Topic 15 Wind Driven Currents

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Topic 15Wind Driven Currents

• GEOL 2503
• Introduction to Oceanography

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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 loopcalled 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 about 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
W E S T E R L I E S
SUBTROPICAL HIGH
A M E R I C A S
E U R O P E A F R I C A
Northeast T R A D E S
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Polar Current
North Atlantic Current
N. Equatorial Current
Equatorial Counter Current
S. Equatorial Current
West Wind Drift
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EKMAN TRANSPORT
A M E R I C A S
E 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
• 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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The West Wind Drift is the only ocean surface
current to completely circle the globe. It thus
has unlimited fetch. It is the northern boundary
of the Southern 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
  gapStrait of Florida
• Florida Current exceeds 3 knots
• Remember Ponce de Leon

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Whats 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
  Earths fresh water

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Numbers millions of cubic meters per second
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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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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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