Mesoscale precipitation variability in the region of the European Alps during the 20th century

The purpose of this study is to construct and evaluate a new gridded analysis of precipitation that covers the entire region of the European Alps (43.2–48.8 ° N, 3.2–16.2 ° E), resolves the most prominent mesoscale variations (grid spacing 25 km) and extends with a monthly time‐resolution over most of the 20th century (1901–90). The analysis is based on a reconstruction using the reduced‐space optimal interpolation technique. It combines data from a high‐resolution network over a restricted time period (1971–90) with homogeneous centennial records from a sparse sample of stations. The reconstructed fields account for 78% of the total variance in a cross‐validation with independent data. The explained variance for individual grid points varies between 60 and 95%, with lower skills over the southern and western parts of the domain. For averages over 100 × 100 km 2 subdomains, the explained variance increases to 90–99%. Comparison of the reconstruction with the CRU05 global analysis reveals good agreement with respect to the interannual variations of large subdomain averages (10 000–50 000 km 2 ), some differences in decadal variations, especially for recent decades, and physically more plausible spatial patterns in the present analysis. The new dataset is exploited to depict 20th century precipitation variations and their correlations with the North Atlantic oscillation (NAO). A linear trend analysis (1901–90) reveals an increase of winter precipitation by 20–30% per 100 years in the western part of the Alps, and a decrease of autumn precipitation by 20–40% to the south of the main ridge. Correlations with the NAO index (NAOI) are weak and highly intermittent to the north and weak and more robust to the south of the main Alpine crest, indicating that changes in the NAOI in recent decades are not of primary importance in explaining observed precipitation changes. Copyright © 2002 Royal Meteorological Society