Comparative Investigation on Meiyu Parameters in Different Subregions of the Changjiang-Huaihe Valley

Using the rotated empirical orthogonal function (REOF) analysis of the averaged June to July rainfall field, the Meiyu amount and the Meiyu onset data (MOD) during the years from 1957 to 2003, the Changjiang-Huaihe Valley is divided objectively into three subregions, namely, the center (CEN), the southeast (SE) and the northwest (NW). The time sequence of the Meiyu onset and ending dates is SE, CEN and NW. It is consistent with the temporal change of geographical location of the rain band.The periodic variations of Meiyu parameters are analyzed using the binomial coefficient weighted average method and the Morlet wave analysis.Result shows that different subregions have different periodic variations. The long-term trend of the Meiyu parameters in the NW is obviously different from that in the CEN and the SE. The Meiyu duration and the Meiyu amount in the CEN and the SE are both increasing, the Meiyu ending dates of the above two regions are delaying. The opposite case is found in the NW. The MOD in the SE is advancing, while there is no apparent trend in the CEN and the SE. The atmospheric circulation patterns related to the Meiyu abnormalities of the three subregions are analyzed by using the NCEP monthly data of height fields during the years from 1957 to 2003. The results point out that the composite differences of the 500 hPa height field in the Northern Hemisphere between the more and the less Meiyu years of the three subregions are all of the wavelike structures with positive and negative values in alternation. The distribution of the positive and negative difference centers in the SE subregion is similar to that of the CEN, but with a more eastward and southward shift than the CEN. The wavelike distribution pattern in the NW subregion is opposite to that in the CEN, and it is also opposite to that in the SE at low latitudes and in the regions along the North Pacific, the North America and the North Atlantic.The distribution patterns of the positive and negative composite difference centers in the years with the earlier and the later MOD in the SE and the NW are both similar to that in the CEN. However, the position in the SE has an eastward deflection and that in the NW has a westward shift. The distribution pattern of NW is opposite to that of the SE.