Ocean Surface Circulation

1. Ocean Surface Circulation Motion in the Ocean, Part I, or Why does the ocean have currents, and why do they move in circles? Jack Barth (barth@coas.oregonstate.edu) NASA web site: http://oceanmotion.org

3. Two types of Ocean Circulation: • Surface Circulation -- Wind-driven • Deep Circulation – Density-driven Density of water is influenced by Temperature and Salinity, so density- driven circulation is often called the “Thermohaline” Circulation Friday’s lecture

4. Atmospheric Circulation

5. Temperature and Pressure • As the Earth’s surface is heated, air is warmed, expands and rises (Low Pressure) • Warm air carries water vapor • In the upper atmosphere the air cools and sinks (High Pressure) • Surface winds blow from High Pressure to Low Pressure • This round-trip is called a “cell”

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

7. Coriolis Deflection • 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!)animation

9. So, in the frame rotating CCW (like northern hemisphere), unforced particle in motion is deflected to the right. If frame rotates CW, motion of particle is to the left (reverse film). Coriolis Force (northern hemisphere) Coriolis Force (southern hemisphere) velocity velocity

10. Coriolis Deflection “During the naval engagement near the Falkland Islands which occurred early in World War I, the British gunners were surprised to see their accurately aimed salvos falling 100 yards to the left of the German ships. The designers of the sighting mechanisms were well aware of the Coriolis deflection and had carefully taken this into account, but they apparently were under the impression that all sea battles took place near 50°N latitude, and never near 50°S latitude. The British shots, therefore, fell at a distance from the targets equal to twice the Coriolis deflection.” Jerry B. Marion, “Classical Dynamics of Particles and Systems”, 2nd edition, 1971.

11. 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

12. Global Wind Circulation westerlies trades trades westerlies

13. 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

14. Surface Ocean Circulation

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

16. Ocean Surface Current Speed cm/second How fast is a cm/second? 100 centimeters in a meter; 1000 meters in a kilometer so 100,000 centimeters per kilometer 24 hrs x 3600 sec/hr = 86,400 sec~100,000 seconds per day 1 cm/second = 1 km/day R. Lumpkin (NOAA/AOML)

17. 106 m3/sec (Sverdrup) = all the rivers

18. Gulf Stream - Benjamin Franklin 1760s Sailing times to and from Europe

19. Gulf Stream from satellite

20. 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

21. Global Wind Circulation westerlies trades trades westerlies

22. Ekman Transport -- moves water 90°to the winds Ekman (1905)

23. Geostrophic Currents

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

25. Northern Hemisphere Gyreswestward intensification ~1000 meters

26. Surface Circulation

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

28. Winter Summer

29. Oregon’s Summer

30. Thanks to Alan Dennis (COAS/OSU)

31. Cold, nutrient-rich water near the Oregon coast: leads to phytoplankton blooms chl (mg/m3) T (ºC) Barth (2007)

32. Equatorial Divergence

33. Equatorial Divergence

34. Antarctic Circulation

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

36. Beach Swap Meets!

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

38. Marine Debris: Pacific Trash

39. What about the debris from the recent Japanese tsunami? AFP-Getty Images US Navy photo

40. How long before debris might reach the US west coast? ~7300 km North Pacific Current ~ 10 cm/s ~ 10 km/day about 2 years for the first of it … but much will sink and enter the North Pacific Garbage Patch Courtesy of N. Maximenko & J. Hafner(UH)

41. Ocean Surface Circulation surface currents driven by winds Coriolis and pressure forces result in oceanic gyres wind-driven currents reach down several 100s of meters up to 1km speeds of 10-100 cm/s (0.1-1.0 m/s ~ 0.2-2 knots); strongest on western sides of ocean basins Ekman flow away from coast leads to coastal upwelling and plankton blooms NASA web site: http://oceanmotion.org