Large scale circulation patterns associated with extreme snowfall events over the Greater Alpine Region

Extreme snowfall events (ESEs) are atmospheric phenomena characterized by a large fingerprint on human society. They impact both mountainous regions and nearby plains, affecting a wide spectrum of services from water availability to road transports. This study provides an extensive analysis of snowfall and extreme snowfall events over the “Greater Alpine Region” (known as GAR, 4-19°E, 43-49°N), a strategic area in the middle of Europe. Snowfall data supplied by two different datasets, namely the monthly observation-based HISTALP (1800-2014) and the daily ECMWF ERA5 Reanalysis (1950-2020), are used to study the variability and trends of snowfall in the November-May snow season, as well as in the winter season (DJF); results show that the average linear trends over the whole region for the seasonal November-May mean snowfall is decreasing (-0.5 mm/decade), as well as the number of snowy days from November to May (-0.8 days/decade); such decreases are related to increasing surface and lower tropospheric temperatures and they are more evident below 2000 m a.s.l. where surface temperatures are closer to the melting point. Beside trends in snowfall, snow variability and ESEs have been also studied in relation to the winter atmospheric Euro-Atlantic variability by using the four recurrent and persistent geopotential height patterns, the so-called Weather Regimes (WRs) which are commonly defined as: positive North Atlantic Oscillation (NAO+), Scandinavian Blocking (SBL), Atlantic Ridge (AR) and negative North Atlantic Oscillation (NAO-). Each one of them affects in a different way the snowfall spatial distribution and ESEs frequency within the GAR. The analysis highlights that NAO+, even though it is the most prevalent in terms of occurrence during winter, is a regime characterized by a reduced frequency of ESEs (on average over GAR reduced by 50%). A significant positive correlation is found instead between ESEs and Atlantic Ridge over the Swiss and Austrian Alps. Also the remaining two WRs (Scandinavian Blocking and NAO-) have a significant impact on snowfall patterns and amount: during their occurrence ESEs increase by almost 60% in Southern Alps and in Po valley, here demonstrating a significant positive correlation.