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 Increased by… Decreased by… Temperature
Solar radiation, esp. near the equator
Heat loss to atmosphere
Salinity
Formation of ice,
evaporation
Precipitation,
ice melt

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