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Ocean surface currents
Distinctive white lines trace the flow of surface currents around the world.
Visualization showing global ocean currents from January 1, 2010, to December 31, 2012, at sea level, then at 2,000 m (6,600 ft) below sea level
Animation of circulation around ice shelves of Antarctica

An ocean current is a continuous, directed movement of seawater generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences.[1] 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 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 Gulf Stream, which, together with its extension the North Atlantic Drift, makes northwest Europe much more temperate for its high latitude than other areas at the same latitude. Another example is Lima, Peru, whose cooler subtropical climate contrasts with that of its surrounding tropical latitudes because of the 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

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.[2][3]

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 (sv), where 1 sv is equivalent to a volume flow rate of 1,000,000 m3 (35,000,000 cu ft) per second.

Surface ocean currents (in contrast to subsurface ocean currents), make up only 8% of all water in the ocean, are generally restricted to the upper 400 m (1,300 ft) of ocean water, and are separated from lower regions by varying temperatures and 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 measured in units of meters per second (m/s) or in knots.[1]

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,[4] and the Coriolis effect plays a major role in their development.[5] 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.[6] In addition, the areas of surface ocean currents move somewhat with the seasons; 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 article: Thermohaline circulation

Further information: Deep ocean water

Deep ocean currents are driven by density and temperature gradients. This 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 upwelling and downwelling. An international program called 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 gradients created by surface heat and freshwater fluxes.[7][8] The adjective thermohaline derives from thermo- referring to temperature and -haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents (such as the Gulf Stream) travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes (forming North Atlantic Deep Water). This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters (with a transit time of around 1000 years)[9] upwell in the North Pacific.[10] 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).

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.
Device to record ocean currents
A recording current meter

Distribution

A 1943 map of the world's ocean currents

Currents of the Arctic Ocean

Currents of the Atlantic Ocean

Currents of the Indian Ocean

Currents of the Pacific Ocean

Currents of the Southern Ocean

Oceanic gyres

Effects on climate and ecology

Ocean currents are important in the study of 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.[11] Cold ocean water currents flowing from polar and sub-polar regions bring in a lot of plankton that are crucial to the continued survival of several key sea creature species in marine ecosystems. 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 life-cycle of the European Eel.

Ocean currents and climate change

As atmospheric temperatures continue to rise, this is anticipated to have various effects on the strength of surface ocean currents, wind-driven circulation and dispersal patterns.[12][13][14] Ocean currents play a significant role in influencing climate, and shifts in climate, in turn, impact ocean currents.[13] Over the last century, reconstructed sea surface temperature data reveal that western boundary currents are heating at double the rate of the global average.[15] These observations indicate that the western boundary currents are likely intensifying due to this change in temperature, and may continue to grow stronger in the near future.[13] Studies investigating international ocean current patterns have also suspected that anthropogenic climate change has accelerated upper ocean currents by 77%.[14] Faster upper ocean currents are often associated with increased vertical stratification, as well as faster and stronger zonal currents.[14]

In addition to water surface temperatures, the wind systems are a crucial determinant of ocean currents.[16] Wind wave systems influence oceanic heat exchange, the condition of the sea surface, and can alter ocean currents.[17] In the North Atlantic, equatorial Pacific, and Southern Ocean, increased wind speeds as well as significant wave heights have been attributed to climate change and natural processes combined.[17] In the East Australian Current, global warming has also been accredited to increased wind stress curls, which intensify these currents, and may even indirectly increase sea levels, due to the additional warming created by stronger currents.[18]

As ocean circulation changes due to climate, typical distribution patterns are also changing. The dispersal patterns of marine organisms depend on oceanographic conditions, which as a result, influence the biological composition of oceans.[12] Due to the patchiness of the natural ecological world, dispersal is a species survival mechanism for various organisms.[19] With strengthened boundary currents moving toward the poles, it is expected that some marine species will be redirected to the poles and greater depths.[12][20] The strengthening or weakening of typical dispersal pathways by increased temperatures are expected to not only impact the survival of native marine species due to inability to replenish their meta populations but also may increase the prevalence of invasive species.[12] In Japanese corals and macroalgae, the unusual dispersal pattern of organisms toward the poles may destabilize native species.[21]

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. Using ocean currents to help their ships into harbor and using currents such as the gulf stream to get back home.[22] The lack of understanding of ocean currents during that time period is hypothesized to be one of the contributing factors to exploration failure. The gulf stream and the Canary current keep western European countries warmer and less variable, while at the same latitude North America's weather was colder.[23] 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 marine power generation, with areas of Japan, Florida and Hawaii being considered for test projects. The utilization of currents today can still impact global trade, it can reduce the cost and emissions of shipping vessels.[24] Ocean currents can also impact the fishing industry, examples of this include the Tsugaru, Oyashio and Kuroshio currents all of which influence the western North Pacific temperature, which has been shown to be a habitat predictor for the Skipjack tuna.[25] It has also been shown that it is not just local currents that can affect a country's economy, but neighboring currents can influence the viability of local fishing industries.[26]

See also

References

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  2. ^ a b "Massive Southern Ocean current discovered". ScienceDaily. Apr 27, 2010.
  3. ^ a b Yasushi Fukamachi, Stephen Rintoul; et al. (Apr 2010). "Strong export of Antarctic Bottom Water east of the Kerguelen plateau". Nature Geoscience. 3 (5): 327–331. Bibcode:2010NatGe...3..327F. doi:10.1038/NGEO842. hdl:2115/44116. S2CID 67815755.
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  9. ^ The global ocean conveyor belt is a constantly moving system of deep-ocean circulation driven by temperature and salinity; What is the global ocean conveyor belt?
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  17. ^ a b Dobrynin, Mikhail; Murawski, Jens; Baehr, Johanna; Ilyina, Tatiana (2015-02-15). "Detection and Attribution of Climate Change Signal in Ocean Wind Waves". Journal of Climate. 28 (4): 1578–1591. Bibcode:2015JCli...28.1578D. doi:10.1175/JCLI-D-13-00664.1. ISSN 0894-8755.
  18. ^ Cai, W.; Shi, G.; Cowan, T.; Bi, D.; Ribbe, J. (2005-12-10). "The response of the Southern Annular Mode, the East Australian Current, and the southern mid-latitude ocean circulation to global warming". Geophysical Research Letters. 32 (23). Bibcode:2005GeoRL..3223706C. doi:10.1029/2005GL024701. ISSN 0094-8276.
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  25. ^ Ramesh, Nandini; Rising, James A.; Oremus, Kimberly L. (2019-06-21). "The small world of global marine fisheries: The cross-boundary consequences of larval dispersal". Science. 364 (6446): 1192–1196. Bibcode:2019Sci...364.1192R. doi:10.1126/science.aav3409. ISSN 0036-8075.
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Further reading