The strength of the North Atlantic Oscillation (NAO) is considered to be the main driver of climate changes over the European and western Asian continents throughout the last millennium. For example, the predominantly warm Medieval Climate Anomaly (MCA) and the following cold period of the Little Ice Age (LIA) over Europe have been associated with long-lasting phases with a positive and negative NAO index. Its climatic imprint is especially pronounced in European winter seasons. However, little is known about the influence of NAO with respect to its eastern extent over the Eurasian continent. Here we present speleothem records (δ13C, δ18O and Sr/Ca) from the southern rim of Fergana Basin (Central Asia) revealing annually resolved past climate variations during the last millennium. The age control of the stalagmite relies on radiocarbon dating as large amounts of detrital material inhibit accurate 230Th dating. Present-day calcification of the stalagmite is most effective during spring when the cave atmosphere and elevated water supply by snow melting and high amount of spring precipitation provide optimal conditions. Seasonal precipitation variations cause changes of the stable isotope and Sr/Ca compositions. The simultaneous changes in these geochemical proxies, however, give also evidence for fractionation processes in the cave. By disentangling both processes, we demonstrate that the amount of winter precipitation during the MCA was generally higher than during the LIA, which is in line with climatic changes linked to the NAO index but opposite to the higher mountain records of Central Asia. Several events of strongly reduced winter precipitation are observed during the LIA in Central Asia. These dry winter events can be related to phases of a strong negative NAO index and all results reveal that winter precipitation over the central Eurasian continent is tightly linked to atmospheric NAO modes by the westerly wind systems.
Bibliographical noteFunding Information:
This research was supported by the Helmholtz Centre Potsdam (GFZ) Global Change Observatory of Central Asia (GCO-CA) , and by the Central Asian Climate Dynamics Project (CADY) , which was funded by Germany's Federal Ministry of Education and Research (BMBF; grant 03G0813A ). JF acknowledges funding by Deutsche Forschungsgemeinschaft (DFG) through grant FO809/4-1 . We are grateful for B. Richert for μXRF scanning, and H. Meyer for water isotope analysis. We would like to thank two anonymous reviewers and Associate Editor, Neil Roberts, for their thorough reviews and helpful comments, which helped to improve the manuscript.
© 2017 Elsevier Ltd