Evolution and drivers of the large-scale surface freeze-back onset in Siberian permafrost regions with the ERA5-Land reanalysis

Abstract. Permafrost is a defining feature of the Arctic and sub-Arctic environments. The extent of the permafrost region accounts for about a quarter of the Northern Hemisphere’s terrestrial surface. While most research on permafrost has focused on the summer season—when the active layer is thawed and carbon emissions peak—the late shoulder season, marking the transition season between summer and winter and ending by surface freeze-back at the large scale, has received less attention. Yet, about 14 % of the annual mean methane emissions from the permafrost occur during the refreezing period of the active layer. Understanding the seasonality, interannual variability and long-term trends of the surface freeze-back is therefore crucial to better constrain the high-latitude atmospheric carbon budget and improve Earth System Model projections. In this study, we analyze the evolution of surface freeze-back onset from 1950 to 2020 using the ERA5-Land reanalysis (0.1° spatial resolution) over a large region of Siberia encompassing the four main permafrost types. We find that surface freeze-back onset has been delayed by five days on average over that 70-yr period. Through spatial regression modeling, we show that, while several climatic and geographic factors influence freeze-back timing, the driving factor at the large scale (~kilometers) is the date when the 2-meter air temperature first falls below 0 °C, followed by the snow cover depth. These findings complement previous research that focused on the small scale (~meters), which emphasized the importance of the vegetation type and the snow cover characteristics at these spatial scales. Our results provide new insights into changes during the late shoulder season in one of the world’s fastest-warming regions and identify key variables to monitor for improving sub-seasonal forecasts that could become relevant to infrastructure upgrade and logistics planning in permafrost-affected areas.