REORGANIZATION OF THE ASIAN MONSOON SYSTEM AT ABOUT 2.6 Ma AGO AND ITS IMPLICATIONS FOR THE RISING OF THE TIBETAN PLATEAU

More and more concentrations have been focused on the uplift of the Tibetan Plateau and its effects on regional and global environment. The development of Asian monsoon has a close relationship with the uplift of the Tibetan Plateau. The understanding for the development of Asian monsoon is the key to the understanding of the process and the mechanism of the environmental evolution of the Asian region and to the understanding of the history of the uplift of the Tibetan Plateau. It is found that a big change of the Asian monsoon system occurred at about 2 6Ma ago. The winter monsoon strengthened (Ding et al.,1992 ), the change of winter monsoon began to in the opposite phase to the summer monsoon in East Asia (An et al., 1998 ), the tropical Southwest monsoon was hard to influence the North China (Li, 1999). However, whether there were changes in direction of winter monsoon and strength of summer monsoon occurred and how to change are still not clear. In this paper, the changes of the Asian monsoon system at about 2 6Ma ago are discussed, based on the reanalysis and combination of the results of atmospheric simulations and the geological records existed. It is suggested that, at about 2 6Ma ago, both winter monsoon and summer monsoon strengthened obviously, because that the amplitude of the climatic change increased a lot—warmer during the warm periods and colder during the cold periods. The direction of winter monsoon in Northern China changed from North—West—West to Northwest or North—West—North, because that the expansion direction of eolian\|deposit\|distribution area in China changed from mainly eastward to mainly southward. The area influenced by subtropical monsoon increased, but the area influenced by tropical monsoon decreased in China, because of the rising of the Tibetan Plateau and the southwest\|ward migration of the center of the Asian Low. At some time of the Pliocene, the tropical monsoon had penetrated onto the Qinghai\|Xizang region and influenced on the North China and Northwest China. However, after 2.6Ma B.P., the effects of the tropical monsoon on Northwest China and the northern part of the Tibetan Plateau decreased. All these suggest that the Tibetan Plateau perhaps reached about 2000m elevation at about 2 6Ma B.P..