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)
Surface winds blow from High P to Low 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 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 Deflection

8. 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

9. Global Wind Circulation

10. 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

11. Surface Ocean Circulation

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

13. 106 m3/sec (Sverdrup) = all the rivers

14. Gulf Stream - Benj Franklin
Sailing times to and from Europe

15. Gulf Stream from satellite

16. 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

17. Global Wind Circulation

18. Geostrophic Currents

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

20. Northern Hemisphere Gyres westward intensification

21. Surface Circulation

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

23. Upwelling Winter winds from the south -- downwelling
Summer winds from the north -- upwelling