Long-term soil temperature dynamics of the Kunlun Pass permafrost region on the Qinghai-Tibetan Plateau

The thermal dynamics was important for permafrost change processes under climate change. However, little studies were focused on the soil thermal dynamics with long-term observed data in the permafrost region on the Qinghai-Tibetan Plateau (QTP). From 2005 to 2017, we have monitored thermal dynamic of active layer overlying permafrost in the Kunlun Pass (CNO6 site) region of the QTP. Results demonstrated that the number of thaw days is lower than the number of freeze days, and the start dates of thawing and freezing were delayed over this period. Moreover, air and soil temperature were all fastest warming in summer at different depths, then in autumn, except in spring and winter which has a cooling trend at some depths. Accordingly, the mean annual soil temperatures exhibited an evident warming trend at different depths. In addition, thawing degree-days (TDD) for air and soil temperature (at 10 cm) showed an increasing trend, whereas the respective freezing degree-days (FDD) had a decreasing trend. The mean freezing and thawing n factor were 1.43 and 0.50 from 2005 to 2017, and the surface offset of the study site ranged from 2.65 to 3.42 degrees C, which was lower than those in the subarctic and Arctic regions. Meanwhile, there was a linear relationship between the TDDs and active layer thickness, and a power function relationship between the TDDa and active layer thickness. The active layer thickness exhibited a significant increase with the rate of 2.4 cm/year from 2005 to 2017. These results can be used to understand the thermal dynamics response to climate change and indicate related changes and differences in permafrost in different permafrost regions.