Ice-wedge temperature trends in East Antarctica: a thermal puzzle?

A monitoring programme on ice-wedge thermal regime has been undertaken from 2004 to 2014 at three sites in northern Victoria Land: Baker Rocks (74°12’27” S; 164°50’01” E; 11 m a.s.l.), Boomerang Glacier (74°30’13” S; 163°50’09” E; 874 m a.s.l.) and Mount Jackman (72°23’07” S; 163°10’49” E; 1326 m a.s.l.). Four sensors took hourly temperature of the air, the ground surface, the ice-wedge top and bottom. In the recording period the mean monthly ground surface temperature (recorded at 2 cm) (GST) ranged between -37.7°C (July 2004) and 9°C (January 2015) at Baker Rocks, -34.4°C (July 2004) and -8.9°C (January 2015) at Boomerang Glacier and -38°C (July 2010) and -0.7°C (December 2012) at Mount Jackman respectively. In the same period the mean monthly air temperature ranged between -34.4°C (July 2004) and 1.3°C (January 2007) at Baker Rocks, -33.5°C (July 2004) and -3.1°C (January 2013) at Boomerang Glacier and -34.7 (July 2010)°C and -7.7°C (January 2014) at Mount Jackman, respectively. Air temperature is always higher than GST in winter, indicating the absence of a significant snow cover. On the other hand, during summer the air temperature is lower than the GST. As for the mean monthly temperature of the ice-wedge top (recorded at -50 cm) ranges between -27.6°C and 0°C at Baker Rocks, between -29.9°C and -4.1°C at Boomerang Glacier and between -36.1°C and -1.5°C at Mount Jackman, respectively. At the ice-wedge bottom (at -80cm), the monthly average temperature is between -26.4°C and -2.6°C at Baker Rocks, between -30°C and -4.6°C at Boomerang Glacier and between -35.1°C and -3.2°C at Mount Jackman, respectively. The thermal offset measured at the top of ice wedges is small, ranging between -2.0 °C and -0.5 °C at Baker Rocks, -0.5 °C and -0.1 °C at Boomerang Glacier and -0.6 °C and -0.1 °C at Mount Jackman, respectively. The active layer has been computed daily for each of the three sites, according to Gold and Lachenbruch (1973). The monthly average thickness of the active layer ranges between 0.17 m and 0.61 m at Baker Rocks, between 0.10 m and 0.71 m at Boomerang Glacier and between 0.01 m and 0.42 m at Mount Jackman. In general the active layer decreases in thickness with increasing altitude. This is confirmed when the active layer computed at Baker Rocks is compared to the one at Mount Jackman. However, this is not the case for the Boomerang Glacier site, where the active layer seems generally thicker that Baker Rocks, despite the higher altitude. Interestingly, the active layer shows a stochastic trend in all sites, indicating a clear non stationary pattern. As for the temperatures, they have strong seasonal unit roots at the daily, weekly and monthly, and quarterly frequencies. Yet, they do not show a significant stochastic trend. To better investigate and understand the reasons for the different pattern of active layer, which has a significant trend, and the stationary GST and air temperature, we compute the thermal diffusivity of the ground at the daily level, using the Carlsaw and Jaeger (1959) formula related to the amplitude attenuation with depth. Monthly mean thermal diffusivity values range between 1.68 x 10-7m2/sec and 3.44 x 10-7 m2/sec at Baker Rocks, between 0.85 x 10-7m2/sec and 18.03 x 10-7m2/sec at Mount Jackman and between 2.07 x 10-7m2/sec and 8.60 x 10-7m2/sec at Boomerang Glacier. They clearly show a seasonal pattern, being in general higher in winter and lower in summer time. This could indicate the presence of higher moisture in summer than in winter, which is less conductive and therefore decreases also the thermal diffusivity. In this work we use a time series approach to model the variability of the observed temperature and active layer series, which is quite new in this field and it represents therefore the main contribution to the existing literature.