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Wind generated current

A Wind generated current is a flow in a body of water that is generated by wind friction on its surface. Wind can generate surface currents on water bodies of any size. The depth and strength of the current depend on the wind strength and duration, and on friction and viscosity losses,[1] but are limited to about 400 m depth by the mechanism, and to lesser depths where the water is shallower.[2] The direction of flow is influenced by the Coriolis effect, and is offset to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. A wind current can induce secondary water flow in the form of upwelling and downwelling, geostrophic flow, and western boundary currents.[1]

Mechanism

Friction between wind and the upper surface of a body of water will drag the water surface along with the wind The surface layer will exert viscous drag on the water just below, which will transfer some of the momentum. This process continues downward, with a continuous reduction in speed of flow with increasing depth as the energy is dissipated. The inertial effect of planetary rotation causes an offset of flow direction with increasing depth to the right in the northern hemisphere and to the left in the southern hemisphere. The mechanism of deflection is called the Coriolis effect, and the variation of flow velocity with depth is called an Ekman spiral. The effect varies with latitude, being very weak at the equator and increasing in strength with latitude. The resultant flow of water caused by this mechanism is known as Ekman transport.[1]

A steady wind blowing across a long fetch in deep water for long enough to establish a steady state flow causes the surface water to move at 45° to the wind direction. The variation in flow direction with depth has the water moving perpendicular to wind direction by about 100 to 150 m depth, and flow speed drops to about 4% of surface flow speed by the depth of about 330 to 400 m where the flow direction is opposite to wind direction, below which the effect of wind on the current is considered negligible. The net flow of water over the effective thickness of the current in these conditions is perpendicular to wind direction. Consistent prevailing winds set up persistent circulating surface currents in both hemispheres, and where the current is bounded by continental land masses, the resulting gyres are restricted in longitudinal extent.[1][2] Seasonal and local winds cause smaller scale and generally transient currents, which dissipate after the driving winds die down.

Real conditions often differ, as wind strength and direction vary, and the depth may not be sufficient for the full spiral to develop, so that the angle between wind direction and surface-water movement can be as small as 15°. In deeper water, the angle increases and approaches 45°. A stable pycnocline can inhibit transfer of kinetic energy to deeper waters, providing a depth limit for surface currents.[1]

The net inward shallow water flow in a gyre causes the surface level to gradually slope upwards towards the centre. This induces a horizontal pressure gradient which leads to a balancing geostrophic flow.[1]

Oceanic wind driven currents

Western boundary

  • 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[3]
  • Agulhas Current – Western boundary current of the southwest Indian Ocean that flows down the east coast of Africa[3]
  • Kuroshio Current – North flowing ocean current on the west side of the North Pacific Ocean[3]

Eastern boundary

  • Benguela Current – The broad, northward flowing ocean current that forms the eastern portion of the South Atlantic Ocean gyre[3]
  • Humboldt Current – A cold, low-salinity eastern boundary current that flows north along the western coast of South America from southern Chile to northern Peru[3]
  • California Current – A Pacific Ocean current that flows southward along the western coast of North America from southern British Columbia to the southern Baja California Peninsula[3]

Equatorial

  • 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[3]
  • 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[3]

Arctic

Atlantic

  • Canary Current – A wind-driven surface current that is part of the North Atlantic Gyre[3]

Pacific

Southern

Oceanic gyres

  • Beaufort Gyre – A wind-driven ocean current in the Arctic Ocean polar region
  • Indian Ocean Gyre – A large systems of rotating ocean currents. The Indian Ocean gyre is composed of two major currents: the South Equatorial Current, and the West Australian Current
  • North Atlantic Gyre – A major circular system of ocean currents
  • North Pacific Gyre – A major circulating system of ocean currents
  • Ross Gyre – A circulating system of ocean currents in the Ross Sea
  • South Atlantic Gyre – The subtropical gyre in the south Atlantic Ocean
  • South Pacific Gyre – A major circulating system of ocean currents
  • Weddell Gyre – One of the two gyres that exist within the Southern Ocean

Lake currents

Local and transient currents

See also

  • Current (stream) – Flow of water in a river due to gravity
  • Downwelling – The process of accumulation and sinking of higher density material beneath lower density material
  • Geostrophic current – An oceanic flow in which the pressure gradient force is balanced by the Coriolis effect
  • Hydrothermal circulation – Circulation of water driven by heat exchange
  • Ocean current – Directional mass flow of oceanic water generated by external or internal forces
  • Thermohaline circulation – A part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes
  • Upwelling – The replacement by deep water moving upwards of surface water driven offshore by wind

References

  1. ^ a b c d e f "Ocean in Motion: Ekman Transport Background". oceanmotion.org. Retrieved 10 October 2020.
  2. ^ a b "Ocean Currents and Climate". earth.usc.edu. Retrieved 10 October 2020.
  3. ^ a b c d e f g h i j "Chapter 9. The surface currents" (PDF). ocean.stanford.edu. Retrieved 10 October 2020.

This page was last updated at 2020-12-23 06:29, update this pageView original page

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