Mediterranean summer air temperature variability and its connection to the large-scale atmospheric circulation and SSTs

The interannual and decadal variability of summer (June to September) air temperature over the Mediterranean area is analyzed for the period 1950 to 1999. The combined influence of the large-scale atmospheric circulation at different levels and thermic predictors (thickness patterns and Mediterranean SSTs) on station temperature data is assessed by means of optimal objective techniques. The validity of the statistical models has been evaluated through cross-validation. Three large-scale predictor fields (300 hPa geopotential height, 700–1000 hPa thickness and SSTs) account for more than 50% of the total summer temperature variability. The positive phase of the first canonical mode is associated with blocking conditions, subsidence and stability related to warm Mediterranean summers. The second CCA mode shows an east–west dipole of the Mediterranean summer air temperature connected by a combination of a trough as well as an extended ridge over the western and eastern parts of the Mediterranean, respectively. Though both modes are found to contribute to long-term summer temperature trends in the 1950–1999 period, it is shown that the first canonical mode is mainly responsible for the 0.4 °C warming (significant at the 95% level) over the last 50 years of the twentieth century. Further, the analysis reveals that the Mediterranean summer temperatures were higher in the 1860s, 1950s and 1990s and lower around 1910 and in the 1970s. A significant temperature increase of 0.5 °C (0.27 °C) is found for the 1900–1999 (1850–1999) period.