Research On The Asian Tropical Summer Monsoon ISO And Its Effect On The Summer Precipitation In East China

The NECP reanalysis data are used in this paper,including wind field,temperature and pressure field,etc.,CMAP precipitation data and the rainfall data of776 stations nationwide.The low-pass filtering method is adopted to extract the30~60-day low-frequency components of each meteorological element(the intra-seasonal oscillation(ISO)mentioned later refers to 30~60-day low-frequency components).Based on the previous studies,the tropical summer monsoon index(TSMI)is defined.It can describe the variation characteristics of tropical South Asian monsoon and East Asian monsoon.When the amplitude intensity of TSMI exceeds a certain threshold,the ISO of the Asian Tropical Summer Monsoon(ATSM)spreads northward with time,which is called "the ATSM surge" in this paper.The fluctuation characteristics of climatic low frequency oscillation(CISO)in the ATSM are discussed by means of synthetic analysis.The climatic circulation variation characteristics of the ATSM surge are obtained in four phases,which are called the development,active,weak and intermittent,and the corresponding variation characteristics of summer precipitation in Eastern China.At the same time,the propagation characteristics of ATSM surge in summer atmospheric circulation are discussed,and the relationship between the ATSM surge activity in early summer and the outbreak process of Asian summer monsoon is discussed,and the inter-annual variation characteristics of the ATSM ISO activity are analyzed,and the reasons for the inter-annual variation of the ATSM ISO are revealed.Lastly,the modulation effect of La Nina events on summer precipitation in China.The results showed:(1)the Asian summer monsoon index(TSMI)was defined in this paper.It can well describe the characteristics of the interannual variation,and indicate the abnormal change of summer rainfall in southern China and the atmospheric circulation characteristics of East Asia.In the climate mean states,when the TSMI changes from negative phase to positive phase,it means the onset of the Asian summer monsoon begin.From the advance process of the low-frequency atmospheric circulation and the low-frequency OLR field,we got the ATSM first breaks out in the Sumatra Peninsula in early summer,followed by the Indo-China Peninsula monsoon,the Bay of Bengal monsoon,the South China Sea monsoon and finally the Indian monsoon.The date the first early ATSM ISO shifting from negative to positive phase is about seven days earlier than the breakout of the ATSM,which has certain forecast indication significance for the prediction of the Asian tropical summer monsoon outbreak.(2)As far as climate characteristics are concerned,in the Indo-Pacific region,the Somali cross-equatorial flow moves northward into the northern hemisphere in the ATSM surge development stage,resulting in low-frequency west wind anomaly on the Arabian Sea,while the Bay of Bengal and the South China Sea are in the grip of the low-frequency east wind abnormal flow.There is no significant rainfall increase in most parts of China.In the active period of the ATSM surge,low-frequency cross-equatorial flow converges in the sea area of the Bay of Bengal,turning into two after propagating westward to the Indo-China Peninsula.One continues to go northward before turning to the South Asian monsoon surge.This northward flow can propagate to the region 25 degrees north latitude,affected by the heat source anomaly of the Qinghai-Tibet Plateau.Another abnormal west wind flow continues to propagate eastward to the South China Sea and the West-Pacific sea area to its east.The South China Sea monsoon trough grows.The PJ wave train is generated in the coast of East Asia,in favor of the front southwest airflow of the South China Sea monsoon trough’s flow to the north.The southern coastal area of China has abnormal rainfall.In the stage when the ATSM surge goes weak,the South Asian monsoon surge,the South China Sea monsoon surge and the abnormal south wind to the west side of the Northwest-Pacific low-frequency anticyclone converge in the region of 30 degrees north latitude,and continue to propagate to the north.The rainfall anomaly in South China moves northward to the Yangtze River area and the rainfall anomaly of South China disappear.In the intermittent stage of the ATSM surge,the West-Pacific subtropical low-frequency anticyclone anomaly extends westward.The eastern part of China is in the grip of the abnormally southerly flow on the west side of the low-frequency anticyclone.The center of rainfall anomaly reaches North China-Korean Peninsula.(3)The ATSM CISO has two fluctuations.It is a complete fluctuation before June to mid-July(early summer).The fluctuation of the ATSM CISO is much weaker from mid-July to August(late summer).After the main reason is the South Asia tropical convection is not active in ATSM surge development phase later summer,which led to the Asian tropical summer monsoon surge in the stage of active and weaken the south Asian summer monsoon CISO transfer is given priority to,west of north pass only to the middle and lower reaches of the Yangtze river region,and in the local didn’t get from a zonally Pacific subtropical summer monsoon CISO supplies.On the other hand,the strength of the south subtropical convective activity will cause the change of the strength of the low-frequency updraft over the main body of the Qinghai-Tibet Plateau,which will cause the change of the meridional circulation on the north side of the Qinghai-Tibet Plateau and the Hadley circulation on the south side of the Qinghai-Tibet Plateau after the south Asian summer monsoon CISO travels north to the Qinghai-Tibet Plateau,and cause the abnormal low-frequency wave activity in the low level of North China and the north region.(4)Based on the analysis of the inter-annual variation characteristics of ATSM ISO,it is found that the activity of the ATSM ISO has increased since the middle of1990 s In this paper,the preliminary analysis of the spring Pacific SST changes on the influence of the interannual variability of ATSM ISO,when the spring in the subtropical Pacific SST and the bigger the temperature difference between the eastern equatorial Pacific SST,Pacific Ocean spring zonal Walker circulation,through the gear coupling,equatorial Indian Ocean zonal monsoon circulation,caused by the south Asian region.As the circulation weakened,the convective activity over the Indian Ocean in the south of the Bay of Bengal in summer weakened the Somali jet in summer,thus resulting in the inactivity of the ATSM surge.In La Nina events,the relationship between the activity of the ATSM surge and the spring Pacific SST was more significant than that in El Nino events.Analysis of individual cases show that La Nina events not only affect on ATSM ISO activity and the adjustment of ATSM ISO path and location,in turn,but also change the pattern of summer precipitation in the north of China under the classic La Nina events by ATSM ISO changing.Therefore,in the case of La Nina event,in addition to considering the influence of La Nina on summer precipitation in China,the ATSM ISO should also be one of the influencing factors to be considered.