Ocean Surface Circulation

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Ocean Surface Circulation

• Motion in the Ocean, Part I, or Why does the
  ocean have currents, and why do they move in
  circles?

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Two types of Ocean Circulation

• Surface Circulation -- Wind-driven
• Deep Circulation -- Density (T,S) driven

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Atmospheric Circulation
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Temperature and Pressure

• As the Earths surface is heated, air is warmed,
  expands and rises (Low P)
• Warm air carries water vapor
• In the upper atmosphere the air cools and sinks
  (High P)
• This round-trip is called a cell

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Things get interesting!

• On a rotating planet, moving objects appear to be
  deflected
• Why is this?

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Coriolis Deflection (aka I Could Care Less
force)

• Apparent force due to Earths rotation
• Deflection in path of motion when viewed from a
  rotating reference frame
• Gustave-Gaspard Coriolis (1835)
• Familiar from merry-go-rounds
• Significant only for large distances (not
  toilets and billiards!)

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Coriolis Deflection
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Consequences of Coriolis

• Moving fluids (atmosphere and ocean) turn to the
  right in the Northern Hemisphere
• Moving fluids (atmosphere and ocean) turn to the
  left in the Southern Hemisphere

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Global Wind Circulation
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Wind-Driven Ocean Circulation

• Steady winds produce waves and set the surface
  water in motion
• Moving water is deflected to the right
  (N.Hemisphere) or left (S.Hemisphere)
• This starts the main gyre motion of the surface
  ocean

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Surface Ocean Circulation
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Main Features

• Five large gyres
• Antarctic Circumpolar Current
• Equatorial Countercurrent
• Velocities vary -- fastest are meters/sec

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106 m3/sec (Sverdrup) all the rivers
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Gulf Stream - Benj Franklin
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Gulf Stream
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So, do the gyres just follow the winds?

• Not exactly! But the winds get the motion in the
  ocean started
• The oceans respond by flowing and turning
• Water piles up in the center of gyres -- several
  meters high

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Ekman Transport -- moves water 90 to the winds
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Upwelling and Oregons Ocean

• Winter winds from the south -- downwelling
• Summer winds from the north -- upwelling

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Winter Summer
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Global Wind Circulation
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Geostrophic Currents
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Coriolis deflection plus the Pressure Gradient
steers the currents around the gyres
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Northern Hemisphere Gyres
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Surface Circulation
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How do we track ocean circulation?

• Fixed Buoys -- measure current speed and
  direction
• Drifters -- travel with the currents and transmit
  their location

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Beach Swap Meets!
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Tracking CurrentsThe Story of the Lost Nikes

• 1 60,000 shoes spilled, May 1990
• 2-8 1990-91
• 9 1993
• 10 1994

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Marine Debris Pacific Trash