Ocean current

The ocean currents.

Corrientes-oceanicas
Ocean surface currents
Distinctive white lines trace the flow of surface currents around the world.
Visualization showing global ocean currents from Jan 01, 2010 to Dec 31, 2012 at sea level then at 2000 meters below sea level.
Animation of circulation around ice shelves of Antarctica.

An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. 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 makes northwest Europe much more temperate than any other region at the same latitude. Other 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.

Function

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
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 (Source:CSIRO)

Surface oceanic currents are sometimes wind driven and develop their typical clockwise spirals in the northern hemisphere and counter-clockwise rotation in the southern hemisphere due to imposed wind stresses. In these wind-driven currents, the Ekman spiral effect results in the currents flowing at an angle to the driving winds. 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 equatorward-flowing branch is broad and diffuse whereas the western poleward flowing branch is very narrow. These western boundary currents (of which the Gulf Stream is an example) are a consequence of the rotation of the Earth.

Deep ocean currents are driven by density and temperature gradients. Thermohaline circulation is also known as the ocean's conveyor belt (which refers to deep ocean density-driven ocean basin currents). These currents, called submarine rivers, flow under 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 being researched using a fleet of underwater robots called Argo.

Surface 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.

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 are found on the surface of an ocean, and are driven by large scale wind currents. They are directly affected by the wind—the Coriolis effect plays a role in their behaviours.[1]

Thermohaline circulation

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.[2][3] 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)[4] upwell in the North Pacific.[5] 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.

Meddes-20060320-browse
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.
Recording Current Meter
A recording current meter

Importance

Ocean currents 1943 (borderless)3
A 1943 map of the world's ocean currents.

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 Portuguese 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 are also very important in the dispersal of many life forms. An example is the life-cycle of the European Eel.

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.

Ocean currents can also be used for marine power generation, with areas off of Japan, Florida and Hawaii being considered for test projects.

See also

References

  1. ^ National Ocean Service (March 25, 2008). "Surface Ocean Currents". noaa.gov. National Oceanic and Atmospheric Administration. Archived from the original on July 6, 2017. Retrieved 2017-06-13.
  2. ^ Rahmstorf, S (2003). "The concept of the thermohaline circulation" (PDF). Nature. 421 (6924): 699. Bibcode:2003Natur.421..699R. doi:10.1038/421699a. PMID 12610602.
  3. ^ 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.
  4. ^ 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?
  5. ^ Primeau, F (2005). "Characterizing transport between the surface mixed layer and the ocean interior with a forward and adjoint global ocean transport model" (PDF). Journal of Physical Oceanography. 35 (4): 545–64. Bibcode:2005JPO....35..545P. doi:10.1175/JPO2699.1.

Further reading

External links

Agulhas Return Current

The Agulhas Return Current (ARC) is an ocean current in the South Indian Ocean. The ARC contributes to the water exchange between oceans by forming a link between the South Atlantic Current and the South Indian Ocean Current. It can reach velocities of up to 4 knots (7.4 km/h; 4.6 mph) and is therefore popular among participants in trans-oceanic sailing races.

Aleutian Current

The Aleutian Current is also called the "Subarctic Current". An eastward flowing ocean current which lies north of the North Pacific Current; it is the northern branch of the Kuroshio Current which moves northeast then east between 40° N and 50° N. As it approaches the coast of North America it divides to form the northward-flowing Alaska Current and the southward-flowing California Current .

Antarctic Circumpolar Current

The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100-150 Sverdrups (Sv, million m³/s), or possibly even higher, making it the largest ocean current. The current is circumpolar due to the lack of any landmass connecting with Antarctica and this keeps warm ocean waters away from Antarctica, enabling that continent to maintain its huge ice sheet.

Associated with the Circumpolar Current is the Antarctic Convergence, where the cold Antarctic waters meet the warmer waters of the subantarctic, creating a zone of upwelling nutrients. These nurture high levels of phytoplankton with associated copepods and krill, and resultant foodchains supporting fish, whales, seals, penguins, albatrosses, and a wealth of other species.

The ACC has been known to sailors for centuries; it greatly speeds up any travel from west to east, but makes sailing extremely difficult from east to west, although this is mostly due to the prevailing westerly winds. Jack London's story "Make Westing" and the circumstances preceding the mutiny on the Bounty poignantly illustrate the difficulty it caused for mariners seeking to round Cape Horn westbound on the clipper ship route from New York to California. The eastbound clipper route, which is the fastest sailing route around the world, follows the ACC around three continental capes – Cape Agulhas (Africa), South East Cape (Australia), and Cape Horn (South America).

The current creates the Ross and Weddell gyres.

Antilles Current

The Antilles Current is 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. The current results from the flow of the Atlantic North Equatorial Current. This current completes the clockwise- cycle or convection (North Atlantic Gyre) that is located in the Atlantic Ocean. It runs north of Puerto Rico, Hispaniola and Cuba, but south to the Bahamas, facilitating maritime communication from across the Atlantic into these islands' northern coasts, and connecting to the Gulf Stream at the intersection of the Florida Strait. Because of its non-dominant pace and rich-nutrient waters, fishermen across the Caribbean Islands use it to fish. It moves almost parallel to the also rich-nutrient Caribbean Current which flows south of Puerto Rico and Cuba, and over Colombia and Venezuela.

Baffin Island Current

Baffin Island Current (or Baffin Current) is an ocean current running south down the western side of Baffin Bay in the Arctic Ocean, along Baffin Island. Its sources are the West Greenland Current and outflow from the Arctic Ocean. Its speed is approximately 17 km (11 mi) per day.

Benguela Current

The Benguela Current is the broad, northward flowing ocean current that forms the eastern portion of the South Atlantic Ocean gyre. The current extends from roughly Cape Point in the south, to the position of the Angola-Benguela front in the north, at around 16°S. The current is driven by the prevailing south easterly trade winds. Inshore of the Benguela Current proper, the south easterly winds drive coastal upwelling, forming the Benguela Upwelling System. The cold, nutrient rich waters that upwell from around 200–300 m depth in turn fuel high rates of phytoplankton growth, and sustain the productive Benguela ecosystem.

California Current

The California Current is a Pacific Ocean current that moves southward along the western coast of North America, beginning off southern British Columbia and ending off southern Baja California Peninsula. It is considered an Eastern boundary current due to the influence of the North American coastline on its course. It is also one of five major coastal currents affiliated with strong upwelling zones, the others being the Humboldt Current, the Canary Current, the Benguela Current, and the Somali Current. The California Current is part of the North Pacific Gyre, a large swirling current that occupies the northern basin of the Pacific.

Caribbean Current

The Caribbean Current is a warm ocean current that transports significant amounts of water and flows northwestward through the Caribbean from the east along the coast of South America and into the Gulf of Mexico. The current results from the flow of the Atlantic South Equatorial Current as it flows north along the coast of Brazil. As the current turns north through the Yucatán Channel, it is renamed the Yucatán Current. The Caribbean Current water comes from the Atlantic Ocean via the North Equatorial, North Brazil, and Guiana Currents. The circulation of the Columbia-Panama Gyre flows counter-clockwise to the Caribbean Current.

Drift diving

Drift diving is a type of scuba diving where the diver is transported by the water movement caused by the tide, an ocean current or in a river. The choice whether to drift dive depends on the purpose of the dive, and whether there is an option. At some sites there is almost always a current running, and at others the strength and direction of water movement may vary with the tide, or other driving forces, like wind or recent rainfall. At some sites there may be considerable variation in visibility and marine life activity based on the speed and direction of flow.The current gives the diver the impression of flying and allows the diver to cover long distances underwater, possibly seeing more habitats and formations than usual. Often drift diving is performed more for the experience of underwater "flight" and less for interactions with underwater life, which, given the speed at which most divers move, are reduced.

East Korea Warm Current

The East Korea Warm Current (동한난류, 東韓暖流) is an ocean current in the Sea of Japan (East Sea). It 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. Between 36° and 38° N, it encounters the North Korea Cold Current and veers southeast into the open sea. The boundary between the two currents fluctuates throughout the year, creating large eddies. As it flows northeastward, the East Korea current eventually rejoins the Tsushima Current. It's very hot and rainy in the Summer.

Gulf Stream

The Gulf Stream, together with its northern extension the North Atlantic Drift, is a warm and swift Atlantic ocean current that originates in the Gulf of Mexico and stretches to the tip of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean. The process of western intensification causes the Gulf Stream to be a northward accelerating current off the east coast of North America. At about 40°0′N 30°0′W, it splits in two, with the northern stream, the North Atlantic Drift, crossing to Northern Europe and the southern stream, the Canary Current, recirculating off West Africa.

The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and the west coast of Europe. Although there has been recent debate, there is consensus that the climate of Western Europe and Northern Europe is warmer than it would otherwise be due to the North Atlantic drift which is the northeastern section of the Gulf Stream. It is part of the North Atlantic Gyre. Its presence has led to the development of strong cyclones of all types, both within the atmosphere and within the ocean. The Gulf Stream is also a significant potential source of renewable power generation.The Gulf Stream is typically 100 kilometres (62 mi) wide and 800 metres (2,600 ft) to 1,200 metres (3,900 ft) deep. The current velocity is fastest near the surface, with the maximum speed typically about 2.5 metres per second (9.0 km/h; 5.6 mph).

Irminger Current

The Irminger Current is a north Atlantic ocean current setting westward off the southwest coast of Iceland. It is composed of relatively warm and saline waters from the eastern North Atlantic that are fed by the North Atlantic Drift. The Irminger Current is part of the North Atlantic subpolar gyre. The current is named after Danish vice-admiral Carl Ludvig Christian Irminger (1802–1888).

Kuroshio Current

The Kuroshio (黒潮), also known as the Black or Japan Current (日本海流, Nihon Kairyū) or the Black Stream, is a north-flowing ocean current on the west side of the North Pacific Ocean. It is similar to the Gulf Stream in the North Atlantic and is part of the North Pacific ocean gyre. Like the Gulf stream, it is a strong western boundary current.

Marine current power

Marine currents can carry large amounts of energy, largely driven by the tides, which are a consequence of the gravitational effects of the planetary motion of the Earth, the Moon and the Sun. Augmented flow velocities can be found where the underwater topography (bathymetry) in straits between islands and the mainland or in shallows around headlands plays a major role in enhancing the flow velocities, resulting in appreciable kinetic energy. The sun acts as the primary driving force, causing winds and temperature differences. Because there are only small fluctuations in current speed and stream location with minimal changes in direction, ocean currents may be suitable locations for deploying energy extraction devices such as turbines. Other effects such as regional differences in temperature and salinity and the Coriolis effect due to the rotation of the earth are also major influences. The kinetic energy of marine currents can be converted in much the same way that a wind turbine extracts energy from the wind, using various types of open-flow rotors.

Mindanao Current

The Mindanao Current is an ocean current along the eastward side of the southern Philippines. It is a narrow, southward flowing current starting at 14°N with 13Sv increasing in strength to 33Sv at 5.5°N. The current splits before the coast of south-east Mindanao where one part flows cyclonically into the Celebes Sea later feeding the North Equatorial Counter Current (NECC) while another part feeds the NECC directly.

North Equatorial Current

The North Equatorial Current is a significant Pacific and Atlantic Ocean current that flows east-to-west between about 10° north and 20° north. It is the southern side of a clockwise subtropical gyre. Despite its name, the North Equatorial Current is not connected to the equator. In both oceans, it is separated from the equatorial circulation by the Equatorial Countercurrent (also known as the North Equatorial Countercurrent), which flows eastward. The westward surface flow at the equator in both oceans is part of the South Equatorial Current.

North Madagascar Current

The North Madagascar Current is an Ocean current near Madagascar. The Madagascar current is split into two current the North Madagascar Current and the East Madagascar Current (EMC). The North Madagascar Current (NMC) flows into the South Equatorial Current just North of Madagascar and is directed into the Mozambique Channel, this connects to the gyre’s equatorial currents into the Agulhas Current off the coast of Southeastern Africa.

South Atlantic Current

South Atlantic Current is an eastward ocean current, fed by the Brazil Current. That fraction of it which reaches the African coast feeds the Benguela Current. It is continuous with the northern edge of the Antarctic Circumpolar Current.

The seafaring is usually easier and thus safer in area of the South Atlantic Current than in the Antarctic Circumpolar Current, though also slower.

Thermohaline circulation

Thermohaline circulation (THC) 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 -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) 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. The water in these circuits transport both energy (in the form of heat) and mass (dissolved solids 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 used to refer to the meridional overturning circulation (often abbreviated as MOC). The term MOC is more accurate and well defined, as it is difficult to separate the part of the circulation which is driven by temperature and salinity alone as opposed to other factors such as the wind and tidal forces. Moreover, temperature and salinity gradients can also lead to circulation effects that are not included in the MOC itself.

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