Topic 15 Wind Driven Currents

1. Topic 15Wind Driven Currents GEOL 2503 Introduction to Oceanography

2. 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

3. 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

7. 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 loop—called a gyre • Several exist, North Atlantic Gyre described above

8. 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

9. Ekman Transport • Ekman Spiral extends to maybe 100-150 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

12. 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

13. North Atlantic Gyre North Atlantic Current A M E R I C A S Gulf Stream E U R O P E A F R I C A Canary Current North Equatorial Current

14. Subpolar Gyre North Atlantic Gyre South Atlantic Gyre Not Developed (Antarctica) Polar Current North Atlantic Current N. Equatorial Current Equatorial Counter Current S. Equatorial Current West Wind Drift

17. EKMAN TRANSPORT A M E R I C A S E U R O P E A F R I C A 2m 1m

19. 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

20. 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

22. 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

23. Continuity Currents • Gulf Stream and Canary CurrentARE NOT WIND DRIVEN • They provide continuity of flow • They complete a circular, clockwise flow centered about 30° N latitude in the northern hemisphere • Several similar currents exist in other oceans

24. 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

26. 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 gap—Strait of Florida • Florida Current exceeds 3 knots • Remember Ponce de Leon

27. What’s 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

28. 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 Earth’s fresh water

29. Numbers = millions of cubic meters per second

30. 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

31. 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.

32. Surface Current 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

33. 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)

36. Langmuir Circulation • When wind blows steadily over water, different kind of circulation can be set up. • This is in addition to Ekman circulation. • Alternate bands of converging and diverging currents form helical cells, parallel to the wind direction. (Helix means spiral, like a corkscrew or a spring.) • The converging currents are plainly visible as bands of seaweed, foam or oil. The diverging currents are areas where plankton are concentrated.

38. http://www.icm.csic.es/geo/gof/out/banco

39. Franklin-Folger Map of Gulf Stream, 1769-1770

40. A 1990 swept 80,000 sneakers off a cargo vessel in the North Pacific. They later washed up on shore, acting as current trackers.

41. 4 6 5 5 5 7 1 2 7 3

42. Agulhas Current

43. Benguela Current

44. Brazil Current

45. California Current

46. Equatorial Countercurrent

47. Kuroshio Current

48. Peru Current