Tree-ring Recorded Hydroclimate Variability Of The Eastern Tibetan Plateau During The Last Millennium

The eastern Tibetan Plateau is the source area of many important rivers in Asia, e.g., the Yangtze River and Yellow River, regional climate change will affect not only this region but also many other regions. Therefore, knowledge about climate variability, especially the hydroclimate change is essential. Here, we developed several tree-ring chronologies at some sites and reconstructed the variablility of streamflow or drought; besides, we also developed a tree-ring network in the eastern Tibetan Plateau by combining some published tree-ring chronologies and performed a climate field reconstruction in this area. The main conclusions are as follows:(1) Based on the Juniperus przewalski tree ring samples collected from the Amne Machin Mountains, we built a chronology, which is reliable from AD 771, and used it to reconstruct the streamflow for the upper reaches of the Yellow River. Over the past millennium, the main low flow periods were identified as AD 1140–1156, 1295–1309, 1473–1500, and 1820–1847, and the main high flow periods were identified as AD 846–873 and1375–1400. Extremely low streamflow over the reconstruction period was seen during the late 15 th century, coinciding with a widespread drought phenomenon, which took place in the northeastern Tibetan Plateau over the same period. Reconstructed streamflow shows significant low-frequency variability, which is in line with drought variability of neighboring regions, as inferred from tree rings and other proxies.(2) Based on tree-ring samples from three Qinghai spruce(Picea crassifolia) sites in the eastern Qilian Mountains, we reconstructed regional PDSI variability from AD 1856-2009. The reconstruction shows droughts occurred in the 1900 s, the 1920 s and early 1930 s, and at the turn of the 21 st century. Multi- taper method spectral analysis indicates the existence of some significant peaks at 2.6, 4.2 and 56.9 years. Drought variations in the study area are significantly correlated with sea surface temperatures in eastern equatorial Pacific Ocean, northern Indian Ocean, eastern China Sea and the Atlantic Ocean, suggesting a possible connection with the El Ni?oSouthern Oscillation, the Asian monsoon and the Westerlies.(3) We developed the first millennium(AD 1009–2010) tree-ring chronology for the eastern Qilian Mountains by using cores from Juniperus przewalski, and used it to reconstruct regional June–July standardized precipitation and evapotranspiration index(SPEI). The record shows that this region has experienced several persistent droughts and pluvials over the past millennium, with significantly drier climate during the fifteenth century and dramatic wetting since the nineteenth century. The low frequency generally agrees with other nearby studies based on both tree-ring data and other proxy data.(4) Using tree-ring width measurements of 155 series from 81 Pinus tabulaeformis trees at three sites in the northern Min Mountains of Northwest China, the SPEI for May–August back to AD 1804 were reconstructed, which account for 64.8% of the variance in the instrumental record. The new reconstruction generally consists with other moisture reconstructions nearby. A notable feature of the SPEI reconstruction is the severity drought in the latter 1990 s and the gradually drying trend in the instrument period, which consist with the records from many areas in northern China and opposite with that in the western Sichuan, southwest China. The reversed response may be because different sub-monsoon systems controlling these areas.(5) Using 119 tree-ring chronologies in the eastern Tibetan Plateau and the point-by-point field reconstruction method, we reconstructed May–July SPEI variability at each grid point. Most of the reconstructions in the North are started before AD1000, while the reconstruction in the South are generally started after 1400 AD. The reconstruction captures the temporal and spatial variability of the instrumental data very well. The drought variability in the south and north of the eastern Tibetan Plateau shown notable different. The regional drought series in the North exhibits significant cycle of about 205 years, and the droughts coincide with the solar minimums. The result of crosswavelet analysis between the regional drought in the North and the solar activity shows in phase at the century and longer cycle. All these results indicate the hydroclimate variability in the North may be connected with the solar activity at century and longer scale.