Using the data from 14 stations, we analyzed trends in the freeze depth and freeze period (including first date, last date, and duration) of frozen soil and examined their relationships to air temperature, thawing index, snow depth, and precipitation, as well as each other.
Line trends were also used to detect trends in other climate variables, including air temperature, thawing index, maximum snow depth, annual precipitation, spring (March, April, and May) precipitation, summer (June, July, and August) precipitation, and autumn (September, October, and November) precipitation at the same locations.
In this study, to explore the potential causes for the observed trends in freeze depth and period since 1960, the averaged time series were related to mean annual air temperature, thawing index, maximum snow depth, and annual, spring, summer, and autumn precipitation.
As shown in Table 5 and Figure 5, from 1960 to 2014, the freeze depth was significantly negatively correlated with air temperature, thawing index, freeze first date, and precipitation, but not with snow depth.
The effects of air temperature, thawing index, and soil moisture indicate that future changes in air temperature and precipitation may have a major influence on freeze depth.
According to Table 5 and Figure 6, the freeze first date was positively correlated with air temperature, thawing index, annual precipitation, and autumn precipitation.
The freeze last date was negatively correlated with air temperature, thawing index, and precipitation and positively associated with freeze depth (Table 5 and Figure 7).
During 1960-2014, the air temperature, thawing index, freeze depth, and annual precipitation had a clear and significant effect on freeze duration, with negative correlations in air temperature, thawing index, and annual precipitation and positive correlations in freeze depth (Table 5 and Figure 8).
The freeze depth and period were strongly affected by air temperature, thawing index, and soil moisture (precipitation), but not by snow.
Caption: Figure 5: Time series of annual air temperature (a), thawing index (b), annual precipitation (c), and freeze first date (d) in TRSR (black line) with the time series of freeze depth (grey line).