Latitudinal characteristics of frozen soil degradation and their response to climate change in a high-latitude water tower

In the context of climate change, the variation of seasonal frozen soil affects the eco-hydrological process in a water tower, which has been widely noted by scientists worldwide. However, the latitudinal characteristics of the temporal and spatial variation of seasonal freezing depth and their response to changing climatic factors need to be strengthened. Therefore, Changbai Mountain, a typical high-latitude water tower, was chosen to analyze the change in temporal and spatial variation of freezing depth and the influence of climatic factors by the modified Mann-Kendll trend test and Generalized Additive Model (GAM) methods. Results showed that the higher the latitude, the greater the freezing depth, the longer the freeze–thaw cycles, the earlier the freeze onset and the later the thaw onset. However, the frozen soil had a clear degradation trend during the period 1960–2018. There was also a significantly latitudinal characteristic. The higher the latitude, the greater the degradation of frozen soil. The downward trend was the largest with −0.35 cm/yr in the high latitude, followed by −0.24 cm/year in the middle-high latitude and the smallest with −0.10 cm/yr in the low latitude. In addition, due to climate change, the period of freeze–thaw cycles has been shortened, the freeze onset was delayed, and thaw onset has been advanced. According to the response of monthly average freezing depth (MAFD) to climatic factors, there is a strong correlation between MAFD and climatic factors in different months. When the soil started to freeze and thaw, temperature was the main factor influencing the change in freezing depth (p < 0.05). It is interesting to note that the air temperature contributed more strongly to the change in MAFD than surface temperature. When the frozen soil was in stable freezing period (from December to March of the following year), the snow cover gradually became the main influencing factor. Snow depth and snow pressure had the greatest contribution to the degradation of frozen soil. The higher the latitude, the longer the duration of influence of snow on frozen soil (explained difference = 20–61%). In addition, wind speed was also an important influencing factor on the change of MAFD in each month. Especially during the thaw period in April and May, wind speed was the most important influencing factor in the high latitude region. This study would be beneficial for the protection of the eco-hydrological cycle in cold region and would provide a basis for the study of seasonal frozen soil.