From left to right: differences of HISTALP spring temperature, total precipitation observations and spring snowfall reconstruction (in absolute values and in percentage) due to the four AMO transitions that occurred in the past 150 years
Figure 3. From left to right: differences of HISTALP spring temperature, total precipitation observations and spring snowfall reconstruction (in absolute values and in percentage) due to the four AMO transitions that occurred in the past 150 years. The periods over which we computed the differences are listed in each row of the plot. Shading is applied over areas where the statistical significance of the differences is below the 95% threshold level according to the Mann–Whitney test.
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.