Class 24/25 -- The Oceans

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Class 24/25 -- The Oceans

• SURFACE CURRENTS
• Major surface currents
• Ekman transport
• Rotary motion - gyres --
• Western boundary currents
• Upwelling and downwelling
• Eddies

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MAJOR SURFACE CURRENT SYSTEMS

• Driving force global winds
• Trade Winds Westerlies
• Deflected by continents and Coriolis effect
• System of E-W currents (wind-driven) and
• N-S "boundary"currents (deflection)

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Ekman Transport In which direction is water
moved when the winds blows across its surface?
1) Coriolis effect Motion of the uppermost
layer is deflected to the right of the winds
force
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• 2) Now consider the next layer deeper
• Motion slower than upper layer
• Force from upper layer is at 45 degrees to the
  wind
• Coriolis effect deflects motion relative to THAT
  direction

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• 3) Successive deeper layers
• Same idea again- and speed keeps decreasing with
  depth

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• 4) What is the overall motion (average for the
  entire depth affected by the wind)? Walfrid
  Ekman did the math...

Ekman Transport- Perpendicular to Wind
Direction!!!!
Ekman Spiral
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Ekman Transport The overall movement of water
is PERPENDICULAR to the wind direction!!!!!!!
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A Detailed Look at the Gyres

• 1. Ekman transport tends to force water into the
  gyre
• This creates a subtle "hill or mound of water.

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North-South currents- Details

• Currents on the western sides
• Strong, narrow, deep
• Example The Gulf Stream

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North-South currents- Details

• Currents on the eastern sides
• Slow, broad, shallow
• Canary Current- Eastern Atlantic

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Why are currents more intense on the western
sides?
1. Trade winds compress currents against
continents there 2. Coriolis effect is greater at
higher latitudes
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UPWELLING AND DOWNWELLING INDUCED BY EKMAN
TRANSPORT
Ekman transport can drive surface waters 1.
apart, --gt upwelling 2. together --gt
downwelling 3. away from coasts --gt
upwelling 4. toward coasts --gt downwelling
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CONVERGENCE
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NORTHWESTERN U.S.
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Regions of upwelling and downwelling

• Equatorial upwelling in the open ocean
• Antarctic Divergence Zone
• Coastal upwelling- west sides of continents (esp.
  South America and Africa)
• Seasonal upwelling and downwelling along some
  coasts due to seasonal wind changes (e.g.,
  California)
• Upwelling Nutrients!

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Equatorial Upwelling
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EDDIES IN SURFACE CURRENTS

• At edges of currents
• Rings of rotating water, 10-100 km wide
• Example In the Gulf Stream
• Cold/Water boundary has curves
• Curves get increasingly pronounced- big loops
• Eddies become pinched-off

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Eddies, Contd

• Waters in eddies have different temperature,
  salinity, etc.
• Rotary motion in eddies can extend to great depths

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