword from the Russian полынья, which refers to a natural ice hole and was adopted in the 19th century by polar explorers to describe navigable portions of the sea.

High-nutrient, low-chlorophyll (HNLC) regions are regions of the ocean where the abundance of phytoplankton is low and fairly constant despite the availability of macronutrients. Phytoplankton rely on a suite of nutrients for cellular function. Macronutrients are generally available in higher quantities in surface ocean waters, and are the typical components of common garden fertilizers. Micronutrients are generally available in lower quantities and include trace metals. Macronutrients are typically available in millimolar concentrations, while micronutrients are generally available in micro- to nanomolar concentrations. In general, nitrogen tends to be a limiting ocean nutrient, but in HNLC regions it is never significantly depleted. Instead, these regions tend to be limited by low concentrations of metabolizable iron. Iron is a critical phytoplankton micronutrient necessary for enzyme catalysis and electron transport.

The Antarctic bottom water (AABW) is a type of water mass in the Southern Ocean surrounding Antarctica with temperatures ranging from −0.8 to 2 °C (35 °F) and absolute salinities from 34.6 to 35.0 g/kg. As the densest water mass of the oceans, AABW is found to occupy the depth range below 4000 m of all ocean basins that have a connection to the Southern Ocean at that level.

The Campbell Plateau is a large oceanic plateau south of New Zealand and the Chatham Rise. It originated in the Gondwanan breakup and is part of Zealandia, a largely submerged continent. The above sea level parts of the plateau — the Bounty Islands, Antipodes Islands, Auckland Islands and Campbell Island — form part of the New Zealand Subantarctic Islands which were declared a World Heritage Site in 1998. Large parts of the Campbell Plateau lie less than 1000 m below sea level. It rises to 500 m at the Pukaki Rise and emerges above sea level at the Auckland and Campbell Islands.

<span class="mw-page-title-main">Subantarctic</span> Term describing the parts of the three largest oceans nearest the Southern Ocean

The sub-Antarctic zone is a region in the Southern Hemisphere, located immediately north of the Antarctic region. This translates roughly to a latitude of between 46° and 60° south of the Equator. 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.

The Tasman Outflow is a water pathway connecting water from the Pacific Ocean and the Indian Ocean. The existence of the outflow was published by scientists of the Australian CSIRO's Division of Marine and Atmospheric Research team in August 2007, interpreting salinity and temperature data captured from 1950 to 2002. The Tasman Outflow is seen as the missing link in the supergyre of the Southern Hemisphere and an important part of the thermohaline circulation.

The Weddell Gyre is one of the two gyres that exist within the Southern Ocean. The gyre is formed by interactions between the Antarctic Circumpolar Current (ACC) and the Antarctic Continental Shelf. The gyre is located in the Weddell Sea, and rotates clockwise. South of the ACC and spreading northeast from the Antarctic Peninsula, the gyre is an extended large cyclone. Where the northeastern end ends at 30°E, which is marked by the southward turn of the ACC, the northern part of the gyre spreads over the Southern Scotia Sea and goes northward to the South Sandwich Arc. Axis of the gyre is over the southern flanks of the South Scotia, America-Antarctic, and Southwest Indian Ridges. In the southern part of the gyre, the westward return flow is about 66 sverdrup (Sv), while in the northern rim current, there is an eastward flow of 61 Sv.

The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of 20,327,000 km² (7,848,000 sq mi), it is regarded as the second-smallest of the five principal oceanic divisions: smaller than the Pacific, Atlantic, and Indian oceans but larger than the Arctic Ocean. Since the 1980s, the Southern Ocean has been subject to rapid climate change, which has led to changes in the marine ecosystem.

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 Antarctic Polar Front zone. This convergence zone is normally located between 50°S and 60°S, hence this is where almost all of the AAIW is formed.

In oceanography, a front is a boundary between two distinct water masses. The formation of fronts depends on multiple physical processes and small differences in these lead to a wide range of front types. They can be as narrow as a few hundreds of metres and as wide as several tens of kilometres. While most fronts form and dissipate relatively quickly, some can persist for long periods of time.

The Northeast Georgia Rise is an oceanic plateau located in the South Atlantic Ocean northeast of South Georgia Island and west of the Falkland Plateau.

Fragilariopsis kerguelensis, is a pennate diatom native to the Southern Ocean. It has been characterized as "the most abundant diatom in the Antarctic Seas".

The Tasmanian Passage, also Tasmanian Gateway or Tasmanian Seaway, is the name of ocean waters between Australia and Antarctica.

Particulate inorganic carbon (PIC) can be contrasted with dissolved inorganic carbon (DIC), the other form of inorganic carbon found in the ocean. These distinctions are important in chemical oceanography. Particulate inorganic carbon is sometimes called suspended inorganic carbon. In operational terms, it is defined as the inorganic carbon in particulate form that is too large to pass through the filter used to separate dissolved inorganic carbon.

Eddy saturation and eddy compensation are phenomena found in the Southern Ocean. Both are limiting processes where eddy activity increases due to the momentum of strong westerlies, and hence do not enhance their respective mean currents. Where eddy saturations impacts the Antarctic Circumpolar Current (ACC), eddy compensation influences the associated Meridional Overturning Circulation (MOC).

The Great Calcite Belt (GCB) refers to a region of the ocean where there are high concentrations of calcite, a mineral form of calcium carbonate. The belt extends over a large area of the Southern Ocean surrounding Antarctica. The calcite in the Great Calcite Belt is formed by tiny marine organisms called coccolithophores, which build their shells out of calcium carbonate. When these organisms die, their shells sink to the bottom of the ocean, and over time, they accumulate to form a thick layer of calcite sediment.

The Southern Ocean overturning circulation is a two-cell system in the Southern Ocean that connects different water basins within the global circulation. It is driven by upwelling and downwelling, which are a result of the physical ocean processes that are influenced by freshwater fluxes and wind stress. The global ocean circulation is an essential mechanism in our global climate system due to its influence on the global heat, fresh water and carbon budgets. The upwelling in the upper cell is associated with mid-deep water that is brought to the surface, whereas the upwelling in the lower cell is linked to the fresh and abyssal waters around Antarctica. Around 27 ± 7 Sverdrup (Sv) of deep water wells up to the surface in the Southern Ocean. This upwelled water is partly transformed to lighter water and denser water, respectively 22 ± 4 Sv and 5 ± 5 Sv. The densities of these waters change due to heat and buoyancy fluxes which result in upwelling in the upper cell and downwelling in the lower cell.

References

Notes

↑ Smith et al. 2013
↑ Donohue, K.A.; et al. (21 November 2016). "Mean Antarctic Circumpolar Current transport measured in Drake Passage". Geophysical Research Letters. 43 (11): 760. Bibcode:2016GeoRL..4311760D. doi: 10.1002/2016GL070319 . hdl: 11336/47067 .
↑ London 1907
↑ Stewart 2007
↑ Orsi, Whitworth & Nowlin 1995 , Introduction, p. 641
1 2 "Explorers marvel at 'Brittlestar City' on seamount in powerful current swirling around Antarctica". 18 May 2008. Retrieved 6 June 2008.
↑ Connolley 2002
↑ Hassold et al. 2009
↑ Barker et al. 2007
1 2 Siegert et al. 2008
↑ Stott 2011 , See "Ancient Current Systems" illustrations at bottom of page
↑ Geerts 1998
1 2 3 Miller 2004 , p. 219
↑ Peloquin & Smith 2007
↑ "The Southern Ocean". GES DISC: Goddard Earth Sciences, Data & Information Services Center. May 2012. Archived from the original on 18 May 2015. Retrieved 13 August 2012.
1 2 Knox 2007 , p. 23
↑ Peck et al. 2010
↑ O'Hara, Rowden & Williams 2008
↑ Rintoul, Hughes & Olbers 2001 , e.g. p. 271
↑ Storer, B.A., Buzzicotti, M., Khatri, H. et al. Global energy spectrum of the general oceanic circulation. Nat Commun 13, 5314 (2022). https://doi.org/10.1038/s41467-022-33031-3. Retrieved 17 September 2022.
↑ Glover et al. 2013

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- Amos, Jonathan (14 August 2013). "Antarctic: Where 'zombies' thrive and shipwrecks are preserved". BBC News.
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- "Antarctica glacier retreat creates new carbon dioxide store; has beneficial impact on climate change". ScienceDaily (Press release). 10 November 2009.
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[1] Smith et al. 2013

[2] Donohue, K.A.; et al. (21 November 2016). "Mean Antarctic Circumpolar Current transport measured in Drake Passage". Geophysical Research Letters. 43 (11): 760. Bibcode:2016GeoRL..4311760D. doi: 10.1002/2016GL070319 . hdl: 11336/47067 .

[3] London 1907

[4] Stewart 2007

[5] Orsi, Whitworth & Nowlin 1995 , Introduction, p. 641

[brittlestar-6] 1 2 "Explorers marvel at 'Brittlestar City' on seamount in powerful current swirling around Antarctica". 18 May 2008. Retrieved 6 June 2008.

[7] Connolley 2002

[8] Hassold et al. 2009

[9] Barker et al. 2007

[Siegert-etal-2008-10] 1 2 Siegert et al. 2008

[11] Stott 2011 , See "Ancient Current Systems" illustrations at bottom of page

[12] Geerts 1998

[Miller-2004-13] 1 2 3 Miller 2004 , p. 219

[14] Peloquin & Smith 2007

[15] "The Southern Ocean". GES DISC: Goddard Earth Sciences, Data & Information Services Center. May 2012. Archived from the original on 18 May 2015. Retrieved 13 August 2012.

[Knox-2007-16] 1 2 Knox 2007 , p. 23

[17] Peck et al. 2010

[18] O'Hara, Rowden & Williams 2008

[19] Rintoul, Hughes & Olbers 2001 , e.g. p. 271

[20] Storer, B.A., Buzzicotti, M., Khatri, H. et al. Global energy spectrum of the general oceanic circulation. Nat Commun 13, 5314 (2022). https://doi.org/10.1038/s41467-022-33031-3. Retrieved 17 September 2022.

[21] Glover et al. 2013

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