Antarctica

Antarctica is a vast island (Figure 16.4), with an area roughly the same as that of South

Figure 16.4 Places on Antarctica and isohyets showing the annual precipitation in terms of the water equivalent, i.e. mm/ a. The numbered places refer to the following: 1. Filchner ice shelf, 2. the highest point on the plateau (3,570 m), 3. Vostok, 4. Byrd, 5. Dome C, 6. Dumont d'Urville, 7. Mt Erebus, 8. Lake Vanda and 9. McMurdo Sound.

Figure 16.4 Places on Antarctica and isohyets showing the annual precipitation in terms of the water equivalent, i.e. mm/ a. The numbered places refer to the following: 1. Filchner ice shelf, 2. the highest point on the plateau (3,570 m), 3. Vostok, 4. Byrd, 5. Dome C, 6. Dumont d'Urville, 7. Mt Erebus, 8. Lake Vanda and 9. McMurdo Sound.

America (south of the equator), twice that of Australia (Note 1.A) and three times the area of permanent ice in the Arctic. It is the world's highest continent, with an average elevation of 2,450 m. There is an eastern part with a coast at about 68°S and containing most of the high land, and a much smaller western part, only 850 m high on average, with a coast at about 75°S, facing the south-east Pacific ocean. So the continent is not symmetric about the Pole. The parts might be thought separated by a line through the Pole, between two broad inlets, the Weddell Sea and the Ross Sea. Each inlet is covered by an immense shelf of partly floating ice, hundreds of metres thick. The larger is the Ross ice shelf (Figure 16.4), whose area is about 530,000 km2 (twice the size of

New Zealand) and thickness 700-250 m (thinner at the seaward edge).

There is a steep coast around the continent, rising to an interior plateau at 3,000-3,500 m above sea-level. Most of the area above sea-level is ice, which is up to 4,500 m thick, so that bedrock is below sea-level in some places. At other places, there are mountains rising to a peak 5,140 m above sea-level, including the active volcano Mt Erebus (at 3,743 m—Figure 16.4). The point furthest from the sea (the so-called 'Pole of Inaccessibility') is Vostok, at 78°S and 3,450 m elevation.

Antarctica carries 90 per cent of the world's glacial ice, and melting of it all would raise the world's sea-level by about 70 m. Glaciers bear the ice slowly towards ice shelves or directly to the ocean, where they crumble off as icebergs (Figure 11.7). The Lambert glacier on the edge of East Antarctica (at 70°E) is 60 km wide and 400 km long, the world's largest.

Sea-ice a few metres thick surrounds the continent (Figure 11.7). It covers an area which fluctuates during the year (Section 11.2), and it is the retreat in late summer, which allows ships to reach McMurdo Sound in the Ross Sea. The sea-ice acts as a giant insulator, blocking heat from the ocean. If it were absent, temperatures in Antarctica would be about 10 K higher.

Winds

The cold (i.e. heavy) air over the Antarctic plateau creates a high pressure at the surface, which leads to cold diffluent winds down to the sea as katabatic winds (Section 14.3). The divergence leads to slow subsidence of the polar atmosphere, as part of the polar cell (Section 12.3). The Coriolis effect is particularly strong at high latitudes, so geostrophic winds flow as southeasterlies from the anticyclone which prevails over Antarctica (Figure 12.7).

The katabatic winds are shallow and strong, especially at the steep coast (Figure 16.5). There

Figure 16.5 Isotherms of annual mean temperature (°C) in Antarctica, and the directions of katabatic winds.

is an annual average of 11.5 m/s at Casey, and winds at Dumont d'Urville (at 67°S, 140°E) are typically 11 m/s, but averaged 29 m/s over March 1951, for instance, and peaked at 89 m/s over two minutes. The winds are usually strongest in winter, but rarely extend beyond 10 km from the shore. Such katabatic winds tend to start and end suddenly, which is characteristic of density currents (Note 14.D). When the winds exceed some threshold, they abruptly become gusty, picking up loose snow and giving rise to blizzards.

The surface wind is much less on the inland plateau. For instance, the average wind speed at the South Pole is 5.3 m/s. There is most movement in summer when the ground inversion is weakest, and so there is more coupling with winds aloft.

Frontal disturbances and deep depressions prevail over the seas around Antarctica. These are followed by cold-air outbreaks which pull air northward from the plateau, forming small polar lows (Section 13-4), mainly at the edge of the sea-ice. These lows occasionally cause winds reaching the strength of those in tropical cyclones, though they are much more shallow.

Temperatures

Average temperatures in Antarctica are around 3 K lower than at the same latitude in the northern hemisphere, because of the altitude (for inland locations) and the insulating effect of the sea-ice. (The warm Gulf Stream in the Atlantic penetrates the Arctic to 80°N, and the large land masses surrounding the North pole warm up considerably in summer.) Figure 16.6 implies a cooling by about 12 K per km of ascent, and more recent measurements give a value of 13 K/km. These values are far greater than the average 4 K/km found elsewhere (Figure 3.5); the difference is due to extra distance from the sea and the strong ground inversion which affects all surface temperatures in Antarctica

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temperature

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100 500 1000 1500 2000

distance from coast: km

100 500 1000 1500 2000

distance from coast: km

Figure 16.6 The connection between annual mean temperature, elevation and distance inland in Antarctica, around 100°E longitude.

(Section 7.6). The inversion means that measurements only 10 m above screen height are often 4 K warmer, and screen measurements are greatly affected by even slight vertical winds bringing warmer air, which sometimes leads to rapid fluctuations of temperature during the day. It may be noted that a temperature gradient of 12 K/km exceeds the dry adiabatic lapse rate of 10 K/km (Section 7.2), so that air descending 1 km is 2 K colder than the atmosphere around, creating a positive feedback which amplifies the katabatic wind.

Daily mean temperatures at the South Pole show a 'summer' of up to -25°C, lasting only from mid-December to mid-January, and a 'winter' of almost constant -60°C, from April to September. There is no clear minimum, i.e. the winter is described as 'coreless'. These temperatures are much lower than the annual range from zero to -35°C at the North Pole, where heat comes from the sea beneath the ice, and from the more frequent south-north winds. Even lower temperatures are encountered at Vostok (Figures 16.4 and 16.6), where the world record minimum was measured (Section 3.2).

Low temperatures mean that the air holds little moisture (Section 6.4). For instance, there is only 0.1-0.4 mm of precipitable water (Note

6.B) on the plateau, and the vapour pressure at the surface is below 1 hPa. As a result, skies over the Antarctic Plateau are usually clear and cloudless, making the area attractive to astronomers (Note 16.C). The clear skies and lack of water vapour also allow unimpeded loss of terrestrial radiation (Table 2.6), which is the explanation for the extremely low temperatures and a negative net-radiation balance (Table 5.2, Figure 5.5). By the same token, the long daylight hours in summer lead to significant warming, notwithstanding a high albedo, which accounts for the large annual range of temperature.

The Earth is nearer the Sun at year's end (during the Antarctic summer) than at mid-year, so that the maximum extra-terrestrial radiation reaching the South Pole is 7 per cent more than that reaching the North (Section 2.2). The clear skies and the elevation of the South Pole mean that sunshine (including its UV component) is almost unattenuated, shining all day for months.

Precipitation

Cold air and clear skies make it the driest continent on Earth. Figure 16.4 shows that the water-equivalent of the annual accumulation of snow is about 400 mm/a around most of the coast, but less than 50 mm/a over the majority of eastern Antarctica. The overall average is about 160 mm/a (Section 10.5).

There are a few so-called dry valleys near sea-level, notably Wright Valley on the east side of McMurdo Sound (Figure 16.4). It is rocky and ice-free because of sublimation into katabatic winds, which often exceed 28 m/s. After descending 3,000 m or so from the central plateau, the air is relatively warm and so dry that the relative humidity is below 10 per cent (Note 7.E). One of the dry valleys contains Lake Vanda (Figure 16.4), which lies under a permanent layer of clear ice 4 m thick. Most of the lake's water is very salty, so that, whenever ice around the lake melts, the fresh meltwater (which has a lower density) floats on top, forming a solar pond (Note 7.I). As a result, the salty water in the lake can reach temperatures above +25°C, heated by sunshine through the layers of ice and meltwater.

Precipitation tends to be most in summer, but falls are too light to allow the visual discernment of annual layering of ice which is possible where snowfalls exceed about 500 mm/a.

About two-thirds of any depletion of Antarctic ice is by calving of icebergs from the ice shelves and (to a minor extent) from glaciers. Because the ice shelves are flat, most icebergs in the southern hemisphere have a table-like top, unlike those in the northern hemisphere. The volume of calving icebergs totals 1,300 km per annum, which is enough to provide water for the entire world population at the current average consumption of 600 litres per head daily.

Apart from icebergs, roughly 6 per cent of the total loss of ice from Antarctica occurs as sublimation into the dry air, and another 13 per cent melts along the coast. The remaining 14 per cent, is blown out to sea as snow in a surface layer a few metres deep during katabatic wind storms, making visibility very poor.

In summary, strong coastal winds and ground inversions are among the main features of Antarctic climates. Others are aridity and extremely low temperatures.

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