第四纪时期东亚季风变化的动力机制

FORCING MECHANISMS OF PALEOMONSOONS OVER EAST ASIA

本文作者主张：古气候演化的动力机制研究应从动力因子和动力过程两个方面人手。通过对比东亚冬季风、夏季风、印度季风及全球冰量变化的时间特征和频率特征，我们得到以下认识：１）大致在过去０．８Ｍａ这个时段，东亚冬、夏季风基本上具同相位、同周期的互为消长演化特征；２）东南季风演化具０．１Ｍａ的主导周期，而印度季风变化则以较短周期为主；３）东亚冬、夏季风与全球冰量变化在时间范畴上可作很好的对比，尤其是这三者均含有主导性的０．１Ｍａ周期。在此基础上，本文提出东亚季风演化的“全球冰量驱动模式”，并初步讨论了东亚季风变化对全球冰量变化响应的动力过程。

In recent years, many records kaye been generated, particularly from the Chinese Loess Plateau, to describe the history of East Asia Monsoon changes in the Quaternary, with few attentions however, paid to the forcing-response processes involved in their long-term cycles. During glaciaLinterglacial variations, the potential importance of the East Asia Monsoon circulations in linking different climatic subsystems in the Northern Hemisphere can be understood from the following aspects:1) as the winter monsoon winds are out from the Siberian region, variations in glaciation conditions in high latitudes can exert a strong control on the intensity of the Siberia High Pressure Cell, and then result in orbital-scale changes in the winter monsoon strength; 2) the southeast monsoon is produced by the coupling of the India Low and the Pacific High, so changes in low-Iatitude solar insolation patterns and SSTs would influence the strength and timing of this summer monsoonal circulation;3) since both the winter and summer monsoon winds can flow across the Equator.they can play a significant role in heat exchange between the two hemispheres; and 4) in the perspective of long-term evolution, the intensity and pattern of the winter and summer monsoonal circulations can be modulated by the vigorous uplifting of the Tibetan Plateau. Therefore, forcing mechanism studies on the East Asia Monsoons would provide a unique opportunity to understand the mutual interactions of different climatic subsystems at least over a hemispheric scale.In this study, we use magnetic susceptibility of loess-soil sequence to represent summer monsoon changes and grain size ratio as the proxy of the winter monsoons intensity. We measured susceptibility of the Xifeng and Weinan sections and grain size of the Weinan and Baoji sections at 10cm intervals above the soil unit S8. These records have a time interval of about 0.8Ma, as indicated by the B/M boundary located between the soils of S7 and Sa in all the three sections. As the Weinan and Xifeng proxy curves can be correlated fairly well to the Baoji grain size record, we tuned the three curves to the orbital time scale recently established with the Baoji grain size record, then resulting in summer and winter monsoon time series of the past 0.8 Ma. We then compared the monsoonal records with the indian Ocean Monsoon and global ice volume changes in the time and frequency perspectives.In this exercise, we recognized the following facts: 1) variations in the East Asia summer and winter monsoons in the last 0.8 Ma were essentially in phase and at the same frequencies; 2) the East Asia summer monsoon changes have dominant 0.1 Ma cycles, whereas the india summer monsoon basically in much higher frequencies; and 3) East Asia scummer and winter monsoon changes can be correlated well with global ice volume variations both in time and frequency domains, and particularly all of them contain predominant 0.1 Ma periods.In order to address these recognized phenomena, we developed a conceptual model which emphasizes the importance of global ice volume changes in determining the strength and timing of the East Asia Monsoon variations. During glacial periods, certain configuration of the Earth's orbits initiated the development of ice-sheets in high latitudes, and by some internal interactions, the ice-volume changes obtained strong 0.1 Ma periodicity in the late Pleistocene. The enhanced ice sheets can then make a downstream cooling to the air masses in the middle latitudes, and so the Siberia High is greatly intensified. Through the interactions between changes in global ice volume and the winter and summer monsoon winds, the two monsoon circulations derived dominant 0.1 Ma cycles.Finally, the conclusion is reached that variations in the monsoonal circulations over East Asia in the last 0.8 Ma were not forced directly by variations in solar insolation pattern, but internally phase-locked by global ice volume changes.