Editing Ocean current (section)

== 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 [[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=etal|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 |s2cid=67815755 |url=https://www.researchgate.net/publication/47656862|hdl-access=free }}</ref>]]
Ocean currents are driven by the wind, by the gravitational pull of the moon in the form of [[tide]]s, and by the effects of variations in water density.<ref>{{Cite web |last=National Oceanic and Atmospheric Administration |date=16 June 2024 |title=What is a current? |url=https://oceanservice.noaa.gov/facts/current.html#:~:text=Ocean%20currents%20are%20driven%20by,or%201.15%20miles%20per%20hour). |access-date=2024-09-03 |website=oceanservice.noaa.gov |language=EN-US}}</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 [[physical unit|units]] of [[Sverdrup|sverdrup (Sv)]], where 1 Sv is equivalent to a [[volume flow rate]] of {{convert|1000000|m3|ft3|abbr=on}} per second.

There are two main types of currents, surface currents and deep water currents. Generally surface currents are driven by wind systems and deep water currents are driven by differences in water density due to variations in water temperature and [[salinity]].<ref>{{Cite web |last=National Oceanic and Atmospheric Administration |date=1 August 2011 |title=Ocean currents |url=https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-currents#:~:text=via%20research%20ship.-,Surface%20currents,influencing%20local%20and%20global%20climate. |access-date=2024-09-14 |website=www.noaa.gov |language=en}}</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 [[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= [[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 [[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|Thermohaline circulation}}
{{Further|Deep ocean water}}[[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.]]
Large scale currents are driven by gradients in water [[density]], which in turn depend on variations in temperature and salinity. This [[thermohaline circulation]] is also known as the ocean's conveyor belt. Where significant vertical movement of ocean currents is observed, this is known as [[upwelling]] and [[downwelling]]. The adjective ''thermohaline'' derives from ''[[wikt:thermo-|thermo-]]'' referring to [[temperature]] and ''{{nowrap|-haline}}'' referring to [[salinity|salt content]], factors which together determine the density of seawater.

The thermohaline circulation is a part of the large-scale ocean circulation that is driven by global [[density gradient]]s created by surface heat and freshwater [[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> [[Wind]]-driven surface currents (such as the [[Gulf Stream]]) travel [[Polar regions of Earth|polewards]] from the equatorial [[Atlantic Ocean]], cooling en route, and eventually sinking at high [[latitude]]s (forming [[North Atlantic Deep Water]]). This dense water then flows into the [[ocean basin]]s. While the bulk of it [[upwelling|upwells]] in the [[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 [[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).

Since the 2000s an international program called [[Argo (oceanography)|Argo]] has been mapping the temperature and salinity structure of the ocean with a fleet of automated platforms that float with the ocean currents. The information gathered will help explain the role the oceans play in the earth's climate.<ref>{{Cite web |last=Scripps Institution of Oceanography, UC San Diego |title=Argo |url=https://argo.ucsd.edu/ |archive-url=https://web.archive.org/web/20240901191501/https://argo.ucsd.edu/ |archive-date=1 September 2024 |access-date=2024-09-05 |website=Argo |language=en-US}}</ref>