Wind-driven Circulation

1. Wind-driven Circulation • Ekman layer • Divergence/convergence • Geostrophy • Major ocean currents

2. Heat enters ocean-atmosphere in tropics, goes out at poles

3. Winds on a NON-rotating earth flow north-south

4. (Pinet)

5. (Pinet)

6. Major ocean current systems

7. Averaged global ocean topography from satellite observations

8. Coriolis Force (Mann and Lazier)

9. Coriolis force is an apparent force • Coriolis force  2 · sin · u, where •  is the Earth’s rotation rate, i.e., •   2/86400 sec-1 , •  is the latitude, and • u is the speed of the object • Coriolis parameter f = 2 · sin • Coriolis force depends on latitude and speed of particle

10. Northern hemisphere situation

11. Inertial Motion in Baltic Sea Inertial Period of 14 hours (time it takes to complete a circle) Inertial Period: T=2/f Latitude =57.8oN f = 2 · sin= 2 · (2/86400) · sin(57.8) = 1.23 x 10-4 sec-1 Inertial Period = 2/f = 14.18 hours In 7 days (15 tick marks) there are 11.8 inertial periods (count the crossings of the trajectory)

12. Wind Stress Direct effect of the wind does NOT extend beyond the top 50-100 meters of the water column

13. Vectors not collinear Vectors still not collinear Vectors collinear Wind drag+ Water drag+ Coriolis = 0 Initially, the velocity is in the direction of the wind stress (or wind drag) But because the forces are not balanced the water moves to right of wind stress, until the vector addition of forces is zero, at which time the velocity is at 45o to the right (in the Northern hemisphere) of the wind stress (or wind drag)

14. DE Ekman layer depth DE50-100m (Pinet)

15. Idealize the conditions in the red box...

16. The red box... Ekman Transport Westerlies Southward transport Northward transport Convergence Trades At the convergence, water piles up and sinks, thus depressing the thermocline and deepening the nutricline! Ekman transport is proportional to wind stress  greater transport for greater wind stress

17. Convergence (sea surface pile-up)

18. Sea surface Pressure gradient force Coriolis force Velocity into page Geostrophic Balance Pressure gradient force balanced by Coriolis force requires that the velocity be into the page, along the pressure lines, not across them (opposite to our daily experience of a “ball rolling downhill”).

19. Geostrophy – a frictionless balance between the pressure gradient And the Coriolis acceleration – generates currents that move Around the ‘hill’

20. Measured average topography of the North Atlantic (red-high)

21. Important points to note • The sea level pile-up is a result of the convergence of the Ekman transport • The Ekman layer is ONLY 50-100 meters thick • The resulting pressure gradient is felt throughout the water column • Thus the geostrophic current occurs over a MUCH greater depth than the depth of the wind-driven layer, as much as the top 200 to 500 meters

22. (Pinet)

24. Major ocean current systems

25. Atlantic temperature and salinity 500m Thicker warm water layer at 30oN (also in Southern Hemisph.)