Thermohaline CirculationThermohaline Circulation

Douglas T. McClureDouglas T. McClure

Intro. to Physical OceanographyIntro. to Physical Oceanography

11/30/0511/30/05

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?

• Conclusions?

In a Nutshell

• “Thermo-” Temperature• “-haline” Salinity

• Temperature & salinity affect density.

• Density differences affect circulation.

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

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

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!

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.

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.

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…

Influence on Climate

• Likely contributes to temperateness of Europe – but how much of this effect can really be attributed to THC?

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…

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.

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