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 mon thly 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 identific ation of 2 major sectors, located north and south of the chain, which exhib it an increase and a more significant decrease, respectively, in precipitat ion during the period examined. The relationship between surface and upper air data is then investigated by means of covariance maps of the precipitat ion principal components (PCs) with the 500 hPa geopotential height monthly anomalies and also by studying the correlation between the same PCs and so me indices of large-scale circulation patterns, such as the North Atlantic Oscillation (NAO) and the Northern Hemisphere blocking frequency. The 2 lea ding precipitation patterns are characterized by significant relationships with large-scale anomalies: the NAO explains most of the Alpine precipitati on variance, and a strong link is also found with Euro-Atlantic blocking. N o significant connection is found between winter Alpine precipitation varia bility and the El Nino signature as deduced by sea-surface temperature anom alies.