The importance of hot summers and individual heatwaves for permafrost in the Swiss Alps

The rapid intensification of permafrost warming and thawing in the Swiss Alps due to anthropogenic climate change is well observed and documented, but the response to atmospheric temperature variability on shorter timescales of days to a season is less explored. Here, we address this research question for an ice-poor permafrost slope on the Swiss mountain peak Schilthorn. Using Swiss Permafrost Monitoring Network (PERMOS) observations, we provide evidence that the year-to-year variability of total atmospheric heat over the snow-free summer period largely determines the year-to-year variability of total heat diagnosed in the active layer from the start of the snow-free summer period until the subsequent spring, since the snowpack effectively decouples the ground from the atmosphere during the rest of the year. With the help of idealized sensitivity simulations with the land surface model SNOWPACK, we further demonstrate that late-summer-to-early-autumn heatwaves increase ground heat at the end of the snow-free period until the subsequent spring more than early-summer heatwaves do, even if the total atmospheric heat over the snow-free period remains the same. The reason is that late atmospheric heat occurs closer to the return date of the insulating snowpack and can thus be better retained by the ground than early heat, which is lost back into the atmosphere long before the snowpack comes back. In essence, we provide evidence that hot summers with individual heatwaves in late summer and early autumn might pose the largest risk for a deepening of the active layer and thus for permafrost warming and thawing, at least for ice-poor permafrost investigated here. Running ground surface models such as SNOWPACK with output from subseasonal and seasonal atmospheric prediction models might thus be beneficial for early warnings of ground temperature and permafrost anomalies.