Map showing 5 circles. The first is between western Australia and eastern Africa. The second is between eastern Australia and western South America. The third is between Japan and western North America. Of the two in the Atlantic, one is in hemisphere.
North Atlantic
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North Atlantic
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North Atlantic
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Indian
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South
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        gyre
Map showing 5 circles. The first is between western Australia and eastern Africa. The second is between eastern Australia and western South America. The third is between Japan and western North America. Of the two in the Atlantic, one is in hemisphere.
World map of the five major ocean gyres

In oceanography, a gyre (/ˈdʒaɪər/) is any large system of circulating ocean surface currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction determine the circulatory patterns from the wind stress curl (torque).[1]

Gyre can refer to any type of vortex in an atmosphere or a sea,[2] even one that is human-created, but it is most commonly used in terrestrial oceanography to refer to the major ocean systems.

Major gyres

The following are the five most notable ocean gyres:[3]

They flow clockwise in the Northern hemisphere, and counterclockwise in the Southern hemisphere.

Other gyres

Tropical gyres

All of the world's larger gyres

Tropical gyres are less unified and tend to be mostly east–west with minor north–south extent.

  • Atlantic Equatorial Current System (two counter-rotating circulations)
  • Pacific Equatorial Current System
  • Indian Monsoon Gyres (two counter-rotating circulations in northern Indian Ocean)[4]

Subtropical gyres

Subtropical gyres are formed by an intricate process involving both Coriolis force and Ekman transport.[5] As global winds, caused by Earth's rotation, blow across the ocean surface they are acted upon by Coriolis causing movement to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. These winds cause frictional surface currents as the wind transfers energy to the ocean allowing the water to move in a circular motion.[6] As Ekman transport acts on these circular currents the net transport of water is actually 90 degrees which drives regions of convergence, allowing water to pile up in the center of the ocean basin forming a bulge.[7]

The center of a subtropical gyre is a high pressure zone, while the outer edges of the gyre are a low pressure zone. This difference in pressure causes a pressure gradient allowing the diffusion of water from the high pressure zone in the bulge to the low pressure zone on the outer edges of the gyre.The movement of water does not flow directly down the bulge in the center but around it due to Coriolis causing circulation around the high pressure zone in a clockwise motion in the northern hemisphere and a counterclockwise motion in the southern hemisphere.Thus, Resulting in the rotation of the gyre. The gyre has a stable circulation of water around it due to the exact balance between Ekman force and Coriolis. These gyres contribute to the Geostrophic Flow of the ocean resulting in the overall Ocean circulation model of the Earth.[8] The movement of subtropical gyres cause areas of downwelling in the ocean resulting in regions of lower productivity.

This build-up of water in the center creates flow towards the equator in the upper 1,000 to 2,000 m (3,300 to 6,600 ft) of the ocean, through rather complex dynamics. This flow is returned towards the pole in an intensified western boundary current. The boundary current of the North Atlantic Gyre is the Gulf Stream, of the North Pacific Gyre the Kuroshio Current, of the South Atlantic Gyre the Brazil Current, of the South Pacific Gyre the East Australian Current, and of the Indian Ocean Gyre the Agulhas Current.

Subpolar gyres

Subpolar gyres form at high latitudes (around 60°). Circulation of surface wind and ocean water is counterclockwise in the Northern Hemisphere, around a low-pressure area, such as the persistent Aleutian Low and the Icelandic Low. Surface currents generally move outward from the center of the system. This drives the Ekman transport, which creates an upwelling of nutrient-rich water from the lower depths.[9]

Subpolar circulation in the southern hemisphere is dominated by the Antarctic Circumpolar Current, due to the lack of large landmasses breaking up the Southern Ocean. There are minor gyres in the Weddell Sea and the Ross Sea, the Weddell Gyre and Ross Gyre, which circulate in a clockwise direction.[3]

Biology in Gyres

An animation of a year in organism density on Earth. The South Pacific Gyre is visibly low (purple) in organism density.

Depending on their location around the world, gyres can be regions of high biological productivity or low productivity. Subtropical gyres are sometimes described as "ocean deserts" or "biological deserts", in reference to arid land deserts where little life exists.[10] Warm subtropical gyres have some of the least productive waters in the ocean.[11] The downwelling of water that occurs in subtropical gyres takes nutrients deeper in the ocean, removing them from surface waters. Organic particles can also be removed from surface waters through gravitational sinking, where the particle is too heavy to remain suspended in the water column.[12] In contrast to subtropical gyres, subpolar gyres can have a lot of biological activity due to upwelling from their cyclonic motion.[13] Subarctic oceanic gyre conditions in the North Atlantic have a "bloom and crash" pattern following seasonal and storm patterns. Highest productivity occurs in boreal spring when there are long days and high levels of nutrients. This is different to the subarctic North Pacific, where almost no phytoplankton bloom occurs and patterns of respiration are more consistent through time than in the North Atlantic.[11]

Ocean gyres typically contain 5-6 trophic levels. The limiting factor for the number of trophic levels is the size of the phytoplankton, which are generally small in nutrient limited gyres. In low oxygen zones, oligotrophs are a large percentage of the phytoplankton.[14]

Climate change

Ocean circulation re-distributes the heat and water-resources, therefore determines the regional climate. For example, the western branches of the subtropical gyres flow from the lower latitudes towards higher latitudes, bringing relatively warm and moist air to the adjacent land, contributing to a mild and wet climate (e.g., East China, Japan). In contrast, the eastern boundary currents of the subtropical gyres streaming from the higher latitudes towards lower latitudes, corresponding to a relatively cold and dry climate (e.g., California).

Currently, the core of the subtropical gyres are around 30° in both Hemispheres. However, their positions were not always there. Satellite observational sea surface height and sea surface temperature data suggest that the world's major ocean gyres are slowly moving towards higher latitudes in the past few decades. Such feature show agreement with climate model prediction under anthropogenic global warming.[15] Paleo-climate reconstruction also suggest that during the past cold climate intervals, i.e., ice ages, some of the western boundary currents (western branches of the subtropical ocean gyres) are closer to the equator than their modern positions.[16][17] These evidence implies that global warming is very likely to push the large-scale ocean gyres towards higher latitudes.[18][19]

The influence of the Coriolis effect on westward intensification

Coriolis effect

Pollution

This section is an excerpt from Garbage patch.[edit]
Trash washed ashore in Hawaii from the Great Pacific Garbage Patch

A garbage patch is a gyre of marine debris particles caused by the effects of ocean currents and increasing plastic pollution by human populations. These human-caused collections of plastic and other debris, cause ecosystem and environmental problems that affect marine life, contaminate oceans with toxic chemicals, and contribute to greenhouse gas emissions. Once waterborne, marine debris becomes mobile. Flotsam can be blown by the wind, or follow the flow of ocean currents, often ending up in the middle of oceanic gyres where currents are weakest.

Within garbage patches The waste is not compact, and although most of it is near the surface of the Pacific, it can be found up to more than 30 metres (100 ft) deep in the water.[20] Patches contain plastics and debris in a range of sizes from Microplastics and small scale plastic pellet pollution, to large objects such as fishing nets and consumer goods and appliances lost from flood and shipping loss.

Garbage patches grow because of widespread loss of plastic from human trash collection systems. The United Nations Environmental Program estimated that "for every square mile of ocean" there are about "46,000 pieces of plastic".[21] The 10 largest emitters of oceanic plastic pollution worldwide are, from the most to the least, China, Indonesia, Philippines, Vietnam, Sri Lanka, Thailand, Egypt, Malaysia, Nigeria, and Bangladesh,[22] largely through the rivers Yangtze, Indus, Yellow, Hai, Nile, Ganges, Pearl, Amur, Niger, and the Mekong, and accounting for "90 percent of all the plastic that reaches the world's oceans".[23][24] Asia was the leading source of mismanaged plastic waste, with China alone accounting for 2.4 million metric tons.[25]

The best known of these is the Great Pacific garbage patch which has the highest density of marine debris and plastic. The Pacific Garbage patch has two mass buildups: the western garbage patch and the eastern garbage patch, the former off the coast of Japan and the latter between Hawaii and California. These garbage patches contain 90 million tonnes (100 million short tons) of debris.[20] Other identified patches include the North Atlantic garbage patch between North America and Africa, the South Atlantic garbage patch located between eastern South America and the tip of Africa, the South Pacific garbage patch located west of South America, and the Indian Ocean garbage patch found east of South Africa listed in order of decreasing size.[26]

See also

Wikimedia Commons has media related to Oceanic gyres.

References

  1. ↑ Heinemann, B. and the Open University (1998) Ocean circulation, Oxford University Press: Page 98
  2. ↑ Lissauer, Jack J.; de Pater, Imke (2019). Fundamental Planetary Sciences : physics, chemistry, and habitability. New York: Cambridge University Press. ISBN 978-1108411981.
  3. 1 2 The five most notable gyres Archived 2016-03-04 at the Wayback Machine PowerPoint Presentation
  4. ↑ "Indian Monsoon Gyres". Archived from the original on 2016-03-03. Retrieved 2008-04-15.
  5. ↑ "Ocean Gyre | National Geographic Society". National Geographic.
  6. ↑ Constantin, Adrian (2018). "Steady Large-Scale Ocean Flows in Spherical Coordinates". Oceanography. 31 (3): 42–50. doi:10.5670/oceanog.2018.308. JSTOR 26509093. S2CID 135278998.
  7. ↑ "Geostrophic Flow". University of Hawaii at Monoa.
  8. ↑ "Ocean Gyre | National Geographic Society". National Geographic.
  9. ↑ Wind Driven Surface Currents: Gyres oceanmotion.org/html accessed 5 December 2021
  10. ↑ Renfrow, Stephanie (2009-02-06). "An Ocean full of Deserts". Earthdata. Retrieved 2022-11-12.
  11. 1 2 Cochran, J. Kirk; Bokuniewicz, Henry J.; Yager, Patricia L., eds. (2019). Encyclopedia of ocean sciences (3 ed.). London, United Kingdom Cambridge, MA, United States: Academic Press is an imprint of Elsevier. pp. 753–754. ISBN 978-0-12-813081-0.
  12. ↑ Gupta, Mukund; Williams, Richard G.; Lauderdale, Jonathan M.; Jahn, Oliver; Hill, Christopher; Dutkiewicz, Stephanie; Follows, Michael J. (2022-10-11). "A nutrient relay sustains subtropical ocean productivity". Proceedings of the National Academy of Sciences. 119 (41): e2206504119. Bibcode:2022PNAS..11906504G. doi:10.1073/pnas.2206504119. ISSN 0027-8424. PMC 9565266. PMID 36191202.
  13. ↑ "Ocean Gyre". education.nationalgeographic.org. Retrieved 2023-11-28.
  14. ↑ Cochran, J. Kirk; Bokuniewicz, Henry J.; Yager, Patricia L., eds. (2019). Encyclopedia of ocean sciences (3 ed.). London, United Kingdom Cambridge, MA, United States: Academic Press is an imprint of Elsevier. p. 578. ISBN 978-0-12-813081-0.
  15. ↑ Poleward shift of the major ocean gyres detected in a warming climate. Geophysical Research Letters, 47, e2019GL085868 doi:10.1029/2019GL085868
  16. ↑ Bard, E., & Rickaby, R. E. (2009). Migration of the subtropical front as a modulator of glacial climate. Nature, 460(7253), 380.
  17. ↑ Wind-driven evolution of the north pacific subpolar gyre over the last deglaciation. Geophys. Res. Lett. 47, 208–212 (2020).
  18. ↑ Climate Change is Pushing Giant Ocean Currents Poleward Bob Berwyn, 26 February 2020 insideclimatenews.org, accessed 5 December 2021
  19. ↑ Major Ocean Currents Drifting Poleward www.loe.org, accessed 5 December 2021
  20. 1 2 "Marine Debris in the North Pacific A Summary of Existing Information and Identification of Data Gaps" (PDF). United States Environmental Protection Agency. 24 July 2015.
  21. ↑ Maser, Chris (2014). Interactions of Land, Ocean and Humans: A Global Perspective. CRC Press. pp. 147–48. ISBN 978-1482226393.
  22. ↑ Jambeck, Jenna R.; Geyer, Roland; Wilcox, Chris (12 February 2015). "Plastic waste inputs from land into the ocean" (PDF). Science. 347 (6223): 769. Bibcode:2015Sci...347..768J. doi:10.1126/science.1260352. PMID 25678662. S2CID 206562155. Retrieved 28 August 2018.
  23. ↑ Christian Schmidt; Tobias Krauth; Stephan Wagner (11 October 2017). "Export of Plastic Debris by Rivers into the Sea" (PDF). Environmental Science & Technology. 51 (21): 12246–12253. Bibcode:2017EnST...5112246S. doi:10.1021/acs.est.7b02368. PMID 29019247. The 10 top-ranked rivers transport 88–95% of the global load into the sea
  24. ↑ Franzen, Harald (30 November 2017). "Almost all plastic in the ocean comes from just 10 rivers". Deutsche Welle. Retrieved 18 December 2018. It turns out that about 90 percent of all the plastic that reaches the world's oceans gets flushed through just 10 rivers: The Yangtze, the Indus, Yellow River, Hai River, the Nile, the Ganges, Pearl River, Amur River, the Niger, and the Mekong (in that order).
  25. ↑ Robert Lee Hotz (13 February 2015). "Asia Leads World in Dumping Plastic in Seas". Wall Street Journal. Archived from the original on 23 February 2015.
  26. ↑ Cózar, Andrés; Echevarría, Fidel; González-Gordillo, J. Ignacio; Irigoien, Xabier; Úbeda, Bárbara; Hernández-León, Santiago; Palma, Álvaro T.; Navarro, Sandra; García-de-Lomas, Juan; Ruiz, Andrea; Fernández-de-Puelles, María L. (2014-07-15). "Plastic debris in the open ocean". Proceedings of the National Academy of Sciences. 111 (28): 10239–10244. Bibcode:2014PNAS..11110239C. doi:10.1073/pnas.1314705111. ISSN 0027-8424. PMC 4104848. PMID 24982135.
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