Backgound And Mechanisms For Persistent Precipitaiton Events In South China

South China(SC),an important sector over East Asia,can be affected by the persistent heavy rainfall,which may be caused by complicated processes in the land-ocean-atmosphere system.By using daily precipitation and daily reanalysis data from 1961 to 2010,the changes of the summer precipitation in SC and the causes of the persistent precipitation in the region are investigated in the persent study.The changes in the structure of the summer precipitation over SC were first analyzed using the concentration index(Q),based on 50a(1961-2010)daily precipitation data collected at 64 rain gauge stations.Then,the distributions,the changes and the trends of different persistent period precipitation were examined.Then,statistical and dynamic diagnosis methods were utilized to expore the causes of the East Asia-Pacific(EAP)teleconnection pattern responsible for the persistent precipitation processes over SC on sub-monthly time scales.The development and maintenance mechanisms for teleconnections were revealed through the tracing and precursor analyses with a lead time of 1-2weeks.From the perspectives of convective activity and sea surface temperature forcing,the background of the EAP teleconnection development and maintenance mechanisms were explored on sub-seasonal time scales.The similarities and differences of the SC EAP events,the Yangtze River Vally(YRV)EAP events and dry SC EAP events in order to fully understand the effect of the EAP teleconnection on the persistent precipitation.The major findings are summarized as follows:(1)The variations of the Q and the total summer rainfall amount during the period 1961-2010 in SC both exhibited an upward trend.At the same time,shorter persistent wet spells showed decrease trend,while longer wet spells increased.(2)The EAP pattern can be well linked to the persistent precipitation processes in SC.About 1 week prior to the onset of the EAP teleconnection pattern,the eastward dispersion of energy in the high-latitudes initiated the formation of a blocking high in the Far East,which was further strengthened through the poleward energy dispersion from the low-latitudes over the Philippines.The western Pacific subtropical high(WPSH)then intensified and extended westwards.Furthermore,a meridional tripole structure of the EAP teleconnection pattern was established.At the same time,the cyclonic circulation over northeastern China introduced cold and dry air to the lower latitudes,merging with the water vapor into SC and thus leading to the heavy precipitation over the region,from along the fringe of the WPSH,the South China Sea,to the Bay of Bengal.There were obvious anomalies of convection in the Philippines,which provided energy for the formation and maintenance of the EAP teleconnection.Negative SST anomalies in the northern Philippine and SC coastal areas were significant signals of the EAP teleconnection on the sub-seasonal scale.The weakening of the negative SST anomaly in these two regions led to the conversion of the cyclonic abnormal circulation around the Philippines into the anticyclonic abnormal circulation.Furthermore,the westward shift of the anticyclonic abnormal circulation led to the westward extension of the WPSH.A poleward energy dispersion form lower latitudes enhanced the Okhotsk blocking markedly and made the mid-latitude trough deepened.(3)Compared with the EAP teleconnection responsible for the precipitation in YRV,the positions of the WPSH and the Meiyu trough were more northerly-located than the SC EAP centers.Such combination of the systems persisted for at least three days,and led to the persistent precipitation processes in YRV.The ocean surface heat conditions in the areas near the South China Sea seemed to be important in affecting the EAP teleconnection pattern for persistent precipitation processes in SC.For dry SC EAP events,the negitive anomaly center located at northeast of China and the north of Japan,the position of the WPSH was easterly-located than for the SC EAP event.So the persistent precipitation cannot occur over the YRV region and the SC reigon.