Seasonality in secular changes and interannual variability of European air temperature during the twentieth century

A gridded terrestrial monthly surface air temperature (AT) data set for 1901 - 2000 from the Climatic Research Unit (CRU), University of East Anglia, is used to investigate seasonality in the long-term AT variability over Europe. Prominent seasonal differences are detected in all considered characteristics of AT variability. Significant warming trends over western and southern Europe are found during summer and fall. In winter, the largest positive trends are observed mostly in southern Europe, whereas during spring, they are detected over Scandinavia and northeastern European Russia. The spatial-seasonal differentiation of warming trends implies that, in different parts of Europe, different seasons play the role as major contributor to the warming trend. The first empirical orthogonal functions (EOF) modes of winter, spring, and fall AT over Europe are associated with the North Atlantic Oscillation (NAO) and explain about 50% of AT variability. For the summer season, the second EOF mode (explaining only 15% of AT variance) might be associated with the summer NAO but has a very different (compared to other seasons) spatial pattern and principal components. The second EOF mode of the winter AT might be linked to the East Atlantic/West Russia (EAWR) teleconnection pattern. Analysis of running correlations between the principal components of the leading EOF modes of AT and the NAO index has revealed nonstationarity of the links between European AT and the NAO and evident seasonality in their long-term changes. Subsequent singular value decomposition analysis performed for the climatic periods of strong/weak links to the NAO revealed considerable interdecadal changes both in the strength and the structure of the links between European AT and regional atmospheric circulation.