The seasonal thawing depth patterns at calm sites in the Upper Kolyma Highlands

The object of the study is the spatial and temporal changes of ground seasonal thawing depth in the Upper Kolyma Highlands, as well as its dependence on various natural factors, such as air temperature, snow accumulation, moisture content and sediment composition, terrain microrelief, and the association with a particular landscape type. Given the degradation of permafrost due to climate change, this topic is important not only from a scientific but also a practical perspective. The seasonally thawed depth affects the choice of construction method when designing engineering structures in the cryosphere and its change can negatively affect their operation. This work is based on empirical data obtained from comprehensive geocryological studies. Field measurements included observations at two sites using the Circumpolar Active Layer Monitoring program (CALM), and microzoning of the sites based on surface-sediment heat exchange conditions. The physical properties of the sediments were determined using field and laboratory methods. Meteorological data were collected based on the authors' own observations of air temperature and snow cover height, as well as the weather stations monitoring network. Differences in seasonal thaw patterns were identified at two sites with similar landscapes, located 20 km apart. These differences are related to the high variability of climatic condition in the mountainous region. At the beginning of the thaw period, the coefficient of variation at the sites is 0.25, decreasing to 0.16–0.20 by September. Peat thickness affects the seasonally thawed depth. Regional formulas have been developed that allow to determine active layer depth for any period based on the sum of positive degree hours. For the poorly studied Upper Kolyma Upland, these results provide a better understanding of the patterns of the spatial and temporal changes of permafrost regime.