Atmospheric circulation and precipitation in Italy for the last 50 years

Daily precipitation series from 75 Italian stations, for the period 1951-95, are clustered into six regions and the average is calculated for each area. For each average series, the seasonal and annual total precipitation and the number of wet days are calculated. The relationship between precipitation and atmospheric circulation is examined using correlation analysis. Atmospheric circulation is represented by five indexes: the well known North Atlantic oscillation index, a blocking frequency index, a Mediterranean oscillation index based on the 500 hPa geopotential record for Algiers, a Western European zonal circulation index (WEZCI) constructed from Madrid+Barcelona and Trondheim+Lund Surface pressure records, and a new Mediterranean circulation index (MCI) constructed from Marseille and Jerusalem Surface pressure records. All the indexes are calculated using monthly series; then seasonal and yearly averages are obtained. The indexes that show the most interesting correlation with total precipitation and number of wet days are WEZCI and MCI. Trend analysis of the index series gives evidence of a signal connected to a strong increase in winter air pressure in the Mediterranean area starting around 1980.