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Ocean Surface Circulation

Ocean Surface Circulation. Motion in the Ocean, Part I, or Why does the ocean have currents, and why do they move in circles?. Two types of Ocean Circulation:. Surface Circulation -- Wind-driven Deep Circulation -- Density (T,S) driven. Atmospheric Circulation. Temperature and Pressure.

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Ocean Surface Circulation

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

  2. Two types of Ocean Circulation: • Surface Circulation -- Wind-driven • Deep Circulation -- Density (T,S) driven

  3. Atmospheric Circulation

  4. Temperature and Pressure • As the Earth’s 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”

  5. Things get interesting! • On a rotating planet, moving objects appear to be deflected • Why is this?

  6. Coriolis Deflection (aka “I Could Care Less” force) • Apparent force due to Earth’s 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!)

  7. Coriolis “movie”

  8. Coriolis Deflection

  9. 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

  10. Global Wind Circulation

  11. 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

  12. Surface Ocean Circulation

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

  14. 106 m3/sec (Sverdrup) = all the rivers

  15. Gulf Stream - Benj Franklin

  16. Gulf Stream

  17. 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

  18. Ekman Transport -- moves water 90° to the winds

  19. Upwelling and Oregon’s Ocean • Winter winds from the south -- downwelling • Summer winds from the north -- upwelling

  20. Winter Summer

  21. Global Wind Circulation

  22. Geostrophic Currents

  23. Coriolis deflection plus the Pressure Gradient steers the currents around the gyres

  24. Northern Hemisphere Gyres

  25. Surface Circulation

  26. How do we track ocean circulation? • Fixed Buoys -- measure current speed and direction • Drifters -- travel with the currents and transmit their location

  27. Beach Swap Meets!

  28. Tracking Currents:The Story of the Lost Nikes • 1:60,000 shoes spilled, May 1990 • 2-8: 1990-’91 • 9: 1993 • 10: 1994

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