Larger variability of winter snow depth promotes the soil thermal regime instability over boreal high latitudes

Abstract. Permafrost degradation in Siberia is assessed as one of the global climate tipping elements. Soil thermal regime instability (STRI) describes the amplitude and frequency of soil temperature extremes at the interannual scale, and such thermal instability can propagate downward through the soil column and affect permafrost thermal conditions, making STRI a useful indicator of permafrost vulnerability. Yet, how STRI changes and the underlying mechanism remain poorly understood. Here, regarding soil temperature variability as a proxy of STRI on interannual scale, and through combining a merged soil temperature dataset with in-situ observations, it was detected that the subsurface (40 and 160 cm) STRI over Siberian continuous permafrost region is higher compared to other regions over boreal high latitudes, most pronounced in winter. Moreover, winter STRI across the Siberia continuous permafrost region has increased suddenly since early 1990s. Increase in STRI is particularly pronounced in regions where soil thermal regime was once stable, with increase rate of STRI exceeding 0.5 °C/10a in some regions. Statistical analyses and numerical experiments reveal that this increased STRI is strongly associated with the larger interannual variability of winter snow depth through its insulation effect, with snow depth accounting for more than 50 % of the STRI increase in most regions, and influence of snow depth on STRI is detectable down to 3.6 m. These findings highlight that snow-soil interactions dominate winter soil thermal dynamics, particularly over the thick snow regions, with potential implications for permafrost stability, carbon dynamics, and ecosystem responses.