Inhomogeneous distributions of Meiyu rainfall in the Jiang-Huai basin, and associated circulation patterns

Using station rainfall data and NCEP/NCAR reanalysis from 1978-2007, we characterize the spatial variations of the heavy rainfall (Meiyu) during June-July over the Jiang-Huai basin (JHB, similar to 28 degrees-34 degrees N and 110 degrees-122 degrees E) in East China and their associated atmospheric circulations. An empirical orthogonal function (EOF) analysis of the Meiyu rainfall revealed 2 other significant modes besides the dominant basin-wide in-phase mode. They include a north-south dipole (out-of-phase) pattern (EOF 2) and a roughly east-west dipole pattern (EOF 3). During the 'south flood and north drought' (SF/ND) phase of EOF 2, the western Pacific subtropical high (WPSH) is displaced southward, which prevents the East Asian summer monsoon from reaching as far north as in normal years. A cyclonic anomaly circulation at 850 hPa occurs over Southeast China and an ascending (descending) anomaly motion south (north) of similar to 30 degrees N is seen over the JHB, which contributes to the excess (lack) of Meiyu rainfall south (north) of similar to 30 degrees N over the JHB during the SF/ND years. During the opposite phase of this mode, these anomaly circulation patterns are roughly reversed. During the 'east wet and west dry' phase of EOF 3, the WPSH is displaced eastward, and a cyclonic anomaly circulation centered south of Japan brings more moisture from the Yellow and East China Seas into the eastern part of the JHB; while southwesterly vapor fluxes past over the western part of the JHB and converges over North China, leaving the western part of the JHB relatively dry. During the opposite phase, the WPSH is shifted northwestward, and the anomaly moisture transports are reversed over the JHB.