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Chapter 7 Ocean Circulation
This chapter discusses ocean circulation patterns driven by surface winds and density differences in deep water masses. Surface currents are separated into two main types - those within the top 10% of the ocean driven by wind patterns, and deeper currents below influenced by density variations. Major circulation features include wind-driven gyres organized into subtropical and subpolar varieties, coastal upwelling and downwelling of nutrient-rich water, and the Gulf Stream. Deep water circulation is termed the global thermohaline circulation and involves the sinking and southward flow of dense water masses.
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Chapter 7 Ocean Circulation
- 1. Chapter 7 OceanCirculation Essentials of Oceanography 7th Edition
- 2. Ocean currents Surface 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 Directmethods Float meters Intentional Inadvertent Propeller meters Indirect methods Pressure gradients Satellites Doppler flow meters Figure 7B
- 4. Surface currents closelyfollow 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 arelarge 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 describesthe speed and direction of flow of surface waters at various depths Factors: Wind Figure 7-6 Coriolis effect
- 9. Ekman transport Ekman transportis the overall water movement due to Ekman spiral Ideal transport is 90º from the wind Transport direction depends on the Figure 7-6 hemisphere Internet visualization
- 10. Geostrophic flow andwestern 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 Warmcurrent 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 Verticalmovement 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 ofupwelling Equatorial upwelling Offshore wind Sea floor obstruction Sharp bend in Equatorial upwelling coastal geometry Figure 7-9
- 16. Antarctic surface circulation Figure 7-13
- 17. Atlantic Ocean surfacecurrents Figure 7-14
- 18. North Atlantic Ocean circulation Figure 7-15
- 19. The Gulf Streamand 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 surfacecurrents 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 inthe 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 1997-98 ElNiño Sea surface temperature anomaly map shows warming during severe 1997-98 El Niño Internet site for El Ni ño visualizations Current state of the tropical Pacific Figure 7-19a
- 26. El Niño recurrenceinterval 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 severeEl Niños Figure 7-21
- 28. Indian Ocean surfacecurrents 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 Conditionsof the deep ocean: Cold Still Dark Essentially no productivity Sparse life Extremely high pressure
- 31. Identification of deepcurrents Deep currents are identified by measuring temperature (T) and salinity (S), from which density can be determined Figure 7-24
- 32. Atlantic Ocean subsurfacewater masses Figure 7-25
- 33. Conveyer-belt circulation Figure 7-27
- 34. End of Chapter7 Essentials of Oceanography 7th Edition