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