Global warming accelerates the upfreezing of permafrost in arid middle Tianshan Mountains

Upfreezing is an important geomorphic process in the periglacial environment. It is a product of cold climate and thus an important part of the cryospheric processes. Based on the long-term positioning observations in the source area of the Urumqi River, this article represents an in-depth discussion of the characteristics of sorted circles and upfreezing mechanisms in the Tianshan Mountains. In the source area of the Urumqi River, the intensity of upfreezing is the highest within the top 25 cm near the surface, while targets with a diameter of 3 cm are least affected by upfreezing. There is no distinct difference between the centre with fine grains and the margin with coarse debris within the same sorted circle in terms of the intensity of upfreezing. The correlation analysis demonstrates that temperature plays an important role in upfreezing and the development of sorted circles. A long-time positioning observation of sorted circles reveals that periglacial landforms are sensitive to regional climate change and respond quickly to the temperature increase of the recent two decades. Enhanced upfreezing was found to be due to increased soil moisture content.