Analysis Of The Meiyu Gernal Circulation Characteristics And Impact Factors Over The Changjiang-Huaihe Valley

The Meiyu over the Changjiang-Huaihe valley is the importantpart constituent of summer rain band in China. There are significant correlations between the abundance or deficiency of the Meiyu rainfall and the formation and continuance of wet and drought disaster over the Changjiang-Huaihe region. So it is valuable to study the abnormal cause of the Meiyu and provide the reference for short-term forecasting on Meiyu.Based on the onset date, the end date, the length, the precipitation and the intensity of Meiyu characteristic datum provided by Jiangsu meteorological administration over the Changjiang-Huaihe valley from 1954 to 2001, the temporal evolution and spatial distribution characters of Meiyu are investigated in terms of harmonic analysis methold, empirical orthogonal function (EOF), rotated empirical orthogonal function (REOF), singular value decomposition (SVD) and so on. The results show that there are significant difference characters of general circulation in the corresponding period between the good meiyu year and poor meiyu year. The precipitation of Meiyu is associated to south Asia high, west Pacific Ocean subtropical high (shortened subtropical high) and sea surface temperature (SST) of Pacific Ocean. The main conclusions are as follows:(1) The Meiyu has the marked temporal evolution and spatial distribution difference. There are prominent inter-annual, inter-decadal, periodic, secular tendency variabilities and catastrophe characters in the precipitation of the Meiyu. The motion of subtropical ridge at 120°E is anastomotic with the inter-penadly variations of average onset date or end date of the Meiyu over the Changjiang-Huaihe region from the forth pentad of June to the second pentad of July.(2) The general circulation phases have notable difference between the good Meiyu year and the poor Meiyu year. There are obvious positive or negative correlations between the precipitation of Meiyu and south Asia high, subtropical high and SST of northwest Pacific Ocean.(3) South Asia high has two equilibrium states located in Iran plateau at 75°E westerly (named west south Asia high pattern) and in the Tibet plateau at 75°E easterly (named east south Asia high pattern). The precipitation of Meiyu is marked increasing (decreasing) over the Changjiang-Huaihe region, which is responsible for the west (east) south Asia high pattern, intensity of polar vortex, subtropical westerly jet, equatorial easterly jet, intensity of subtropical high, water-vapor transfer of the air current spanning the eqator in the southern hemisphere and southwest air flow from the subtropical high westerly, the intensity of convergence over the Changjiang-Huanhe, pumping action in the altitude-low level, ascending motion and convective activity are amplification (weaken), and vice versa.(4) There is dramatic positive or negative correlation between the abundance or deficiency of the Meiyu rainfall over the Changjiang-Huaihe valley and the SST variation of northwest Pacific Ocean. The results show that the precipitation of Meiyu is increasing (decreasing) in the corresponding period when SST of north Pacific Ocean in the west wind drift from Jannuary to March or SST of west Pacific Ocean in the black current and warm pool from June to July is warmer (colder) than average. The results of SVD diagnostic analysis are consistent with those of composite analysis.