Study On The Influence Of Low-frequency Oscillation In The Past 30 Years On The Flood And Drought Years In The Pre-flood Season Of South China

Using 753 station's daily precipitation data in China,NCEP/NCAR daily reanalysis data from 1980 to 2017 and HYSPLIT data of NOAA driving track model,the lead-lag correlations between 10-20-d low-frequency(LF)precipitation and LF water vapor transport are comparatively analyzed in the pre-flood season of South China during typical flood and drought years,and verified using Hybrid Single Particle Lagrangian Integrated Trajectory model.On this basis,the influence of the southern hemisphere LF circulation system and cross-equatorial flow on the LF precipitation in flood and drought years in the pre-flood season of South China is further studied,and a conceptual model of the southern hemisphere LF weather is established.The main conclusions are as follows:(1)During both the flood and drought years,the primary LF water vapor sources include the south side of Lake Baikal,the northern Sea of Japan and the Yellow Sea,the South China Sea and the western Pacific Ocean.The southern Sea of Japan is another water vapor source in the flood years.In addition,there are two southwest warm water vapor flows(WVFs)and one southeast cross-equatorial WVF(CEWVF)in flood years in the LF phase of heavy precipitation.While in drought years,there is only one warm WVF in the southwest direction.(2)during both the flood and drought years,the key LF water vapor circulation systems primarily include the Lake Baikal anticyclone(LBAC),the Northeast China cyclone(NECC),the Philippine Sea anticyclone(PSAC)and the South China cyclone(SCC).The Australia anticyclone(AAC)is another key LF water vapor circulation system in the flood years.In both the flood and drought years,notable predictive signals start appearing at approximately-4 d.However,there is a difference in the time of occurrence of similar predictive signals.Some predictive signals only appear in the flood years.(3)A conceptual model of LF weather in the southern hemisphere affecting the LF precipitation in flood and drought years in the pre-flood period of South China is established.In the flood(drought)years,-4 d(-6 d),the LF Australia high was transformed into a positive phase,and the southeast LF CEF was initially formed.At-2 d(-4 d),the LF Australia high was further strengthened,and the southeast LF CEF was the strongest.At 0 d(-2 d),the LF Australian high reached the strongest,the southeast LF CEF began to weaken,at the same time,the LF Mascarene high reached the strongest,and the southwest LF CEF was initially formed.At +2 d(0 d),the LF Mascarene high began to weaken,the southwest LF CEF reached its strongest,and the southeast LF CEF disappeared.No matter in flood or drought years,the southeast LF CEF caused by the strengthening of LF Australia high on the weak basis is the strongest,and the southwest LF CEF caused by the weakening of LF Mascarene high on the strong basis is the strongest.