Roles Of Moisture Sources And Transport In Precipitation Variabilities During The Flood Season Over East China And Related Climate Dynamics

Based on observational station data from data archives of China Meteorological Administration and reanalysis data from National Centers for Environmental Prediction(NCEP)and European Centre for Medium-Range Weather Forecasts(ECMWF),the moisture sources of precipitation in three subregions of East China(South China(SC),the middle and lower Yangtze River valley(YRV),and North China(NC))are studied in this paper using the hybrid single-particle Lagrangian integrated trajectory(HYSPLIT)model,results of which contribute to a better understanding of the atmospheric water cycle over East China.Moreover,the intra-seasonal variations of moisture transport over East China are analyzed.Then,the conception of the“Cumulative Effect”of torrential rain(CETR)is proposed.Finally,this paper studied the inter-annual and inter-decadal variations of moisture transport over East China during flood season,and further revealed underlying mechanisms,which is related to the air-sea interaction activities.The results are summarized as follows.(1)Using the HYSPLIT model,fixed atmospheric moisture sources are defined:land area source(LD source),East China source(EC source,except for the target region),Indian Ocean source(IO source),Pacific Ocean source(PO source),South China Sea source(SCS source),and South China source(SC source).Sea source moisture transport accounts for more than 75%of the total precipitation during the preflood season,which is the most important moisture source in South China.For April,southeasterly moisture transport,which includes PO source and SCS source moisture transports,is the major moisture transport over SC.Then,the IO source and SCS source precipitation increased significantly during May.During June,the IO source precipitation accounted for 43.75%of the total precipitation,thus dominating precipitation over SC.In addition,there is a significant difference in the interdecadal variation in the preflood season precipitation before and after the Indian summer monsoon onset.Before the Indian summer monsoon onset,the land-sea thermal contrast decreased,which led to a significant interdecadal reduction in the IO source,PO source,and SCS source moisture transports.After the Indian summer monsoon onset,the increase in IO source moisture transport is the primary cause of the interdecadal increase in precipitation over SC.(2)To expand torrential rain,which is a meso-and microscale weather process,to a meso-and long-scale weather process,in this paper,we choose SC as a sample region and propose the conception of the“Cumulative Effect”of torrential rain(CETR)by using daily precipitation observational data from 740 stations.Through a statistical analysis of the observations,three indexes—continuous time(L_d),control area(A_r),and precipitation contribution rate(Q_s)—are used to define the CETR and indicate the torrential rain processes.The results shown that the CETR influences and even determines the amount and distribution of precipitation over SC.In addition,there is a significant difference in the spatial distribution of the heavy CETR events.Based on the center position of the CETR,the middle region displayed middle-region-heavy CETR events while the western region displayed west-region-heavy CETR events.El Ni?o events in the previous period(December,January,February,March(DJFM))are major extra-forcing factors of middle-region heavy CETR events.The major extra-forcing factor of a west-region-heavy CETR is the negative anomaly in the southern Tropical Indian Ocean during the previous period(DJFM).This factor is beneficial for strengthening the cross-equatorial flow and westerly winds from the Bay of Bengal to the South China Sea and early SCS summer monsoon onset.(3)Reanalysis data collected by ERA-Interim 4 times daily from 1979 to 2014 are used to analyze the main atmospheric moisture sources of cumulative effect of torrential rain(CETR)events during the preflood season(1 April to 30 June)over SC.Using HYSPLIT model the major atmospheric moisture sources of 66 CETR events for the period of 1979–2014 are identified and compared.The results indicate that during the preflood season,IO source moisture transport accounts for 30%of the atmospheric moisture generated in CETR events,thus dominating precipitation over SC.The water vapor transport from the LD source(14%),PO source(10%),SCS source(25%),and SC source(20%)also play important roles in preflood season CETR events.Furthermore,a significant transition in water vapor transport occurred in the 1990s,during which moisture transport from the Indian Ocean increased significantly,while moisture transport from the western Pacific Ocean significantly declined.In addition,a significant positive anomaly in PO source and SCS source moisture transport spurred CETR events in the east,while a significant positive anomaly in IO source moisture transport facilitated CETR events in the west.(4)The SC precipitation over the core monsoon zone is influenced by the IO source moisture transport(52.4%).The southwesterly moisture transport plays the most important role in the interannual varibiliy of the SC precipitaion over the core monsoon zone.For the YRV,the IO source accounts for 38.4%of precipitation over the core monsoon zone,thus dominating the interannual variability in the summer precipitation.The land evapotranspiration,which includes the EC source moisture(38.9%)and NC source moisture(21.8%),is the major source of moisture for NC precipitation over the core monsoon zone.The interannual varibility of the NC precipitation is dominated by the evapotranspiration over SC and south YRV.In addition,for the East China,the anomalous anticyclone over the western North Pacific dominating the release site of southwesterly moisture transport.At the same time,because of the anomalous anticyclone,the westward moisture transport from the PO source cannot arrive directly in East China,which leads to the decrease of southeasterly moisture transport precipitation.(5)The precipitation of the IO and SCS sources accounts for 43.73%and 23.45%of the SC summer precipitation,respectively,thus dominating the interannual and interdecadal variabilities in the SC summer precipitation.For the YRV,the summer precipitation is influenced by the EC source precipitation(22.41%),YRV source precipitation(23.97%),IO source moisture(27.78%)and SCS source moisture(14.40%).The interannual and interdecadal variabilities in precipitation are influenced by all moisture sources except the LD source and PO source moisture transports.The land evapotranspiration,which includes the EC source moisture(39.04%)and NC source moisture(22.04%),is the major source of moisture for summer precipitation over NC.In addition,the westward moisture transport from the PO source does not directly arrive in East China during El Ni?o events.Simultaneously,most of the northward moisture is released over the western YRV.Warming in the tropical Indian Ocean is responsible for decreases in precipitation from the IO source,PO source,and SCS source.In addition,the significant increase in the PO source precipitation over the eastern YRV and the significant decrease in the IO source precipitation over the western YRV were induced by the Pacific decadal oscillation phase transition from warm to cold in approximately 1999.