Study On The Process Of The Meiyu And Its Relationships With The Circulation

By using the data of 17 stations' precipitation in middle and lower reaches of Yangtze River and the NCEP's daily 500hpa height, this article using the chosen criterion of Meiyu ascertains the initial and final date of Meiyu after choosing out a credible criterion in some existing criterion of Meiyu. In this article, the essential characteristic during Meiyu and how to improve the criterion of Meiyu, the effect process of circulation during Meiyu, the characteristic of precipitation's distributing and the character between the precipitation and height are construed. The result indicate that:1) In interdecadal time scale, the subtropical high intensifying, instead of the ridge line, the line 582 can be used as the referenced criterion to verdict the Meiyu posture. 2) There is three basic precipitation regions in middle and lower reaches of Yangtze River. They have visible homologous relationship with the period of Meiyu in the temporal variation, In the late 1980's, the characteristic of precipitation in middle and lower reaches of Yangtze River begins to altering. 3) The initial and final date of the rainband in middle and lower reaches of Yangtze River have the obvious characteristic in interannual change in summer. The precipitation in middle and lower reaches of Yangtze River response to the positive abnormal in Eurasian continent and it also has the relationship with the interannual variability of the middle and high latitude of north Atlantic SST and the west Pacific warm pool. 4) During the Meiyu, the precipitation in middle and lower reaches of Yangtze River has the relationship with southern hemisphere circulation in early April and May. When the meridional circulation in middle and high latitude of South Pacific in April and May is rather obvious and the South Pole eddy is rather strong, the precipitation in middle and lower reaches of Yangtze River may be bigger than ever. When the early AAO is rather strong, the precipitation in middle and lower reaches of Yangtze River in the latter June and July would be enhanced, v.v.. The variation in the early AAO beget the rainband latitudinal discrepance of the China summer monsoon by means of impacting on the intensity and position of northwest Pacific subtropical high in the late summer. 5) Pointing out a circulation process which may influence the Meiyu in the corresponding period. When south Indian Ocean in southern hemisphere-zonal (meridional) circulation developing in middle latitude and in high latitude of Australian, the precipitation in middle and lower reaches of YangtzeRiver rather big (small) after some relative process affecting. 6) The main teleconnection structure (EAP, EU, WP) which affect the Meiyu in middle and lower reaches of Yangtze River is found by means of the varieties of teleconnection from beginning to the end of Meiyu. And the precipitation modes' characteristic of stations is the same as the representative variety during the Meiyu: mostly centralized along the coast, and affecting by the subtropical high, the rainband moves west and north in late of Meiyu. When the intensity of Meiyu increasing, the continental low in the northwest of Meiyu rainband is getting stronger and the west Pacific subtropical high is getting weaker. These results give out a brief summary of the variation about Meiyu in 1953-2000. But some conclusions need to be clarified by using the dynamics theory in future.