Part III: Antarctica and Academe - Scott Polar Research Institute

Antarctic Convergence). This is a front that marks the point on the surface of the
ocean at which the cold Antarctic surface water flowing northward meets warmer
sub-Antarctic water flowing southwards from the Atlantic, Pacific and Indian
Oceans. Even in summer the polar surface water temperatures are close to freezing.
Although this front is mobile, with eddies and loops that span a zone as wide as l50
km, it has a fairly constant mean position from year to year at an average latitude of
about 50°S. The seas south of the Polar Front extend over 35 million km 2 and
comprise about a tenth of the world ocean. The cold air and the salt expressed when
salt water freezes to form sea ice means that this ocean contains the coldest and
densest sea water on earth, the Antarctic Bottom Water. As it sinks it flows over the
ocean floors and depresses the temperature of at least 55-60% of the total volume of
all the oceans to less than 2°C. This is only one of the ways in which it affects the
climate of the Earth, counterbalancing the effect of the tropics. Cold Antarctic water
is also well-oxygenated and aerates the world ocean.
The Southern Ocean is not contained within the circle formed by the Antarctic Polar
Front, but extends further north than this, although by common consent among
oceanographers its northern boundary remains undefined. The term Antarctic Ocean
has been used for the waters south of the Antarctic Polar Front, which is bounded to
the south by the Antarctic Continent. Some 45% of the coastline of that continent is in
the form of floating ice shelves some 200 m thick. The atmospheric circulation
influences the ocean in two main ways. As the katabatic winds spread out from the
icy dome of Antarctica they impose temperature and salinity alterations on the ocean
near the continent, inducing upwelling of warmer and saltier sub-surface water as
the colder, less salty and therefore lighter surface water is blown out to sea. A few
hundred kilometres further north at the zone where the more southern easterly
winds, which maintain the East Wind Drift, and the westerly winds which maintain
the Antarctic Circumpolar Current (or West Wind Drift) meet, there is a complicated
shear zone. This the Antarctic Divergence, is also another zone of upwelling where
the cold, relatively fresh, surface water is replaced continually by deeper water that is
2-3°C warmer and more saline; here the surface water diverges, two-thirds flowing
northwards and one third southwards. Upwelling is an important process in the
ocean because it brings nutrients to the surface, where the upwelled water resides for
two years on average, while its nutrients are taken up by the phytoplankton. The
total vertical upwelling in this divergence zone is estimated at 45 million m 3 per
second (astonishingly a third of the total horizontal transport in the Antarctic
Circumpolar Current!) and it is clearly a very major phenomenon in the world ocean.
In fact, everything in the Antarctic seems to be on a grand scale.
The so-called "Roaring Forties and "Furious Fifties" result from the dynamic effects
between the atmospheric high pressure cells characteristic of the "thirties" latitudes
and the low pressure cells near the coast of Antarctica. This causes strong winds from
the west, with a strength of 40 knots or more, that push the sea surface into waves up
to 20 m high and drive the Antarctic Circumpolar Current. It is by far the windiest
and has the highest waves of all the oceans. Computer models suggest a greater
velocity for this current than it actually has, because it is braked by the extensive
north-south system of ridges in the ocean floor and to the east of New Zealand and
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