The climate in the Antarctic
Several factors, like high altitude, low solar
radiation, the Antarctic Circumpolar Current and
reflectivity of the ice (80% of light radiation is
reflected back into the atmosphere), combine to make
this the coldest of all Earth’s continents.
Here you will find information on the climate but also geology, ice and the
Southern Ocean …
Temperature
Temperatures are relatively “mild” on
the coast: -10°C, on average the
seasonal extreme varying from 0 to
-30°C.
However,
they
plummet
dramatically the further the distance
towards the heart of the continental
ice sheet. The annual average is
between -20°C at 1000 m altitude and
-55°C at the Russian research station
Vostok (3500 m): -30°C for the
warmest 2 months (January-February)
and an average of -60°C the rest of
the year with lowest point at –89.3°C, the lowest temperature ever measured
on the Earth’s surface (at Vostok). At this temperature, any person dressed
in normal clothing would die of cold in less than a minute.
Winds
The average wind speed is quite moderate in the central areas (10 to 20
km/h) but stronger at the coast (30 to 70 km/h) where winds gusting to
record speeds are possible: 320 km/h has been measured at the Dumont
d’Urville station.
Precipitation
It hardly snows in the Antarctic, seemingly a great paradox. Atmospheric
depressions cannot easily penetrate into the interior of the continent and
most precipitation thus occurs on the coast. The interior of the landmass
is like a desert: over a surface area of 5 million km², annual
precipitation is less than the equivalent of 5 cm of water and often less
than 2 cm! It must have taken Nature a long time to build up the several-
kilometre thick layer that constitutes the ice sheet.
Ice cap and sea-ice
The Antarctic’s surface is 98% covered over with ice. Radar and satellite
studies, combined with field observations, yield useful data for defining
the topography of the ice sheet and determining ice thickness and
monitoring any changes and developments.
The ice sheet: ice cap
The average thickness of the ice sheet is 1300 m in the West Antarctic and
2200 m in the Eastern part. Its maximum thickness reaches nearly 5000 m.
Its base therefore lies more than 2500 m below sea level. It is fed by snow
falls which are more abundant in the coastal regions. Precipitation in the
interior of the continent is even only half that received by the hottest
deserts. Year after year, the snow accumulates and changes into compact
ice. Under the combined effect of this extra load and gravity, the ice
flows away from the centre of the ice cap towards the continental
periphery. These movements are very weak at the centre of the ice sheet, a
few tens of centimetres per year, then pick up speed, reaching about 100
metres per year, in the voluminous glaciers which, at the coast, drain the
continental ice out to the ocean.
Ice-shelves and icebergs
The ice sheet extends in some areas as immense ice-shelves, spreading and
floating on the Southern Ocean, and whose surface areas combined exceed 1.5
million km². The 3 largest ice-shelves are Amery, Ronne and Ross, with an
area almost as large as France.
As they advance onto the ocean, ice-shelves fragment as blocks that can
attain over 400 m thickness and can form tabular icebergs. Some can even be
larger than Corsica.
Sea-ice
Every winter, the ocean around
the Antarctic freezes. Seawater’s salinity means that
surface freezing begins at
–1.8°C. Ice at the formation
stage is fragile and is
destroyed at every storm.
Subsequently, as the cold
intensifies, the ice in the sea
thickens to turn into the seaice with average thickness 40-60
cm. The maximum extent of the
sea-ice, in September, can reach
20 million km², which doubles
the frozen area of the Southern
Hemisphere.
In the midst of expanses of sea-ice there are persistent or recurrent zones
of open water called polynyas. These are linked to upwelling of warmer
water from the depths. The most extensive is located is the eastern part of
the Weddell Sea, measuring 250 000 km².
In February, nearing the end of the Austral summer, the sea-ice is
completely broken up and most of the coasts are therefore freely
accessible. Which means however that access to the continent is possible
only during 2 to 3 months of summer. And that is uncertain because the
condition of the ice formations depends heavily on the vagaries of the
climate.
The annual rhythm of formation and melting of this immense quantity of ice
bears a strong influence on the global ocean circulation, the heat
exchanges between ocean and the atmosphere and the biology of the oceans of
the Southern Hemisphere.
Southern Ocean
From an oceanographic point
of view, the Antarctic Ocean or Southern Ocean is defined as an ocean
traversed by the Antarctic circumpolar current, which moves from West to
East around the Antarctic continent. Contrary to the other oceans, which
are defined as expanses of water delimited by the continents, the Southern
Ocean is the only one to be defined as a mass of water that surrounds a
continent. This ring of water stretches to south of the 60th parallel South
and over the whole of Earth’s circumference.
History
The name “Southern Ocean” and the boundary of 60°S were adopted in 2000
following a proposal and opinion poll organized by the International
Hydrographic Organization (IHO). Before that, the waters covered by the
Southern Ocean were considered to be southern parts of the Atlantic, Indian
and Pacific Oceans, even if the term already existed. According to this
definition its surface area exceeds 20 million km².
Ocean circulation
The circulation of this ocean is considered to be the main driving force
behind the formation of deep-moving bodies of water in the World ocean,
cooling more than half its volume by about 2°C. Water masses differing in
temperature and salinity circulate from and around the continent, in
equilibrium with the uprising of deep Atlantic water arriving from the
Arctic. The Antarctic convergence is the best natural marker of the upper
boundary of the Southern Ocean: it is a distinct region in the centre of
the Antarctic Circumpolar Current which separates the very cold surface
waters to the south from the warmer subantarctic waters to the north. This
phenomenon effectively isolates the continent even further from the heat
transport and exchange operating in the middle latitudes. Another current
circulates even closer to the Antarctic: the Antarctic Coastal Current,
which circulates from east to west, separated from the circumpolar current
by the Antarctic Divergence.
Bathymetry
Southern Ocean is deep, between 4000 and 5000 m over most of its extent,
with only few shallower areas. The maximum depth is reached at the South
Sandwich trench (60°S, 24°W), at 7235 m. The Antarctic continental shelf is
generally narrow. The part near the coastline coasts is also deep: from 400
to 800 m whereas the global average is only 133 m.
Climate
The sea temperature varies between about –1.8°C and 10°C. The oceanic zone
covering from about 40°S to the Antarctic Circle is subjected to the
strongest winds ever known on Earth. There is no lack of evocative terms
for the conditions encountered when sailing to the Far South: through
regions of the “Roaring forties” or the “Furious fifties“. In winter, the
ocean freezes beyond 65°S in the Pacific sector, and from 55°S in the
Atlantic sector, lowering the surface temperatures to well under 0°C and
forming the sea-ice.
Geology
The Antarctic came into being at least 3.8 billion years ago (Ga), the age
of the most ancient rocks found there. Its growth then continued according
to all the build-up and breakup processes undergone by a succession of
supercontinents: Columbia (1.8 – 1.6 Ga BP), Rodinia (1.3 – 0.93 Ga),
Pannotia (0.68 – 0.55 Ga), then Gondwana (0.45 – 0.25 Ga). The breaking-up
of Gondwana gradually led the Antarctic to become a separate entity,
beginning its drift towards the South Pole around 60 million years (Ma) BP.
Some 30 Ma ago, the Antarctic had become separate and the opening-up of the
Drake passage, south of Cape Horn, generated a circumpolar current which
isolated the continent climatically by blocking the passage of temperate
influences from the other oceans: Atlantic, Indian and Pacific.
It was around 14 Ma BP that the Antarctic became an ice sheet: a continent
topped with a thick cap of ice. Nevertheless at depth the ice masses do
flow, towards the ocean, and those in place today are considered to be no
more than 900 000 years old.
There are two major geological domains in the Antarctic:
The oldest is on the East and consists of continental remnants in the form
of nuclei of rocks called cratons surrounded by belts built up during
continental-drift induced collisions and overlap. Erosion planed down all
the top parts of these configurations. All that now remains are the
deepest folded zones: indeed the “roots” of ancient mountain chains.
The western part, which has been reworked several times (by fusion then
magmatism); hence it is younger. Two present-day mountain chains govern
its morphology:
> The West Antarctic Cordillera, heavily marked by volcanoes and
earthquakes, whose origin is comparable with that observed in the Andes.
> The Transantarctic Mountains generated by a rift (tearing of the
lithosphere) cuts into the continent over 3 000 km and is punctuated with
volcanoes. The best known of these, Mount Erebus, is still active and
continually emits chlorine: possible influence on the ozone layer remains
to be determined.