Persistent Heavy Rainfall In The Early Flood Season In South China And Its Relationship With Tropical Atmospheric Intraseasonal Oscillatio

Based on daily precipitation data supplied by the Chinese meteorological administration,hourly reanalysis datasets provided by the ECMWF and daily outgoing long wave radiation supplied by the NOAA,the climatic characteristics and circulation situation of the persistent rainstorm in South China(SC)from April to June in 1979-2020 were systematically discussed.The interaction between specific humidity and circulation field at the background-scale,the intra-seasonal-scale and the synoptic-scale,and its influence on persistent heavy precipitation over the SC during the April-June rainy season were quantitatively diagnosed and analyzed.On this basis,the possible influence mechanism of tropical atmospheric intra-seasonal oscillation on the development and anomalous persistence of rainstorm over the SC during the April-June rainy season was emphatically discussed.The main results are as follows:(1)From 1979 to 2020,persistent heavy rainfall events(PHREs)over the SC during the April-June rainy season have occurred seventy times,mostly from mid-May to late-June,exhibiting significant synoptic-scale disturbances and intra-seasonal oscillation(10-30-day)features.Its frequency and intensity have annual differences.Vertically integrated moisture flux convergence(VIMFC)can well represent the variation of the PHREs.Through the multi-scale quantitative diagnosis of precipitation rate,it is found that the PHREs over the SC during the April-June rainy season is mainly affected by low-frequency-scale circulation disturbance(10–30-day),background water vapor and background circulation(greater than 30 days),and water vapor convergences play a major role.(2)The background scale and low-frequency scale precipitation components mainly contribute to the PHREs,while the synoptic scale precipitation component contributes more to the peak value of the PHREs.In the years with frequent PHREs,the background water vapor in SC is more abundant.The intensity of the background southwest winds transporting water vapor is stronger.At the same time,there is a strong low-frequency disturbance kinetic energy,and the low-level water vapor converges to a large number of local areas,making the upward movement continuously strengthened.In addition,the active synoptic scale disturbance kinetic energy in the middle and high latitudes is conducive to the continuous southward movement of dry and cold air,which significantly affects the PHREs in SC.Besides,the quasi-biweekly oscillation of the tropical atmosphere originating from the westward or northwestward propagation of the Northwest Pacific are closely related to the PHREs over the SC during the April-June rainy season.(3)In addition to the impact of atmospheric low-frequency oscillation,the occurrence of the PHREs are more likely to be triggered by strong background scale disturbance,which have a very important impact on the duration and frequency of rainstorm during the April-June rainy season.For the pre-summer rainstorm lasting with longer than 5 days in SC,the role of background water vapor and background circulation are strengthened,and the convergence of background water vapor are dominant,while the secondary contribution of the general PHREs become a synoptic scale circulation disturbance.Besides,the general PHREs are closely related to the westward quasi-biweekly oscillation of the tropical atmosphere.In addition to the quasibiweekly oscillation of the tropical atmosphere spreading to the west,the long-term PHREs over the SC during the April-June rainy season also have the characteristics of eastward intraseasonal oscillation of the tropical atmosphere.(4)During the long continuous rainstorm,from the Indian Peninsula,the Bay of Bengal to the SC are the positive anomaly area of background specific humidity.The strong background westerly winds continuously transport a large amount of water vapor to SC,creating a very favorable warm and humid environment for the continuous heavy precipitation.At the same time,under the control of the strong low-frequency anomalous cyclonic circulation in SC,the local low-frequency disturbance kinetic energy is significantly increased,which is conducive to the local horizontal convergence and vertical transport of water vapor.The South China Sea Northwest Pacific atmospheric quasi-biweekly oscillation contributes to the construction of the large-scale meridional vertical circulation circle between South China Sea and SC by promoting low latitude circulation anomalies.In addition,the low-latitude large-scale average circulation is further developed in energy conversion,which is conducive to the strengthening and western extension of the western Pacific sub-height active here during the April-June rainy season.The low-frequency activity of tropical convection makes the warm and humid air flow from the South China Sea continuously transported to SC,and the strong and continuous water vapor transport promotes the occurrence and continuation of extreme precipitation.The restrained convection activity caused by the cold sea surface temperature anomaly in the northwest Pacific promotes the formation of the abnormal anticyclone in the northwest Pacific,and the convergence of cold and warm advection causes the continuous release of unstable energy,which is conducive to the enhancement and continuation of rainstorm.In contrast,the background of the general PHREs is weaker than that of humidity,and the lack of closed low-frequency abnormal cyclonic circulation makes it impossible to generate vertical motion conditions conducive to the sustained maintenance of the rainstorm.Its lowfrequency convective activity intensity is significantly weaker than the long-term PHREs,and the low-level synoptic scale water vapor convergence(divergence)area is similar to the longterm PHREs,but the intensity is significantly enhanced.It shows that when there is a strong synoptic scale disturbance in SC,it is conducive to the production of the PHREs in the preflood season,but the long-term anomaly maintenance of rainstorm mainly depends on the strength of the background scale disturbance.During the pre-summer general PHREs,the SC is mainly affected by the significant convection suppression center that is constantly deepening and developing over the Philippine Sea and moving westward.Its unusually strong lowfrequency southerly winds continuously transport water vapor to the SC.However,compared with the long-term PHREs,the intensity and spatial scale of the longitudinal vertical circulation of the general PHREs are small,and low-frequency water vapor transmission is obviously insufficient.