Chapter 7 Ocean Circulation


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Chapter 7 Ocean Circulation

  1. 1. Chapter 7 Ocean CirculationEssentials of Oceanography 7th Edition
  2. 2. Ocean currentsSurface currents Affect surface water within and above the pycnocline (10% of ocean water) Driven by major wind belts of the worldDeep currents Affect deep water below pycnocline (90% of ocean water) Driven by density differences Larger and slower than surface currents
  3. 3. Measuring surface currentsDirect methods Float meters Intentional Inadvertent Propeller metersIndirect methods Pressure gradients Satellites Doppler flow meters Figure 7B
  4. 4. Surface currents closely follow global wind belt patternTrade winds at 0-30ºblow surface currentsto the eastPrevailing westerliesat 30-60º blowcurrents to the west Figure 7-3
  5. 5. Wind-driven surface currents Figure 7-4
  6. 6. Current gyresGyres 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. 7. Current gyresGyres (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. 8. Ekman spiralEkman spiraldescribes the speedand direction of flowof surface waters atvarious depthsFactors: Wind Figure 7-6 Coriolis effect
  9. 9. Ekman transportEkman transport is theoverall water movementdue to Ekman spiralIdeal transport is 90ºfrom the windTransport directiondepends on the Figure 7-6hemisphereInternet visualization
  10. 10. Geostrophic flow and western intensificationGeostrophic flowcauses a hill to form insubtropical gyresThe center of the gyreis shifted to the westbecause of Earth’srotationWestern boundarycurrents are intensified Figure 7-7
  11. 11. Western intensification of subtropical gyresThe western boundary currents of allsubtropical gyres are: Fast Narrow DeepWestern boundary currents are also warmEastern boundary currents of subtropicalgyres have opposite characteristics
  12. 12. Currents and climateWarm currentwarms air highwater vapor humidcoastal climateCool current coolsair low water vapor dry coastal climate Figure 7-8a
  13. 13. Upwelling and downwellingVertical 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. 14. Coastal upwelling and downwellingEkman transport moves surface water awayfrom shore, producing upwellingEkman transport moves surface watertowards shore, producing downwelling Figure 7-11
  15. 15. Other types of upwellingEquatorialupwellingOffshore windSea floorobstructionSharp bend in Equatorial upwellingcoastal geometry Figure 7-9
  16. 16. Antarctic surface circulation Figure 7-13
  17. 17. Atlantic Ocean surface currents Figure 7-14
  18. 18. North Atlantic Ocean circulation Figure 7-15
  19. 19. The Gulf Stream and sea surface temperaturesThe Gulf Stream isa warm, westernintensified currentMeanders as itmoves into theNorth AtlanticCreates warm andcold core rings Figure 7-16
  20. 20. Pacific Ocean surface currents Figure 7-17
  21. 21. El Niño-Southern Oscillation (ENSO)El Niño = warm surface current inequatorial eastern Pacific that occursperiodically around ChristmastimeSouthern Oscillation = change inatmospheric pressure over Pacific Oceanaccompanying El NiñoENSO describes a combined oceanic-atmospheric disturbance
  22. 22. Normal conditions in the Pacific Ocean Figure 7-18a
  23. 23. El Niño conditions (ENSO warm phase) Figure 7-18b
  24. 24. La Niña conditions (ENSO cool phase; opposite of El Niño) Figure 7-18c
  25. 25. The 1997-98 El NiñoSea surfacetemperature anomalymap shows warmingduring severe 1997-98El NiñoInternet site for El Niño visualizationsCurrent state of the tropical Pacific Figure 7-19a
  26. 26. El Niño recurrence intervalTypical recurrence interval for El Niños =2-12 yearsPacific has alternated between El Niño andLa Niña events since 1950 Figure 7-20
  27. 27. Effects of severe El Niños Figure 7-21
  28. 28. Indian Ocean surface currents Northeast monsoon Southwest monsoon Figure 7-23
  29. 29. Deep currentsDeep 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. 30. Deep ocean characteristicsConditions of the deep ocean: Cold Still Dark Essentially no productivity Sparse life Extremely high pressure
  31. 31. Identification of deep currentsDeep currentsare identified bymeasuringtemperature (T)and salinity (S),from whichdensity can bedetermined Figure 7-24
  32. 32. Atlantic Ocean subsurface water masses Figure 7-25
  33. 33. Conveyer-belt circulation Figure 7-27
  34. 34. End of Chapter 7Essentials of Oceanography 7th Edition
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