东亚冬、夏古季风变化的相位差及热带太平洋在季风变化中的驱动作用

斋堂黄土剖面古气候记录表明：１)冬、夏季风在千年尺度上的变化存在相位差；２)冬、夏季风记录在变化趋势上具有互为消长的关系，而在变化幅度和频率方面则有明显的差异。这表明，冬、夏季风在千年尺度上的变化是分别由不同的因素和过程所控制。我们提出热带太平洋海气相互作用系统在东亚季风变化过程中起着直接的驱动作用。

PHASE-DIFFERENCE BETWEEN SUMMER AND WINTER PALEOMONSOON VARIATIONS OVER EAST ASIA AND THE TROPICAL PACIFIC FORCING OF MONSOON EVOLUTION

The factors and mechanisms governing palaeomonsoon changes over East Asia have been a focus having attracted wide attention of Quaternary scientists. For the changes on time scales of 10 to 100kyr, Ding et al. suggest global ice volume rather than the variations in solar insolation pattern as the direct forcing. For the millennial flactuations, however, there is no appropriate explaination due to the lack of high resolution palaeomonsoon records.Here we present a high resolution paleoclimatic record based on the analysis of magnetic susceptibility, grain size, and chemical composition of the Zhaitang loess sequence in Beijing. Samples were taken at 2cm intervals. The chronology of this sequence is obtained by a correlation between the magnetic susceptibility of the loess sequence and oxygen isotope record in deep sea sdiments.Results derived from the analysis reveal the following characteristics of East Asia monsoon variations during the late Pleistocene:(1) There is an apparent phase difference between the summer and winter monsoonal changes. TIs difference was observed in marine oxygen isotopic stage 5 and the transitional interval from stage 4 to stage 3, where the changes of winter monsoon lead those of summer monsoon.(2) Both the Summer and winter monsoon changes are punctuated with frequent flactuations during the last glaciation. There are four intervals with stronger winter monsoon and weaker summer monsoon which can be correlated with the Heinrich events (H-3～H-6). (3) Differences of the summer and winter monsoon changes are distingushable in some aspects. For example, summer monsoon shows significant variations on timescales of 10kyr whereas the millennial fluctuations in winter monsoon are more obvious. Additionally, summer and winter monsoon variations are not always matched each other.These scenarios indicate that summer and winter monsoon over East Asia are not dominanted by the same factor and mechanism.We propose that:First, the coupled changes in both external and internal boundary forcings are the main factor causing the variations in summer and winter monsoon over East Asia. It is known that East Asia monsoon is resulted from sea-land thermal difference between Euroasian continent and the western Pacific ocean. During glacial period, summer solar insolation as the external boundary forcing decreased in high latitudes with larger amplitude than in low latitudes. This larger decrease makes the thermal difference between land (mainly in high latitudes) and sea (mainly in low latitudes) amplified during winter season but reduced during summer season. As internal boundary forcings, SSTs, the exposure of continental shelves, ice sheets, albedo etc.,are all coupled with the external forcing in strengthening winter monsoon and weakening summer monsoon. For the period of interglaciation, however, the case is opposite.Second, the clues of Heinrich events in palaeomonsoon records indicate that the North Atlantic force pacing summer and winter monsoon variation is cannelled by different pathways. It has been suggested that the signals of Heinrich events are transmitted to the winter monsoon by the westerlies. We speculate that the signals in summer monsoon are derived from the reorganization of the Global Thermohaline Conveyor Belt. A plausiable mechanism hidden in it could be that, during the intervals of the Heinrich events, the Conveyor Belt is cut of T (or changes to a shallow one), and this mode shift results in the decreases in the intensity of upwelling over east Pacific and in SSTs gradients between east and west tropical Pacific. These decreases lead to the weakened latitudinal circulation and tropic convection, and which further cause the reductions in the intensity of West Pacific Subtropic High and East Asia monsoon in summer season.Third, the summer-winter monsoon phase difference implies different mechanisms controlling the variations in summer and winter monsoon over East Asia. The different mechanisms are stemmed from the different sensitivity in response to boundary forcing change between high l