Seasonal Evolution Of The Intraseasonal Variability Of China Summer Precipitation

Seasonal cycle of China summer precipitation has significant impacts on its subseasonal predictability.This work investigates the seasonal evolution of the intraseasonal variability?ISV?of China summer precipitation and its associated low-frequency atmospheric circulations.The results of this study indicate that:?1?Following the seasonal evolution of monsoon precipitation,the seasonal evolution of the ISV activity can be separated to three stages:early summer stage from 1^stMay to 14^thJune,Meiyu season stage from 15^thJune to 15^thJuly,and late summer stage from 16^thJuly to 31^stAugust.The ISV of China summer precipitation mainly prevails over South China to the south of the Yangtze River in early summer,and it prevails around the mid-lower reaches of the Yangtze River Basin and Huaihe River during the Meiyu season.In late summer,the maximum ISV activity retreats to the coastal region of southeastern China and eastern Tibetan Plateau.The ISV activity in the Meiyu season is stronger than those in early and late summers.?2?There are two common leading ISV modes:a uniform mode?UM?over southeastern China and a dipole mode?DM?between coastal southeastern China and mid-lower reaches of the Yangtze River Basin,which occur during all three stages with a dominant period of 8-15days in the early and late summers and 8-30 days during Meiyu season.These two modes show southward propagation in early summer,but they are independent from each other in the other two stages.?3?There are two low-frequency signals that affect the ISV of China summer precipitation:low-tropospheric tropical quasi-biweekly oscillation and upper-tropospheric mid-latitude wave trains.In early summer,both UM and DM are only related to mid-latitude wave train,and no preceding signal is found in the tropics.During the Meiyu season,the preceding tropical intraseasonal signal from the western tropical Pacific provides another predictability source for UM and DM,in addition to the mid-latitude wave train.In late summer,the effect of preceding mid-latitude wave train is weak,both UM and DM are mainly related to the strong tropical cyclone-like intraseasonal signal.?4?Changes of these low-frequency signals in different stages are attributed to the changes of mean states.The weak ISV over the Western North Pacific in early summer before the formation of monsoon trough will limit the effect of tropical signal on the ISV of China summer monsoon.In contrast,during the Meiyu season and late summer,preceding tropical signals are observed when the Western North Pacific monsoon trough becomes strong.In the late summer,the northward shift and weaken of upper-tropospheric westerly jet prohibits the effect of mid-latitude wave train,which has less effect on the ISV of China summer precipitation.