The history and variability of the East Asian paleomonsoon climate
Introduction
The East Asian monsoon is an integral part of the global climatic system. Monsoon climates, and especially monsoon-associated precipitation, are important to the maintenance of living environments and socially sustainable development in populous regions of East Asia. As a result, they have attracted the attention of many climatologists and geographers (e.g. Gao and Xu, 1962; Ye and Gao, 1979; Zhang and Liu, 1992). The importance of monsoon research has been acknowledged by Chinese Quaternary scientists (Yang and Xu, 1985; Li et al., 1988), but until the late 1980s, they did not realise that East Asian monsoon evolution is a principal and direct factor that controls palaeoenvironmental changes in East Asia. This contention has been derived from geological and biological studies of the well-known loess-paleosol sequences in central China. Loess is dust transported and deposited from its source areas by the northerly winter monsoon, while the development of interbedded paleosols is closely associated with the southerly moisture-bearing summer monsoon (An et al., 1990a, An et al., 1991a, An et al., 1991b). Much research has been carried out on paleomonsoon records of various types, and has provided a framework of dynamic control for East Asian monsoon evolution over the last 130 kyr and 20 kyr, and lead to the hypothesis that the monsoon is the controlling force for palaeoenvironment changes in East Asia. As a result, the study of East Asian paleomonsoon changes has been an area of increasing interest in the last few years (Porter et al., 1992; Shi et al., 1992; Zhou et al., 1992; Ding et al., 1992; An et al., 1993; Liu and Ding, 1993; Rutter and Ding, 1993; Ding et al., 1994; Zhou et al., 1994; Porter and An, 1995; Wang et al., 1995; Xiao et al., 1995; An and Porter, 1997; Vandenberghe et al., 1997; Huang et al., 1997; Thompson et al., 1997; An and Thompson, 1998; Zhou et al., 1998; Xiao et al., 1998; Zhang et al., 1998).
Palaeomonsoon and palaeoenvironment research in China currently targets two research areas: (1) the examination of high-resolution records such as loess, lake, palaeo-ocean, ice core and cave deposits for the last glacial cycle, the last deglaciation and the Holocene; as well as tree rings and historical documents for the last 2000 years, to evaluate climatic instability and its possible relation to the East Asian monsoon climate; (2) the extraction of information dealing with the inception and/or evolution process of the East Asian monsoon from the eolian sequences deposited over the last 7 Ma in the Chinese Loess Plateau, exploration of the relationship between East Asian monsoon evolution, global ice volume and uplift of the Tibetan Plateau (An et al., 1999), and understanding of the contribution of the East Asian monsoon and this plateau uplift to the global climatic system. Spatially, palaeomonsoon research has extended from the Loess Plateau to the Chinese marginal seas and the West Pacific Warm Pool (WPWP). Specific attention has been given to the frontal migration of the monsoon regime for the last 130 ka, and the diachroneity of the Holocene optimum (An et al., in press; Sun et al., 1996). It is clear that the study of the East Asian monsoon system should be placed within the context of global change. A better investigation of the response and contribution of the East Asian monsoon to the integrated behaviour of the entire global climatic system will result in a better understanding of processes involved in the evolution of the East Asian monsoon itself.
East Asian monsoon circulation is closely associated with climatic features of northern high latitudes, and is also linked with the equatorial ocean and the Southern Hemisphere (Zhao and Wang, 1979; Wang and Li, 1982; Tao and Chen, 1988; Chen et al., 1991). The transportation of water vapour and heat from the equatorial ocean to the middle and high latitudes is of vital importance in understanding global water patterns and global changes (Broecker, 1994a, Broecker, 1994b; Thompson et al., 1997). This paper will first examine its modern characteristics, and then will review the history and variability of the East Asian monsoon regime, its relationship with the palaeoclimates of the northern high latitudes, low latitude oceans and the Southern Hemisphere. This may provide important information for the prediction of future climatic changes in East Asia and the world.
Section snippets
East Asian monsoon
The East Asian monsoon regime is a sub-system of the Asian monsoon circulation. It affects an area to the east of the Bay of Bengal and the Tibetan Plateau. Climatic observations indicate that it is independent of, but interacts with, the Indian monsoon (Gao and Xu, 1962; Chen et al., 1991; Zhang and Liu, 1992). The East Asian monsoon is formed as a result of thermal differences between the Asian landmass and the Pacific Ocean, and is further enhanced by the thermal and dynamic effect of the
The history of the East Asian monsoon climate
Loess deposits are widespread in central China, covering an area of about 500,000 km2 with a thickness of 150–300 m. Geological, biological and chemical evidence derived from loess-paleosol sequences for the last 2.5 Ma in the Loess Plateau (Fig. 3) (Liu, 1985; Kukla, 1987; Kukla et al., 1988; Kukla and An, 1989) indicates that loess is the deposition of dust transported by the northerly winter monsoon. Particle-size, aeolian flux and detrital quartz composition can be used as proxy indices of the
Variability of the East Asian Paleomonsoon Climate
Multi-monsoon climate cycles characterised by the alternating dominance of warm-humid and dry-cold conditions recorded in the loess-paleosol sequences in central China correlate well with multi-glaciation cycles recorded in deep sea sediments. High-resolution loess records as well as lacustrine evidence also indicate that monsoonal variations exhibt both orbital periodicities of 10 kyr scale and sub-orbital oscillations of 1 kyr scale, which agree well with high-frequency Late Quaternary climatic
Comparison and teleconnection of palaeoclimates between the Southern and Northern Hemispheres
It is still an open question as to how the palaeoclimates of the Southern and Northern Hemispheres are correlated and connected together. An understanding of the palaeoclimatic signals from the Southern Ocean and the Southern Hemisphere that had effects on the East Asian monsoon is attempted here. Further understanding of some oceanic–atmospheric processes will be attempted by examining palaeoclimatic data obtained from the marginal seas of China, the WPWP, and northern Australia.
During the
Concluding remarks
Long-term variations of the East Asian monsoon regime are a reflection of the interaction between atmosphere, land, ocean and ice, and are an expression of their combined effects within the boundary conditions imposed by the East Asian continent and changing solar insolation. High-resolution measurements of proxy indices of monsoon climate from loess-paleosol-red clay sequences suggest that the East Asian monsoon may have commenced at least 7.2 Ma ago. The history of the East Asian monsoon can
Acknowledgments
I thank Dr. John Head, Dr. A.J.T. Jull, Asso. Prof. Mingyang Chen, Prof. Xiaodong Liu, Dr. Li Li, Dr. Hongbo Zheng, Mr. Youbin Sun, Mr. Yanjun Cai, Dr. Donghuai Sun, Dr. Huayu Lu, Prof. Weijian Zhou and Prof. Xiaoye Zhang for their kind help and assistance. Prof. Xiangjun Sun generously provided some unpublished data. Prof. John Chappell and Prof. John Dodson are thanked for their valuable discussions and comments. CAS, SSTC, CNSF and NSF provided financial support.
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