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 prin- cipal components (PCs) with the 500 hPa geopotential height monthly anomalies and also by study- ing 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 anom- alies: 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 vari- ability and the El Nino signature as deduced by sea-surface temperature anomalies.