Regional extreme climate events on the northeastern Tibetan Plateau since AD 1450 inferred from tree rings

Qilian juniper (Juniperus przewalskii Kom.) is a widely distributed tree species growing on south-facing slopes in the northeastern Tibetan Plateau in arid northwestern China. We established a tree-ring width network based on two new chronologies and four previously published chronologies. Correlation and response function analyses demonstrate that precipitation positively influences radial growth. Despite of minor differences in local climate-growth relations, precipitation for the annual window between previous July and current June shows consistent positive correlations with ring width at all study sites. Similar to the so called 'pointer year' approach, 'anomalous' growth years were defined to extract extreme climate events for the period AD 1450-2006. We defined a dryness-wetness grade series with five grades of climate events inferred from anomalous year analysis. During the last 50 years, the frequency of wet events increased and that of drought events decreased noticeably, implying that the probability of occurrence of dry years in the northeastern Tibetan Plateau will further decrease in the future if regional warming continues. Combining our proxy records with a historical dryness-wetness record from eastern China, we mapped dryness-wetness patterns over large parts of China. By analyzing the atmospheric pressure patterns at the 850 hPa level over China for selected extreme event years, we found that the confluence of cold and hot air is a precondition for a flood event in the northeastern Tibetan Plateau. Thus, a counter-clockwise atmospheric circulation centered in south of Lake Baikal only occurs in flood event years. (C) 2010 Elsevier B.V. All rights reserved.