The Decadal-multidecadal Variability And Mechanism Of East Asian Summer Monsoon Precipitation In The Past Millennium

Exploring the variation characteristics of the East Asian Summer Monsoon(EASM)precipitation on the decadal-multidecadal scale in characteristic periods of the past millennium is the scientific basis for identifying whether the current East Asian Summer Monsoon precipitation variation is abnormal in a longer time scale.In this study,we adopted the Multi-model Ensemble Mean(MME),which is the ensemble mean of 7 full-forcing simulation experiments from CMIP5/PMIP3.Based on the comparison of simulated results with observed data and proxy records,we verified the MME’s performance of simulating the climate change of the Medieval Climatic Anomaly(MCA,851-1250 AD),the Little Ice Age(LIA,1451-1850 AD)and the Present Warm Period(PWP,1901-2000 AD)in the past millennium.And this paper discusses the temporal variation characteristics and spatial differences of East Asian Summer Monsoon precipitation on the decadal-multidecadal scale,and discusses the impact mechanism of external forcing factors and the internal variability of the climate system on East Asian Summer Monsoon precipitation in the context of different characteristic periods.The main conclusions are as follows:(1)The East Asian Summer Monsoon precipitation sequence simulated by MME has obvious decadal-multidecadal variation characteristics.In addition,the 84.3-year and 63.9-year interdecadal periods of the East Asian Summer Monsoon precipitation sequence simulated by MME are mainly reflected in the Yangtze River region,while the 42.1-year and27.8-year interdecadal periods are mainly reflected in North China.On the spatial distribution characteristics,in the past thousand years of strong EASM years,the East Asian Summer Monsoon precipitation simulated by MME generally showed a distribution structure of "positive north and negative south".With sufficient water vapor,the anticyclonic circulation in the Northwest Pacific will inhibit the production of local precipitation,and jointly promote the northward shift of the East Asian Summer Monsoon precipitation belt.The distribution structure of summer precipitation with "positive south and negative north",the EASM region prevails with a northerly wind,which is not conducive to the deepening of oceanic water vapor.At the same time,the abnormal cyclonic circulation over the Northwest Pacific promotes the increase of local precipitation,which leads to the EASM.The overall precipitation belt moves southward.(2)The external mode of surface temperature simulated by MME is mainly affected by the increase of solar radiation,and shows the characteristics of high temperature in the East Asian continental region as a whole.The anticyclonic airflow further strengthens the southwesterly wind in the East Asian Summer Monsoon region,corresponding to the external pattern of East Asian Summer Monsoon precipitation of "positive north and negative south".The internal model of surface temperature simulated by MME shows the characteristics of local high temperature in the East Asian continental region.This "negative-positivenegative" surface temperature distribution pattern from south to north is closely related to the Pacific Ocean process.And the internal mode of the 850 h Pa wind field shows that the East Asian Summer Monsoon area is mainly southerly winds in the east of 110°E,and the cyclonic circulation anomalies are exhibited in the area south of 20°N.The corresponding internal mode of East Asian Summer Monsoon precipitation shows the precipitation distribution structure of "negative north-positive south".(3)According to the simulation results of MME,in different characteristic periods,when the Pacific Decadal Oscillation(PDO)is in the positive phase,the thermal contrast between the MCA East Asia and the Pacific Ocean decreases,corresponding to the weakened EASM;The 850 h Pa wind field influenced by the positive phase of PDO showed an abnormal anticyclonic circulation in the Yangtze River Basin.The precipitation in the Yangtze River Basin decreased significantly,and the precipitation in the areas on both sides of the Yangtze River Basin showed an opposite phase change;the warming of the LIA East Asian land area enhanced the EASM,the 850 h Pa wind field is mainly southerly winds in the East Asian monsoon region,and the precipitation is mainly concentrated in the south of the Yangtze River,while the precipitation on both sides is less;the PWP sea-land heat contrast decreases,corresponding to the weakened EASM.The wind field at 850 h Pa is mainly southwesterly,and the precipitation is anomalous in northeastern China and south of the Yangtze River.(4)On the multi-decadal temporal change,by comparing the reconstructed sequence of external forcing factors with the East Asian Summer Monsoon precipitation sequence simulated by MME,it can be found that the changes of MCA solar radiation and volcanic activity have a significant negative relationship with the changes of East Asian Summer Monsoon precipitation.On the multi-decadal time scale,the LIA East Asian Summer Monsoon precipitation and the reconstructed sequence of external forcing factors have a similar correlation as the MCA.The influence of precipitation plays a dominant role;the correlation between PWP East Asian Summer Monsoon precipitation and changes in GHGs concentrations increases significantly.On the multi-decadal spatial variation,the simulation results of the CESM single-factor sensitivity experiment were used to explore the effects of different external forcing factors on the multi-decadal spatial variation of subsummer monsoon precipitation.From the perspective of different characteristic periods,the spatial distribution of East Asian Summer Monsoon precipitation on the multi-decadal time scale of MCA and LIA is mainly affected by volcanic eruptions;the multi-decadal spatial variation characteristics of PWP East Asian Summer Monsoon precipitation are mainly affected by changes in greenhouse gases concentrations.