1. Topic 15 Wind 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 E U R O P E A F R I C A A M E R I C A 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 E U R O P E A F R I C A
A M E R I C A S
E U R O P E A F R I C A
North Atlantic Current
North Equatorial Current
Gulf
Stream
Canary
Current

14. North Atlantic Gyre
South Atlantic Gyre
Subpolar 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 Current ARE 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 knot (about
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 = 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.

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

40. A 1990 swept 80,000 sneakers off a cargo vessel in the North Pacific
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. 1 2 3 4 5 6 7

42. Agulhas Current

43. Benguela Current

44. Brazil Current

45. California Current

46. Equatorial Countercurrent

47. Kuroshio Current

48. Peru Current