Spatial and temporal variations in the temperature regime in the Mediterranean and their relationship with circulation during the last century

Temperatures in 22 grid boxes across the Mediterranean were subjected to principal component analysis (PCA). Four factors were retained, dividing the Mediterranean into four different subregions in each season (only three in spring). PC1 and PC2 defined the eastern and western Mediterranean, respectively, in all seasons. PC3 defined the central Mediterranean in summer and autumn, the Balkans in winter and a combination of both regions in spring. Finally, PC4 defined the Balkans in summer and autumn and the central Mediterranean in winter. Time series analysis of PC scores enabled the determination of discontinuity time points when abrupt temperature changes occurred. The pressure patterns associated with high temperatures in each region were defined over a major part of the northern hemisphere. The circulation associated with each factor in each season was determined from meridional (MI) and zonal (ZI) circulation indices. A seesaw-like oscillation, namely the Mediterranean Oscillation, was found between the eastern and western Mediterranean. Favourable circulation for high temperatures in one part of the Mediterranean is associated with unfavourable circulation in the other part and vice versa.