Spatial and temporal variability of ground surface temperature and active layer thickness at the margin of maritime Antarctica, Signy Island

A CALM grid with a data logger system to monitor the active layer thermal regime was established on Signy Island (60°43′S, 45°38′Wat 80 m a.s.l.) in December 2005. The active layer at each of the 36 nodes of the grid was monitored measuring the ground temperature at least at 4 different depths between 0.02 and 0.4 m at the end of the summer season. In addition, within the grid, we selected four sites closely spaced (in a ray of 25 m) three of which with the same topographical characteristics (north facing aspect) but different vegetation coverage (one bare ground, BG1 and two sites with different vegetation: Andreaea sp. and Sanionia uncinata) and the fourth (BG2) it is as BG1 a bare ground but with south facing aspect. In particular, 4 thermistors were located at depths of 0.02, 0.3, 0.6, and 0.9 m at BG2 and at the Andreaea sp site, 9 thermistors at 0.02, 0.3, 0.6, 1, 1.2, 1.4, 1.6, 2, and 2.5 mat BG1 and at 0.02 and 0.6 mof depth at Sanionia site. Generally, with the same aspect, a thick vegetation cover (as in Sanionia site) provides a greater insulative effect than a thinner vegetation cover (as in Andreaea site) or bare ground (BG1) because vegetation both shades and insulates the ground resulting in a reduction in summer heat flux. Ground Surface Temperature (GST) was colder and more buffered in spring and summer under the vegetated ground than in BG1, although the coldest GST and lowest Thawing Degree Days (TDD) were recorded at BG2 and related to its southern aspect. Our data confirm that air temperature is the main driver of GST, as already reported both in the Arctic and Antarctic. We also found that the effect of air temperature changes seasonally, being drastically reduced in winter and, to a lesser extent, in fall and spring, when there is generally thin snow cover (b30 cm). During the summer, when snow cover is usually absent, the air temperature is the dominant driver, although incoming radiation also had an effect on the northern exposed bare ground and to a lesser extent on the vegetated and southerly exposed bare ground. The active layer ranges between 81 and 185 cm on the 4 continuously monitored sites and, considering the sites with the same aspect, it is thicker under bare ground (between 10% up to more than 100%) than under vegetated ground, confirming previous observations in the Arctic and Antarctic. However at our sites, climate forcing has no effect on the active layer thickness, enhancing the role of soil properties including the periods of high moisture content and lateral flow of water. The lack of a statistically significant regressions between GST and active layer thickness could be due to the limited study period (four years) and/or to the variationwith timeof changes in soil characteristics such as soilmoisture, and the possible occurrence of non-conductive heat transfer processes including the lateral flow of water. Further data are required to understand the role of moisture and possible ground water circulation within the active layer to explain the unexpected strong dichotomy between the GST regime and active layer thickness.

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