| Climate changes over the past 2000 years are important part of the research for the "Past Global Changes"(PAGES) and the "Climate variability and predictability"(CLIVAR), two major international research projects. Especially, climatic changes of the past 1000 years are an important hinge period which links up instrumental data and proxy records. Northern Hemisphere temperature reconstructions reveal significant fluctuations during this period, such as the Medieval Warm Period, the Little Ice Age and the unprecedented 20th century warming, which are focused by the climatologists/meteorologists worldwide, however, little is known about the characters in monsoon. The Asian monsoon system affects more than half of humanity worldwide and its changes appear to have played a major role in shaping significant regional societal changes, yet the dynamical processes that govern its complex spatiotemporal variability are not sufficiently understood to model and predict its behavior, due in part to inadequate annually-resolved long-term climate observations. Furthermore, as changes in Asian monsoon link directly to high-northern latitude climates and also to Southern climates, via across-equatorial air flows, it is possible to decipher the precise relationship between the two hemisphere climates and to understand mechanisms of the monsoon variability. As the only energy supplier for the earth, the sun is reasonably assumed to be the important forcing of the globe climate change. However, the causality relationship between monsoon changes and solar irradiance remains uncertain. Stalagmite record has the precise and independent chronology and reserves integrated information. Therefore, stalagmite has become a major data source for continent paleoclimate reconstruction.In this dissertation, annually resolved oxygen isotope records of two stalagmites from Dongge Cave in southeast Guizhou province, based on 20 high-precision U/Th dates,1903 annual layers and 1746 oxygen data, provide a continuous history of the Asian monsoon over the past 1026 years. Uranium-series age dating and counts of annual growth bands enable an excellent age calibration. The new calcite?18O records, including the previously reported Dongge records(DA and D15), are very similar in timing, shape, and amplitude on inter-decadal to centennial scale, indicating that the speleothems most likely grew under isotopic equilibrium conditions and sample specific noise did not blur the climatic signal. Calibrated with the local instrumental meteorological records, shifts in stalagmite?18O largely reflect changes in?18O values of cave drip water. Furthermore, the Dongge?18O records exhibit a close similarity to the amount of summer monsoon rainfall and various types of monsoon index, supporting the ideal that the?18O record is a proxy for Asian monsoon intensity.The oxygen isotope profile clearly shows the transition at?1321 AD from a generally stronger monsoon Medieval Warm Period to a weaker monsoon Little Ice Age that lasted from approximately AD 1321-1633. The weak monsoon Little Ice Age is characterized by two weaker monsoon events and a relative strong monsoon event. The former weak monsoon event is 17 years, the latter is 77 years. The gradually decrease in monsoon rainfall since the 1960s is also obvious in stalagmite?18O records from the Middle East to northern China, indicating that changes in speleothem?18O largely represent variations of precipitation isotopic composition associated with large-scale summer monsoon circulations. In our research, most of the weak monsoon periods were characterized by popular unrest, suggesting that climate played an important role on the demise of Chinese Dynasties.Comparisons of the Dongge cave records with the precisely dated contemporaneous records from Cascayunga Cave in Peru, show that minima in?18O (wet periods, intense Asian monsoon) at our site are synchronous with maxima in?18O (dry periods, weak South American Monsoon) in Northeast Peru (within precise dating errors) and vice versa. This anti-phased precipitation relationship between two low-latitude locations may be inter-hemispheric in extent, based on comparison with records from other sites. Monsoon precipitation anti-phasing may be related to north-south shifts in the mean position of the intertropical convergence zone (ITCZ). The variations of the solar energy maybe cause the shifts of the ITCZ.Time series of Asian monsoon intensity, solar radiation and Northern Hemisphere surface temperature were used to analyze their causality relationship with?200 periodic oscillations by wavelet transform. The results indicate that the Asian monsoon intensity is temporally and inversely correlated to the solar radiation and the Northern Hemisphere surface temperature. This is also supported by the pattern matching as well as the cross-spectral analysis of monsoon data and solar activity records. It is suggested that a complex causality relationship between climate and solar activity. The dynamics of the Southern Ocean, especially the heat reservoir effect, may play an important role to modulate this relationship. |
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