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Ocean Circulation

Ocean Circulation

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Ocean Circulation

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  1. Ocean Circulation

  2. Ocean Circulation • Ocean currents are affected by two kinds of forces; the primary forces that start water moving and determines its velocity. • Secondary forces are factors that influence the direction and nature of its flow.

  3. Ocean Circulation • Primary forces are the stress of wind blowing over the water, thermal expansion and contraction of water as well as different densities between layers. • Secondary forces and factors are the Coriolis effect, gravity, friction, and the shape of the ocean basins themselves.

  4. Ocean Circulation • About 10% of the water in the world ocean is involved in surface currents. • Surface currents are considered horizontally flowing water in the uppermost 400 meters (1,300 ft.) of the ocean’s surface. • Most surface currents move water above the pycnocline.

  5. Surface Currents • Solar heating causes water to expand slightly. Because of this, sea level near the equator is about 8 centimeters higher than sea level in temperate oceans. • This global difference creates a very slight slope, and warm equatorial water flows downhill (poleward) in response to gravity.

  6. Surface Currents • The difference in temperature between the poles and the equator is not the major primary force responsible for surface currents • The major primary force for surface currents is the winds. • As winds blow over the surface of the ocean, energy from the wind is transferred to the water buy the shear effect.

  7. Surface Currents • The wind blowing over the water puts the water beneath it into motion. • As a rule of thumb, the friction of wind blowing for at least 10 hours will cause surface water to flow downwind at about 2% of wind speed. • Because of the Coriolis effect, Northern Hemisphere currents flow to the right of the wind direction.

  8. Surface Currents • Continents and the shape of ocean basins often block continuous flow and help deflect the moving water into a circular pattern. • This flow around the periphery of an ocean basin is called a gyre.

  9. Surface Currents • Oceanographers subdivide the North Atlantic Gyre into four different currents. • The water in the gyres flow around the periphery of the ocean basin because of the Coriolis effect. • One might expect the water within a gyre to pile up in the center and stop moving as a result of the Coriolis effect.

  10. Surface Currents • But it does not, due to another phenomenon. • It was discovered in the early part of the twentieth century that the topmost layer of water in the ocean in the Northern Hemisphere flows at about 45-degrees to the right of the wind direction.

  11. Surface Currents

  12. Surface Currents • The surface layer puts the layer below into motion and that layer is deflected 45-degrees to the right of the layer below it. • The same thing happens in the layer below that, and so on, to a depth of about 100 meters (330 feet) at mid-latitudes. • Because of frictional losses each lower layer moves more slowly than the layer above it.

  13. Surface Currents • The resulting situation is known as the Ekman spiral. • The net motion of the water down to abut 100 meters, after allowances for the summed effects of Ekman spiral is known as Ekman transport. • In theory Ekman transport in the Northern Hemisphere is 90-degrees to the right of the wind direction at the surface.

  14. Surface Currents • Theoretically the currents could be deflected by the Coriolis and the water pile up in the center of the ocean and the circulation stop. • A deviation in the theory occurs because of an interaction between the Coriolis and the pressure gradient.

  15. Surface Currents • As the water is deflected to the right by the Coriolis it builds up a hill of water in the middle of the gyre. • After a point the force of gravity (pressure) forces the water down the hill, still being deflected by the Coriolis. • A balance is developed between the force of the Coriolis building the hill and gravity pulling the water down.

  16. Surface Currents • The phenomenon just described is called a geostrophic current. • Yes, there really is a hill near the middle of the North Atlantic, centered in the area of the Sargasso Sea. • It is not a steep mountain of water. Its maximum height is about 2 meters (6.5 feet).

  17. Gyres • Within the subtropical gyres there are two main types of currents. • They are western boundary currents and eastern boundary currents. • The fastest and deepest currents are found at the western boundary of ocean basins (off the east coast of continents).

  18. Western Boundary Currents • Narrow, fast, deep western boundary currents take warm water toward the poles. • There are five large western boundary currents; the Gulf Stream, the Kuroshio Current, the Brazil Current, the Agulhas Current, and the East Australian Current.

  19. Western Boundary Currents • Western boundary currents tend to meander as they travel poleward. • The looping meanders sometimes connect to from turbulent rings, or eddies, that trap cold or warm water in their centers and then separate from the main flow.

  20. Western Boundary Currents • An example; cold-core eddies form in the Gulf Stream as it meanders eastward upon leaving the coast of North America off Cape Hatteras. • Warm-core eddies can form north of the Gulf Stream when the warm current loops into the cold water lying to the north.

  21. Eastern Boundary Currents • There are five eastern boundary currents at the eastern edge of the ocean basins. • They are; the Canary Current, the Benguela Current, the California Current, the West Australian Current and the Peru or Humboldt Current.

  22. Eastern Boundary Currents • Eastern boundary currents carry cold water equatorward; they are shallow and broad, sometimes more than 1,000 kilometers (620 miles) across. • Their boundaries are not well defined and eddies do not tend to form.

  23. Western Intensification • Western boundary currents are concentrated because of the converging flow of the trade winds. • The trade winds blow water westward where it piles up. This heightens the Coriolis effect, the water turns to the right and heads north.

  24. Western Intensification • Another reason this water piles up on the western side of the ocean basin is the rotation of the Earth. • These forces combine to produce a phenomena known as the western intensification of currents.

  25. Change Disk