Ocean Circulation

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Ocean Circulation
Atmospheric circulation is directly tied to ocean
circulation in distributing the worlds energy
budget.
Circulation in the oceans is very different that
that of the atmospheres in several ways.
The atmosphere is heated from below and the ocean
from above where 90 is absorbed in the upper
100m.
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Because warmer surface water is less dense than
cooler water below, this increases stability.
However, because surface heating also increases
evaporation, the water becomes more saline and
thus more dense.
The thermal inertia of the ocean prevents it from
responding directly to changing insolation on a
small scale the way that air is affected.
It is the movement of the atmosphere that forces
the movement of surface waters. The movement of
one fluid (air) against the other (water) results
in friction/drag (known as wind stress) on the
surface that pushes it along as a wind-drift
current.
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The Coriolis effect turns water to the right as
the wind pushes it such that currets are at
20-25 angles from the direction of the wind.
This results in the large semi-circular gyres in
each ocean basin.
Why would water pile up in the middle of these
gyres?
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Real world surface currents approximate the
simple model. There are some further
complications to consider for deeper water
currents.
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The Ekman spiral leads to Ekman transport of
water at a 90 angle to the wind that forces the
motion.
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Upwelling and Downwelling
In areas of convergence, water piles up and the
surface layer thickens causing downwelling. In
areas of divergence, the surface water moves away
and the surface layer thins causing upwelling.
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Western boundary currents tend to be narrow and
fast because of the vorticity effect.
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The Gulf Stream is a great example of just such a
current.
How was it discovered?
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Circulation of the Deep Ocean
Salinity is the driving mechanism for circulation
in the deep ocean.
Salinity is the number of grams of salt dissolved
in one kilogram of water.
Thermohaline circulation- driven by temperature
and salinity.
Vertical structure of the deep ocean
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Latitudial differences
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Bottom-Water Formation- begins with the formation
of cold and/ or salty water (that is relatively
dense) at high latitudes. This is the densest
water in the oceans.
There are two primary ways to form dense bottom
water.
Sea Ice formation- when seawater freezes, the
salt is excluding leaving extra salty water
behind. This is obviously cold and dense and
therefore sinks.
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Antarctic Bottom Water (AABW)- forms in the
Weddell Sea and circles Antarctica eventually
seeping north into all ocean basins finally
reaching 45-50 in about 250 years according to
14C data.
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North Atlantic Deep Water (NADW)- this water has
a more complex formation than AABW. The warm
salty Gulf Stream travels north and cools off.
While jus chilln the water bumps into water from
the Greenland and Norwegian Seas that has formed
sea ice.
This results in formation of about half of all
the deep sea water. It mingles with AABW and then
spreads into the deep basins around the world.
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NADW mingles with AABW and then spreads into the
deep basins around the world.
Flow at 4000m
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Thermohaline Conveyor Belt
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So, what happens if this flow stops?
Predicted change in mean annual temperatures 30
years after shutdown of circulation.
Has it ever happened in the past?
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Yes, it has happened before
Abu Hureyra
Mesopotamia (Sumer)
Ur
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Abu Hureyra stood close to the river floodplain
and in close proximity to picket forests. People
lived on hackberries, wild plums, nuts, and
gazelle meat. The global warming that sustained
this environment was interrupted.
The Laurentide Ice sheet that covered eastern
North America for 100,000 years was rapidly
melting. Enormous volumes of fresh water flowed
into the North Atlantic. Icebergs broke off from
the eastern margins of the ice cap and deposited
as much as 1/2 meter of small rocks (IRDs) from
the Hudson Bay region onto the floor of the
Labrador Sea.
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As this ice sheet melted, Lake Agassiz filled
the Depression left by the retreating ice cap. At
first, the melt water spilled into the
Mississippi watershed and flowed into the Gulf of
Mexico.
12,000 years ago, the shrinking of ice front
opened up a new channel to the east.
Lake Agassiz dropped rapidly as the water shot
across southern Canada into what is now the St.
Lawrence valley.
An enormous surge of fresh water washed into the
already dilute Labrador Sea.
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Laurentide Ice Sheet
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Lake Agassiz boots into the Atlantic
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Within a short time, the downwelling that carried
salt into the deep ocean and and it's movement
southward stopped altogether.
The warm conveyor belt that had nourished global
warming for 3000 years had abruptly shut down.
Far below the surface of the Labrador Sea, Salt
ceased to flow away from the northern ocean.
Global warming stopped perhaps within a few
years. The northern and glaciers advanced again,
plunging the world into a 600 year cold snap.
Then suddenly, Atlantic down welling suddenly
resumed, and the conveyor belt switched on.
10,000 years of warmer conditions began and
continue to this day.
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This event, called the Younger Dryas, brought
severe drought to the Euphrates Valley. Cold
wind, reduced rainfall, and much depleted nut
harvests changed Abu Hureyra very quickly. The
nut-rich forests retreated almost 100 km away
from the village.
The people turned to wild cereal grasses and
drought resistant clovers to supplement the now
meager nut harvests. 1000 years earlier, these
people could have dispersed into smaller groups
and relied on ties with kin to see them through
the lean years.
It could happen again thanks to global warming!