CHAPTER 7 Ocean Circulation

1. CHAPTER 7 Ocean Circulation

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 http://www.global-greenhouse-warming.com/images/thermohaline.jpg

4. Measuring surface currents • 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 http://static.howstuffworks.com/gif/ocean-current-4.jpg

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. 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

13. 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

14. http://maritime.haifa.ac.il/departm/lessons/ocean/ 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

15. 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 http://maritime.haifa.ac.il/departm/lessons/ocean

16. Eastern Boundary Currents • Eastern side of ocean basins • Tend to have the opposite properties of Western Currents • Slow • Wide • Shallow • Cold http://maritime.haifa.ac.il/departm/lessons/ocean

18. 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

19. 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

20. 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

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

22. 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

23. Antarctic circulation • Antarctic Circumpolar Current (West Wind Drift) • Encircles Earth • Transports more water than any other current Fig. 7.14

24. 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

25. North Atlantic Subtropical Gyre • Sargasso Sea • Center of gyre – eddy • Sargassum “weed” accumulates in convergence • Provides habitat for many marine animals http://www.safmc.net/Portals/0/Sargassum/Sargassum_Ross1.jpg http://upload.wikimedia.org/wikipedia/commons/b/b4/Sargasso.png

26. 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

27. Warm core ring Warm core ring forming Cold core rings http://sam.ucsd.edu/sio210/gifimages http://ocw.mit.edu/NR/rdonlyres/Global

28. Warm core ring off “Loop Current” in the Gulf of Mexico http://storm.rsmas.miami.edu/~nick/

29. In 2005, winds of Hurricanes Katrina and Rita increase as they pass over warm loop current in Gulf of Mexico http://en.wikipedia.org/wiki/ http://ccar.colorado.edu/~leben/

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

31. 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

32. 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

33. 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

34. 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

35. 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

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

37. 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

38. “Severe” (1997) and “Mild” (2002) El Nino http://science.nasa.gov/headlines/

39. Freeze probability lower during La Nina Freeze probability slightly higher during El Nino

40. Thermohaline circulation • Caused by density differences due to temp and salinity in different areas • Below the pycnocline • 90% of all ocean water • Slow velocity http://www.john-daly.com/polar

41. Polar circulation • Bottom water formation • Movement caused by differences in density (temperature and salinity) • Formed by freezing saltwater  dense brine sinks • Cooler seawater denser • Saltier seawater denser http://www.geology.um.maine.edu/ges121/lectures/19-ocean-conveyor

42. Thermohaline circulation Fig. 7.26

43. Thermohaline circulation • Selected deep-water masses • Antarctic Bottom Water • North Atlantic Deep Water • Antarctic Intermediate Water • Oceanic Common Water • Cold surface seawater sinks at polar regions and moves equatorward

44. Conveyor-belt circulation • Combination deep ocean currents and surface currents Fig. 7.27

45. Deep ocean currents • Cold, oxygen-rich surface water to deep ocean • Dissolved O2 important for life and mineral processes • Deep ocean currents do bring water across the equator • Changes in thermohaline circulation can cause global climate change • Example: warmer surface waters with increased melting of ice caps  less dense surface waters • Bottom water will not form and sink  Gulfstream may be altered  long-term cooling, particularly in northern Europe • less oxygen deep ocean

46. Misconceptions – what have we learned makes these statement false? • Earth events taking place within the global environment are not interconnected, such as El Nino is not important to people living in the midwest. • Humans are the only cause of global warming. • The greenhouse effect will cause all living things to die.