Observed trends in surface freezing/thawing index over the period 1987–2005 in Mongolia

Mongolia is located in central Asia at the latitudes between 41°35′N and 52°09′N and at the longitudes between 87°44′E and 119°56′E, and occupies a territory of about 1.56 million km² in area. The Mongolian relief is characterized by relatively high elevation, which is 1580 m above the sea level on average in this landlocked country. Owing to the continental cold and dry climate with severe winters in Mongolia, approximately 67% of the country is underlain by permafrost (Tumurbaatar and Mijiddorj, 2006). During the last 60 years, the mean annual air temperature in Mongolia has significantly increased by 1.66 °C (Nandintsetseg et al., 2007) and the warming trend was predicted to continue in the following 80 years (Sato and Kimura, 2006). In the context of great warming occurring in the last decades, the noticeable increase in permafrost temperatures and active layer thickness has been observed in Mongolia (Sharkhuu et al., 2007, Sharkhuu et al., 2008a, Zhao et al., 2010). The prominent permafrost degradation has exerted profound influence on the stream hydrology, taiga forest distribution, ecosystems, and sustainable development in Mongolia (Bohannon, 2008, Dulamsuren et al., 2010).

The annual freezing and thawing indices of the ground surface are one of the most important parameters to assess the permafrost and seasonally frozen ground distribution and to estimate the seasonal freeze and thaw depth in cold regions (Anisimov et al., 2007, Frauenfeld et al., 2004, Nelson and Outcalt, 1987, Shiklomanov and Nelson, 2002, Zhang et al., 2005), applied extensively in engineering designs in cold regions (Lunardini, 1981), and considered as a useful indicator of climate change as well (Frauenfeld et al., 2007). Freezing index reflects combined magnitude and duration of air or surface temperatures below freezing during given cold season and is one of the major parameters used to assess the ground-freezing potential of a given climate. The most important applications of freezing index are to determine the seasonal ground frost depth, to estimate the effect of freezing conditions on the soil foundations of structures, and to provide implications for design of road pavement and maintenance operations of infrastructures in cold regions (Schmidlin and Dethier, 1985, Steurer, 1996). Similarly, a close relationship between thawing index and active layer thickness has been demonstrated in many studies (Brown et al., 2000, Frauenfeld et al., 2004, Hinkel and Nelson, 2003, Nelson et al., 1998, Romanovsky and Osterkamp, 1997). In Geocryology, the square root of surface annual thawing index is used to directly determine active layer thickness in Stefan equation (Nelson and Outcalt, 1983, Nelson and Outcalt, 1987, Zhang et al., 2005). An important parameter, n-factor, was introduced to reflect the relationship between the air temperature and the ground surface temperature in cold regions (Lunardini, 1978). The surface freezing and thawing indices are essential variables involved in the calculation of the n-factor.

There are two kinds of freezing and thawing indices in terminology of climatology; air freezing/thawing index and surface freezing/thawing index. Generally, the air and surface freezing/thawing indices are defined as the cumulative number of degree-days below/above 0 °C for air temperature and ground surface temperature respectively during a given time period. There are a lot of studies to introduce the freezing and thawing indices of air temperature (Frauenfeld et al., 2007, Hanson et al., 2010, Jiang et al., 2008, Schmidlin and Dethier, 1985, Steurer, 1996, Wu et al., 2008). The widely applied air freezing/thawing index during the past decades was described by Frauenfeld et al. (2007), while at present there are relatively few literature demonstrating the surface freezing and thawing indices because of unavailability of consecutive instrumental records of long-term ground surface temperature time series.

In this paper we document the variations of surface freezing/thawing index in Mongolia, on the basis of analysis of continuous instrumental records of monthly ground surface temperature series at 20 meteorological stations relatively evenly distributed in this country from 1987 to 2006. The 3-year observation of ground surface temperature acquired by infrared sensor and thermistor sensor at the depth of 5 cm at a representative automatic weather station is used to assess the validity of using monthly ground surface temperature data in the calculation of the approximate annual surface freezing/thawing index. The estimation of variations in the annual freezing/thawing index would be counted on to enhance our understanding of climate change and variations in thermal regime of ground surface near the southern boundary of Siberian permafrost regions.