The start of frozen dates over northern permafrost regions with the changing climate

Climate warming exerts important impacts on the freeze-thaw cycle in permafrost regions. Although increasing attention has been paid on understanding the responses of spring thawing to climate change, the mechanisms controlling the global interannual variability of the start date of permafrost frozen (SOF) remain unclear. Using long term SOF derived from the freeze–thaw Earth system data record (FT-ESDR) over 1979-2020, and analytical techniques, including partial correlation, ridge regression, path analysis, the Random Forest (RF) and SHapley Additive exPlanations (SHAP), we explored the responses of SOF to multiple climate factors, including warming (surface and air temperature), start date of permafrost thawing (SOT), soil properties (soil temperature and volume of water), and the snow depth water equivalent (SDWE). In summary, while climate warming exerted predominant influence on SOF, SOT in spring also played a significant role in shaping SOF variability. Among the observed 65.9% significant correlations between SOT and SOF, 79.3% were positive, indicating that an earlier thawing would contribute to an earlier frozen in winter. Machine learning analysis reinforced the significance of SOT as the second most crucial determinant of SOF. To elucidate the mechanism behind the SOT-SOF relationship, path analysis was employed, revealing that changes in soil temperature had the maximum impact on this relationship, irrespective of the permafrost type. Finally, we analyzed the temporal changes in these responses using the moving window approach and found an increasing influence of soil warming on SOF. In conclusion, these findings offer valuable insights for understanding and predicting variations in SOF in the context of future climate change.