| Summary: | Global climate change affects the soil temperature and moisture in permafrost soils thus influencing active layer freeze-thaw processes. While warming of Arctic permafrost has been well documented, changes in the soil energy content and seasonal freeze-thaw processes have generally received less attention. In this study, we focus on the variability of the active layer freeze-back onset and duration at an Arctic research site on Samoylov Island at the Lena Delta in Northeastern Siberia. Samoylov is underlain by continuous permafrost and characterized by a polygonal tundra landscape. Continuous measurements of soil parameters and meteorological conditions have been conducted at this site since 1998. We analyzed data on soil temperature and moisture from 2012 to 2021 with respect to the polygon’s microtopography. We estimated the timing of freeze-back at different depths in both the elevated rim and water-logged center of the polygon. During the study period, the freeze-back onset was relatively stable with an interannual variability of approximately 10 days. We observed a clear increase in freeze-back duration at all measured depths of both, the polygon rim and center. Close to the surface, the duration increased by more than 200%. Due to differences in soil moisture, freeze-back at the polygon center started approximately one week later and took about 10 days longer than at the rim. The results of our study highlight the importance of changes in freeze-thaw processes in addition to soil temperature trends in gaining a more complete understanding of active layer conditions.
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