Geography Reference
In-Depth Information
sinks and then flows south forming the main deep water current of
the whole Atlantic. There have been concerns (most clearly exag-
gerated in the Hollywood movie
The Day After Tomorrow
) that ice
melt in the North Atlantic region caused by global warming might
supply lots of fresh water which would reduce the salinity of the
Gulf Stream water so that it is not dense enough to sink. This would
then cause the whole thermohaline circulation system to cease oper-
ating, or severely weaken, thereby reducing heat transfer from the
equator resulting in much colder climate conditions at higher lati-
tudes. For example, the current mild climate of north-west Europe
could resemble the much colder climate of north-east Canada (e.g.
Labrador). This would lead to further feedbacks as reflection of the
Sun's energy by enhanced snow and ice cover in Europe would
cool the climate further. There is some evidence that this slowing of
the thermohaline circulation has occurred in the past and this is dis-
cussed in Chapter 2.
Sinking water has to be balanced by rising water coming to the
surface. Deep water upwelling occurs thousands of kilometres away
from the sinking zones. Along several eastern edges of continents
where surface water is driven offshore by winds this surface water
is replaced by deep water from below. This upwelled water is often
rich in nutrients that have fallen to the depths of the ocean. When
these nutrients reach the surface at upwelling zones, and where
there is sufficient light, the nutrients can be utilised by plankton
which in turn can maintain rich fisheries. One such example of a
rich upwelling zone is off the coast of Peru.
INTERANNUAL CLIMATE VARIABILITY
While there are rich fisheries off the coast of Peru because of an
important upwelling zone, these fisheries often collapse because of
a specific event which directly demonstrates the feedbacks between
the oceans and the atmosphere in controlling the climate. This
event is known as the
El Niño Southern Oscillation
and this
seems to occur about once every five years (but sometimes after
three to seven years).
The Southern Oscillation is characterised by an exchange of air
between the south-east Pacific (high pressure) and the Indonesian
equatorial region (low pressure). Most of the time the trade winds
