A Tree-Ring Based Reconstruction of Balkan Temperatures Back to Medieval Times Reveals a Robust Pan-European Summer Teleconnection Mode

             The dominant atmospheric circulation pattern that governs European summer climate is a seesaw between the geopotential height fields over northwestern versus southeastern Europe: when a low occurs over the British Isles, a blocking-like pattern dominates the Balkans, and vice versa.  One phase of this seesaw was exemplified by the summer of 2012, when anomalously wet and cold conditions over northwestern Europe co-occurred with dry and hot conditions over souteastern Europe.             We here present a summer temperature reconstruction (1274-2009 CE) for the southeastern node of this teleconnection pattern.  This reconstruction is based on maximum latewood density (MXD) measurements of 111 Pinus heldreichii tree cores from a high-elevation site in the Pirin Mountains in Bulgaria. Our reconstruction explains 41% of the variance in interannual summer temperature variability in the Balkans as instrumentally recorded over the twentieth Century.             Fluctuations in our Bulgarian reconstruction correspond to summer temperature variability in southeastern Europe, but opposing temperature variability patterns are manifested over the British Isles and southern Scandinavia.  The strong and consistent anti-phase relationship between our reconstruction and a reconstruction of the summer North Atlantic Oscillation (sNAO) suggests that the sNAO pattern is a main driver of the teleconnection between summer temperatures in southeastern versus northwestern Europe. This teleconnection was well pronounced during the summer of 2012 and has been stable since at least 1700 CE. Dr. Valerie Trouet (Assistant Professor at the University of Arizona) is a dendroclimatologist who uses tree rings to reconstruct historical climate and examine how climate dynamics interact within ecosystems.  Dr. Trouet earned her doctorate from Katholieke Universiteit Leuven in Belgium (2004) and worked as a post-doctoral researcher in the Department of Geography at PennState and as a research scientist at the Swiss Federal Institute for Forest, Snow, and Landscape Research before moving to her current position in the Laboratory of Tree-Ring Research.