Study region with the station distribution located over the Italian regions of Friuli (red squares), Trentino (green circles) and Lombardia (blue diamonds), and over Switzerland (red crosses) and France (blue crosses)
Figure 1. Study region with the station distribution located over the Italian regions of Friuli (red squares), Trentino (green circles) and Lombardia (blue diamonds), and over Switzerland (red crosses) and France (blue crosses). The size of the markers represents approximately the radius of influence of the gridding procedure (see section 2). The gray contour line represents the 1000 m elevation of the orography.
Global warming is believed to be responsible for the reduction of snow amount and duration over the Alps. In fact, a rapid shortening of the snowy season has been measured and perceived by ecosystems and society in the past 30 years, despite the large year-to-year variability. This trend is projected to continue during the 21st century in the climate change scenarios with increasing greenhouse gas concentrations. Superimposed on the long-term trend, however, there is a low-frequency variability of snowfall associated with multi-decadal changes in the large-scale circulation. The amplitude of this natural low-frequency variation might be relatively large, determining rapid and substantial changes of snowfall, as recently observed. This is already known for winter snowfall over the Alps in connection with the recent tendency toward the positive phase of the North Atlantic Oscillation. In this study, we show that the low-frequency variability of Alpine spring snowfall in the past 150 years is affected by the Atlantic Multi-decadal Oscillation (AMO), which is a natural periodic fluctuation of Northern Atlantic sea surface temperature. Therefore, the recently observed spring snowfall reduction might be, at least in part, explained by the shift toward a positive AMO phase that happened in the 1990s.