1. The Surface Currents An Introduction to the World’s Oceans Sverdrup et al. - Chapter Nine - 8th Ed.

2. Surface Currents The Ekman spiral and Ekman transport –Coriolis force –Frictional coupling –Wind and surface water transport Ocean gyres –Wind patterns and Ekman transport Geostrophic flow –Convergent and divergent Ekman transport –Force balance: Coriolis force and pressure force –Using subsurface density distribution to deduce volume transport in the geostrophic flow

3. Fig. 9.1

4. Fig. 9.2

5. Fig. 9.3

6. Oceanic gyres.

7. Fig. 9.4

8. TOPEX/Poseidon satellite eye view of the North Atlantic “hill”

9. Wind-Driven Ocean Currents Wind-driven currents and continuity currents: see map in Figure 9.5 Pacific Ocean currents Atlantic Ocean currents Indian Ocean currents Arctic Ocean currents

10. Fig. 9.5

11. Fig. 9.6

12. Current Flow Current speed –Relation to wind speed –Acceleration through narrow gaps –Volume transport by ocean currents

13. 9.3 Current Flow, continued Western intensification –Stronger, deeper, and narrower currents on western side of oceans –Factors that play key roles in western intensification: Eastward turning of the Earth Increase of Coriolis effect with latitude Changing strength of wind with latitude Friction between land masses and ocean water currents –Stronger geostrophic currents implies steeper sea surface slope

14. Eddies Fast-moving current moves into slower-moving water Currents develop meanders that can break off to form eddies –Trapped warm or cold water. Warm eddy circulates clockwise (to the right. Cold eddy circulates counterclockwise (to the left) Can last for years (3 or so) and disturb the abyssal plain. Eddies and main flow energy dissipation Measuring eddies –Satellites measure sea surface height to map geostrophic eddies –Floats monitor deep water eddies

15. Fig. 9.7

16. Gulf Stream with eddies: 3. warm core 4. cold core Gulf Stream with eddies: 3. warm core 4. cold core

17. Fig. 9.9

18. Convergence and Divergence Langmuir cells –Caused by strong winds –Mix surface waters –Organize the distribution of organic matter in convergent and divergent zones Permanent zones (see map) –Convergences (low in nutrients and aquatic life) The tropical convergence The subtropical convergences The Arctic and Antarctic convergences –Divergences (upwelling delivers nutrients to surface waters) Two tropical divergences The Antarctic divergence

19. Fig. 9.10

21. Convergence and Divergence Seasonal zones –Examples from the west coast of North America: Southerly alongshore winds produce onshore Ekman transport and downwelling in winter Northerly alongshore winds produce offshore Ekman transport and upwelling in summer

25. Changing Circulation Patterns Global currents –Major changes in ocean circulation associated with glacial and interglacial periods - Milankovitch Cycles –Ice cores and sediment cores record changes North Pacific Oscillations –Pacific Decadal Oscillation (PDO) Ocean: current shifts and changes occur in water temperature Atmosphere: changes in surface pressure, winds, and precipitation North Atlantic Oscillations –Complex interaction between the ocean and atmosphere Pools of anomalously cold and low-salinity surface water associated with harsh winters in Europe Life spans of anomalies: 4-10 years

26. Fig. 9.14

28. Fig. 9.16

29. 9.7 Measuring the Currents Current measurements fall into two groups: –Following water parcels Drifting buoys are tracked using Global Positioning Systems (GPS) and satellites –Measuring speed and direction at a fixed point: Current meters Doppler effect method Satellites measure sea surface height to deduce geostrophic flows

31. 9.8 Practical Considerations: Energy from the Currents Massive reservoir of energy (280 trillion watt-hours) Indirect source of solar energy No cost effective method of harnessing energy at present time

32. The End