Antarctic Circumpolar Current

Image:Antarctic Circumpolar Current.jpg

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.

The current is accompanied by three fronts: the Subantarctic front (SAF), the Polar front (PF), and the Southern ACC front (SACC).{{Harvnb|Stewart|2007}} Furthermore, the waters of the Southern Ocean are separated from the warmer and saltier subtropical waters by the subtropical front (STF).{{Harvnb|Orsi|Whitworth|Nowlin|1995|loc=Introduction, p. 641}}

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.

The circumpolar current is driven by the strong westerly winds in the latitudes of the Southern Ocean.

File:thermohaline_circulation.svg [{{filepath:thermohaline_circulation.svg}} (animation)] ]]

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 {{Convert|4|km/h|abbr=on}} over the Macquarie Ridge south of New Zealand.{{cite web |url=http://www.eurekalert.org/pub_releases/2008-05/coml-ema051208.php |title=Explorers marvel at 'Brittlestar City' on seamount in powerful current swirling around Antarctica |access-date=6 June 2008 |date=18 May 2008 }} 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.{{Harvnb|Connolley|2002}} 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.

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, following the fragmentation of the Antarctic land bridge. The Tasmanian Seaway separates East Antarctica and Australia, and is reported to have opened to water circulation 33.5 million years ago (Ma).{{Harvnb|Hassold|Rea|Pluijm|Parés|2009}} 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;{{Harvnb|Barker|Filippelli|Florindo|Martin|2007}} estimates of the opening of the Drake passage are between 20 and 40 Ma.{{Harvnb|Siegert|Barrett|Deconto|Dunbar|2008}} 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.{{Harvnb|Stott|2011|loc=See "Ancient Current Systems" illustrations at bottom of page}}

File:S2004341153052.L2 HCHL.MalvinasCurrent.jpg transports nutrient-rich cold waters from the ACC north toward the Brazil–Malvinas Confluence. Phytoplankton chlorophyll concentration are shown in blue (lower concentrations) and yellow (higher concentrations).]]

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.{{Harvnb|Geerts|1998}} 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.{{Harvnb|Miller|2004|p=219}} 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.

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.

Phytoplankton blooms are believed to be limited by irradiance in the austral (southern hemisphere) spring, and by biologically available iron in the summer.{{Harvnb|Peloquin|Smith|2007}} 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.{{cite web |url=http://disc.sci.gsfc.nasa.gov/education-and-outreach/additional/science-focus/locus/tutorials/module6.shtml |title=The Southern Ocean |publisher=GES DISC: Goddard Earth Sciences, Data & Information Services Center |date=May 2012 |access-date=13 August 2012 |archive-url=https://web.archive.org/web/20150518090656/http://disc.sci.gsfc.nasa.gov/education-and-outreach/additional/science-focus/locus/tutorials/module6.shtml |archive-date=18 May 2015 }} 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.{{Harvnb|Knox|2007|p=23}}

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.

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.{{Harvnb|Peck|Barnes|Cook|Fleming|2010}}

An expedition in May 2008 by 19 scientists{{Harvnb|O'Hara|Rowden|Williams|2008}} 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. After studying the circumpolar current it is clear that it strongly influences regional and global climate as well as underwater biodiversity.{{Harvnb|Rintoul|Hughes|Olbers|2001|loc=e.g. p. 271}} The subject has been characterized recently as "the spectral peak of the global extra-tropical circulation at ≈ 10^4 kilometers".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.

The current helps preserve wooden shipwrecks by preventing wood-boring "ship worms" from reaching targets such as Ernest Shackleton's ship, the Endurance.{{Harvnb|Glover|Wiklund|Taboada|Avila|2013}}

The "State of the Cryosphere" report found, that the Antarctic Circumpolar Current became weaker. By 2050 it expected to lose 20% of its strength with "widespread impacts on ocean circulation and climate." The Weddell Sea Bottom Water has lost 30% of its volume in the latest 32 years, and the Antarctic Bottom Water is expected to shrink. This will impact ocean circulation, nutrients, heat content and carbon sequestration.{{cite book |title=State of the Cryosphere 2024 Lost Ice, Global Damage |date=November 2024 |publisher=International Cryosphere Climate Initiative |pages=x (in the beginning), 8, 48, 52 |url=https://articles.unesco.org/sites/default/files/medias/fichiers/2024/11/State%20of%20the%20Cryosphere%20Report%202024.pdf |access-date=20 November 2024}} UNESCO mentions that the report in the first time "notes a growing scientific consensus that melting Greenland and Antarctic ice sheets, among other factors, may be slowing important ocean currents at both poles, with potentially dire consequences for a much colder northern Europe and greater sea-level rise along the U.S. East Coast."{{cite web |title=State of the Cryosphere Report 2024 Lost Ice, Global Damage |url=https://www.unesco.org/en/articles/state-cryosphere-report-2024 |website=UNESCO |access-date=20 November 2024}} The findings were bolstered by a 2025 study published in Environmental Research Letters.{{cite journal |last1=Sohail |first1=Taimoor |last2=Gayen |first2=Bishakhdatta |last3=Klocker |first3=Andreas |title=Decline of Antarctic Circumpolar Current due to polar ocean freshening |journal=Environmental Research Letters |date=3 March 2025 |volume=20 |issue=3 |page=034046 |doi=10.1088/1748-9326/adb31c |doi-access=free |bibcode=2025ERL....20c4046S }}

{{Reflist|30em}}

{{Refbegin|30em}}

• {{Cite journal

| last1 = Barker | first1 = P. F.

| last2 = Filippelli | first2 = G. M.

| last3 = Florindo | first3 = F.

| last4 = Martin | first4 = E. E.

| last5 = Scher | first5 = H. D.

| title = Onset and Role of the Antarctic Circumpolar Current

| year = 2007 | journal = Deep-Sea Research Part II | volume = 54 | issue = 21 | pages = 2388–2398

| url = http://www.earth-prints.org/bitstream/2122/2184/1/1214.pdf

| doi = 10.1016/j.dsr2.2007.07.028 | bibcode = 2007DSRII..54.2388B}}

• {{Cite journal

| last = Connolley | first = W. M.

| title = Long-term variation of the Antarctic Circumpolar Wave

| year = 2002 | journal = Journal of Geophysical Research | volume = 107 | issue = C4

| page = 8076

| doi = 10.1029/2000JC000380 | bibcode=2002JGRC..107.8076C| citeseerx = 10.1.1.693.4116}}

• {{Cite web

| last = Geerts | first = B.

| title = Antarctic sea ice: seasonal and long-term changes

| year = 1998 | publisher = Department of Atmospheric science, University of Wyoming

| url = http://www-das.uwyo.edu/~geerts/cwx/notes/chap11/sea_ice.html | access-date = 29 December 2016

}}

• {{Cite journal

| last1 = Glover | first1 = A. G.

| last2 = Wiklund | first2 = H.

| last3 = Taboada | first3 = S.

| last4 = Avila | first4 = C.

| last5 = Cristobo | first5 = J.

| last6 = Smith | first6 = C. R.

| last7 = Kemp | first7 = K. M.

| last8 = Jamieson | first8 = A. J.

| last9 = Dahlgren | first9 = T. G.

| title = Bone-eating worms from the Antarctic: the contrasting fate of whale and wood remains on the Southern Ocean seafloor

| year = 2013 | journal = Proceedings of the Royal Society B | volume = 280 | issue = 1768

| doi = 10.1098/rspb.2013.1390 | page=20131390 | pmid=23945684 | pmc=3757972}}

• {{cite news |first=Jonathan |last=Amos |date=14 August 2013 |title=Antarctic: Where 'zombies' thrive and shipwrecks are preserved |work=BBC News |url=https://www.bbc.co.uk/news/science-environment-23682521}}
• {{Cite journal

| last1 = Hassold | first1 = N. J. C.

| last2 = Rea | first2 = D. K.

| last3 = Pluijm | first3 = B. A. van der

| last4 = Parés | first4 = J. M.

| title = A physical record of the Antarctic Circumpolar Current: late Miocene to recent slowing of abyssal circulation

| year = 2009 | journal = Palaeogeography, Palaeoclimatology, Palaeoecology | volume = 275 | issue = 1–4

| pages = 28–36

| url = http://www.globalchange.umich.edu/Ben/Publications/09_ppp_hassold.pdf

| doi = 10.1016/j.palaeo.2009.01.011 | bibcode = 2009PPP...275...28H

}}

• {{Cite book

| last = Knox | first = G. A.

| title = Biology of the Southern Ocean

| year = 2007 | publisher = CRC Press | series = CRC Marine Biology Series

| isbn = 9780849333941 }}

• {{Cite web

|last=London

|first=Jack

|author-link=Jack London

|title=Make Westing

|year=1907

|publisher=Literature Collection

|url=http://www.literaturecollection.com/a/london/149

|archive-url=https://archive.today/20130127235537/http://www.literaturecollection.com/a/london/149

|archive-date=27 January 2013

|access-date=29 December 2016

}}

• {{Cite book

| last = Miller | first = C. B.

| title = Biological Oceanography

| year = 2004 | publisher = Blackwell Publishing

| isbn = 9780632055364 }}

• {{Cite journal

| last1 = O'Hara | first1 = T. D.

| last2 = Rowden | first2 = A. A.

| last3 = Williams | first3 = A.

| title = Cold-water coral habitats on seamounts: do they have a specialist fauna?

| year = 2008 | journal = Diversity and Distributions | volume = 14 | issue = 6 | pages = 925–934

| doi = 10.1111/j.1472-4642.2008.00495.x | s2cid = 86154159

| doi-access = free | bibcode = 2008DivDi..14..925O

}}

• {{Cite journal

| last1 = Orsi | first1 = A. H.

| last2 = Whitworth | first2 = T.

| last3 = Nowlin | first3 = W. D. Jr.

| title = On the meridional extent and fronts of the Antarctic Circumpolar Current

| year = 1995 | journal = Deep-Sea Research | volume = 42 | issue = 5

| pages = 641–673

| url = http://www.ccpo.odu.edu/~klinck/Reprints/PDF/orsiDSR95.pdf

| doi = 10.1016/0967-0637(95)00021-W| bibcode = 1995DSRI...42..641O}}

• {{Cite journal

| last1 = Peck | first1 = L. S.

| last2 = Barnes | first2 = D. K. A.

| last3 = Cook | first3 = A. J.

| last4 = Fleming | first4 = A. H.

| last5 = Clarke | first5 = A.

| title = Negative feedback in the cold: ice retreat produces new carbon sinks in Antarctica

| year = 2010 | journal = Global Change Biology | volume = 16 | issue = 9 | pages = 2614–2623

| url = https://www.researchgate.net/publication/229907687

| access-date = 29 December 2016

| doi = 10.1111/j.1365-2486.2009.02071.x | bibcode = 2010GCBio..16.2614P| s2cid = 85748895

}}

• {{cite press release |date=November 10, 2009 |title=Antarctica glacier retreat creates new carbon dioxide store; has beneficial impact on climate change |website=ScienceDaily |url=https://www.sciencedaily.com/releases/2009/11/091109121117.htm}}
• {{Cite journal

| last1 = Peloquin | first1 = J. A.

| last2 = Smith | first2 = W. O. Jr.

| title = Phytoplankton blooms in the Ross Sea, Antarctica: Interannual variability in magnitude, temporal patterns, and composition

| year = 2007 | journal = Journal of Geophysical Research: Oceans | volume = 112 | issue = C08013

| pages = C08013

| url = http://www.ccpo.odu.edu/~klinck/Reprints/PDF/peloquinJGR07.pdf

| doi = 10.1029/2006JC003816 | bibcode=2007JGRC..112.8013P| doi-access = free}}

• {{Cite book

| last1 = Rintoul | first1 = S. R.

| last2 = Hughes | first2 = C.

| last3 = Olbers | first3 = D.

| chapter = The Antarctic Circumpolar Current System

| chapter-url = http://epic.awi.de/2649/1/Rin8888b.pdf

| title = Ocean Circulation and Climate

| editor1-last = Siedler | editor1-first = G.

| editor2-last = Church | editor2-first = J.

| editor3-last = Gould | editor3-first = J.

| year = 2001 | publisher = Academic Press | location = New York

| isbn = 0-12-641351-7 | hdl = 10013/epic.13233

}}

• {{Cite journal

| last1 = Siegert | first1 = M. J.

| last2 = Barrett | first2 = P.

| last3 = Deconto | first3 = R. M.

| last4 = Dunbar | first4 = R.

| last5 = Cofaigh | first5 = C. O.

| last6 = Passchier | first6 = S.

| last7 = Naish | first7 = T.

| title = Recent Advances in Understanding Antarctic Climate Evolution

| year = 2008 | journal = Antarctic Science | volume = 20 | issue = 4 | pages = 313–325

| url = http://works.bepress.com/robert_deconto/18/

| doi = 10.1017/S0954102008000941 | bibcode = 2008AntSc..20..313S| citeseerx = 10.1.1.210.9532 | s2cid = 18274113

}}

• {{Cite web

| last1 = Smith | first1 = R.

| last2 = Desflots | first2 = M.

| last3 = White | first3 = S.

| last4 = Mariano | first4 = A. J.

| last5 = Ryan | first5 = E. H.

| title = The Antarctic Circumpolar Current

| year = 2013 | publisher = Rosenstiel School of Marine & Atmospheric Science

| url = http://oceancurrents.rsmas.miami.edu/southern/antarctic-cp.html | access-date = 29 December 2016

}}

• {{Cite web

|last=Stewart

|first=R. H.

|title=Deep Circulation in the Ocean: Antarctic Circumpolar Current

|year=2007

|publisher=Department of Oceanography, Texas A&M University

|url=http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_04.htm

|archive-url=https://archive.today/20120713080518/http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_04.htm

|archive-date=13 July 2012

|access-date=29 December 2016

}}

• {{Cite web

| last = Stott | first = L. D.

| title = Ocean Currents and Climate

| date = 2011 | publisher = Department of Earth Sciences, University of Southern California

| url = http://earth.usc.edu/~stott/Catalina/Oceans.html | access-date = 29 December 2016

}}

{{Refend}}

{{Ocean}}

Category:Currents of the Southern Ocean

Category:Geography of the Southern Ocean

Category:Climate of Chile

Category:Subantarctic