Wind generated current

Last updated

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]

Contents

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]

Boundary currents

The world's largest ocean gyres Currents.svg
The world's largest ocean gyres

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

Eastern boundary currents are relatively shallow, broad and slow-flowing currents on the eastern side of oceanic basins along the western coasts of continents. Subtropical eastern boundary currents flow equatorward, transporting cold water from higher latitudes to lower latitudes; examples include the Benguela Current, the Canary Current, the Humboldt Current, and the California Current. Coastal upwelling caused by offshore flow due to Ekman transport where the prevailing wind parallels the shoreline brings nutrient-rich water into eastern boundary current regions, making them highly productive areas.

Western boundary currents are warm, deep, narrow, and fast flowing currents that form on the west side of ocean basins due to western intensification. They carry warm water from the tropics poleward. Examples include the Gulf Stream, the Agulhas Current, and the Kuroshio.

Western intensification is an effect on the western arm of an oceanic current, particularly a large gyre in an ocean basin. The trade winds blow westward in the tropics. The westerlies blow eastward at mid-latitudes. This applies a stress to the ocean surface with a curl in north and south hemispheres, causing Sverdrup transport toward the tropics. Conservation of mass and potential vorticity cause that transport to be balanced by a narrow, intense poleward current, which flows along the western coast, allowing the vorticity introduced by coastal friction to balance the vorticity input of the wind. The reverse effect applies to the polar gyres – the sign of the wind stress curl and the direction of the resulting currents are reversed. The principal west side currents (such as the Gulf Stream of the North Atlantic Ocean) are stronger than those opposite (such as the California Current of the North Pacific Ocean).

Wind driven upwelling

When the net Ekman transport along a coastline is offshore, a compensatory inflow is possible from below, which brings up bottom water, which tends to be nutrient rich as it comes from the poorly lit regions where photosynthesis is insignificant.

Effects of equatorial upwelling on surface chlorophyll concentrations in the Pacific ocean Equatorial upwelling zone in the Pacific ocean.gif
Effects of equatorial upwelling on surface chlorophyll concentrations in the Pacific ocean

Upwelling at the equator is associated with the Intertropical Convergence Zone (ITCZ) which moves seasonally, and consequently, is often located just north or south of the equator. Easterly trade winds blow from the Northeast and Southeast and converge along the equator blowing West to form the ITCZ. Although there are no Coriolis forces present along the equator, upwelling still occurs just north and south of the equator. This results in a divergence, with denser, nutrient-rich water being upwelled from below. [3]

Oceanic downwelling

Downwelling in the ocean Downwelling NADW.svg
Downwelling in the ocean

Downwelling occurs at anti-cyclonic places of the ocean where warm core rings cause surface convergence and push the surface water downwards, or wind drives the sea towards a coastline. Regions that have downwelling generally have lower productivity because the nutrients in the water column are utilized but are not resupplied by nutrient-rich water from deeper below the surface. [4]

Oceanic wind driven currents

Western boundary

Eastern boundary

Equatorial

Arctic

Atlantic

Pacific

Southern

Oceanic gyres

Lake currents

Local and transient currents

See also

Related Research Articles

Downwelling The process of accumulation and sinking of higher density material beneath lower density material

Downwelling is the process of accumulation and sinking of higher density material beneath lower density material, such as cold or saline water beneath warmer or fresher water or cold air beneath warm air. It is the sinking limb of a convection cell. Upwelling is the opposite process and together these two forces are responsible in the oceans for the thermohaline circulation. The sinking of cold lithosphere at subduction zones is another example of downwelling in plate tectonics.

Upwelling The replacement by deep water moving upwards of surface water driven offshore by wind

Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton. Due to the biomass of phytoplankton and presence of cool water in these regions, upwelling zones can be identified by cool sea surface temperatures (SST) and high concentrations of chlorophyll-a.

Ocean current Directional mass flow of oceanic water generated by external or internal forces

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.

Physical oceanography The study of physical conditions and physical processes within the ocean

Physical oceanography is the study of physical conditions and physical processes within the ocean, especially the motions and physical properties of ocean waters.

The Cromwell Current is an eastward-flowing subsurface current that extends the length of the equator in the Pacific Ocean.

Ocean gyre Any large system of circulating ocean currents

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

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

The California Current is a cold water Pacific Ocean current that moves southward along the western coast of North America, beginning off southern British Columbia and ending off southern Baja California Sur. 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.

Ekman spiral A structure of currents or winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary

The Ekman spiral is a structure of currents or winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary. It derives its name from the Swedish oceanographer Vagn Walfrid Ekman. The deflection of surface currents was first noticed by the Norwegian oceanographer Fridtjof Nansen during the Fram expedition (1893–1896) and the effect was first physically explained by Vagn Walfrid Ekman.

Equatorial Counter Current A shallow eastward flowing current found in the Atlantic, Indian, and Pacific Oceans

The Equatorial Counter Current is an eastward flowing, wind-driven current which extends to depths of 100-150m in the Atlantic, Indian, and Pacific Oceans. More often called the North Equatorial Countercurrent (NECC), this current flows west-to-east at about 3-10°N in the Atlantic, Indian Ocean and Pacific basins, between the North Equatorial Current (NEC) and the South Equatorial Current (SEC). The NECC is not to be confused with the Equatorial Undercurrent (EUC) that flows eastward along the equator at depths around 200m in the western Pacific rising to 100m in the eastern Pacific.

North Pacific Gyre A major circulating system of ocean currents

The North Pacific Gyre (NPG) or North Pacific Subtropical Gyre (NPSG), located in the northern Pacific Ocean, is one of the five major oceanic gyres. This gyre covers most of the northern Pacific Ocean. It is the largest ecosystem on Earth, located between the equator and 50° N latitude, and comprising 20 million square kilometers. The gyre has a clockwise circular pattern and is formed by four prevailing ocean currents: the North Pacific Current to the north, the California Current to the east, the North Equatorial Current to the south, and the Kuroshio Current to the west. It is the site of an unusually intense collection of man-made marine debris, known as the Great Pacific Garbage Patch.

The Sverdrup balance, or Sverdrup relation, is a theoretical relationship between the wind stress exerted on the surface of the open ocean and the vertically integrated meridional (north-south) transport of ocean water.

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

The Atlantic meridional overturning circulation (AMOC) is the zonally-integrated component of surface and deep currents in the Atlantic Ocean. It is characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, and a southward flow of colder, deep waters that are part of the thermohaline circulation. These "limbs" are linked by regions of overturning in the Nordic and Labrador Seas and the Southern Ocean. The AMOC is an important component of the Earth's climate system, and is a result of both atmospheric and thermohaline drivers.

Ekman transport Net transport of surface water perpendicular to wind direction

Ekman transport is part of Ekman motion theory, first investigated in 1902 by Vagn Walfrid Ekman. Winds are the main source of energy for ocean circulation, and Ekman Transport is a component of pink- driven ocean current. Ekman transport occurs when ocean surface waters are influenced by the friction force acting on them via the wind. As the wind blows it casts a friction force on the ocean surface that drags the upper 10-100m of the water column with it. However, due to the influence of the Coriolis effect, the ocean water moves at a 90° angle from the direction of the surface wind. The direction of transport is dependent on the hemisphere: in the northern hemisphere, transport occurs at 90° clockwise from wind direction, while in the southern hemisphere it occurs at 90° anticlockwise. This phenomenon was first noted by Fridtjof Nansen, who recorded that ice transport appeared to occur at an angle to the wind direction during his Arctic expedition during the 1890s. Ekman transport has significant impacts on the biogeochemical properties of the world's oceans. This is because they lead to upwelling and downwelling in order to obey mass conservation laws. Mass conservation, in reference to Ekman transfer, requires that any water displaced within an area must be replenished. This can be done by either Ekman suction or Ekman pumping depending on wind patterns.

A subsurface current is an oceanic current that runs beneath surface currents. Examples include the Equatorial Undercurrents of the Pacific, Atlantic, and Indian Oceans, the California Undercurrent, and the Agulhas Undercurrent, the deep thermohaline circulation in the Atlantic, and bottom gravity currents near Antarctica. The forcing mechanisms vary for these different types of subsurface currents.

Boundary current Ocean current with dynamics determined by the presence of a coastline

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.

Indian Monsoon Current The seasonally varying ocean current regime found in the tropical regions of the northern Indian Ocean

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. The Somali Current, which is strongly linked to the Indian monsoon, is also discussed in this article.

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, and deep ocean.

Beaufort Gyre A wind-driven ocean current in the Arctic Ocean polar region

The Beaufort Gyre is one of the two major ocean currents in the Arctic Ocean, it is roughly located north of the Alaskan and Canadian coast. In the past, Arctic sea-ice would circulate in the Beaufort gyre up to several years, leading to the formation of very thick multi-year sea-ice. Due to warming temperatures in the Arctic, the gyre has lost an extensive amount of ice, practically turning what used to be a nursery for sea-ice to mature and grow into the thickest and oldest ice of the Arctic Ocean into a "graveyard" for older ice.

Somali Current An ocean boundary current that flows along the coast of Somalia and Oman in the Western Indian Ocean

The Somali Current is a cold ocean boundary current that runs along the coast of Somalia and Oman in the Western Indian Ocean and is analogous to the Gulf Stream in the Atlantic Ocean. This current is heavily influenced by the monsoons and is the only major upwelling system that occurs on a western boundary of an ocean. The water that is upwelled by the current merges with another upwelling system, creating one of the most productive ecosystems in the ocean.

In oceanography, Ekman velocity – also referred as a kind of the residual ageostropic velocity as it derivates from geostrophy – is part of the total horizontal velocity (u) in the upper layer of water of the open ocean. This velocity, caused by winds blowing over the surface of the ocean, is such that the Coriolis force on this layer is balanced by the force of the wind.

References

  1. 1 2 3 4 5 6 "Ocean in Motion: Ekman Transport Background". oceanmotion.org. Retrieved 10 October 2020.
  2. 1 2 "Ocean Currents and Climate". earth.usc.edu. Retrieved 10 October 2020.
  3. ↑ Jennings, S.; Kaiser, M.J.; Reynolds, J.D. (2001). Marine Fisheries Ecology. Oxford: Blackwell Science Ltd. ISBN   0-632-05098-5.
  4. ↑ "Ocean Motion : Definition : Wind Driven Surface Currents - Upwelling and Downwelling" . Retrieved 12 March 2016.
  5. 1 2 3 4 5 6 7 8 9 10 "Chapter 9. The surface currents" (PDF). ocean.stanford.edu. Retrieved 10 October 2020.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.