Teleconnections Of The Asian-Pacific Summer Monsoon And Their Associations With The Summer Precipitation In China

The interactions among the Asian-Pacific monsoon subsystems have significant impacts on the climatic regimes in the monsoon region and even the whole world.The teleconnections of the Indian summer monsoon(ISM),the East Asian summer monsoon (EASM) and the western North Pacific summer monsoon(WNPSM),the subsystems of the Asian-Pacific summer monsoon,in the Asian-Pacific monsoon region,and their associations with the summer precipitation in China are studied in this paper.The ISM,EASM and WNPSM are significantly independent in the date of onset,the northward advancing march and their structures,but also they are closely linked with each other to some extent.Four different teleconnection modes are found in the Asian-Pacific monsoon region,which reveal clearly the interactions among the ISM,EASM and WNPSM,in different space and time.On the other hand,we also explain the reasons for the existence of each teleconnection from the aspects of atmospheric circulation,water vapor transport and numerical simulation,etc. Finally,the consistent interannual variation of the Asian-Pacific monsoon is performed through 850hPa wind,water vapor transport and precipitation from an overall perspective, which provides more valuable reference for the Asian summer monsoon precipitation forecasts in the future.The major conclusions are as follows:(1) In the period of the Asian monsoon onset,the date of ISM onset is two weeks earlier than the beginning of the Meiyu over the Yangtze River Basin,and a teleconnection mode is set up from the southwestern India via the Bay of Bengal(BOB) to the Yangtze River Basin and southern Japan,i.e.the "southern" teleconnection of the Asian summer monsoon. In the two weeks,Asian monsoon circulation has experienced a series of important changes: ISM setting up,the South Asia High(SAH) moving northward,the onset vortex occurring, the stronger tropical westerly belt extending eastward,and the western Pacific subtropical high(WPSH) jumping northeastward,etc.Consequently,the upper westerly jet stream and the low level jet have been coupled over the East Asia,while Yangtze River Valley happens to lie at the ascending motion area between the upper and low level jet,i.e.right of the entrance of the upper jet stream and left of the low level jet.Such a structure of the vertical circulation can trigger the Meiyu onset in Yangtze River Valley.(2) In the Asian monsoon culmination period,the precipitation of the Yangtze River Basin is influenced significantly by the WNPSM through their teleconnection relationship, and is negatively related to the WNPSM rainfall.It is found from the climatological intraseasonal scale that in the period of the weak WNPSM,the Yangtze River rainfall is influenced simultaneously by the westerly climatological intraseasonal oscillation(CISO) from the tropical west Pacific and the easterly CISO from the tropical Indian Ocean altogether,giving rise to more rainfall than normal there.(3) In contrast to the rainfall over the Yangtze River Basin,the precipitation of northern China(from the 4^th pentad of July to the 3^rd pentad of August) is positively related to the WNPSM.The water vapor transport which is brought by the abnormally strong southeastern monsoon has important impact on the precipitation anomaly in northern China.Through the intraseasonal scale analysis,the reason that more rainfall than normal in northern China may be the mutual influences of northerly CISO from the tropical western Pacific and the southerly CISO from the high latitude region.(4) The summer rainfall in northern China has also a positive correlation with the ISM. During the peak period of ISM,another teleconnection pattern is formed from Northwest India via the Tibetan Plateau to northern China,i.e.the "northern" teleconnection of the Asian summer monsoon.The Indian low usually dominantly impacts on the intensity of the ISM. When the Indian low deepens,the low troughs in mid-high latitudes are also frequently strengthened,and the ridge of the WPSH extends westwards,which is favorable for more rainfall there than normal. (5) The subsystems of the Asia-Pacific summer monsoon have a concurrent variation at the interannual scale,with the oscillation cycle of 2 to 3 years.In other words,when one of the subsystems of the Asian summer monsoon is stronger than normal and the other monsoon subsystems will also be stronger than normal in the same year,and vice versa.The consistent interannual variation of the summer monsoon that is performed by the water vapor transport is more remarkable than by the 850hpa wind.The tropospheric biennial oscillation(TBO) may be an inherent cycle in the Asian-Pacific summer monsoon system,which is closely associated with the atmosphere-ocean interactions over the warm ocean surfaces.