The Evolution Of East Asian Summer Monsoon And Drought/Flood Distribution On Eastern China During The Last Millennium

The activities (strength, course and so on) of East Asian summer monsoon (EASM) have great influence on the summer precipitation over eastern China. So, it is important to study the long-term variation of EASM. Due to the observe data only have 50-60 period, the study of the long-term variation of EASM is restricted. This paper analyzed the coupling relationship between the drought/flood grades and the East Asian summer sea-level pressure (SLP) by using the history of climate proxy data (for example drought/flood grades, dry/wet series and so on) and summer average SLP. Based on this relationship, the East Asian summer SLP are reconstructed over the past 500 or 1000 years. According to the reconstructed EASM indices, the long-term evolution of EASM and its associations with drought/flood distribution in eastern China are discussed. Besides, the reconstructed SLP, simulated SLP and Asian Pacific Oscillation (APO) are compared. The mainly conclusions are as follows:(1) The typical spatial distribution and interdecadal variations of drought/flood grads in the last 159 years and precipitation in the last 50 years over eastern China are basically the same, which indicate that drought/flood grads can reflect the main features of drought/flood changes over eastern China. The interdecadal variation of drought/flood distribution patterns over eastern China had a close relationship with that of the East Asian summer monsoon defined in this study, although their correlation was not stable and had a significant difference in interdecadal phase change.(2) Based on the closely relationship between drought/flood distribution over eastern China and the key areas of the EASM (i.e. China and some part of the Northwest Pacific) during 1850-2008, the SLP is reconstructed back to 1470. The tests show that the key areas of the EASM are more credible than the other areas for the reconstructed SLP. The analysis shows that the primary patterns of the reconstructed SLP were differences between high-latitude area and middle-latitude area as well as differences between sea and land during 1470-2008. The EASM is drived by the heat difference between sea and land. As one of the mainly air-pressure activity center, the Tibet Plateau plays an important role on the activities of EASM(3) Compared the characteristic of time/spatial distribution between the reconstructed SLP and simulated, the results show that the primary patterns of the stimulated SLP are differences between sea and land during 1470-1999. The EASM defined by the reconstructed SLP shows obviously periodically. The strength of reconstructed EASM is stronger during mid-16th century and the early 17th century and subsequently weaker during 17th century. The EASM experienced 4 modulations from weaker start during 18th century and 3 changes from weaker to stronger during 19th century. During 20th century, the EASM has obvious change of the "weaker-stronger-weaker". The primary differences between the reconstructed and simulated EASM appeared at the end of 16th and 18th century, and the trend is basically consistent in the other times.(4) The active centers of six drought/flood grads are defined by using the large contours of the six REOF modes in eastern China. They are Northeast China, North China, Changjiang River-huaihe River valley, South China, Northwest China and Southwest China. The response of EASM is obviously in South China and Changjiang River-huaihe River valley, and instability in Northwest China and Southwest China. The stronger strength of EASM may result in less rainfall over South China and North China, and more rainfall over Changjiang River-huaihe River valley. The weaker strength of EASM may result in more rainfall over South China and North China, and less rainfall over Changjiang River-huaihe River valley. The six regional dry/wet series and the drought/flood grades over South China and Changjiang River-huaihe River valley are basically the same during the last 530a. Then, based on the closely relationship between SLP and dry/wet series, the grid data of Eastern Asian summer SLP is reconstructed. The results show that the key areas of the EASM are more credible than the other areas for the reconstructed SLP during the last 1000a.(5) From the millennium time scale, the trend of EASM index experienced nine times turning. The most significant vibration of the EASM strength is occurred in 1330s.(6) The evolution trend of APO index has obviously characteristics of stage, i.e. the APO is stronger than normal during 1631-1796 and 1921-1985 and it is weaker than normal during 1470-1630 and 1797-1920. On 60-80 periodic components, the interdecadal evolution trend between APO and SLP has difference before 1797 and accordance after 1797.