Insight into subsurface - quantification of alpine heat waves and their impact on high mountain permafrost

In recent years the changing state of the cryosphere has been one of the most visually striking effects of climate change in mountainous terrains, gathering increased attention of not only the scientific community but the general public. Ice loss in the subsurface, caused by a warming ground thermal regime, is not directly visible such as retreating glaciers or annual snow cover changes, but it can have major impacts on ground stability.
Heat waves may contribute twofold to cryospheric changes: (1) as contributors to the general warming trend and (2) by (potentially) irreversibly changing the ground ice content through excessive amounts of heat penetrating the ground during such an event. Here, we focus on the second aspect and its impact on mountain permafrost. Although climatological research provides several tools for heat wave analysis, the application of (often regional) studies to the sparsely available borehole data and discrete meteorological monitoring networks are rare.
We employ the Heat Wave Magnitude Index daily (HWMId) metric to analyse temperature data from several Swiss Permafrost Monitoring Network (PERMOS) and MeteoSwiss stations near well-studied permafrost monitoring sites in the Alps. Historical and reconstructed data are used to determine specific temperature thresholds per site, accounting for local conditions (such as geomorphology, geology or ice content) therefore a systematic heat wave definition can be applied uniformly across all locations.
HWMId is compared to the changes in ground moisture content and observed changes in the permafrost body derived from borehole data. In addition, ice content is independently estimated from time series of 2-dimensional geophysical data, namely seismic refraction tomography and electrical resistivity tomography jointly inverted by petrophysical joint inversion. Initial results from the analysis of decade-long time series show correspondence between ground resistivity decrease with a general increasing trend in heat wave occurrences and intensity. Moreover heat waves precondition the permafrost for further thawing in subsequent years. Resilience of permafrost to the heat wave events in different landforms brings important implications for slope stability and safety of communities and infrastructure in mountainous regions.