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Lecture 8: The Oceans (2)
EarthsClimate_Web_Chapter.pdf, p. 24-27
Deep-Ocean Circulation
The thermohaline circulation =
the density-driven circulation
(thermo- for heat, and -haline for
salt).
Other names:
the global ocean conveyor belt, or
meridional overturning
circulation (MOC).
Wind driving and density driving of the oceanic circulation are closely
coupled and cannot be easily separated.
Global Conveyor Belt
1. Warm and salty
shallow water from
Tropics is carried
northward through
Atlantic
2. Water cools in North
Atlantic and sinks
east of Greenland as
cold and salty dense
water
3. North Atlantic Deep
Water flows toward
the Equator and into
the South Atlantic,
where it meets yet
colder and denser
Antarctic Bottom
Water flowing toward
the Equator
2
1
3
Global Conveyor Belt (Cont’d)
4. Combined flow eastward
below Africa and into the
Pacific Ocean
5. Deflected by Asia
6
5
7
6. Journey through Pacific
causes warming and slow
rise to surface
4
7. Warm and less dense
Pacific surface waters
move south and west to
converge toward Equator
as shallow surface water
to continue warming
Global Conveyor Belt (Cont’d)
8. 1,000 years for a complete
cycle
9. Conveyor Belt aided by
imbalance between loss
of water by evaporation in
Atlantic and gain of water
by precipitation and runoff
10. Bering Strait prevents
free exchange between
Arctic and Pacific Oceans
 inhibits Arctic-Pacific
Conveyor Belt
Impacts on global climate
1. Supplying heat to the polar
regions, and regulating
sea ice there
2. Determining the
concentration of carbon
dioxide in the atmosphere
3. Warming Western Europe
by about 2°C relative to
the similarly located west
coast of Canada
4. 12,000 years ago, meltwater from ice sheets disrupted deep water
formation and subsidence in the North Atlantic and caused a cooling
climate period in Europe known as the Younger Dryas.
Global warming and shutdown of THC
Hypothesis:
global warming  melting Greenland ice sheet, increasing precipitation
and river discharge  interrupt THC  cooling in Europe
Summary:
• What role does deep-ocean circulation play in climate change?
– Transport heat from tropics to higher latitude and from surface to
deep ocean.
• Where does surface water sink to deep ocean and where does
deep ocean water surfaces globally?
– Surface water mainly sinks to deep ocean in sub-polar North
Atlantic and Arctic ocean, deep ocean water surfaces in eastern
ocean boundary, Antarctic coast and the equatorial upwelling
regions.
• What change might occur to the thermohaline circulation in a
warmer climate? How would such a change influence the global
climate?
– Melting of ice and warming of surface temperature would reduce
density of the sea water in the sub-polar North Atlantic and Arctic
oceans, consequently weakens the thermohaline circulation.
Should this occur, it might cause abrupt climate change in the
northern hemisphere middle and high latitudes.