1. The Oceans in Motion
Subsurface Currents

2. Subsurface Currents Mechanics Deep water formation
The Importance of the Global Conveyer Belt

3. Mechanics subsurface currents = thermohaline circulation
90% ocean water moved by subsurface currents
Density driven currents
Gravity is the driving force
Heavier, denser H2O sinks due to gravity
density of H2O
cold vs. warm ‘thermo’
fresh vs. salty ‘haline’
dense H2O sinks and less dense H2O rises
(convection on a global scale)
overturn and mixing of the water column, creates an
unstable H2O column

4. Mechanics (cont’d) What is a stable water column?
See Fig. 7.4 (Intro 7e) or 7.2a (Fund. 4e)
Salinity increases with depth, temperature decreases with depth
A stable water column is layered or stratified, like a three layered cake

5. Mechanics (cont’d)
Now let’s look at an unstable water column
Salinity uniform with depth, temperature uniform with depth
An unstable water column is not stratified, it is well mixed
Dense water continually sinking

6. Mechanics (cont’d) Where is coldest surface water?
at the poles
Where is saltiest surface water?
At the poles, the water column is unstable and is well mixed because of sinking cold and salty water
NOAA

7. Deep Water Formation Where is deep water formed?
South Pole off coast of Antarctica
Antarctic Bottom Water (AABW)
1°C, 34.7 ppt
densest water in the ocean
North Pole off coast of Greenland
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)

8. Deep Water Formation
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
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
Starts all over again called the “Great Global Conveyor Belt”
this is how H2O circulates throughout the world’s oceans
One turn of the global conveyor belt takes between yrs
Global Conveyor Belt
American Museum of Natural History

9. 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
There is vigorous mixing at the poles (water column is
unstable)
dense surface water is sinking at the poles (very cold
surface waters with polar ice caps)
thermohaline circulation is initiated
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)
oceans acting as a ‘heat pump’ to warm the land masses  

10. The Importance of the Global Conveyer Belt (cont’d)
OFF  
As it has been in the past- the global conveyor belt is turned OFF
There is no vigorous mixing at the poles (water column is stable)
there is no dense water sinking at the poles
(surface waters warmed, polar ice caps melt)
thermohaline circulation is not initiated
the global conveyor belt ‘switch’ is turned off
There is no heat pump to warm the land masses
much colder in northern Europe

11. 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
right now we’re in an ice age- we have been for the past 2 million years
last glacial stage ended 11,000 ybp
after last glacial stage ended there was a 2000 year warming period
interrupted by a 700 year return to glacial stage conditions
how do we know? pollen records
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”

12. The Importance of the Global Conveyer Belt (cont’d)
What do we think happened to cause the ‘Younger Dryas’?
As earth warmed during warming 2000 year warming period
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:
We also have records of a prolonged period of cold in northern
Europe from
known as the “Little Ice Age”
could have been caused by an interruption or slow-down in
thermohaline circulation, conveyor belt slowed down (sluggish)