Study On The Characteristics And Classifications Of Rainy Season Process In Southern China

Droughts and floods often occur in the rainy season over China,every rainy season onset plays an important role as an important signal for flood and drought prevention.Demand for forecasting the onset of each rainy season is growing.At the same time,the transformation of fast and slow rainy season process in eastern China will directly affect integral deployment of prevention and control of droughts and floods in China.This paper integrates onset dates of first rainy season in South China(ODFRS＿SC)and Meiyu(OD＿Meiyu)over middle and lower reaches of the Yangtze River to classify types of rainy season processes in southern China and discusses the main atmospheric circulation anomalies and SST characteristics affecting each type.Research provides a new perspective to deeply understand multiple patterns of the main rainy season in southern China and find possible forecast factors.The main conclusions of this paper are as follows.(1)This paper analyzes the differences of precipitation,circulation and SST signals from March to April in early and late onset of flooding years based on ODFRS＿SC from 1961 to 2021.Result shows that while the easterly jet over the200 h Pa layer is weakened,cyclonic circulation anomaly exist on eastern Tibetan Plateau,potential height of the East Asian tropical ocean region is positively spaced and East Asian monsoon trough is strong,leading to the increase of the longitudinal pressure gradient in East Asia,so that southwest water vapor transport and local convergence are enhanced and ODFRS＿SC tends to be early;when the quasi-biweekly low-frequency disturbance signal from western Europe is transmitted eastward on the way of westly in early April,it will cause the local potential height and vorticity changed in East Asia,which lead to water vapor transport enhancement and wind convergence in South China,and conducive to the early ODFRS＿SC.The SST forcing signal is mainly from the southern hemisphere subtropical Indian Ocean and tropical Pacific Ocean,where the relationship between ODFRS＿SC and SIOD signal appears interdecadal strengthening,and the relationship with the SST gradient between eastern and western tropical Pacific Ocean appears interdecadal weakening.(2)Comparing precipitation evolution between years of early and late OD＿Meiyu over middle and lower reaches of the Yangtze River,it is found that there are significant differences in precipitation from May 5 pentad to June 4 pentad,with more(less)precipitation in the early(late)years on early stage of Meiyu and no significant differences on mid-late stages;in onset early years,western Pacific subtropical high(WPSH)westward and northward extension significantly in June,East Asia monsoon trough was strong,Somali trans-equatorial flow continued to be strong,while the opposite was true in years of onset late;OD＿Meiyu was also significantly influenced by eastward transmitted low frequency wave train in the middle and high latitudes of Eurasia.The El Ni(?)o signal in the previous winter interacts with the tropical Atlantic and Indian Ocean SSTs,which power can continue into summer and affect OD＿Meiyu.The interdecadal relationships between OD＿Meiyu with NINO3.4 index in the previous winter,the TNA index in spring and the IOBW index reach their strongest at the beginning of this century.(3)There are complex interdecadal transitions in the relationship between the ODFRS＿SC and OD＿Meiyu,so combination of ODFRS＿SC and OD＿Meiyu indices is used to classify the types of rainy season processes in southern China into four categories: early-type(early for ODFRS＿SC and OD＿Meiyu),late-type(late for ODFRS＿SC and OD＿Meiyu),first-phase early and second-phase late type(early for ODFRS＿SC and late for OD＿Meiyu)as well as first-phase late and second-phase early type(late for ODFRS＿SC and early for OD＿Meiyu).a phased increase in precipitation occurs in years with faster processes and vice versa.In early-type years,the intensity of East Asian subtropical jet(EASJ)is strong in the early stage of the process,WPSH intensity is strong in early stage and the first northward jump of which is early in late stage,while the opposite is true in late-type years.In the first-phase early and second-phase late type years,the position of EASJ continues to be southward,WPSH intensity is strong in early stage and the first northward jump is southward,which is the opposite in the first-phase early and second-phase late type years.So that,the abnormality of the strength and position of EASJ mainly affects integral advance of the process,such as early-type(late-type);the abnormality of WPSH intensity and position mainly causes sudden changes between early and late stages of the process,such as first-phase early and second-phase late type(first-phase late and second-phase early type).The relationship between the rainy season process in southern China and tropical SST evolution is not significant,and relationship between different rainy season processes and ENSO evolution signal is more complicated.