View source for Earth:Ocean current

{{short description|Directional mass flow of oceanic water generated by external or internal forces }}
[[File:Corrientes-oceanicas.png|thumb|upright=2|Ocean surface currents]]
[[File:Perpetual Ocean.ogv|thumb|Distinctive white lines trace the flow of surface currents around the world.]]
[[File:Ocean flows at surface and 2000 meters below sea level.webm|thumb|Visualization showing global ocean currents from January{{nbsp}}1, 2010, to December{{nbsp}}31, 2012, at sea level, then at {{cvt|2000|m|ft}} below sea level]]
[[File:Circulation of Ocean Currents Around the Western Antarctic Ice Shelves.ogv|thumb|Animation of circulation around [[Earth:Ice shelf|ice shelves]] of Antarctica]]
An '''ocean current''' is a continuous, directed movement of [[Earth:Seawater|seawater]] generated by a number of forces acting upon the water, including wind, the Coriolis effect, [[Physics:Breaking wave|breaking wave]]s, [[Physics:Cabbeling|cabbeling]], and temperature and [[Earth:Salinity|salinity]] differences.<ref name="NOAAs National Ocean Service 2009" /> Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents are primarily horizontal water movements.

An ocean current flows for great distances and together they create the global conveyor belt, which plays a dominant role in determining the [[Earth:Climate|climate]] of many of Earth's regions. More specifically, ocean currents influence the temperature of the regions through which they travel.  For example, warm currents traveling along more temperate coasts increase the temperature of the area by warming the sea breezes that blow over them.  Perhaps the most striking example is the [[Earth:Gulf Stream|Gulf Stream]], which, together with its extension the North Atlantic Drift, makes northwest Europe much more [[Earth:Temperate climate|temperate]] for its high latitude than other areas at the same latitude. Another example is Lima, Peru, whose cooler [[Earth:Subtropics|subtropical climate]] contrasts with that of its surrounding tropical latitudes because of the [[Earth:Humboldt Current|Humboldt Current]]. Ocean currents are patterns of water movement that influence climate zones and weather patterns around the world. They are primarily driven by winds and by seawater density, although many other factors – including the shape and configuration of the ocean basin they flow through – influence them. The two basic types of currents – surface and deep-water currents – help define the character and flow of ocean waters across the planet.

== Causes ==
[[File:CSIRO ScienceImage 11128 The bathymetry of the Kerguelen Plateau in the Southern Ocean governs the course of the new current part of the global network of ocean currents.jpg|thumb|right|The [[Earth:Bathymetry|bathymetry]] of the Kerguelen Plateau in the Southern Ocean governs the course of the Kerguelen deep western boundary current, part of the global network of ocean currents.<ref name=Massive>{{cite web |title=Massive Southern Ocean current discovered |url=https://www.sciencedaily.com/releases/2010/04/100427101234.htm |website=ScienceDaily |date=Apr 27, 2010}}</ref><ref name=Fukamachi>{{cite journal |display-authors=0|last1=Yasushi Fukamachi, Stephen Rintoul |title=Strong export of Antarctic Bottom Water east of the Kerguelen plateau |journal=Nature Geoscience |date=Apr 2010 |volume=3 |issue=5 |pages=327–331 |doi=10.1038/NGEO842 |bibcode=2010NatGe...3..327F |hdl=2115/44116 |url=https://www.researchgate.net/publication/47656862|hdl-access=free }}</ref>]]
Ocean dynamics define and describe the motion of water within the oceans. Ocean temperature and motion fields can be separated into three distinct layers: mixed (surface) layer, upper ocean (above the thermocline), and deep ocean. Ocean currents are measured in units of [[Sverdrup|sverdrup (sv)]], where 1 sv is equivalent to a volume flow rate of {{convert|1000000|m3|ft3|abbr=on}} per second.

'''Surface ocean currents''' (in contrast to ''[[Earth:Subsurface ocean current|subsurface ocean current]]s''), make up only 8% of all water in the ocean, are generally restricted to the upper {{convert|400|m|ft|abbr=on}} of ocean water, and are separated from lower regions by varying temperatures and [[Earth:Salinity|salinity]] which affect the density of the water, which in turn, defines each oceanic region. Because the movement of deep water in ocean basins is caused by density-driven forces and gravity, deep waters sink into deep ocean basins at high latitudes where the temperatures are cold enough to cause the density to increase.
Surface currents are measued in units of meters per second (m/s) or in [[Physics:Knot (unit)|knots]].<ref name="NOAAs National Ocean Service 2009">{{cite web | title=What is a current? | website=NOAA's National Ocean Service | date=2009-03-01 | url=https://oceanservice.noaa.gov/facts/current.html | access-date=2023-03-14}}</ref>

=== Wind-driven circulation ===
Surface oceanic currents are driven by wind currents, the large scale prevailing winds drive major persistent ocean currents, and seasonal or occasional winds drive currents of similar persistence to the winds that drive them,<ref name="NatGeo Current" >{{cite web |url=https://www.nationalgeographic.org/encyclopedia/current/#:~:text=or%20as%20lightning.-,Air%20Currents,flow%20in%20a%20certain%20direction |title=Current |publisher=National Geographic |website=www.nationalgeographic.org |date=2 September 2011 |access-date=7 January 2021 }}</ref> and the Coriolis effect plays a major role in their development.<ref>{{cite web|url=https://dashamlav.com/ocean-currents-world-map-types-causes-characteristics/|title=Ocean Currents of the World: Causes|date=29 August 2020|access-date=2020-11-20}}</ref> The [[Physics:Ekman spiral|Ekman spiral]] velocity distribution results in the currents flowing at an angle to the driving winds, and they develop typical clockwise spirals in the northern hemisphere and counter-clockwise rotation in the southern hemisphere.<ref name=NNOAACurrents>
{{cite web |url= http://oceanservice.noaa.gov/education/kits/currents/05currents1.html |title = Surface Ocean Currents |last = National Ocean Service |date = March 25, 2008 |website= noaa.gov |publisher= [[Organization:National Oceanic and Atmospheric Administration|National Oceanic and Atmospheric Administration]] |access-date = 2017-06-13 |url-status= live |archive-url= https://web.archive.org/web/20170706062957/https://oceanservice.noaa.gov/education/kits/currents/05currents1.html |archive-date = July 6, 2017}}</ref>
In addition, the areas of surface ocean currents move somewhat with the [[Physics:Season|season]]s; this is most notable in equatorial currents.

Deep ocean basins generally have a non-symmetric surface current, in that the eastern equator-ward flowing branch is broad and diffuse whereas the pole-ward flowing western boundary current is relatively narrow.

=== Thermohaline circulation ===
{{main|Earth:Thermohaline circulation}}
{{Further|Earth:Deep ocean water}}
Deep ocean currents are driven by [[Physics:Density|density]] and temperature gradients. This [[Earth:Thermohaline circulation|thermohaline circulation]] is also known as the ocean's conveyor belt. These currents, sometimes called submarine rivers, flow deep below the surface of the ocean and are hidden from immediate detection. Where significant vertical movement of ocean currents is observed, this is known as [[Earth:Upwelling|upwelling]] and [[Earth:Downwelling|downwelling]]. An international program called [[Earth:Argo (oceanography)|Argo]] began researching deep ocean currents with a fleet of underwater robots in the 2000s.

The thermohaline circulation is a part of the large-scale ocean circulation that is driven by global [[Density gradient|density gradient]]s created by surface heat and freshwater [[Physics:Flux|flux]]es.<ref>{{cite journal|last=Rahmstorf|first=S|title=The concept of the thermohaline circulation|journal=Nature|volume=421|page=699|year=2003|url=http://www.pik-potsdam.de/~stefan/Publications/Nature/nature_concept_03.pdf|pmid=12610602|issue=6924|doi=10.1038/421699a|bibcode = 2003Natur.421..699R |s2cid=4414604|doi-access=free}}</ref><ref>{{cite journal|last=Lappo|first=SS|title=On reason of the northward heat advection across the Equator in the South Pacific and Atlantic ocean|journal=Study of Ocean and Atmosphere Interaction Processes|year=1984|pages=125–9|publisher=Moscow Department of Gidrometeoizdat (in Mandarin)}}</ref> The adjective ''thermohaline'' derives from ''thermo-'' referring to [[Physics:Temperature|temperature]] and ''{{nowrap|-haline}}'' referring to [[Earth:Salinity|salt content]], factors which together determine the density of sea water. [[Earth:Wind|Wind]]-driven surface currents (such as the [[Earth:Gulf Stream|Gulf Stream]]) travel [[Earth:Polar regions of Earth|polewards]] from the equatorial [[Earth:Atlantic Ocean|Atlantic Ocean]], cooling en route, and eventually sinking at high [[Earth:Latitude|latitude]]s (forming [[Earth:North Atlantic Deep Water|North Atlantic Deep Water]]). This dense water then flows into the ocean basins. While the bulk of it [[Earth:Upwelling|upwells]] in the [[Earth:Southern Ocean|Southern Ocean]], the oldest waters (with a transit time of around 1000 years)<ref>The global ocean conveyor belt is a constantly moving system of deep-ocean circulation driven by temperature and salinity; [http://oceanservice.noaa.gov/facts/conveyor.html What is the global ocean conveyor belt?]</ref> upwell in the North Pacific.<ref>{{cite journal|doi=10.1175/JPO2699.1|last=Primeau|first=F|title=Characterizing transport between the surface mixed layer and the ocean interior with a forward and adjoint global ocean transport model|journal=Journal of Physical Oceanography|volume=35|issue=4|pages=545–64|year=2005|bibcode=2005JPO....35..545P|s2cid=130736022 |url=https://escholarship.org/content/qt5f76r4wn/qt5f76r4wn.pdf?t=n3tp5j}}</ref> Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth's oceans a global system. On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of the circulation has a large impact on the [[Earth:Climate|climate]] of the Earth. The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt. On occasion, it is imprecisely used to refer to the meridional overturning circulation, (MOC).

[[File:Meddes-20060320-browse.jpg|thumb|Coupling data collected by NASA/JPL by several different satellite-borne sensors, researchers have been able to "break through" the ocean's surface to detect "Meddies" – super-salty warm-water eddies that originate in the Mediterranean Sea and then sink more than a half-mile underwater in the Atlantic Ocean. The Meddies are shown in red in this scientific figure.]]
[[File:Recording Current Meter.jpg|thumb|left|70px|A recording current meter|alt=Device to record ocean currents]]

== Distribution ==
<!-- the major currents of the world ocean and their interconnection -->
[[Image:Ocean currents 1943 (borderless)3.png|thumb|right|250px|A 1943 map of the world's ocean currents]]

Currents of the [[Earth:Arctic Ocean|Arctic Ocean]]
* {{annotated link|Earth:Baffin Island Current}}
* {{annotated link|Earth:Beaufort Gyre}}
* {{annotated link|Earth:East Greenland Current}}
* {{annotated link|Earth:East Iceland Current}}
* {{annotated link|Earth:Labrador Current}}
* {{annotated link|Earth:North Icelandic Jet}}
* {{annotated link|Earth:Norwegian Current}}
* {{annotated link|Earth:Transpolar Drift Stream}}
* {{annotated link|Earth:West Greenland Current}}
* {{annotated link|Earth:West Spitsbergen Current}}

Currents of the [[Earth:Atlantic Ocean|Atlantic Ocean]]
* {{annotated link|Earth:Angola Current}}
* {{annotated link|Earth:Antilles Current}}
* {{annotated link|Earth:Atlantic meridional overturning circulation}}
* {{annotated link|Earth:Azores Current}}
* {{annotated link|Earth:Benguela Current}}
* {{annotated link|Earth:Brazil Current}}
* {{annotated link|Earth:Canary Current}}
* {{annotated link|Earth:Cape Horn Current}}
* {{annotated link|Earth:Caribbean Current}}
* {{annotated link|Earth:East Greenland Current}}
* {{annotated link|Earth:East Iceland Current}}
* {{annotated link|Earth:Equatorial Counter Current}}
* {{annotated link|Earth:Falkland Current}}
* {{annotated link|Earth:Florida Current}}
* {{annotated link|Earth:Guinea Current}}
* {{annotated link|Earth:Gulf Stream}}
* {{annotated link|Earth:Irminger Current}}
* {{annotated link|Earth:Labrador Current}}
* {{annotated link|Earth:Lomonosov Current}}
* {{annotated link|Earth:Loop Current}}
* {{annotated link|Earth:North Atlantic Current}}
* {{annotated link|Earth:North Brazil Current}}
* {{annotated link|Earth:North Equatorial Current}}
* {{annotated link|Earth:Norwegian Current}}
* {{annotated link|Earth:Portugal Current}}
* {{annotated link|Earth:South Atlantic Current}}
* {{annotated link|Earth:South Equatorial Current}}
* {{annotated link|Earth:West Greenland Current}}
* {{annotated link|Earth:West Spitsbergen Current}}

Currents of the [[Earth:Indian Ocean|Indian Ocean]]
* {{annotated link|Earth:Agulhas Return Current}}
* {{annotated link|Earth:Equatorial Counter Current}}
* {{annotated link|Earth:Indian Monsoon Current}}
* {{annotated link|Earth:Indonesian Throughflow}}
* {{annotated link|Earth:Somali Current}}
* {{annotated link|Earth:South Equatorial Current}}

Currents of the [[Earth:Pacific Ocean|Pacific Ocean]]
* {{annotated link|Earth:Alaska Current}}
* {{annotated link|Earth:Aleutian Current}}
* {{annotated link|Earth:California Current}}
* {{annotated link|Earth:Cape Horn Current}}
* {{annotated link|Earth:Cromwell Current}}
* {{annotated link|Earth:Davidson Current}}
* {{annotated link|Earth:East Australian Current}}
* {{annotated link|Earth:East Korea Warm Current}}
* {{annotated link|Earth:Equatorial Counter Current}}
* {{annotated link|Earth:Humboldt Current}}
* {{annotated link|Earth:Indonesian Throughflow}}
* {{annotated link|Earth:Kamchatka Current}}
* {{annotated link|Earth:Kuroshio Current}}
* {{annotated link|Earth:Mindanao Current}}
* {{annotated link|Earth:North Equatorial Current}}
* {{annotated link|Earth:North Korea Cold Current}}
* {{annotated link|Earth:North Pacific Current}}
* {{annotated link|Earth:Oyashio Current}}
* {{annotated link|Earth:South Equatorial Current}}
* {{annotated link|Earth:Subtropical Countercurrent}}
* {{annotated link|Earth:Tasman Front}}
* {{annotated link|Earth:Tasman Outflow}}

Currents of the [[Earth:Southern Ocean|Southern Ocean]]
* {{annotated link|Earth:Antarctic Circumpolar Current}}
* {{annotated link|Earth:Tasman Outflow}}

[[Physics:Ocean gyre|Oceanic gyres]]
* {{annotated link|Earth:Beaufort Gyre}}
* {{annotated link|Earth:Indian Ocean Gyre}}
* {{annotated link|Earth:North Atlantic Gyre}}
* {{annotated link|Earth:North Pacific Gyre}}
* {{annotated link|Earth:South Atlantic Gyre}}

== Effects on climate and ecology ==
Ocean currents are important in the study of [[Earth:Marine debris|marine debris]], and vice versa. These currents also affect temperatures throughout the world. For example, the ocean current that brings warm water up the north Atlantic to northwest Europe also cumulatively and slowly blocks ice from forming along the seashores, which would also block ships from entering and exiting inland waterways and seaports, hence ocean currents play a decisive role in influencing the climates of regions through which they flow. Cold ocean water currents flowing from polar and sub-polar regions bring in a lot of [[Earth:Plankton|plankton]] that are crucial to the continued survival of several key sea creature species in [[Earth:Marine ecosystem|marine ecosystem]]s. Since plankton are the food of fish, abundant fish populations often live where these currents prevail.

Ocean currents are also very important in the dispersal of many life forms. An example is the [[Biology:Eel life history|life-cycle of the European Eel]].

== Economic importance ==
Knowledge of  surface ocean currents is essential in reducing costs of shipping, since traveling with them reduces fuel costs. In the wind powered sailing-ship era, knowledge of wind patterns and ocean currents was even more essential. A good example of this is the Agulhas Current (down along eastern Africa), which long prevented sailors from reaching India. In recent times, around-the-world sailing competitors make good use of surface currents to build and maintain speed.
Ocean currents can also be used for [[Earth:Marine current power|marine power generation]], with areas  of Japan, Florida and Hawaii being considered for test projects.

== See also ==
* {{annotated link|Earth:Currentology}}
* {{annotated link|Earth:Deep ocean water}}
* {{annotated link|Biology:Fish migration}}
* {{annotated link|Earth:Geostrophic current}}
* {{annotated link|Earth:List of ocean circulation models}}
* {{section link|Marine habitats|Ocean currents}}
* {{annotated link|Earth:Marine current power}}
* {{annotated link|Physics:Ocean gyre}}
* {{annotated link|Earth:Physical oceanography}}
* {{annotated link|Earth:Subsurface ocean current}}
* {{annotated link|Earth:Thermohaline circulation}}
* {{annotated link|Earth:Volta do mar}}

== References ==
{{Reflist}}

==Further reading==
* {{Cite journal | last1 = Hansen | first1 = B. | title = Already the day after tomorrow? | journal = Science | volume = 305 | date = 2004 | pages = 953–954 | doi = 10.1126/science.1100085 | pmid = 15310882 | last2 = Østerhus | first2 = S | last3 = Quadfasel | first3 = D | last4 = Turrell | first4 = W | issue = 5686| s2cid = 12968045 }}
* {{Cite journal | last = Kerr | first = Richard A. | title = A slowing cog in the North Atlantic ocean's climate machine | journal = Science | volume = 304 | date = 2004 | pages = 371–372 | doi = 10.1126/science.304.5669.371a | pmid = 15087513 |  issue = 5669 | s2cid = 42150417 }}
* {{Cite journal | last=Munday | first=Phillip L. | author2 = Jones, Geoffrey P. | author3 = Pratchett, Morgan S. | author4 = Williams, Ashley J. | title=Climate change and the future for coral reef fishes | journal=Fish and Fisheries | volume=9 | issue=3 | date=2008 | pages=261–285 | doi=10.1111/j.1467-2979.2008.00281.x | doi-access=free }}
* {{Cite journal | last=Rahmstorf | first=S. | title=Thermohaline circulation: The current climate | journal=Nature | volume=421 | pages=699 | date=2003 | doi=10.1038/421699a | pmid=12610602 | issue=6924 | bibcode=2003Natur.421..699R| s2cid=4414604 | doi-access=free }}
* {{Cite journal | last=Roemmich | first=D. | title=Physical oceanography: Super spin in the southern seas | journal=Nature | volume=449 | date=2007 | pages=34–35 | doi=10.1038/449034a | pmid=17805284 | issue=7158 | bibcode=2007Natur.449...34R| s2cid=2951110 }}

== External links ==
* [https://earth.nullschool.net/#current/ocean/surface/currents/winkel3/ Current global map of sea surface currents]

{{Ocean}}
{{Physical oceanography}}

[[Category:Ocean currents| ]]
[[Category:Oceanographical terminology]]
[[Category:Physical oceanography]]

{{Sourceattribution|Ocean current}}