Thermohaline Circulation

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.

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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.

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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_circulat
  ion
• 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.h
  tml
• http//www.whoi.edu/institutes/occi/currenttopics/
  abruptclimate_curry_testim.html