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CHAPTER 7 Ocean Circulation. Fig. CO7. Ocean currents. Large-scale moving seawater Surface ocean currents Transfer heat from warmer to cooler areas Similar to pattern of major wind belts Affect coastal climates Deep ocean currents Provide oxygen to deep sea Affect marine life.

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CHAPTER 7 Ocean Circulation

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    1. CHAPTER 7 Ocean Circulation Fig. CO7

    2. Ocean currents • Large-scale moving seawater • Surface ocean currents • Transfer heat from warmer to cooler areas • Similar to pattern of major wind belts • Affect coastal climates • Deep ocean currents • Provide oxygen to deep sea • Affect marine life

    3. Types of ocean currents • Surface currents • Wind-driven • Primarily horizontal motion • Deep currents • Driven by differences in density caused by differences in temperature and salinity • Vertical and horizontal motions

    4. Measuring surface currents Fig. 7.1a • Direct methods • Floating device tracked through time • Fixed current meter • Indirect methods • Pressure gradients • Radar altimeters • Satellites measuring bulges which are due to shape of ocean flow and currents • Doppler flow meter (Acoustic Doppler Current Profiler) • Measures shift in frequency of sound waves to determine current movement

    5. Measuring deep currents • Floating devices tracked through time • Chemical tracers (radioactive isotopes) • Tritium • Chlorofluorocarbons • Characteristic temperature and salinity • Arrays of bottom monitors and cables

    6. Surface currents • Frictional drag between wind and ocean • Wind plus other factors such as… • Distribution of continents • Gravity • Friction • Coriolis effect cause • Gyres or large circular loops of moving water

    7. Ocean Gyres • World’s 5 subtropical gyres: • North Atlantic Gyre • South Atlantic Gyre • North Pacific Gyre • South Pacific Gyre • Indian Ocean Gyre

    8. When talking about location of currents, we refer to the ocean basin – not the land! • For instance, the Gulf Stream is a Western Boundary current of the North Atlantic • Even though it runs along side the eastern US

    9. Ocean gyres 3 2 4 1 4 2 • Subtropical gyres • Center of each is about 30o N or S • Major parts • Equatorial currents flow west • Western Boundary currents flow north or south • Northern or Southern Boundary currents – push water east • Eastern Boundary currents flow north or south 3

    10. Other surface currents • Equatorial countercurrents flow east, counter to equatorial currents in the subtropical gyres • As trade winds push equatorial current of subtropical gyre west, water builds up in western part of ocean basin • Since coriolis effect is minimal at equator, that build-up of water then flows east at equator • Subpolar gyres flow eastward Fig. 7.5

    11. Other factors affecting surface currents • Ekman transport • Geostrophic currents • Western intensification of subtropical gyres • All of these are connected Fig. 7.5

    12. Ocean Circulation

    13. Ekman spiral and transport • Surface currents move at angle to wind • Ekman spiral describes speed and direction of seawater flow at different depths • Each successive layer moves increasingly to right (N hemisphere) • Initial push of water and then Coriolis effect comes into play

    14. Ekman spiral and transport • Average movement of seawater under influence of wind affected by Coriolis effect • 90o to right of wind in Northern hemisphere • 90o to left of wind in Southern hemisphere

    15. Ekman Spiral and Coastal Upwelling/Downwelling

    16. Geostrophic flow • Ekman transport piles up water within subtropical gyres • Surface water flows downhill (gravity) and • Also to the right (Coriolis effect) • Balance of downhill and to the right causes net geostrophic flowaround the “hill” Fig. 7.8

    17. Western intensification • Top of hill of water displaced toward west due to Earth’s rotation •  water piles up on western side of gyre • Western boundary currents intensified • Faster • Narrower • Deeper • Warm

    18. Eastern Boundary Currents • Eastern side of ocean basins • Tend to have the opposite properties of Western Currents • Slow • Wide • Shallow • Cold

    19. Ocean currents and climate • Warm ocean currents warm air at coast • Creates warm, humid air on coast • Humid climate on adjoining landmass • Cool ocean currents cool air at coast • Cool, dry air • Dry climate on adjoining landmass August temperatures February temperatures

    20. Ocean currents and climate August temperatures February temperatures Fig. 7.9

    21. Diverging surface seawater • Divergence • winds move surface seawater away from geographical equator due to Coriolis effect • Deeper seawater (cooler, nutrient-rich) flows up to replace surface water • Equatorial Upwelling • High biological productivity Fig. 7.10

    22. Converging surface seawater • Convergence • surface seawater moves towards an area • Surface seawater piles up • Nutrient depleted surface water moves downward • Downwelling • Low biological productivity • Occurs at center of gyres Fig. 7.11

    23. Coastal upwelling and downwelling • Winds blowing down coasts • Ekman transport moves surface seawater… • onshore (downwelling) or… • offshore (upwelling) Fig. 7.12

    24. Upwelling at higher latitudes • Remember there is no pycnocline at the poles • Therefore, water moving towards the poles cools and becomes more dense, like all of the other water at the poles • This allows there to be significant vertical mixing • Very productive waters – this is where large animals (such as whales) go to feed

    25. Antarctic circulation • Antarctic Circumpolar Current (West Wind Drift) • Encircles Earth • Transports more water than any other current • East Wind Driftfrom polar easterlies • Antarctic Divergencefrom opposite directions of west and east wind drifts • Antarctic Convergence piles up and sinks below warmer sub-Antarctic waters Fig. 7.14

    26. Atlantic Ocean circulation • North Atlantic Subtropical Gyre • Gulf Stream (GS) • North Atlantic Current • Canary Current (C) • North Equatorial Current (NE) • Merges with S. Equatorial current to form Antilles and Caribbean currents  • Converge to form Florida Current (F) thru Florida Strait • May form Loop current into Gulf of Mexico • Atlantic Equatorial Counter Current (EC)– between North and South Equatorial currents No. Atlantic current F

    27. • North Atlantic Subtropical Gyre • Sargasso Sea • Center of gyre – eddy • Sargassum “weed” accumulates in convergence • Provides habitat for many marine animals

    28. Fig. 7.16

    29. Atlantic Ocean circulation • South Atlantic Subtropical Gyre • South Equatorial Current (SE) • Brazil Current (Br) • Antarctic Circumpolar Current (WW) • Benguela Current (Bg) Fig. 7.14

    30. Gulf Stream • Best studied • Meander is a bend in current  may pinch off into a loop • Warm-core rings form to north • Cold-core rings form to south • Unique biological populations Fig. 7.17b

    31. Warm core ring Warm core ring forming Cold core rings

    32. Core ring vertical profiles

    33. Warm core ring off “Loop Current” in the Gulf of Mexico

    34. In 2005, winds of Hurricanes Katrina and Rita increase as they pass over warm loop current in Gulf of Mexico

    35. Other North Atlantic currents • Labrador Current • Irminger Current • Norwegian Current • North Atlantic Current

    36. Climate effects of North Atlantic currents • Gulf Stream warms East coast of U.S. and Northern Europe • North Atlantic and Norwegian Currents warm northwestern Europe • Labrador Current cools eastern Canada • Canary Current cools North Africa coast

    37. Pacific Ocean circulation • North Pacific subtropical gyre • Kuroshio • North Pacific Current • California Current • North Equatorial Current • Alaskan Current Fig. 7.19

    38. Figure 7.19

    39. Pacific Ocean circulation • South Pacific subtropical gyre • East Australian Current • Antarctic Circumpolar Current • Peru Current • South Equatorial Current • Equatorial Counter Current

    40. Atmospheric and oceanic disturbances in Pacific Ocean – El Niño-Southern Oscillation (ENSO) • Relationship of sea surface temp and high altitude pressure • Normal conditions • Air pressure across equatorial Pacific is higher in eastern Pacific • Strong southeast trade winds • Pacific warm pool on western side • Thermocline deeper on western side • Upwelling off the coast of Peru bring nutrient-rich waters to surface As, you are looking at these figures pay attention to where thermocline is (remember that raising thermocline leads to upwelling which brings nutrients and oxygen-rich waters up to surface for organisms

    41. Normal conditions Fig. 7.20a

    42. El Niño-Southern Oscillation (ENSO) • Warm (El Niño) • High pressure in eastern Pacific weakens • Weaker trade winds • In strong El Nino events, trade winds can actually reverse • Warm pool migrates eastward • Thermocline deeper in eastern Pacific • Downwelling lowers biological productivity in East Pacific • Corals particularly sensitive to warmer seawater • Fish kills

    43. El Niño and La Niña

    44. El Niño-Southern Oscillation (ENSO) • Warm (El Niño) • Produces heavy rains and flooding in equatorial western South America • Droughts in western Pacific • Droughts in Indonesia and Northern Australia

    45. Cool phase (La Niña) • Increased pressure difference across equatorial Pacific – overshoots return to normal from El Niño • Stronger trade winds • Stronger upwelling in eastern Pacific • Shallower thermocline • Higher biological productivity • Cooler than normal seawater • Higher than normal hurricane season in Atlantic

    46. El Niño-Southern Oscillation (ENSO)Cool phase (La Niña) Fig. 7.20c

    47. ENSO events • El Niño warm phase about every 2 to 10 years • Highly irregular • Phases usually last 12 to 18 months Fig. 7.22

    48. ENSO events • Strong events effect global weather • 1982-83, 1997-98 • Mild events only effect weather in equatorial South Pacific • 2002-2003, 2004-2005, 2006-2007 • Effects of strong event can vary • Can be drier than normal or wetter than normal • Can be warmer than normal or cooler • Can produce thunderstorms and tornados in midwest • Can produce droughts in west • Pushes jet stream eastward, pushing Atlantic hurricanes east, away from U.S. • Less hurricanes reach US during El Nino, more during La Nina • Flooding, drought, erosion, fires, tropical storms, harmful effects on marine life • Examples: coral reef death in Pacific, crop failure in Philippines, increased cyclones in Pacific, drought in Sri Lanka Fig. 7.21

    49. “Severe” (1997) and “Mild” (2002) El Nino