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Thermohaline Circulation

Thermohaline Circulation. Douglas T. McClure Intro. to Physical Oceanography 11/30/05. Overview. What is Thermohaline Circulation? Why does it happen? How is it affected by climate? How does it affect climate? How has it changed in the past? How might it change in the future?

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Thermohaline Circulation

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  1. Thermohaline Circulation Douglas T. McClure Intro. to Physical Oceanography 11/30/05

  2. Overview • What is Thermohaline Circulation? • Why does it happen? • How is it affected by climate? • How does it affect climate? • How has it changed in the past? • How might it change in the future? • Conclusions?

  3. In a Nutshell • “Thermo-”  Temperature • “-haline”  Salinity • Temperature & salinity affect density. • Density differences affect circulation.

  4. Effects of Temperature & Salinity • Warm water expands  less dense  rises • Salt dissolves in water  denser  sinks • Anomalies: • In fresh water: density greatest at 4oC • In salt water: density continues to increase down to freezing point at –1.8oC

  5. Factors Affecting T & S

  6. The Big, Oversimplified Picture • A typical water molecule might spend ~1000 years in the deep current before resurfacing on the other side of the world!

  7. Pattern in the Atlantic • Warm water in Atlantic moves North (20 Sv, 1 pettawatt), cools, and sinks at high latitude, forming NADW. • Specifically, most NADW forms during winter in Labrador and Greenland Seas. • NADW slides South along the ocean bottom.

  8. Completing the Cycle • South of Africa, water can spread East. • Gradually spreads, warms, and rises in the Indian and Pacific oceans. • Travels back West to complete the cycle.

  9. Sensitivity to Climate • Critical phenomenon: formation of NADW • Effects of a warmer North Atlantic climate: • Rainfall UP  Density DOWN • Ice melt UP  Density DOWN • Temperature UP  Density DOWN • Evaporation UP  Density UP • Water in the North Atlantic may be more stable, less apt to sink and form NADW…

  10. Influence on Climate • Likely contributes to temperateness of Europe – but how much of this effect can really be attributed to THC?

  11. Historical Changes • Location and degree of NADW formation has varied significantly in the past. • Younger Dryas: a brief return to glacial temperatures shortly after the last ice age. • What might have caused this? Large-scale melting of glaciers  Freshwater runoff into North Atlantic  Thermohaline shutdown? Maybe…

  12. Summary and Conclusions • Variations in temperature and salinity  large-scale flow of water masses. • Influenced by climate, and in turn influences climate to some extent. • Future is unclear: many variables are involved, and the ultimate result depends on their complicated interdependencies.

  13. References • http://en.wikipedia.org/wiki/Thermohaline_circulation • http://www.ngdc.noaa.gov/paleo/ctl/thc.html • http://www.cru.uea.ac.uk/cru/info/thc/ • http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html • http://www.whoi.edu/institutes/occi/currenttopics/abruptclimate_curry_testim.html

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