Impact Of Atmospheric Circulation Patterns Over East Asia On Summer Precipitation In Eastern China

East Asia is one of the most typical monsoon climatic zones in the world,and the summer precipitation over this region mainly contributes to flooding disasters in eastern China,which has attracted much attention from governments and public service.Atmospheric circulation acts as a bridge between external forcings and drought and flooding,and external forcings indirectly affect drought and flooding through atmospheric circulation.This thesis mainly focuses on the summer atmosphere circulation patterns over East Asia and their impacts on summer precipitation in eastern China to provide valuable references for drought and flooding forecasting in eastern China.In this thesis,we firstly investigated the interdecadal and interannual variations of the East Asia–Pacific(EAP)pattern and their impacts on precipitation and surface air temperature in eastern China.Furthermore,we proposed the new circulation patterns over East Asia,and identified the impact of the occurrence and persistence of the circulation patterns on total precipitation and different intensity precipitation in eastern China.The main conclusions are as follows:(1)The intensity and location of the EAP pattern anomaly centers occurred obvious interdecadal changes in 1984/1985.During 1961–1984,the fast decaying El Ni?o produced the warm sea surface temperature(SST)anomalies and significant anomalous southerly winds over the tropical Indian Ocean(TIO).This change in the surface winds increased evaporation,which,in turn,was unfavorable for the development and maintenance of the warm TIO anomalies in the following summer.Although the warm TIO SST anomalies suppressed the convection over the northwestern Pacific,the anomalous descending motion center was located more eastward and less significant.Consequently,the anomaly centers of the EAP pattern was located eastward.During 1985–2013,the slow decaying El Ni?o induced the significant warm SST anomalies over the TIO and cold SST anomalies over the northwestern Pacific in spring.Moreover,the significant anomalous easterly windsfrom the northwestern Pacific to northern Indian Ocean decreased evaporation,which then produced warm SST anomalies over the TIO in the following summer.The summer warm TIO SST anomalies were stronger than those during 1961–1984,and hence significantly suppressed convection over the northwestern Pacific.Specially,the anomalous descending motion over the northwestern Pacific was located more westward and stronger,as compared to that during 1985–2013.Therefore,the anomaly centers of the EAP pattern shifted westward.(2)The summer atmospheric circulation over East Asia has three main patterns:the EAP,Baikal Lake-Okhotsk Sea(BLOS)and eastern China-northern Okhotsk Sea(ECNOS)patterns.The positive EAP pattern significantly increases precipitation in the mid-to-lower reaches of the Yangtze River valley(MLRYR)and favors cooling north of the Yangtze River and warming south of the Yangtze River.During the El Ni?o decaying summer,the warm TIO SST anomalies suppress convection over the northwestern Pacific through the Ekman divergence induced by a Kelvin wave and excite the EAP pattern.The positive BLOS pattern produces below-average precipitation south of the Yangtze River and robust cooling in northeastern China.This anomalous sea ice contributes to a Rossby wave activity flux originating from the Greenland Sea,which propagates eastward along the mid-high latitudes of Eurasia.The positive ECNOS pattern leads to below-average precipitation and significant warming in northeastern China.The reduced soil moisture associated with the earlier spring snowmelt enhances surface warming over mid-latitudes of Asia and the later spring snowmelt increases soil moisture and leads to surface cooling over high latitudes of Asia,both of which are responsible for the formation of the ECNOS pattern.(3)The three summer atmospheric circulation over the mid-latitudes of East Asia are present the monopole(MOP),north–south out-of-phase dipole(NSD)and east–west out-of-phase dipole(EWD)patterns.An increase in the persistence of the positive MOP pattern enhances precipitation in the MLRYR.The high frequency of the negative MOP is more favorable for below-average precipitation in the MLRYR,as compared to its long persistence.An increase in the frequency and persistence of the positive NSD pattern leads to above-average precipitation in northeastern China,with the higher frequency also inducing a clear reduction in precipitation in southern China.Meanwhile,a higher frequency of the negative NSD pattern produces less precipitation in northeastern China,while the longer persistence dramaticallyintensifies precipitation in southern China.Lastly,for the EWD pattern,an increase in the frequency and particularly the persistence of the positive EWD pattern favors below-average precipitation in northeastern China.In contrast,an increase in the frequency and persistence of the negative EWD pattern is conducive to above-average precipitation in northeastern China,with the higher frequency also significantly reducing precipitation in the MLRYR.The results indicate that the frequency and persistence of each pattern have different impact on precipitation in eastern China.(4)The following results are obtained in association with high occurrence of each circulation pattern.For the positive MOP pattern,below-average light precipitation in northern and northeastern China is affected by increasing lower-tropospheric air temperature(LTAT)and decreasing lower-tropospheric precipitable water content(LTPW),and above-average moderate-to-heavy precipitation occur in the MLRYR.The negative MOP pattern reduces moderate-to-heavy precipitation in the MLRYR.In the positive NSD phase,the combined effects of colder LTAT and higher LTPW intensify light precipitation in northeastern China,and the spatial distributions of above-average moderate and heavy precipitation in the MLRYR are more elongated than those of the positive MOP pattern.The opposite conditions prevail in the negative NSD phase.In the positive EWD phase,the decrease in LTPW reduces light and moderate precipitation in northern and northeastern China,and above-average moderate and heavy precipitation occurs in southern China.In the negative EWD phase,below-average light precipitation in northern and northeastern China is due to warming,and below-average moderate-to-heavy precipitation extends from the western region of southern China to the MLRYR.