Topic 15Wind Driven Currents

GEOL 2503Introduction to Oceanography

Surface Currents

• Wind sets surface water in motion• Friction, not density as in thermohaline

circulation, transfers energy from wind to water

• Horizontal circulation of the surface water layer

• Wind-driven circulation

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Coriolis Effect

• Very important, as in atmospheric circulation

• Water deflected 45° to the right of the wind in Northern Hemisphere

• Water deflected 45° to the left of the wind in Southern Hemisphere

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Gyres

• Northeast trade winds drive water to west• North America forces water to north• Westerly trades drive water to east• Europe and Africa force water south• Forms a continuous loop—called a gyre• Several exist, North Atlantic Gyre

described above

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Ekman Spiral

• Wind-driven surface water drives water just below

• Lower layer also deflected by Coriolis Effect

• That layer drives the next lower layer, and so on

• Each successive layer also deflected by Coriolis Effect

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Ekman Transport

• Ekman Spiral extends to maybe 100-150 meters

• At depth, water is flowing opposite to wind• Overall, the average flow of the wind-

driven water is 90° to the right of the wind in the Northern Hemisphere

• Compare to surface water which moves 45° to the right of the wind

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Wind Belts

SUBTROPICAL HIGH

A M

E R

I C

A S

E U

R O

P E

A F R I C

A

W E S T E R L I E S

Northeast T R A D E S

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North Atlantic GyreA

M E

R I

C A

SE

U R

O P

E A F R

I C A

North Atlantic Current

North Equatorial Current

GulfStream

CanaryCurrent

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North Atlantic Gyre

South Atlantic Gyre

Subpolar Gyre

Not Developed(Antarctica)

Equatorial Counter Current

N. Equatorial Current

S. Equatorial Current

West Wind Drift

North Atlantic Current

Polar Current

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EKMAN TRANSPORTA

M E

R I

C A

SE

U R

O P

E A F R

I C A

2m 1m

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Effects of Ekman Transport

• 1. Surface water piled into center of gyres• 2. Water also pushed to west end of gyres because

Coriolis force increases with latitude

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Major North Atlantic Currents

• North Equatorial Current flows to west pushed by northeast tradewinds

• North Atlantic Current (or Drift) flows to east pushed by westerly trade winds

• Gulf Stream connects them in the west• Canary Current connects them in the east

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Other North Atlantic Currents

• Florida Current from Gulf of Mexico through Strait of Florida

• Labrador and East Greenland Currents flow south from Arctic Ocean driven by polar easterlies

• Norwegian Current flows north into Arctic Ocean

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Continuity Currents

• Gulf Stream and Canary Current ARE NOT WIND DRIVEN

• They provide continuity of flow• They complete a circular, clockwise flow

centered about 30° N latitude in the northern hemisphere

• Several similar currents exist in other oceans

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Equatorial Countercurrents

• Current moving opposite to wind-driven currents

• Between North and South Equatorial Currents

• Below the doldrums• Helps return accumulated surface water

eastward across each ocean

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Current Speed

• About 1/100th of wind speed• Typically around 0.25-1.0 knot (about

0.1-0.5 m/s)• Flow is faster when water forced through

narrow gap—Strait of Florida• Florida Current exceeds 3 knots• Remember Ponce de Leon

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What’s a knot?

• 1 knot = 1 nautical mile per hour• 1 nautical mile = 1 minute of latitude• A nautical mile is slightly longer than a land

mile• 1 nautical mile = 1.151 land miles

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Current Volume

• Ocean surface currents move huge volumes of water

• Gulf Stream carries 500 times more water than the Amazon River

• For comparison, the Amazon River carries 1/4th of Earth’s fresh water

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Numbers = millions of cubic meters per second29

Surface Currents and Climate

• Currents flowing from the tropics carry warm water and keep land warmer

• Currents flowing from the high latitudes carry cold water and keep land cooler

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Cold versus Warm Currents

Ocean surface currents moving away from the tropics carry warm water and moderate the climate. Ocean surface currents moving toward the tropics carry cool water and cool the climate.

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Surface Current Eddies

• Ocean surface currents meander (curve and wind back and forth) just like rivers on land

• Sometimes meanders break off and form small separate loops called eddies

• Eddies move with circular motion

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Gulf Stream Eddies

• Gulf Stream separates cold water near land from warm water more seaward

• If an eddy forms around cold water to the west, the eddy will be rotating counterclockwise (cold-core eddy)

• If an eddy forms around warm water to the east, the eddy will be rotating clockwise (warm-core eddy)

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Langmuir Circulation• When wind blows steadily over water, different

kind of circulation can be set up. • This is in addition to Ekman circulation.• Alternate bands of converging and diverging

currents form helical cells, parallel to the wind direction. (Helix means spiral, like a corkscrew or a spring.)

• The converging currents are plainly visible as bands of seaweed, foam or oil. The diverging currents are areas where plankton are concentrated.

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http://www.icm.csic.es/geo/gof/out/banco 38

Franklin-Folger Map of Gulf Stream, 1769-1770

A 1990 swept 80,000 sneakers off a cargo vessel in the North Pacific. They later washed up on shore, acting as current trackers.

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Agulhas Current 42

Benguela Current 43

Brazil Current 44

California Current 45

Equatorial Countercurrent 46

Kuroshio Current 47

Peru Current 48