Climatic features of atmospheric stability in the Mediterranean region (1948-2006): spatial modes, inter-monthly and inter-annual variability

The predominant spatial modes of atmospheric stability over the Mediterranean region are revealed by applying factor analysis and cluster analysis on the daily 2.5 degrees x 2.5 degrees grid point values of K-index for the period 1948-2006 . At first, S-mode factor analysis is applied on the K-index time series in order to reduce the dimensionality of the data set. The k-means cluster analysis is then applied to the factor scores time series, and the days are classified into 12 clusters with distinct and characteristic spatial distributions of the K-index. The 12 spatial modes revealed are associated with the prevailing atmospheric circulation types and/or the thermal and humidity characteristics of the lower and the middle troposphere. The inter-monthly variations of their frequency reveal the seasonal factors that contribute to their predominance. The seasonality of the 12 clusters shows that three of them prevail mainly in winter, three of them prevail mainly in summer, and the other six clusters present a relatively even seasonal distribution. The inter-annual variations of their frequency exhibit considerable long-term changes, providing also some possible indications of the oncoming climatic change. The continuous decrease of the typical high instability pattern of late spring and early summer and the recent increase of one of the typical high instability winter patterns provide some useful information about climate variability in the Mediterranean. Copyright (c) 2007 Royal Meteorological Society