Top panel: time-series of snowfall HISTALP reconstruction (dashed lines) and the direct observations (solid lines) of March and April mean snowfall averaged over Friuli (in red), Trentino (in green) and Lombardia (in blue), in mm of equivalent water per day
Figure 2. Top panel: time-series of snowfall HISTALP reconstruction (dashed lines) and the direct observations (solid lines) of March and April mean snowfall averaged over Friuli (in red), Trentino (in green) and Lombardia (in blue), in mm of equivalent water per day. Central panel: the same for Switzerland (in red) and France (in blue). Bottom panel: 11 years running mean time-series computed over Switzerland (in red) and the French Alps (in blue). The black line represents the 11 years running mean spring AMO index, of which we plot the inverse to allow an easier comparison with the spring snowfall data. The yellow regions highlight the Atlantic Multi-decadal Oscillation (AMO) transitions from cold to warm periods in the past 150 years, as defined in the text.
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.