Ocean surface currentsDistinctive white lines trace the flow of surface currents around the world.Visualization showing global ocean currents from January1, 2010, to December31, 2012, at sea level, then at 2,000 m (6,600 ft) below sea levelAnimation of circulation aroundice shelves ofAntarctica
Anocean current is a continuous, directed movement ofseawater generated by a number of forces acting upon the water, including wind, theCoriolis effect,breaking waves,cabbeling, and temperature andsalinity differences.[1]Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents move both horizontally, on scales that can span entire oceans, as well as vertically, with vertical currents (upwelling anddownwelling) playing an important role in the movement of nutrients and gases, such as carbon dioxide, between the surface and the deep ocean.
Ocean currents flow for great distances and together they create theglobal conveyor belt, which plays a dominant role in determining theclimate 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 theGulf Stream, which, together with its extension theNorth Atlantic Drift, makesnorthwest Europe much moretemperate for its high latitude than other areas at the same latitude. Another example isLima, Peru, whose coolersubtropical climate contrasts with that of its surrounding tropical latitudes because of theHumboldt Current.
The largest ocean current is theAntarctic Circumpolar Current (ACC), a wind-driven current which flows clockwise uninterrupted around Antarctica. The ACC connects all the ocean basins together, and also provides a link between the atmosphere and the deep ocean due to the way water upwells and downwells on either side of it.
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 influence them – including the shape and configuration of theocean basin they flow through. The two basic types of currents – surface and deep-water currents – help define the character and flow of ocean waters across the planet.the ocean current is divided in to two warm ocean current and cold ocean current
Ocean currents are driven by the wind, by the gravitational pull of the moon in the form oftides, and by the effects of variations in water density.[4]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 inunits ofsverdrup (Sv), where 1 Sv is equivalent to avolume flow rate of 1,000,000 m3 (35,000,000 cu ft) 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 andsalinity.[5]
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,[6] and theCoriolis effect plays a major role in their development.[7] TheEkman spiral velocity distribution results in the currents flowing at an angle to the driving winds, and they develop typical clockwise spirals in thenorthern hemisphere and counter-clockwise rotation in thesouthern hemisphere.[8]In addition, the areas of surface ocean currents move somewhat with theseasons; 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 flowingwestern boundary current is relatively narrow.
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 waterdensity, which in turn depend on variations in temperature and salinity. Thisthermohaline circulation is also known as the ocean's conveyor belt. Where significant vertical movement of ocean currents is observed, this is known asupwelling anddownwelling. The adjectivethermohaline derives fromthermo- referring totemperature and-haline referring tosalt 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 globaldensity gradients created by surface heat and freshwaterfluxes.[9][10]Wind-driven surface currents (such as theGulf Stream) travelpolewards from the equatorialAtlantic Ocean, cooling en route, and eventually sinking at highlatitudes (formingNorth Atlantic Deep Water). This dense water then flows into theocean basins. While the bulk of itupwells in theSouthern Ocean, the oldest waters (with a transit time of around 1000 years)[11] upwell in the North Pacific.[12] 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 theclimate 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 themeridional overturning circulation, (MOC).
Since the 2000s an international program calledArgo 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.[13]
Ocean currents 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.[14] Ocean currents are important in the study ofmarine debris.[15][16]
Plankton are dispersed by ocean currents.
Upwellings and cold ocean water currents flowing from polar and sub-polar regions bring in nutrients that supportplankton growth, which are crucialprey items for several key species inmarine ecosystems.[17]
Ocean currents are also important in the dispersal and distribution of many organisms, including those withpelagic egg or larval stages.[18] An example is thelife-cycle of the European Eel. Terrestrial species, for example tortoises and lizards, can be carried on floating debris by currents to colonise new terrestrial areas andislands.[18]
The continued rise of atmospheric temperatures is anticipated to have various effects on the strength of surface ocean currents, wind-driven circulation and dispersal patterns.[19][20][21] Ocean currents play a significant role in influencing climate, and shifts in climate in turn impact ocean currents.[20]
Human-induced climate change is leading to long-term alterations in ocean and atmospheric circulation. The accumulation of greenhouse gases traps extra heat within the Earth's system, causing both the atmosphere and oceans to warm. Notably, over 90% of this trapped heat is absorbed by the oceans. There are signs that crucial circulation patterns are shifting, with growing evidence suggesting that the Atlantic Meridional Overturning Circulation may be slowing down.
Over the last century, reconstructed sea surface temperature data reveal that westernboundary currents are heating at double the rate of the global average.[22] 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.[20] There is evidence that surface warming due to anthropogenicclimate change has accelerated upper ocean currents in 77% of the global ocean.[21] Specifically, increased verticalstratification due to surface warming intensifies upper ocean currents, while changes in horizontal density gradients caused by differential warming across different ocean regions results in the acceleration of surfacezonal currents.[21]
There are suggestions that theAtlantic meridional overturning circulation (AMOC) is in danger of collapsing due to climate change, which would have extreme impacts on the climate of northern Europe and more widely,[23][24][25] although this topic is controversial and remains an active area of research.[26][27][28] The "State of the cryosphere" report, dedicates significant space to AMOC, saying it may be en route to collapse because of ice melt and water warming. In the same time, theAntarctic Circumpolar Current (ACC) is also slowing down and is expected to lose 20% of it power by the year 2050, "with widespread impacts on ocean circulation and climate".[29]UNESCO mentions that the report in the first time "notes a growing scientific consensus that melting Greenland and Antarctic ice sheets, among other factors, may be slowing important ocean currents at both poles, with potentially dire consequences for a much colder northern Europe and greater sea-level rise along the U.S. East Coast."[30]
In addition to water surface temperatures, the wind systems are a crucial determinant of ocean currents.[31] Wind wave systems influence oceanic heat exchange, the condition of the sea surface, and can alter ocean currents.[32] 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.[32] In theEast Australian Current, global warming has also been accredited to increasedwind stresscurl, which intensifies these currents, and may even indirectly increase sea levels, due to the additional warming created by stronger currents.[33]
As ocean circulation changes due to climate, typical distribution patterns are also changing. Thedispersal patterns of marine organisms depend on oceanographic conditions, which as a result, influence the biological composition of oceans.[19] Due to the patchiness of the natural ecological world, dispersal is a species survival mechanism for various organisms.[34] With strengthened boundary currents moving toward the poles, it is expected that some marine species will be redirected to the poles and greater depths.[19][35] 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 theirmeta populations but also may increase the prevalence ofinvasive species.[19] In Japanese corals and macroalgae, the unusual dispersal pattern of organisms toward the poles may destabilize native species.[36]
Knowledge of surface ocean currents is essential in reducing costs of shipping, since traveling with them reduces fuel costs. In the wind poweredsailing-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.[37] 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.[38] A good example of this is theAgulhas 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 formarine 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.[39]
Skipjack tuna fishery in Indonesia.
Ocean currents can also impact thefishing industry, examples of this include theTsugaru,Oyashio andKuroshio currents all of which influence the western North Pacific temperature, which has been shown to be a habitat predictor for theSkipjack tuna.[40] 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.[41]
^Lappo, SS (1984). "On reason of the northward heat advection across the Equator in the South Pacific and Atlantic ocean".Study of Ocean and Atmosphere Interaction Processes. Moscow Department of Gidrometeoizdat (in Mandarin):125–9.
^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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