The influence of wave trains in mid-high latitudes on persistent heavy rain during the first rainy season over South China

Based on daily precipitation data from the Chinese Meteorological Administration and reanalysis data from the National Centers for Environmental Prediction-Department of Energy, the character of low-frequency precipitation variability during the first rainy season (April–June) over South China and its corresponding atmospheric circulations in the mid-high latitudes are investigated. The results show that the precipitation anomalies during this period exhibit obvious quasi-biweekly oscillation (QBWO) features, with a period of 8–24 days. The influence of wave trains in the mid-high latitudes to low-frequency persistent heavy rain event (PHR-LF event, the 8–24-day filtered precipitation larger than one standard deviation of filtered time series and persisting at least three days over South China) is further discussed. During the first rainy season over South China, there are two low-frequency wave trains in the mid-high latitudes associated with the PHR-LF event—the wave train crossing the Eurasian continent and the wave train along the subtropical westerly jet. Analysis of wave activity flux indicates that the wave energy disperses toward eastern China along these two low-frequency wave trains from north to south and from west to east, and then propagates downward over South China. Accordingly, the disturbance of the relative vorticity of the cyclonic anomalies over eastern China is strengthened, which enhances the meridional gradient of relative vorticity. Owing to the transport of low-frequency relative vorticity and geostrophic vorticity by meridional wind, the ascending motion over South China intensifies and lasts for a long time, triggering a PHR-LF event. In addition, the tropical system is also a key factor to PHR-LF event. The QBWO of the convection over the South China Sea provide moisture for PHR-LF events, maintaining persistent rainfall and vertical ascending motion over South China.