1. Chapter 7 Ocean Circulation
Essentials of Oceanography
7th Edition

2. Ocean currents Surface currents Deep currents
Affect surface water within and above the pycnocline (10% of ocean water)
Driven by major wind belts of the world
Deep currents
Affect deep water below pycnocline (90% of ocean water)
Driven by density differences
Larger and slower than surface currents

3. Measuring surface currents
Direct methods
Float meters
Intentional
Inadvertent
Propeller meters
Indirect methods
Pressure gradients
Satellites
Doppler flow meters
Figure 7B

4. Surface currents closely follow global wind belt pattern
Trade winds at 0-30º blow surface currents to the east
Prevailing westerlies at 30-60º blow currents to the west
Figure 7-3

5. Wind-driven surface currents
Figure 7-4

6. Current gyres Gyres are large circular-moving loops of water
Subtropical gyres
Five main gyres (one in each ocean basin):
North Pacific
South Pacific
North Atlantic
South Atlantic
Indian
Generally 4 currents in each gyre
Centered at about 30º north or south latitude

7. Current gyres Gyres (continued) Subpolar gyres
Smaller and fewer than subtropical gyres
Generally 2 currents in each gyre
Centered at about 60º north or south latitude
Rotate in the opposite direction of adjoining subtropical gyres

8. Ekman spiral
Ekman spiral describes the speed and direction of flow of surface waters at various depths
Factors:
Wind
Coriolis effect
Figure 7-6

9. Ekman transport
Ekman transport is the overall water movement due to Ekman spiral
Ideal transport is 90º from the wind
Transport direction depends on the hemisphere
Internet visualization
Figure 7-6

10. Geostrophic flow and western intensification
Geostrophic flow causes a hill to form in subtropical gyres
The center of the gyre is shifted to the west because of Earth’s rotation
Western boundary currents are intensified
Figure 7-7

11. Western intensification of subtropical gyres
The western boundary currents of all subtropical gyres are:
Fast
Narrow
Deep
Western boundary currents are also warm
Eastern boundary currents of subtropical gyres have opposite characteristics

12. Currents and climate
Warm current  warms air  high water vapor  humid coastal climate
Cool current  cools air  low water vapor  dry coastal climate
Figure 7-8a

13. Upwelling and downwelling
Vertical movement of water ()
Upwelling = movement of deep water to surface
Hoists cold, nutrient-rich water to surface
Produces high productivities and abundant marine life
Downwelling = movement of surface water down
Moves warm, nutrient-depleted surface water down
Not associated with high productivities or abundant marine life

14. Coastal upwelling and downwelling
Ekman transport moves surface water away from shore, producing upwelling
Ekman transport moves surface water towards shore, producing downwelling
Figure 7-11

15. Other types of upwelling
Equatorial upwelling
Offshore wind
Sea floor obstruction
Sharp bend in coastal geometry
Equatorial upwelling
Figure 7-9

16. Antarctic surface circulation
Figure 7-13

17. Atlantic Ocean surface currents
Figure 7-14

18. North Atlantic Ocean circulation
Figure 7-15

19. The Gulf Stream and sea surface temperatures
The Gulf Stream is a warm, western intensified current
Meanders as it moves into the North Atlantic
Creates warm and cold core rings
Figure 7-16

20. Pacific Ocean surface currents
Figure 7-17

21. El Niño-Southern Oscillation (ENSO)
El Niño = warm surface current in equatorial eastern Pacific that occurs periodically around Christmastime
Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El Niño
ENSO describes a combined oceanic-atmospheric disturbance

22. Normal conditions in the Pacific Ocean
Figure 7-18a

23. El Niño conditions (ENSO warm phase)
Figure 7-18b

24. La Niña conditions (ENSO cool phase; opposite of El Niño)
Figure 7-18c

25. The El Niño
Sea surface temperature anomaly map shows warming during severe El Niño
Internet site for El Niño visualizations
Current state of the tropical Pacific
Figure 7-19a

26. El Niño recurrence interval
Typical recurrence interval for El Niños = 2-12 years
Pacific has alternated between El Niño and La Niña events since 1950
Figure 7-20

27. Effects of severe El Niños
Figure 7-21

28. Indian Ocean surface currents
Northeast monsoon
Southwest monsoon
Figure 7-23

29. Deep currents Deep currents: Form in subpolar regions at the surface
Are created when high density surface water sinks
Factors affecting density of surface water:
Temperature (most important factor)
Salinity
Deep currents are also known as thermohaline circulation

30. Deep ocean characteristics
Conditions of the deep ocean:
Cold
Still
Dark
Essentially no productivity
Sparse life
Extremely high pressure

31. Identification of deep currents
Deep currents are identified by measuring temperature (T) and salinity (S), from which density can be determined
Figure 7-24

32. Atlantic Ocean subsurface water masses
Figure 7-25

33. Conveyer-belt circulation
Figure 7-27

34. Essentials of Oceanography 7th Edition
End of Chapter 7
Essentials of Oceanography
7th Edition