WAVE-MEAN FLOW INTERACTION AND FORMATION OF BLOCKING--PERSISTENT ANOMALOUS WEATHER IN CHINA IN THE SUMMER OF 1980*

In the summer of 1980,serious persistent abnormal weather occurred over vast areas in China.While record-breaking cold and flood were observed in the reaches of the Changjiang and Huaihe Rivers,severe hot wave and drought dominated the entire northern China.The long-lasting disastrous weather is mainly due to the stable development and maintenance of blocking anticyclone over the northeastern Asia.This study aims at the understanding of the roles of time-varying weather system transport in the formation of the blocking.It was shown that during this period,there appeared continuous generation of synoptic-scale perturbations along the strong baroclinic zone over Europe and the western Asia.While such perturbations propagated eastward,energy conversion occurred.At equivalent barotropic layer,with weak dissipation,such energy conversion was subjected to the so-called bi-directional principle:while the energy of the synoptic-scale system cascaded to smaller scale system,a much larger portion was transferred to the blocking system with larger scale.Potential vorticity diagnoses also revealed that the transient weather systems played the roles of maintaining the mean anticyclonic vorticity to the south,and mean cyclonic vorticity to the north,of the westerly jet,and exciting strong anticyclonic vorticity growth and corresponding geopotential height increase in high latitude area downstream of the westerly diffluence region.The research also showed that,the intensity of the forcing of the blocking formation via wave-mean flow interaction in this Asian case was much stronger than that occurring in the western Europe in the summer of 1976.It was therefore concluded that when persistent abnormal weather in the northern China was studied,in addition to the subtropical weather systems,attention should also be drawn to the development of baroclinic zone over Europe and the western Asia,and the propagation and transfer properties of the synoptic systems embedded in the baroclinic zone.