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Directional mass flow of oceanic water generated by external or internal forces
An ocean current flows for great distances and together they create the global conveyor belt, which plays a dominant with 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 makes northwest Europe much more temperate than any other region at the same latitude. Another example is Lima, Peru, where the climate is cooler, being sub-tropical, than the tropical latitudes in which the area is located, due to the effect of the Humboldt Current. Ocean currents are patterns of water movement that influence climate zones and weather patterns around the world. They're 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.
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 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 currents, which 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.
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, and the Coriolis effect plays a major role in their development. 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.
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.
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. Deep ocean currents are currently[when?] being researched using a fleet of underwater robots called Argo.
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. The adjective thermohaline derives from thermo- referring to temperature and 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) upwell in the North Pacific. 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.
Angola Current – A temporary ocean surface current. It is an extension of the Guinea Current, flowing near western Africa's coast
Antilles Current – A highly variable surface ocean current of warm water that flows northeasterly past the island chain that separates the Caribbean Sea and the Atlantic Ocean
Atlantic meridional overturning circulation – system of currents in the Atlantic Ocean, having a northward flow of warm, salty water in the upper layers and a southward flow of colder, deep waters that are part of the thermohaline circulation
Azores Current – A generally eastward to southeastward-flowing current in the North Atlantic, originating near the Grand Banks of Newfoundland where it splits from Gulf Stream
Benguela Current – The broad, northward flowing ocean current that forms the eastern portion of the South Atlantic Ocean gyre
Brazil Current – A warm current that flows south along the Brazilian south coast to the mouth of the Río de la Plata
Canary Current – A wind-driven surface current that is part of the North Atlantic Gyre
Falkland Current – A cold water current that flows northward along the Atlantic coast of Patagonia as far north as the mouth of the Río de la Plata
Florida Current – A thermal ocean current that flows from the Straits of Florida around the Florida Peninsula and along the southeastern coast of the United States before joining the Gulf Stream near Cape Hatteras
Guinea Current – A slow warm water current that flows to the east along the Guinea coast of West Africa
Gulf Stream – A warm, swift Atlantic current that originates in the Gulf of Mexico flows around the tip of Florida, along the east coast of the United States before crossing the Atlantic Ocean
Irminger Current – A north Atlantic current setting westward off the southwest coast of Iceland
Labrador Current – A cold current in the Atlantic ocean along the coasts of Labrador, Newfoundland and Nova Scotia
Lomonosov Current – A deep current in the Atlantic Ocean. from the coast of Brazil to the Gulf of Guinea
Loop Current – Ocean current between Cuba and Yucatán Peninsula
North Atlantic Current – A powerful warm western boundary current in the north Atlantic Ocean that extends the Gulf Stream northeastward
North Brazil Current – A warm current that is part of the southwestern North Atlantic Gyre which begins by splitting from the Atlantic South Equatorial Current and flows aling the northwest coast of Brazil until it becomes the Guiana Current
North Equatorial Current – A Pacific and Atlantic Ocean current that flows east-to-west between about 10° north and 20° north on the southern side of a clockwise subtropical gyre
Norwegian Current – A current that flows northeasterly along the Atlantic coast of Norway into the Barents Sea
Portugal Current – A weak ocean current that flows south along the coast of Portugal
Indian Monsoon Current – The seasonally varying ocean current regime found in the tropical regions of the northern Indian Ocean
Indonesian Throughflow – Ocean current that provides a low-latitude pathway for warm, relatively fresh water to move from the Pacific to the Indian Ocean
Leeuwin Current – A warm ocean current which flows southwards near the western coast of Australia. It rounds Cape Leeuwin to enter the waters south of Australia where its influence extends as far as Tasmania
Madagascar Current – The Madagascar current is split into two currents, the North Madagascar Current and the East Madagascar Current
Mozambique Current – A warm ocean current in the Indian Ocean flowing south along the African east coast in the Mozambique Channel
North Madagascar Current – an Ocean current near Madagascar that flows into the South Equatorial Current just North of Madagascar and is directed into the Mozambique Channel
Somali Current – An ocean boundary current that flows along the coast of Somalia and Oman in the Western Indian Ocean
South Equatorial Current – Ocean current in the Pacific, Atlantic, and Indian Ocean that flows east-to-west between the equator and about 20 degrees south
Cromwell Current – An eastward-flowing subsurface current that extends along the equator in the Pacific Ocean
Davidson Current – A coastal countercurrent of the Pacific Ocean flowing north along the western coast of the United States from Baja California, Mexico to northern Oregon
East Australian Current – The southward flowing western boundary current that is formed from the South Equatorial Current reaching the eastern coast of Australia
East Korea Warm Current – An ocean current in the Sea of Japan which branches off from the Tsushima Current at the eastern end of the Korea Strait, and flows north along the southeastern coast of the Korean peninsula
Weddell Gyre – One of the two gyres that exist within the Southern Ocean
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. 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.
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 marine power generation, with areas of Japan, Florida and Hawaii being considered for test projects.
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