A classification of the precipitation patterns during the Yangtze-Huaihe meiyu period for the recent 52 years

According to the daily meiyu period precipitation data from 1954 to 2005, the distribution of the predominant precipitation patterns in the Yangtze-Huaihe region is analyzed by the rotated empirical orthogonal function (REOF) method. The results show that there mainly are four kind meiyu precipitation modes in the region, namely, the Yangtze-Huaihe-type, south of the Yangtze River-type, Huaihe River-type, as well as Dongting Lake and Poyang Lake type. During the recent 52years, the occurrence probability of the precipitation modes of the Huaihe River-type becomes increasing and the probability of the other three modes is almost the same as each other. All of them are of 6-7-year and quasi -20- year periodical oscillations as suggested by use of the method of the maximum entropy spectrum with the Dongting Lake and the Poyang Lake type having an additional 2-3 -year period. It is worthwhile to note that the south of the Yangtze Rivertype tends to decrease while the Huaihe River type to increase since 1990. Based further on the REOF, the subdivision of the precipitation distribution pattern has been obtained by use of the method of the system cluster analysis. The precipitation distribution of the Yangtze-Huaihe type meiyu can be divided into 5 categories of pattern. The first and the second precipitation categories are the rich and poor meiyu pattern in the whole Yangtze -Huaihe region, respectively. The other three categories are characterized by rich in the south and poor in the north (RMSPN), rich in the north and poor in the south (RMNPS), rich in the south and north and poor in the middle (RMSNPM), respectively. They account for 15.4%, 40.4%, 15.4%, 19.2% and 9.6% of the total years, respectively. And each precipitation pattern has its different meiyu parameter characteristics.For example, the earlier onset date, later end date, longer meiyu duration and stronger rainfall are characteristic of the rich meiyu pattern, but the reverse conditions are of the poor one. As long as the rain belt pattern is concerned, the RMSNPM shows an earlier onset time, and the RMNPS does a later one with the RMSPN of the earliest onset time. All kinds of the distribution are corresponding with the geography location. Furthermore, in addition to the obvious interannual characteristics, the meiyu precipitation pattern also takes on the interdecadal variation. Both the poor meiyu and the RMNPS pattern can been found mainly before 1970s. Then the poor meiyu pattern occurred during 1970s-1990s. And the rich meiyu and the RMSPN pattern are mostly presented in 1990s, while the poor meiyu and the RMNPS pattern are met since 2000. In the end, according to the NCEP/NCAR reanalysis data, the different atmospheric circulation features, including the location of the South Asian High, the upper-level westerly and easterly jet, the permanence and maintenance of the western North Pacific subtropical high and the mid high latitude blocking high, the vertical velocity, as well as the water vapor transport during the summer monsoon, are discussed for the 5 kind meiyu precipitation patterns, respectively.