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Ocean Current Switching: Can changes in ocean circulation cause rapid climate change?

Ocean Current Switching: Can changes in ocean circulation cause rapid climate change?. Richard Karsten Department of Mathematics and Statistics rkarsten@acadiau.ca. IB Camp, Wolfville August 20, 2008. Climate change, Hollywood style.

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Ocean Current Switching: Can changes in ocean circulation cause rapid climate change?

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  1. Ocean Current Switching:Can changes in ocean circulation cause rapid climate change? Richard Karsten Department of Mathematics and Statistics rkarsten@acadiau.ca IB Camp, Wolfville August 20, 2008

  2. Climate change, Hollywood style

  3. Premise: Climate change might occur rapidly, causing untold destruction This year, a sweater won't do. Whoever said "Tomorrow is another day"... didn't check the weather. Nature has spoken.

  4. The villain: the Gulf Stream Wikipedia Synopsis: The movie is based on the idea that the Gulf Stream (or North Atlantic drift), an ocean current which circulates warm water from the tropics to the Northern Hemisphere, is disrupted by the melting of the polar ice caps. This leads to catastrophic changes in the Earth's climate, as the temperature of the Earth's atmosphere stabilizes into a new pattern.

  5. A little more science on the big screen … Melt water from Greenland, because of its lower salinity, could then halt the currents that keep northern Europe warm and quickly trigger dramatic local cooling there. (source: Wikipedia)

  6. Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Is any of this realistic?

  7. Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Is any of this realistic?

  8. Australia Ocean and weather: Nova Scotia 19o 15o www.nsac.ns.ca

  9. Australia El Niño Ocean and climate: El Nino La Niña earthobservatory.nasa.gov

  10. The villain: the Gulf Stream British Geomorphological Research Group www.bgrg.org

  11. Hopedale, 55º27’ N Stornoway,58º54’ N

  12. The ocean stores a lot of heat. A column of ocean water only 3 m thick contains as much heat capacity as the full atmosphere above (Gill, 1982). Bordeaux Halifax Depth 5000 m

  13. The ocean stores the heat created by global warming. S. Levitus, J. Antonov, and T. Boyer, GRL 2005

  14. Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Is any of this realistic?

  15. Ocean currents The strongest currents in the ocean are a result of the wind blowing over the surface of the ocean.

  16. Average surface winds

  17. A rotating earth changes dynamics: Coriolis force To observers in the Northern Hemisphere, rotation of the earth deflects motion to the right.

  18. Wind forcing: Flow is 90o to the right! 50 m

  19. Wind-driven circulation H

  20. Gyre flow H

  21. Changes in the density of sea water can also drive ocean circulation. • Vertical Motion: Light water rises to the top and heavy water sinks. • Density changes are due to changes in temperature (thermo) and salinity (haline). Thermohaline circulation

  22. Warm water low density (light) Cold Water high density (heavy) Fresh Water low density Salty Water high density Sea water density

  23. Water properties Temperature Salinity

  24. Water properties: Salinity North South meridional depth Temperature Salinity North South meridional depth

  25. Cooling at the surface can create heavy water at the surface, which then “sinks.” Heat loss Convection and overturning The net effect is an overturning circulation.

  26. Warm water pulled north by cooling and sinking. Gulf Stream Gulf Stream transports heat north Warm water “pushed” north by wind. (after W. Broecker, modified by E. Maier-Reimer).

  27. www.travelblog.org/Bloggers/Dan--NZ/ www.theworldwidegourmet.com Australia Gulf Stream and climate

  28. Thermohaline circulation (Meridional overturning circulation)

  29. Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Is any of this realistic?

  30. Melting Ice • when ice melts in turns into COLD,FRESH water • If this water enters the north Atlantic it can prevent the sinking of the salty surface waters

  31. Melting Ice

  32. Water properties: Salinity zonal Global warming is melting polar ice caps, which should make the northern waters fresher. meridional North South meridional depth

  33. Fresh water from glacial melt makes surface water fresher. Heat loss Fresh water shuts off convection X If the water is fresh enough, it doesn’t become heavy so it doesn’t sink.

  34. x Thermohaline circulation (Meridional overturning circulation)

  35. Can the ocean affect climate? • What is the Gulf Stream and how does it affect climate? • Can melting icecaps move ocean currents? • Is the effect of the Gulf Stream changing due to global warming? Is any of this realistic?

  36. Is the Gulf Stream changing? Weaker Gulf Stream threatens Britain's climate By Mark Henderson, Science Correspondent London Times, November 30, 2005 An ominous current event Matthew Hart Globe and Mail, January 14, 2006

  37. Evidence of freshening  http://www.whoi.edu/mr/pr.do?id=5098

  38. Evidence of freshening  (B. Dickson, et. al., in Nature, April 2002)

  39. North Atlantic water has become fresher, possibly reducing the thermohaline circulation. X • If the warm water is no longer pulled into the North Atlantic, it should return south at shallower depths. Northern waters are fresher (after W. Broecker, modified by E. Maier-Reimer).

  40. Are there changes in the circulation?

  41. Shallow return flow has increased by almost 50%. Deep return flow has reduced by about 30%. Measurement line Surface warm flow has decreased slightly

  42. The results are a surprise to scientists in the field. • Modelling suggests that increase of fresh water flows large enough to shut down the thermohaline circulation would be an order of magnitude greater than currently estimated to be occurring. • The Bryden results could be caused by natural variation, or "noise", that is, coincidence. • If the results are correct, perhaps thermohaline circulation reductions will not have the drastic effects that have been predicted on European cooling. Reaction:Climate change: A sea changeby Quirin Schiermeier in Nature 2006

  43. “…the seemingly dramatic reduction discovered two years ago in the strength of the Atlantic meridional overturning circulation (MOC) is easily within the range of huge seasonal variability.” More … Ocean circulation noisy, not stalling by Quirin Schiermeier in Nature 2007

  44. Ocean Current Switching:Can changes in ocean circulation cause rapid climate change?

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