环境地球化学国家重点实验室知识库

Significant changes in the global atmospheric and oceanic circulation system occurred during the Younger Dryas cold period. Several researchers have demonstrated a weakening of intensity of the Indian Ocean Summer Monsoon during that period. However, the exact characteristics of the East Asian Summer Monsoon still remain vague. Here we present a late-glacial precipitation proxy record of the East Asian Summer Monsoon, based on the peat cellulose delta C-13 found in Hani, Northeast China. Both the peat cellulose record and a pollen record from Lake Sihailongwan sediment indicate an abrupt increase in precipitation in the region during the Younger Dryas period. These results support the occurrence of wet conditions in the north and of dry conditions in the south of the Chinese Mainland during that period. By examining the activity of the East Asian Summer Monsoon on an interannual timescale, we propose a theory for the anomalous precipitation distribution that we attribute to the occurrence of an El Nino-like phenomenon in the Equatorial Pacific Ocean during the Younger Dryas. In this case, the intensity of the Western Pacific subtropical high may strengthen, and its position over the western Pacific Ocean may move northward. This could cause an enhancement of the East Asian Summer Monsoon and migration of a monsoonal rainbelt towards the northern region of the Chinese mainland, resulting in a precipitation distribution of wet conditions in the north and dry conditions in the south. Therefore, this anomalous rainfall distribution should be considered to indicate the strengthening of the East Asian Summer Monsoon, in anti-phase with the Indian Ocean Summer monsoon that weakened during the same period. This agrees with the previously revealed anti-phase variations of the two monsoons during the ice-rafted debris cold events of the North Atlantic Ocean. It appears that, in relation to the abrupt temperature drop in the Northern Hemisphere on centennial to millennial time scales, anti-phase variations of the two Asian summer monsoons occurred.