The Impact Of Synoptic-scale Wave-train On Summer Rainfall In The Middle And Lower Reaches Of The Yangtze River

Based on the NCEP/NCAR daily reanalysis datasets(2.5°×2.5°), the daily precipitation datasets of743weather stations in China and outgoing long-wave radiation(OLR) data from NOAA during1979～2007, using correlation analysis, synthetic analysis et al. conventional statistic methods, the correlation coefficients between the pentad average rainfall over the middle and lower reaches of the Yangtze River in June and the pentad average850hPa zonal wind are computed, it points out that there is a northwest-southeast wave-train from high latitude to low latitude in June. Then, analyze the relationship between the synoptic-scale wave-train and precipitation in June and summer (June-August) low-frequency rainfall over the Yangtze River. The main conclusions are as follows:(1) The correlation coefficients between precipitation and850hPa zonal wind are calculated in June. The result shows that positive and negative high-value centers alternate with arrangement from high latitude to low latitude, namely wave-train characteristic with "-+-" phase, passing the Yellow River basin, the Yangtze River and the Philippines from the North to the South. The composite analysis of85OhPa zonal wind and E-P flux in typical drought or flood pentads suggests that when the precipitation is little in the Yangtze River in June,850hPa zonal wind field exists three obvious high value areas which performance as wave-train characteristic with"+-+"phase propagating from northwest to southeast, the E-P flux field exists a corresponding wave-train signal, when more precipitation, there are also three obvious high value areas, but with"-+-"phase, the characteristic of wave-train and E-P flux are relatively weak.(2) Through synthetic analysis and a case study, the results indicate that when the precipitation is little in the Yangtze River in June, there is a northwest-southeast wave-train in the east Asia and west Pacific from high to low latitude. When the wave-train exists, the Yangze River is easy to be drought. However, its flood is mainly due to the summer monsoon, the influence of wave-train is very weak. Moreover, the new wave-train’s extended distance (passing the Yellow River basin, the Yangtze River and the Philippines from the north to the south) and time of duration is both shorter than EAP or OKJ wave-train, it is stronger only below500hPa.(3) Defines an index to describe the strength of wave-train, then investigates its impact on rainfall in the Yangtze River in June. The results show strong negative correlation between them, in other words, the bigger the index, the less the rainfall. When the wave-train exists, the convection is very active in the South China Sea (SCS) and west Pacific, and restrained in the Yangtze River. Moreover, abnormal cyclone circulation in SCS and west Pacific plays a role and goes against the subtropical high getting into SCS. These factors are not conducive to the precipitation in the Yangtze River. Similarly, defines a wave-train index of June in every year to investigate the wave-train’s monthly scale characteristic. The results show that the new wave-train is reflected in monthly scale, but not as obvious as OKJ wave-train. The two kinds of waves show opposite influence on precipitation in the Yangtze River. (4) It is found by power spectral analysis that summer precipitation in the Yangtze River has a main period:30～60day. Synthetic analysis of the low frequency wind fields in the whole period shows that the east Asian summer monsoon with abnormal anticyclone/cyclone appears alternately in the middle and lower Yangtze River, the north of the South China Sea and the western Pacific region in low layer. It spreads to the southeast coast of China with east-west type of30-60d oscillation, which modulates the western Pacific subtropical high getting into (out of) SCS or the Yangtze River. At the top, the circulation type is coupling the low layer circulation, which regulates the South Asia High anomalous moves east (west).These factors may form an environment which is conducive to precipitation or not.(5) Analyze the characteristics of low frequency zonal wind and E-P flux in1-8phase, the results suggest that the low frequency zonal wind does not show the distribution characteristic of wave-train structure throughout the whole oscillation cycle. It proves that the impact of wave-train on summer low-frequency rainfall is little, but very significant in June. In addition, the low frequency E-P flux fields show that when the precipitation is little, the E-P flux is much greater through the Yangtze River from high latitude to low latitude. Especially in the negative phase of low frequency precipitation (except for7th phase), the wave activity flux is significantly increased, its vector direction is partial east. It points out that E-P flux plays a weakened or strengthened role in precipitation.