Spatiotemporal Analysis of Snow Trends in the Austria Alps and their Relationship to Weather Patterns

In this study, a set of consistent and reliable long-term time series of snow depth observations on a daily scale, from selected sites across the Austrian Alps is used. The Central Institute for Meteorology and Geodynamics (ZAMG) and the Hydrographical Central Bureau of Austria (HZB) collected the data, enabling to investigate time series covering a period from the late nineteenth century until today. Spatiotemporal characteristics of seasonal snow depth observations were analyzed by using the method of principal component analysis (PCA). In order to derive regions with similar temporal evolution we regionalized the time series based on their spatial variability patterns for the period 1961-2010. In some cases, the regionalisation was limited due to sparse data coverage. The non-parametric Mann-Kendall test were used to assess the significance of trends of several snow indices, e.g. snow depth, maximum snow depth, snow cover duration, at monthly and seasonal time scales. We removed lag-1 serial correlations from the snow data by the trend-free pre-whitening approach. For the monthly and seasonal time series during the period 1961-2010, negative trends in snow indices were significant at the 95% confidence level, primarily at stations in the Western and Southern part of Austria. Correlation between snow observations and gridded winter air temperature and precipitation fields (HISTALP data set) show that increased air temperatures and decreasing precipitation during the 1990s, yield to a pronounced reduction in both snow depth and snow duration. In fact, the results indicate major shifts of the snow depth and snow cover duration around the 1970s and especially the 1990s, which are predominantly causing these trends. Decreasing trends of snow-depth in the West and South of Austria, as well as stable snow depth trends in the Northeast are well correlated to changing weather patterns, using an objective weather type classification system. In particular, changes in the frequency of weather types with cyclones moving from South towards the Alps are relevant for snow depth trends, which is also captured by respective changes of the NAOI.