The climate in the Antarctic

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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

Ant-Concordia-Science-AlexSALAM-2009-IPEV (4)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

Ant-DumontDUrville-Paysage-CyrilGALLUT-2014-IPEV-1-8

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

OLYMPUS DIGITAL CAMERAEvery winter, the ocean around the Antarctic freezes. Sea-water’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 sea-ice 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

Ant-Astrolabe-Paysage-EduardoDAFORNO-2014-IPEV-117From 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.