Observed winter Alpine precipitation variability and links with large-scale circulation patterns

The winter precipitation variability over the Alpine region is described by a standard principal component analysis (PCA), performed starting from monthly precipitation anomalies for the 1971-1992 winters. With respect to the temporal variability, significant trends are found over some areas within the domain. In particular, the Alpine orography signature enables identification of 2 major sectors, located north and south of the chain, which exhibit an increase and a more significant decrease, respectively, in precipitation during the period examined. The relationship between surface and upper air data is then investigated by means of covariance maps of the precipitation principal components (PCs) with the 500 hPa geopotential height monthly anomalies and also by studying the correlation between the same PCs and some indices of large-scale circulation patterns, such as the North Atlantic Oscillation (NAO) and the Northern Hemisphere blocking frequency. The 2 leading precipitation patterns are characterized by significant relationships with large-scale anomalies: the NAO explains most of the Alpine precipitation variance, and a strong link is also found with Euro-Atlantic blocking. No significant connection is found between winter Alpine precipitation variability and the El Nino signature as deduced by sea-surface temperature anomalies.