The Beast from the East - winter atmospheric blocking over Eastern Europe during the Late Holocene and its role in regional climate variability

Several recent cold winters in North America and Western Europe has drawn attention on the possible increase in the frequency and/or intensity of extreme events in the mid-latitude Northern Hemisphere. Whether these could result form a strengthening or weakening of the circumpolar vortex and/or shift in the position of the North Atlantic storm track is still a matter of hot debate. A less known player in this conundrum is the dynamics of the Siberian High, one of the major semi–permanent and quasi–stationary weather systems in the Northern Hemisphere; active in winter and associated with dense and cold air masses over Asia and East Europe. The causes behind the variability of the of the Siberian High (strengthening and south and westwards expansion) are still poorly understood, yet important in the context of future climatic changes expected in the core area of its manifestation. In this context, we present here an overview of the present and past (~5000 years) dynamics of the Siberian High, based on 1) modern climate data from Asia and Eastern Europe and 2) proxy-based reconstructions of winter climatic conditions (temperature and precipitation amount). Our analysis starts with a instrumental-based investigation of the mechanisms behind the onset, strengthening and westward expansion of the high-pressure cell centered over North Asia. We further construct and test several hypotheses behind these mechanisms and test them by analyzing the dynamics of winter conditions during several episodes of particularly cold events in the Northern hemisphere (at 4.2 ka BP, 2.8 ka BP, 1.3 ka BP, 0.8-0.2 ka BP). We tentatively suggest that high insolation gradients between summer and winter in the high–latitudes of the Northern Hemisphere could result in the weakening of the polar vortex and increase in the meandering behavior of the jet that leads to an early onset of winter in North Asia. The expanding snow cover reinforces the strength of the Siberian High, leading to its expansion towards south and west and thus bringing colder conditions in West Asia and Europe. Future Arctic amplification could result in a higher frequency of similar behavior of the climate system, thus leading to more frequent and stronger cold spells across Europe.