Ocean gyre

In oceanography, a gyre (/ˈdʒaɪər/) is any large system of circulating ocean currents, particularly those involved with large wind movements. Gyres are caused by the Coriolis effect; planetary vorticity along with horizontal and vertical friction, determine the circulation patterns from the wind stress curl (torque).[1]

The term gyre can be used to refer to any type of vortex in an atmosphere or a sea,[2] even one that is man-made, but it is most commonly used in terrestrial oceanography to refer to the major ocean systems.

Oceanic gyres
The five major ocean gyres

Major gyres

The following are the five most notable ocean gyres:[3]

Other gyres

Tropical gyres

Currents
All of the world's larger gyres

Tropical gyres are less unified and tend to be mostly east-west with minor north-south extent.

  • Atlantic Equatorial Current System (two counter-rotating circulations)
  • Pacific Equatorial Current System
  • Indian Monsoon Gyres (two counter-rotating circulations in northern Indian Ocean)[4]

Subtropical gyres

The center of a subtropical gyre is a high pressure zone. Circulation around the high pressure is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere, due to the Coriolis effect. The high pressure in the center is due to the westerly winds on the northern side of the gyre and easterly trade winds on the southern side. These cause frictional surface currents towards the latitude at the center of the gyre.

This build-up of water in the center creates flow towards the equator in the upper 1,000 to 2,000 m (3,300 to 6,600 ft) of the ocean, through rather complex dynamics. This flow is returned towards the pole in an intensified western boundary current. The boundary current of the North Atlantic Gyre is the Gulf Stream, of the North Pacific Gyre the Kuroshio Current, of the South Atlantic Gyre the Brazil Current, of the South Pacific Gyre the East Australian Current, and of the Indian Ocean Gyre the Agulhas Current.

Subpolar gyres

Subpolar gyres form at high latitudes (around 60°). Circulation of surface wind and ocean water is counterclockwise in the Northern Hemisphere, around a low-pressure area, such as the persistent Aleutian Low and the Icelandic Low. Surface currents generally move outward from the center of the system. This drives the Ekman transport, which creates an upwelling of nutrient-rich water from the lower depths.[5]

Subpolar circulation in the southern hemisphere is dominated by the Antarctic Circumpolar Current, due to the lack of large landmasses breaking up the Southern Ocean. There are minor gyres in the Weddell Sea and the Ross Sea, the Weddell Gyre and Ross Gyre, which circulate in a clockwise direction.[3]

Climate change

Recently, stronger winds, especially the subtropical trade winds in the Pacific ocean have provided a mechanism for vertical heat distribution. The effects are changes in the ocean currents, increasing the subtropical overturning, which are also related to the El Niño and La Niña phenomena. Depending on natural variability, during La Niña years around 30% more heat from the upper ocean layer is transported into the deeper ocean.[6] Several studies in recent years, found a multidecadal increase in OHC of the deep and upper ocean regions and attribute the heat uptake to anthropogenic warming.[7]

The influence of the Coriolis effect on westward intensification

Coriolis effect14

Pollution

Ocean gyres are known to collect pollutants. The Great Pacific Garbage Patch in the central North Pacific Ocean is a gyre of marine debris particles and floating trash halfway between Hawaii and California, and extends over an indeterminate area of widely varying range depending on the degree of plastic concentration used to define it. An estimated 80,000 metric tons of plastic inhabit the patch, totaling 1.8 trillion pieces. 92% of the mass in the patch comes from objects larger than 0.5 centimeters.

A similar patch of floating plastic debris is found in the Atlantic Ocean, called the North Atlantic garbage patch. The patch is estimated to be hundreds of kilometers across in size, with a density of over 200,000 pieces of debris per square kilometer.

See also

References

  1. ^ Heinemann, B. and the Open University (1998) Ocean circulation, Oxford University Press: Page 98
  2. ^ Lissauer, Jack J.; de Pater, Imke (2019). Fundamental Planetary Sciences : physics, chemistry, and habitability. New York, NY, USA: Cambridge University Press. ISBN 9781108411981.
  3. ^ a b The five most notable gyres PowerPoint Presentation
  4. ^ Indian Monsoon Gyres
  5. ^ Wind Driven Surface Currents: Gyres
  6. ^ Balmaseda, Trenberth & Källén (2013). "Distinctive climate signals in reanalysis of global ocean heat content". Geophysical Research Letters. 40 (9): 1754–1759. Bibcode:2013GeoRL..40.1754B. doi:10.1002/grl.50382.
  7. ^ Abraham; et al. (2013). "A review of global ocean temperature observations: Implications for ocean heat content estimates and climate change". Reviews of Geophysics. 51 (3): 450–483. Bibcode:2013RvGeo..51..450A. CiteSeerX 10.1.1.594.3698. doi:10.1002/rog.20022.

External links

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.

Anticyclone

An anticyclone (that is, opposite to a cyclone) is a weather phenomenon defined by the United States National Weather Service's glossary as "a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere". Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows such as tropical cyclones, due to descending cool air from the backside of upper troughs such as polar highs, or from large scale sinking such as the subtropical ridge.

The evolution of an anticyclone depends upon variables such as its size, intensity, and extent of moist convection, as well as the Coriolis force.

Bartolomeu Dias

Bartolomeu Dias (; Portuguese: [baɾtuluˈmew ˈdi.ɐʃ]; Anglicized: Bartholomew Diaz; c. 1450 – 29 May 1500), a nobleman of the Portuguese royal household, was a Portuguese explorer. He sailed around the southernmost tip of Africa in 1488, the first European to do so, setting up the route from Europe to Asia later on. Dias is the first European during the Age of Discovery to anchor at what is present-day South Africa.

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.

Boundary current

Boundary currents are ocean currents with dynamics determined by the presence of a coastline, and fall into two distinct categories: western boundary currents and eastern boundary currents.

Index of physics articles (O)

The index of physics articles is split into multiple pages due to its size.

To navigate by individual letter use the table of contents below.

Indian Monsoon Current

The Indian Monsoon Current refers to the seasonally varying ocean current regime found in the tropical regions of the northern Indian Ocean. During winter, the flow of the upper ocean is directed westward from near the Indonesian Archipelago to the Arabian Sea. During the summer, the direction reverses, with eastward flow extending from Somalia into the Bay of Bengal. These variations are due to changes in the wind stress associated with the Indian monsoon. The seasonally reversing open ocean currents that pass south of India are referred to as the Winter Monsoon Current and the Summer Monsoon Current (alternately, the Northeast Monsoon Current and the Southwest Monsoon Current). The Somali Current, which is strongly linked to the Indian monsoon, is also discussed in this article.

Indian Ocean Gyre

The Indian Ocean gyre, located in the Indian Ocean, is one of the eighteen major oceanic gyres, large systems of rotating ocean currents, which together form the backbone of the global conveyor belt. The Indian Ocean gyre is composed of two major currents: the South Equatorial Current, and the West Australian Current.

Normally moving counter-clockwise, in the winter the Indian Ocean gyre reverses direction due to the seasonal winds of the South Asian Monsoon. In the summer, the land is warmer than the ocean, so surface winds blow from the ocean to the land. However, during the winter, these temperatures reverse, making the winds blow from the land to the ocean. Because most of the air pressure gradient is retained behind the Tibetan plateau, air pressure gradients over the Indian Ocean and the gyre are small. This results in winds of moderate strength, due to the protection from the full force winds blowing off the Mongolian high pressure region. Because of these moderate, dry winds, the Winter Monsoon season in the Indian Ocean region is the dry season for most of Southern Asia. Due to this seasonal wind cycle, the currents of the Indian Ocean, which make up the Indian Ocean gyre, are directly affected, causing reversal.Like the other gyres, it contains a garbage patch. A garbage patch is a suspended region of marine debris within the water column that circulate the gyre constantly. The Indian Ocean’s garbage patch covers a massive area: at least five million square kilometers (two million square miles). As garbage patches such as these circulate for long periods of time, they cause inorganic toxins to enter the food chain due to solar breakdown of plastics in the water. In the Indian Ocean gyre, the garbage patch has been more of a mystery. Having just been discovered in 2010, it is still under research. It is known, however, that like most garbage patches, it is very fluid, and changes with the seasons, making its location difficult to pinpoint. It seems to circulate with the Indian Ocean gyre, from the Australian side to the African side, down the African coast, and then back to Australia. According to the team who discovered the garbage patch, the full rotation of the gyre’s garbage patch takes about six years, until it reaches the center of the gyre, where it may remain indefinitely.

Indian Ocean garbage patch

The Indian Ocean garbage patch, discovered in 2010, is a gyre of marine litter suspended in the upper water column of the central Indian Ocean, specifically the Indian Ocean Gyre, one of the five major oceanic gyres. The patch does not appear as a continuous debris field. As with other patches in each of the five oceanic gyres, the plastics in it break down to ever smaller particles, and to constituent polymers. As with the other patches, the field constitutes an elevated level of pelagic plastics, chemical sludge, and other debris; primarily particles that are invisible to the naked eye. The concentration of particle debris has been estimated to be approximately 10,000 particles per square kilometer.A similar patch of floating plastic debris in the Pacific Ocean, the Great Pacific garbage patch, was predicted in 1985, and discovered in 1997 by Charles J. Moore as he passed through the North Pacific Gyre on his return from the Transpacific Yacht Race. The North Atlantic garbage patch was discovered in 2010.

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.

North Atlantic Current

The North Atlantic Current (NAC), also known as North Atlantic Drift and North Atlantic Sea Movement, is a powerful warm western boundary current within the Atlantic Ocean that extends the Gulf Stream northeastward.The NAC originates from where the Gulf Stream turns north at the Southeast Newfoundland Rise, a submarine ridge that stretches southeast from the Grand Banks. The NAC flows northward east of the Grand Banks, from 40°N to 51°N, before turning sharply east to cross the Atlantic. It transports more warm tropical water to northern latitudes than any other boundary current; more than 40 Sv in the south and 20 Sv as it crosses the Mid-Atlantic Ridge. It reaches speeds of 2 knots near the North American coast. Directed by topography, the NAC meanders heavily, but in contrast to the meanders of the Gulf Stream, the NAC meanders remain stable without breaking off into eddies.The colder parts of the Gulf Stream turn northward near the "tail" of the Grand Banks at 50°W where the Azores Current branches off to flow south of the Azores. From there the NAC flows northeastward, east of the Flemish Cap (47°N, 45°W). Approaching the Mid-Atlantic Ridge, it then turns eastward and becomes much broader and more diffuse. It then splits into a colder northeastern branch and a warmer eastern branch. As the warmer branch turns southward, most of the subtropical component of the Gulf Stream is diverted southward, and as a consequence, the North Atlantic is mostly supplied by subpolar waters, including a contribution from the Labrador Current recirculated into the NAC at 45°N.West of Continental Europe, it splits into two major branches. One branch goes southeast, becoming the Canary Current as it passes northwest Africa and turns southwest. The other major branch continues north along the coast of Northwestern Europe.

Other branches include the Irminger Current and the Norwegian Current. Driven by the global thermohaline circulation, the North Atlantic Current is part of the wind-driven Gulf Stream, which goes further east and north from the North American coast across the Atlantic and into the Arctic Ocean.

The North Atlantic Current, together with the Gulf Stream, have a long-lived reputation for having a considerable warming influence on European climate. However, the principal cause for differences in winter climate between North America and Europe seems to be winds rather than ocean currents (although the currents do exert influence at very high latitudes by preventing the formation of sea ice).

Ocean current

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.

Retroflect

Retroflection is the movement of an ocean current that doubles back on itself.

Ross Sea

The Ross Sea is a deep bay of the Southern Ocean in Antarctica, between Victoria Land and Marie Byrd Land and within the Ross Embayment, and is the southernmost sea on Earth. It derives its name from the British explorer James Ross who visited this area in 1841. To the west of the sea lies Ross Island and Victoria Land, to the east Roosevelt Island and Edward VII Peninsula in Marie Byrd Land, while the southernmost part is covered by the Ross Ice Shelf, and is about 200 miles (320 km) from the South Pole. Its boundaries and area have been defined by the New Zealand National Institute of Water and Atmospheric Research as having an area of 637,000 square kilometres (246,000 sq mi).The circulation of the Ross Sea is dominated by a wind-driven ocean gyre and the flow is strongly influenced by three submarine ridges that run from southwest to northeast. The circumpolar deep water current is a relatively warm, salty and nutrient-rich water mass that flows onto the continental shelf at certain locations. The Ross Sea is covered with ice for most of the year.

The nutrient-laden water supports an abundance of plankton and this encourages a rich marine fauna. At least ten mammal species, six bird species and 95 fish species are found here, as well as many invertebrates, and the sea remains relatively unaffected by human activities. New Zealand has claimed that the sea comes under its jurisdiction as part of the Ross Dependency. Marine biologists consider the sea to have a high level of biological diversity and it is the site of much scientific research. It is also the focus of some environmentalist groups who have campaigned to have the area proclaimed as a world marine reserve. In 2016 an international agreement established the region as a marine park.

Sargasso Sea

The Sargasso Sea () is a region of the North Atlantic Ocean bounded by four currents forming an ocean gyre. Unlike all other regions called seas, it has no land boundaries. It is distinguished from other parts of the Atlantic Ocean by its characteristic brown Sargassum seaweed and often calm blue water.The sea is bounded on the west by the Gulf Stream, on the north by the North Atlantic Current, on the east by the Canary Current, and on the south by the North Atlantic Equatorial Current, a clockwise-circulating system of ocean currents termed the North Atlantic Gyre. It lies between 70° and 40° W, and 20° to 35° N, and is approximately 1,100 km wide by 3,200 km long (700 by 2,000 miles). Bermuda is near the western fringes of the sea.All of the currents deposit the marine plants and refuse which they are carrying into this sea, yet the ocean water in the Sargasso Sea is distinctive for its deep blue color and exceptional clarity, with underwater visibility of up to 61 m (200 ft). It is also a body of water that has captured the public imagination, and so is seen in a wide variety of literary and artistic works and in popular culture.

South Atlantic Gyre

The South Atlantic Gyre is the subtropical gyre in the south Atlantic Ocean. In the southern portion of the gyre, northwesterly (or southeastward-flowing) winds drive eastward-flowing currents that are difficult to distinguish from the northern boundary of the Antarctic Circumpolar Current. Like other oceanic gyres, it collects vast amounts of floating debris.

Undersea mountain range

Undersea mountain ranges are mountain ranges that are mostly or entirely underwater, and specifically under the surface of an ocean. If originated from current tectonic forces, they are often referred to as a mid-ocean ridge. In contrast, if formed by past above-water volcanism, they are known as a seamount chain. The largest and best known undersea mountain range is a mid-ocean ridge, the Mid-Atlantic Ridge. It has been observed that, "similar to those on land, the undersea mountain ranges are the loci of frequent volcanic and earthquake activity".

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

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