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The Oceans in Motion
Subsurface Currents
Subsurface Currents
1. Mechanics
2. Deep water formation
3. The Importance of the Global
Conveyer Belt
Mechanics
subsurface currents = thermohaline circulation
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90% ocean water moved by subsurface currents
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Density driven currents
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Gravity is the driving force
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Heavier, denser H2O sinks due to gravity
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density of H2O
cold vs. warm ‘thermo’
fresh vs. salty ‘haline’
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dense H2O sinks and less dense H2O rises
(convection on a global scale)
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overturn and mixing of the water column, creates an
unstable H2O column
Mechanics (cont’d)
What is a stable water column?
See Fig. 7.4 (Intro 7e) or 7.2a (Fund. 4e)
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Salinity increases with depth, temperature decreases with depth
A stable water column is layered or stratified, like a three layered cake
Mechanics (cont’d)
Now let’s look at an unstable water column
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Salinity uniform with depth, temperature uniform with depth
An unstable water column is not stratified, it is well mixed
Dense water continually sinking
Mechanics (cont’d)
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Where is coldest surface water?
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Where is saltiest surface water?
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at the poles
at the poles
At the poles, the water column is unstable and is
well mixed because of sinking cold and salty water
NOAA
Deep Water Formation
Where is deep water formed?
1.
South Pole off coast of Antarctica
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Antarctic Bottom Water (AABW)
~ 1°C, 34.7 ppt
~ densest water in the ocean
2.
North Pole off coast of Greenland
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North Atlantic Deep Water (NADW)
~ 3°C, 34.9 ppt
~ very dense but not as dense as AABW
NADW and AABW form
at surface, sink and then
spread out in horizontal
direction at the bottom of
the ocean
See Fig. 7.6 (Intro 7e) or 7.5 (Fund. 4e)
Deep Water Formation
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NADW forms, sinks, flows southward along Atlantic Basin, once reaches
Antarctica, flows eastward along continent, mixes with AABW, both H2O
masses flow northward into the Indian Ocean and Pacific Oceans
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These deep waters gradually warm and mix with other waters and are
upwelled in upwelling regions (esp Equatorial upwelling zones) and flow back
toward the N. Atlantic
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Starts all over again called the “Great Global Conveyor Belt”
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this is how H2O circulates throughout the world’s oceans
One turn of the global conveyor belt takes between 1000-1500yrs
Global Conveyor Belt
American Museum of Natural History
The Importance of the Global Conveyer Belt
Why is this important to know about?
It is a “heat pump” that sometime exists and sometimes doesn’t
Two conditions – ON or OFF
ON
As it is today – the global conveyor belt is turned ON
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There is vigorous mixing at the poles (water column is
unstable)
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dense surface water is sinking at the poles (very cold
surface waters with polar ice caps)
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thermohaline circulation is initiated
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this is the ‘switch’ that turns the conveyor belt on
As the global conveyor belt returns water to the poles (i.e. via surface
currents), the oceans give off the heat picked up at the lower
latitudes to the land masses at the higher latitudes (i.e. northern Europe)
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oceans acting as a ‘heat pump’ to warm the land masses
The Importance of the Global Conveyer Belt (cont’d)
OFF
As it has been in the past- the global conveyor belt is turned OFF
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There is no vigorous mixing at the poles (water column is stable)
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there is no dense water sinking at the poles
(surface waters warmed, polar ice caps melt)
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thermohaline circulation is not initiated
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the global conveyor belt ‘switch’ is turned off
There is no heat pump to warm the land masses
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much colder in northern Europe
The Importance of the Global Conveyer Belt (cont’d)
Example of when the global conveyor belt has been turned off:
“Younger Dryas”-discovered about 20 years ago
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right now we’re in an ice age- we have been for the past 2 million years
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last glacial stage ended 11,000 ybp
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after last glacial stage ended there was a 2000 year warming period
interrupted by a 700 year return to glacial stage conditions
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how do we know? pollen records
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forests grew for 2000 years, abruptly stopped (no pollen records)
and then resumed growing about 700 years later (pollen records again)
cold snap period known as the ‘Younger Dryas”
The Importance of the Global Conveyer Belt (cont’d)
What do we think happened to cause the ‘Younger Dryas’?
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As earth warmed during warming 2000 year warming period
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the surface waters also warmed
polar ice caps melted (surface waters less salty)
northern Atlantic surface water less dense
no vigorous mixing, interruption in thermohaline circulation
global conveyor belt turned off
no heat transfer to northern Europe
ice sheets, no forests grow
Another example of when the global conveyor belt is turned off:
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We also have records of a prolonged period of cold in northern
Europe from 1650-1850
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known as the “Little Ice Age”
could have been caused by an interruption or slow-down in
thermohaline circulation, conveyor belt slowed down (sluggish)