East Asian Summer Monsoon And Its Association With Anomalies In The Mid-high Latitude Atmospheric Circulation And External Forcing

Using the NCEP/NCAR and JRA-25reanalysis data from1979to2009, this studyrevealed dominant modes of summer season (June-August)850hPa wind field variabilityover East Asia by means of the complex vector empirical orthogonal function method. Thisthesis deeply analyzed space-time distribution characteristics of the former two EASM modes,and its effect on the precipitation in summer. Moreover, the external forcing exerting possibleeffects on EASM were discussed. The main conclusions are as follows：(1) The leading EASM mode is constituted by two similar sub-modes or their linearcombination, which were revealed by NCEP/NCAR reanalysis data. The real part and theimaginary part of the leading complex principal component can be used as intensity index todescribe two similar sub-modes, which are named M11and M12respectively.The correlationcoefficient between M11and M12are0.77(excluding trend).When M11and M12are in thepositive phase,850hPa wind field anomalies potray a East Asia/Pacific (EAP)teleconnection pattern from the western North Pacific (WNP), across Japan to the OkhotskSea. M11corresponding to anomalous500hPa geopotential height fields show a anomalouszonal teleconnection pattern in northern Eurasia and a EAP teleconnection wave train, whileM12only correspond to an EAP wave train. The distribution of summer precipitationanomalies association with M11and M12both show a significant reduction in the middle andlower reaches of the Yangtze River and north of the Northeast, whereas precipitation over eastof the Northeast、the southeast coast and of the west Southwest increase. In M11and M12positive phase period, sea level pressure (SLP) are significantly lower in the WNP, whileoutgoing longwave radiation (OLR) anomalies in Philippines Sea and South China Sea (SCS)are significantly negative, the convection heating enhance. JRA-25reanalysis data showsimilar results.(2)M11is closely related to ENSO in the previous winter. When M12is in its positivephase, in the previous winter, tropical India Ocean (TIO) and tropical Pacific SST anomaliescoexist. Meanwhile ENSO is in the most robust stage, TIO SST anomalies also exceed95%significance level. Late spring, ENSO decay rapidly, whereas the TIO SST anomalies peak. In summer, ENSO vanished, while TIO SST anomalies sustain the influences on the EASM.JRA-25reanalysis data also show similar results. Using partial correlation analysis, weconcluded the tropical Pacific SST and TIO SST anomaly basically developed independentlyin the previous season, moreover they both exerted their influences on EASM。(3) The second EASM mode revealed by two sets of reanalysis data underwent oneinter-decadal shift in the early1990s, which were consistent with the inter-decadal shift ofChinese summer rainfall in the early1990s. The second EASM mode is closely related to themid-high latitude atmospheric circulation anomalies, corresponding anomalous500hPageopotential height fields show a anomalous quasi-zonal teleconnection pattern in northernEurasia, whereas the distribution of summer precipitation anomalies correspond to ameridional dipole pattern. Along with inter-decadal shift of EASM, after the early1990s themajority of summer precipitation in northern China decreased, especially in north of theNortheast and the area around105oE between the Yangtze and Yellow River region; whilesummer precipitation increased significantly in South China and the Huaihe River Basin.From the perspective of dynamic, the characteristics of inter-decadal shift of Chinese summerprecipitation are described. The difference distribution of summer500hPa geopotential heightfields between two period (1993–2009and1979–1992) show northern Eurasian zonalteleconnection pattern, then the difference distribution of summer850hPa wind fields showthis structure that there are two anomalous anti-cyclonic circulations in southeast of LakeBaikal and south of Japan, while there are two anomalous cyclonic circulations in southernChina and sea of Okhotsk.(4) We discuss the reasons for inter-decadal shift of EASM. Sea surface temperature(SST) in the northwestern Pacific, north Indian Ocean and part of high latitude ocean（NorthAtlantic and North Pacific）, as well as spring Eurasia snow water equivalent changedsignificantly in the early1990s, spring Arctic sea ice occurred one inter-decadal shift in theearly1990s. These external forcing might be the causes of interdecadal shift of EASM in theearly1990s. The role of these external forcing in interdecadal shift of EASM is unclear andfurther study is essential.