View source for Physics:Ocean gyre

{{short description|Any large system of circulating ocean surface currents}}
In [[Physics:Oceanography|oceanography]], a '''gyre'''<!-- gyre is bolded per WP:MOSBOLD as this article is a redirect target for "gyre" more generally, and not merely ocean gyres --> ({{IPAc-en|ˈ|dʒ|aɪər}}) is any large system of circulating [[Earth:Ocean current|ocean surface current]]s, particularly those involved with large [[Earth:Wind|wind]] movements. Gyres are caused by the Coriolis effect; planetary [[Vorticity|vorticity]], horizontal friction and vertical friction determine the circulatory patterns from the ''[[Physics:Wind stress|wind stress]] [[Curl (mathematics)|curl]]'' ([[Physics:Torque|torque]]).<ref>Heinemann, B. and the Open University (1998) ''Ocean circulation'', Oxford University Press: Page 98</ref>

''Gyre'' can refer to any type of [[Vortex|vortex]] in an atmosphere or a sea,<ref name=lissauer2019>{{Cite book |title=Fundamental Planetary Sciences : physics, chemistry, and habitability |last1=Lissauer|first1=Jack J. |last2=de Pater|first2=Imke |year=2019 |publisher=Cambridge University Press |isbn=978-1108411981 |location=New York}}</ref> even one that is human-created, but it is most commonly used in terrestrial [[Physics:Oceanography|oceanography]] to refer to the major [[Earth:Ocean|ocean]] systems.

== Major gyres ==
The following are the five most notable ocean gyres:<ref name="SIO210slides">[http://www-pord.ucsd.edu/~ltalley/sio210/Wind_circulation_NAtl_SHem/lecture_natl_shem.htm The five most notable gyres] {{Webarchive|url=https://web.archive.org/web/20160304034127/http://www-pord.ucsd.edu/~ltalley/sio210/Wind_circulation_NAtl_SHem/lecture_natl_shem.htm |date=2016-03-04 }} PowerPoint Presentation</ref>
* [[Earth:Indian Ocean Gyre|Indian Ocean Gyre]]
* [[Earth:North Atlantic Gyre|North Atlantic Gyre]]
* [[Earth:North Pacific Gyre|North Pacific Gyre]]
* [[Earth:South Atlantic Gyre|South Atlantic Gyre]]
* South Pacific Gyre

They flow [[Physics:Clockwise|clockwise]] in the Northern hemisphere, and counterclockwise in the Southern hemisphere.

== Other gyres ==

=== Tropical gyres ===
[[File:Currents.svg|thumb|250px|right|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){{citation needed|date=October 2019}}
* Pacific Equatorial Current System{{citation needed|date=October 2019}}
* Indian Monsoon Gyres (two counter-rotating circulations in northern Indian Ocean)<ref>{{Cite web |url=http://www.mar.dfo-mpo.gc.ca/science/ocean/BedfordBasin/Papers/Longhurst1998/Provinces/BGCP_table.htm |title=Indian Monsoon Gyres |access-date=2008-04-15 |archive-date=2016-03-03 |archive-url=https://web.archive.org/web/20160303174143/http://www.mar.dfo-mpo.gc.ca/science/ocean/BedfordBasin/Papers/Longhurst1998/Provinces/BGCP_table.htm |url-status=dead }}</ref>

=== Subtropical gyres ===
Subtropical gyres are formed by an intricate process involving both [[Physics:Coriolis force|Coriolis force]] and [[Physics:Ekman transport|Ekman transport]].<ref>{{cite web |url=https://education.nationalgeographic.org/resource/ocean-gyre |website=National Geographic|title=Ocean Gyre &#124; National Geographic Society }}</ref> 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.<ref>{{cite journal |last1=Constantin |first1=Adrian |title=Steady Large-Scale Ocean Flows in Spherical Coordinates |journal=Oceanography |year=2018 |volume=31 |issue=3 |pages=42–50 |doi=10.5670/oceanog.2018.308 |jstor=26509093 |s2cid=135278998 |doi-access=free }}</ref> 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.<ref>{{cite web |title=Geostrophic Flow |url=https://manoa.hawaii.edu/exploringourfluidearth/physical/atmospheric-effects/effect-surface-currents |website=University of Hawaii at Monoa}}</ref>

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.<ref>{{cite web |url=https://education.nationalgeographic.org/resource/ocean-gyre |website=National Geographic|title=Ocean Gyre &#124; National Geographic Society }}</ref> The movement of subtropical gyres cause areas of [[Earth:Downwelling|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 {{convert|1000|to|2000|m|ft|abbr=on}} of the ocean, through rather complex dynamics. This flow is returned towards the pole in an intensified western [[Earth:Boundary current|boundary current]]. The boundary current of the [[Earth:North Atlantic Gyre|North Atlantic Gyre]] is the [[Earth:Gulf Stream|Gulf Stream]], of the [[Earth:North Pacific Gyre|North Pacific Gyre]] the [[Earth:Kuroshio Current|Kuroshio Current]], of the [[Earth:South Atlantic Gyre|South Atlantic Gyre]] the [[Earth:Brazil Current|Brazil Current]], of the South Pacific Gyre the [[Earth:East Australian Current|East Australian Current]], and of the [[Earth:Indian Ocean Gyre|Indian Ocean Gyre]] the Agulhas Current.{{citation needed|date=October 2019}}

=== Subpolar gyres ===
<!-- Subpolar gyre redirects to this section -->
Subpolar gyres form at high latitudes (around 60°). Circulation of surface wind and ocean water is counterclockwise in the Northern Hemisphere, around a [[Physics:Low-pressure area|low-pressure area]], such as the persistent [[Earth:Aleutian Low|Aleutian Low]] and the [[Earth:Icelandic Low|Icelandic Low]]. Surface currents generally move outward from the center of the system. This drives the [[Physics:Ekman transport|Ekman transport]], which creates an [[Earth:Upwelling|upwelling]] of nutrient-rich water from the lower depths.<ref>[http://oceanmotion.org/html/background/wind-driven-surface.htm Wind Driven Surface Currents: Gyres] ''oceanmotion.org/html'' accessed 5 December 2021</ref>

Subpolar circulation in the southern hemisphere is dominated by the [[Earth:Antarctic Circumpolar Current|Antarctic Circumpolar Current]], due to the lack of large landmasses breaking up the [[Earth:Southern Ocean|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.<ref name="SIO210slides"/>

== Biology in Gyres ==
[[File:Mollweide Cycle.gif|thumb|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 [[Earth:Aridity|arid]] land [[Earth:Desert|deserts]] where little life exists.<ref name="Renfrow 20093">{{cite web |last=Renfrow |first=Stephanie |date=2009-02-06 |title=An Ocean full of Deserts |url=http://www.earthdata.nasa.gov/learn/sensing-our-planet/an-ocean-full-of-deserts |access-date=2022-11-12 |website=Earthdata}}</ref> Warm subtropical gyres have some of the least productive waters in the ocean.<ref name=":32">{{Cite book |title=Encyclopedia of ocean sciences |date=2019 |publisher=Academic Press is an imprint of Elsevier |isbn=978-0-12-813081-0 |editor-last=Cochran |editor-first=J. Kirk |edition=3 |location=London, United Kingdom Cambridge, MA, United States |pages=753–754 |editor-last2=Bokuniewicz |editor-first2=Henry J. |editor-last3=Yager |editor-first3=Patricia L.}}</ref> 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.<ref>{{Cite journal |last1=Gupta |first1=Mukund |last2=Williams |first2=Richard G. |last3=Lauderdale |first3=Jonathan M. |last4=Jahn |first4=Oliver |last5=Hill |first5=Christopher |last6=Dutkiewicz |first6=Stephanie |last7=Follows |first7=Michael J. |date=2022-10-11 |title=A nutrient relay sustains subtropical ocean productivity |journal=Proceedings of the National Academy of Sciences |language=en |volume=119 |issue=41 |pages=e2206504119 |doi=10.1073/pnas.2206504119 |issn=0027-8424 |pmc=9565266 |pmid=36191202|bibcode=2022PNAS..11906504G }}</ref> In contrast to subtropical gyres, subpolar gyres can have a lot of biological activity due to upwelling from their cyclonic motion.<ref>{{Cite web |title=Ocean Gyre |url=https://education.nationalgeographic.org/resource/ocean-gyre |access-date=2023-11-28 |website=education.nationalgeographic.org |language=en}}</ref> 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.<ref name=":32" />

Ocean gyres typically contain 5-6 [[Earth:Trophic level|trophic levels]]. The limiting factor for the number of trophic levels is the size of the [[Earth:Phytoplankton|phytoplankton]], which are generally small in nutrient limited gyres. In low oxygen zones, [[Biology:Oligotroph|oligotrophs]] are a large percentage of the phytoplankton.<ref>{{Cite book |title=Encyclopedia of ocean sciences |date=2019 |publisher=Academic Press is an imprint of Elsevier |isbn=978-0-12-813081-0 |editor-last=Cochran |editor-first=J. Kirk |edition=3 |location=London, United Kingdom Cambridge, MA, United States |pages=578 |editor-last2=Bokuniewicz |editor-first2=Henry J. |editor-last3=Yager |editor-first3=Patricia L.}}</ref>

== 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 [[Physics:Sea surface temperature|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.<ref>Poleward shift of the major ocean gyres detected in a warming climate. Geophysical Research Letters, 47, e2019GL085868 {{doi|10.1029/2019GL085868}}</ref> 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.<ref>Bard, E., & Rickaby, R. E. (2009). Migration of the subtropical front as a modulator of glacial climate. Nature, 460(7253), 380.</ref><ref>Wind-driven evolution of the north pacific subpolar gyre over the last deglaciation. Geophys. Res. Lett. 47, 208–212 (2020).</ref> These evidence implies that global warming is very likely to push the large-scale ocean gyres towards higher latitudes.<ref>[https://insideclimatenews.org/news/26022020/climate-oceans-weather-fishing-gyres-gulf-stream-sea-level/ Climate Change is Pushing Giant Ocean Currents Poleward] Bob Berwyn, 26 February 2020 ''insideclimatenews.org'', accessed 5 December 2021</ref><ref>[https://www.loe.org/shows/segments.html?programID=20-P13-00013&segmentID=3 Major Ocean Currents Drifting Poleward] ''www.loe.org'', accessed 5 December 2021</ref>

==The influence of the Coriolis effect on westward intensification==
[[File:Coriolis effect14.png|thumb|Coriolis effect]]

== Pollution ==
{{excerpt|Garbage patch}}

== See also ==
* [[Earth:Anticyclone|Anticyclone]]
* [[Physics:Cyclone|Cyclone]]
* [[Earth:Ecosystem of the North Pacific Subtropical Gyre|Ecosystem of the North Pacific Subtropical Gyre]]
* [[Physics:Eddy (fluid dynamics)|Eddy]]
* [[Physics:Fluid dynamics|Fluid dynamics]]
* [[Earth:Geostrophic current|Geostrophic current]]
* Skookumchuck
* ''[[Earth:Volta do mar|Volta do mar]]''
* [[Physics:Whirlpool|Whirlpool]]

== References ==
{{reflist}}

== External links ==
* [http://5gyres.org 5 Gyres – Understanding Plastic Marine Pollution]
* [http://oceanmotion.org/html/background/wind-driven-surface.htm Wind Driven Surface Currents: Gyres]
* [https://web.archive.org/web/20131102195719/http://www-pord.ucsd.edu/~ltalley/sio210/Global_circulation/ SIO 210: Introduction to Physical Oceanography – Global circulation]
* [https://web.archive.org/web/20120806231748/http://www-pord.ucsd.edu/~ltalley/sio210/Wind_circulation_NAtl_SHem/ SIO 210: Introduction to Physical Oceanography – Wind-forced circulation notes]
* [http://sam.ucsd.edu/sio210/lect_6/lecture_6.html SIO 210: Introduction to Physical Oceanography – Lecture 6]
* [http://www.physicalgeography.net/fundamentals/8q.html Physical Geography – Surface and Subsurface Ocean Currents]
* [https://web.archive.org/web/20080506085457/http://eros.eas.gatech.edu/npgo/ North Pacific Gyre Oscillation] — Georgia Institute of Technology
* {{Skeptoid|id=4132|number=132|title=The Sargasso Sea and the Pacific Garbage Patch|date=16 December 2008}}

{{Ocean}}
{{Physical oceanography}}

[[Category:Aerodynamics]]
[[Category:Fluid dynamics]]

{{Sourceattribution|Ocean gyre}}