The climate of Antarctica embraces the most extreme cold conditions found on Earth. Antarctica is cold because the solar radiation is only 16% of that at equatorial regions, and also because of the high average surface elevation of the ice sheet, which in places exceeds 4,000 m. Temperatures as low as -89°C have been recorded at Vostok (Fig. 2.1), and -49°C at the South Pole. However, mean annual air temperatures increase nearer the coast where land is exposed, and in the northernmost areas (-25°C at Mt. Fleming at the head of Wright Dry Valley near the edge of the Polar Plateau, -20°C at Vanda Station in the Dry Valleys, -18°C at McMurdo Station on Ross Island, -15°C at Hallett Station). Further north, in coastal areas of East Antarctica, warmer climates are found (MacNamara 1973; Burton and Campbell 1980). At Davis Station in the Vestfold Hills, mean annual temperature is -10.2°C, while at Molodezhnaya and Casey (Fig. 2.1) similar temperatures to those at Davis Station are experienced.
Air temperatures directly influence permafrost properties, with the active layer thickness decreasing from around 80 to 100 cm in the warmer coastal and northern regions to 2 cm or less in the cold inland high-elevation sites (Fig. 2.2) following the
Fig. 2.2 Hourly temperature records from Marble Point (solid line; 70 m above sea level (asl), measurement at 7.5 cm) and Mount Fleming (dashed line; 2,000 m asl, measurement at 2 cm) from December 4 2002 to February 12 2003. The records illustrate the large difference that site climate has on soil thermal properties
Fig. 2.2 Hourly temperature records from Marble Point (solid line; 70 m above sea level (asl), measurement at 7.5 cm) and Mount Fleming (dashed line; 2,000 m asl, measurement at 2 cm) from December 4 2002 to February 12 2003. The records illustrate the large difference that site climate has on soil thermal properties adiabatic lapse rate (Campbell and Claridge 2006). Other soil thermal properties related to geographic differences in climate include the length of the thaw period, the number of thaw days during summer, the number of freeze/thaw cycles that occur and the length of time that the soil may be continuously above freezing. At Marble Point, for example [approximately 70 m above sea level (asl) and permafrost table at 60 cm], the thaw period (measured at 7.5 cm depth) extended over 70 days, there were 34 freeze-thaw cycles and 16 days when the soil temperature was continuously above 0°C (Fig. 2.2). By contrast, at Mt. Fleming (2,000m asl, permafrost table approximately 2 cm) the thaw period, measured at 2 cm depth, extended over 31 days, but with only 6 days in which soil temperature was briefly above 0°C.
The mean annual precipitation over Antarctica averages around 50 mm per year, with least falling inland and most in coastal locations. In the McMurdo Dry Valleys, one of the driest areas of Antarctica, precipitation averaged 13 mm per year on the valley floor near Lake Vanda and 100 mm per year in nearby upland mountains. Around the periphery of East Antarctica, precipitation is much higher, with 650 mm per year at Molodezhnaya in Enderby Land (MacNamara 1973). The precipitation normally falls as snow, and little is available for direct soil moistening because of ablation and evaporation. Despite the minimal amounts of soil moistening, distinct soil climate zones, based on moisture availability, have been recognized (ultraxerous, xerous, xerous to subxerous, oceanic subxerous and moist zones; Campbell and Claridge 1969). Soils of the ultraxerous zone are found in arid inland areas, rarely if ever have liquid water present, and have ground temperatures that are seldom above freezing point. At the other extreme, moist soils in coastal environments may be moistened at the soil surface, and ground temperatures remain above freezing point for periods throughout the year.
In Antarctica, the soil climate and permafrost properties are strongly influenced by the surface radiation balance, since the soil thermal regime is consequent upon the gains and losses of radiation from the soil surface. Surface radiation balance investigations for soils at several sites were reported by Balks et al. (1995), MacCulloch (1996) and Campbell et al. (1997), who found that soils with dark-coloured surfaces had low albedo values (approximately 5% at Scott Base) while soils with light-coloured surfaces had much higher albedo values (26% at Northwind Valley). Differences such as these, when coupled with available soil moisture, translate into appreciable differences in the diurnal soil thermal regime and permafrost characteristics. At Bull Pass in Wright Valley, for example, a soil surface with approximately 50% dark-coloured clasts had summer soil temperatures (measured at 2 cm) up to 5°C higher (max 17°C) than in adjacent soil with a light-coloured surface, while the mean annual soil temperature at that depth was 0.25°C greater than for the light coloured soil.
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