Analysis On Low Frequency Oscillation Characteristics And Impact Factors Of Persistent Heavy Rainfall Over South China

Based on daily rainfall data collected at observational stations in China,NCEP/NCAR reanalysis data,NOAA daily outgoing long wave radiation(OLR)and sea surface temperature(SST)data,and the Real-time Multivariate(RMM)MJO index from Australian Meterological Bureau from May to August during 1982-2011,the basic statistical characteristics of 12-30d persistent heavy rainfall over South China(SC)are analyzed by selecting low frequency rain events.The corresponding low frequency atmospheric circulation pattern,and the source and propagation of low frequency signals are revealed using the phase-composite method.The possible impacts and approaches of the low frequency air-sea interaction,the propagation activity of tropical Madden-Julian oscillation(MJO)are also investigated.The main conclusions are as follows:Summer rainfall over SC has a significant characteristic of low frequency(12-30d)oscillation,and the maximum center of the precipitation and rainfall intensity are both located in the southeast coastal areas of SC.Lasting days of persistent heavy rainfall are from 3 days to 6 days,and the 3-4 days lasting events occur most.The precipitation of 4 days lasting events takes up the largest ratio of all low frequency rain events.The event number of persistent heavy rainfall of SC gets it peak around the end of pre-rainy season(June).The rain band spreads from the southeast to the northwest of SC during the persistent heavy rainfall period.Accompanied with the persistent heavy rainfall,a strong low frequency cyclone controls SC and adjacent sea area,leading to strong ascending motion.Meanwhile,a low frequency anticyclone is located over the region from the South China Sea(SCS)to the Philippines Sea(PS)in the lower atmosphere.The strong southwesterly winds continuously bring water vapor from the north of the Bay of Bengal and the SCS to SC,leading to water vapor convergence over there.The low frequency signal in the lower atmosphere originates from the northwestward propagation of the low frequency oscillation over the SCS-PS and the tropical western Pacific,which is associated with the westward extension and eastward retreat of the western Pacific subtropical high.In the upper atmosphere,a strong low frequency cyclone centered over the north of SC around(22°-45°N,95°-130°E)is coupled with a low frequency anticyclone located over the Bay of Bengal to the SCS,which provides a strong divergent environment over SC,and it is favorable for the enhancement of convergence in the lower atmosphere and low frequency ascending motion.As a result,persistent heavy rainfall events enhance and maintain over SC.The low frequency signal in the upper atmosphere comes from the southeastward propagation of the low frequency Rossby wave trains.In the process of the low frequency rain,accompanied with the evolution of the low frequency circulation in the lower atmosphere,the low frequency sea surface temperature anomaly(SSTA)reflects obvious low frequency oscillation.What's more,the evolution of the sea surface temperature and the atmospheric circulation in the lower atmosphere presents the phase difference of ?/2,which means the existence of the low frequency air-sea interaction.The low frequency SSTA can be modulated via the process of cloud radiation and heat fluxes driven by the low frequency circulation anomaly,while the low frequency SSTA can significantly influence the atmospheric circulation in turn by changing the stability in the lower atmosphere.Such the low frequency air-sea interaction process favor the propagation of the low frequency signal in the lower atmosphere,thereby affects the development and termination of 12-30d persistent heavy rainfall over SC.The propagation activity of tropical strong MJO has an obvious modulating effect on the 12-30d persistent heavy rainfall in summer over SC.When the convective center of MJO is mainly over the Indian Ocean,the anomalous circulation pattern makes for the occurence of the persistent heavy rainfall.Especially in phase 1,the western Pacific subtropical high extends westward,and the positive anomaly of the geopotential height exists over the tropical western Pacific,accompanied with a strong anomalous anticyclone,which makes anomalous southwester control SC,leading to strong water vapor transportation to SC,and anomalous water vapor convergence over SC.Meanwhile,obvious ascending motion exists over SC,which is favorable for the upward transportation of water vapor and the development of the convective activity.Therefore,the number of days of 12-30d persistent heavy rainfall over SC gets it peak in the phase 1 of strong MJO.When the convective center of MJO gets to the western Pacific,the anomalous circulation can restrain the occurrence of persistent heavy rainfall,particularly in phase 7,the anomalous circulation pattern is opposite to that in phase 1,so the number of days of 12-30d persistent heavy rainfall is least in phase 7.