Quasi-biweekly Oscillation Of Persistent Heavy Rain Over China In Summer

The quasi-biweekly oscillation of persistent heavy rain over Southeast China (SC)、Yangtze-Huaihe river basin(YHB) in1979-2009and the associated low frequency circulation are studied using daily rainfall station data in China, OLR data and NCEP/NCAR reanalysis dataset. The source and propagation of the low frequency signal are investigated by composite method. The quasi-biweekly oscillation of sea surface temperature(SST) in key area and its possible connection to persistent heavy rain over YHB are also studied.Results show that the summer rainfall over both SC and YHB have the characteristics of quasi-biweekly oscillation. According to this, we define the persistent heavy rain(low frequency rain events) of these two area. The event number of persistent heavy rain of SC gets it peak around June10and July1. While in YHB the persistent heavy rain occurs frequently in June and July. Lasting days of persistent heavy rain in these two area are from3days to8days and the3days lasting events occur most. The precipitation of five-day low frequency rain events takes up the largest ratio of all low frequency events.During the period of persistent heavy rain over SC, in the low atmosphere a strong low frequency cyclonic circulation is located over SC, while a strong low frequency anticyclone is over South China Sea (SCS) to Western Pacific region. The persistent low frequency water vapor is supplied from the Bay of Bengal-. SCS and the Philippine Sea, and converges over the south of the Yangtze River. At the same time, a low frequency anticyclone controls Northeast China in the upper troposphere. This circulation cooperating with a low frequency anticyclone west of it and a low frequency cyclone located over the coastal water, is favorable to a divergence environment in SC. Thus, the upward motion is enhanced. About7days before the low frequency rainfall, there was a low frequency anticyclone over about150oE in the lower level and it strengthened and propagated to the coastal water of Southeast China, while the low frequency cyclone over the SCS propagated northwestward to SC. At the same time, the western Pacific subtropical high has an apparently westward extension.The low and high circulations cooperating with each other result in the low frequency rainfall over SC.Accompanied with the persistent heavy rain, a strong low frequency cyclonic is located over YHB and a strong low frequency anticyclone is over South China Sea (SCS) to Philippines Sea (PS) region in the lower troposphere. At the same period, the water vapor transportation to the west of the low frequency anticyclone takes water vapor to YHB. The southeast propagation of low frequency oscillation over the loop of Yellow River and the northwest propagation of low frequency oscillation over the northwestern Pacific at850hPa, are two dominant signals of low frequency circulation associated with persistent heavy rain. In the upper troposphere, a strong low frequency cyclone centered about80oE-100oE,37.5oN is located over the northwest of YHB. Meanwhile a low frequency anticyclone is located over northwest of the cyclone. Thus, a low frequency wave train is formed from Northwest China to the SCS to PS region. Before the peak of persistent heavy rain, the Western Pacific subtropical high and the South Asia high get closer to each other, and then departure to each other gradually after the peak of persistent heavy rain.Further, the low frequency SST over northwestern Pacific has a significant positive correlation with low frequency rainfall in YHB. On the quasi biweekly scales, between ocean and atmosphere is negative feedback relationship which can maintain the low frequency oscillation in the atmosphere and provides a good condition for persistent heavy rain over YHB...