The role of blocking in the summer 2014 collapse of Etesians over the eastern Mediterranean

We investigate the dynamical harbingers leading to the remarkable summer 2014 decline of the northerly flow (Etesians) over the eastern Mediterranean. From mid-July to mid-August four distinct episodes of unseasonal southerly flow were identified and associated with upper level troughs over central Europe and the Balkans. These features developed as repeated episodes of wave breaking, leading to blocking over Europe in July, and triggered equatorward streamers of high potential vorticity. During July a twofold increase in blocking occurrence against climatology was identified over parts of Europe and was part of a five-wave hemispheric pattern featuring abundant high-latitude blocking also over central Asia, the central Pacific, and western Atlantic. Overall, the frequent European blocking resulted in the southward displacement of the midlatitude storm track toward the Balkans and the relaxation of the traditional sharp east-west pressure gradient that triggered the collapse of Etesians. The bifurcation of the midlatitude jet caused by blocking led to the intensification of the westerly flow over the Mediterranean, accompanying the passing disturbances farther to the north, which combined with the weak Etesians resulting in a dramatic modification of the large-scale circulation over the Mediterranean Basin.