Australia: The Land Where Time Began |
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Global
Thermohaline Circulation The thermohaline circulation is the circulation of
the waters of the entire global ocean, that is a slow, churning current
that is driven by variations in density caused by differences of
temperature and salinity, rather than by wind as are most ocean
currents. Located mainly in polar oceans, it involves surface water
gaining salinity from the formation of ice, and during the process of
freezing the salt is driven out of the ice and back to the sea. A slow
flow of water is thus sucked in towards the poles from the tropics,
which carries heat and salt. The shape of the continents and the
Coriolis force associated
with the rotation of the Earth and the shape of the continents modifies
the flow. The surface flow is a slow flow which mimics in
some respects circulation that is wind-driven, though in this case the
circulation would continue in the absence of wind. In the northern
Indian Ocean and the North Pacific there are 2 broad areas where
upwelling takes place. In these 2 areas deep water is brought to the
surface and then slowly flows to the south and west. The Pacific water
flows through the East Indies and into the Indian Ocean water which it
joins to flow around the Cape of Good Hope then northwards to the
tropical Atlantic. It gathers more water in the tropical Atlantic from
the Gulf of Mexico then moves to the northeast across the North
Atlantic, just as the Gulf Stream and North Atlantic Current do. The
flow continues to the north towards the Arctic Ocean but then vanishes.
Somewhere the water sinks. A slow flow of deep water can be detected
moving southwards through the North Atlantic and South Atlantic, and
when it reaches the Indian Ocean and the Pacific Ocean it completes a
single full journey along a great conveyer belt after about 1,000 years.
It is called the Great Ocean Conveyor Belt, though the flow is much more
complex than this, as it is broken up into a series of cells. Overall it
is an inexorable slow flow of water that is driven by the very
fundamental driving forces of heating, cooling, evaporation and the
rotation of the Earth. The cogs driving the Great Ocean Conveyor are the
forces of upwelling which brings deep water to the surface in the Indian
Ocean and Pacific Ocean, and the downwelling forces causing downwelling
of surface water. When considering the case of changes taking place in
the polar regions these broad areas of upwelling are ignored while the
driving forces of the more concentrated sinking are considered which
occurs in the northern Atlantic. The question to be answered is where
and why does it happen. It has been found that it happens only in 2
locations that are surprisingly small in extent (Marshall & Schott,
1999). One is in the centre of the Labrador Sea, where the cold winter
winds blowing off Labrador and Greenland chill the surface water. The
density of the cooling water increases through the winter and eventually
gains enough density to sink to great depths. The volume of the water
that is sinking crucially depends on the air temperature during winter,
and varies greatly from year to year. Wadhams says the other area of
downwelling is more interesting as the process involves sea ice. This is
a tiny area in the Greenland Sea, located at 75oN 0oW,
and changes at this critical site affect the entire world.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |