Possible Linkage Between Winter Extreme Precipitation Events Over China And Synoptic-scale Transient Eddy Activity

Association of winter extreme precipitation events over China with synoptic-scale transient eddies is examined by using ERA40and NCEP/NCAR daily reanalysis data and daily rainfall data of756stations in China. Firstly, we focus on the possible linkage between extreme precipitation events in the middle and lower reaches of the Yangtze River and synoptic-scale transient eddy activity. Secondly, the scope of this research is expanded to further discussion the relationship between extreme rainfall events in southern China and transient eddies. Finally, transient eddy activity in typical years is analyzed as well. Our results have been summarized as follows:(1) There is a close linkage between extreme precipitation events in the middle and lower reaches of the Yangtze River and transient eddies. In winter, two branches of synoptic-scale transient eddies are accompanied by subtropical jet and polar-front jet, respectively. It is demonstrated that southern branch of transient eddies performs differently during high and low frequent years of extreme precipitation events. Overall, transient eddies become active, strong and spread continuously with long duration over Eurasia during high frequent extreme precipitation years, and vice versa for the low frequent ones. Moreover, intensity and energy of transient eddies both experience a similar variation with the daily frequency of extreme precipitation. Meanwhile, the northern branch of transient eddies and water vapor transport by transient eddies are favorable to the occurrence of extreme precipitation events.(2)Similarly, extreme rainfall events in southern China also have some connection with synoptic-scale transient eddy activity. Generally, during high frequent extreme rainfall years, transient eddies propagate more coherent, sustained and stronger with larger zonal component alone the propagation direction. While in the low frequent years, transient eddy becomes relatively weaker with larger meridional component in its propagation. On a daily scale, variation of daily extreme rainfall events is basically the same with the changes of transient eddy activity and intensity. Based on the analysis of the situation in the most frequent winter (i.e.1982/1983), we speculated that both the southern and northern branches of transient eddies play a role in the occurrence of extreme precipitation events over southern China.(3) Through a comparison between eddy activities in typical positive and negative years, it is found that, during these years, the frequency and intensity of extreme precipitation have a good consistency with transient eddy activity. Besides, downstream propagation of transient energy is likely to provide a favorable condition for the occurrence of extreme precipitation. In addition, transient eddy activity over the region named storm track may have a significant impact on the formation of wide range extreme precipitation events in China.Therefore, there is a close linkage between extreme precipitation events and synoptic-scale transient eddies, the propagation of transient eddies may be an important factor impacting extreme precipitation events, which provides a new perspective for the causes of extreme precipitation.