A comparative study of the interannual variation of summer rainfall anomolies between the Huaihe Meiyu season and the Jiangnan Meiyu season and their climate background.

Based on monthly rainfalls of the 160 China stations, the NCEP/NCAR reanalysis dataset and the HadISST1 dataset for 1951-2007, the generally considered Meiyu is divided into the Huaihe Meiyu and the Jiangnan Meiyu according to the location of maximum precipitation. A comparison study is then performed between these two Meiyu components for the June-July mean situation. It shows that both the Huaihe Meiyu and the Jiangnan Meiyu are characterized by obvious interannual variability. In addition, the Huaihe (Jiangnan) Meiyu underwent significant increased (decreased) trend since the late 1990s although no significant general trend is found for the period 1951-2007. The circulation features associated with the two Meiyu components are then investigated through composite analyses. It reveals that when the Huaihe Meiyu is anomalously strong, the geopotential height field is characterized by double blockings over the midlatitudes, which are located to the east of the Urals and to the east of the Okhotsk Sea, respectively. In contrast, the variation of the western Pacific subtropical high (WPSH) is weak and insignificant. At upper troposphere, the East Asian jet stream shifts southward slightly, with weak westerly anomalies located just above the Huaihe valley, which may cause anomalous ascending motion in the Huaihe valley through the secondary circulation at the entrance of the jet stream. In addition, the anomalous low-level convergence zone induced by the cold northerlies from the north and the warm southwesterlies from the west side of the WPSH are also located around this area. Therefore, all of these circulation features favor more precipitation in the Huaihe valley. When the Jiangnan Meiyu is anomalously strong, the midlatitude geopotential height field is also characterized by double blockings, with the locations extending much westward especially for the one around the Okhotsk Sea. Meanwhile, the WPSH is anomalously strong and also extends westward, with the anomalous positive center around north of Philippines. This implies the amplification of the anticyclone around Philippine, which favors the convergence around south China. On the other hand, the East Asian jet stream shifts much southward compared with the Huaihe Meiyu situation, which may cause anomalous ascending motion around south China through the secondary circulation at the entrance of the jet stream. Therefore, these circulation features favor more precipitation in south China, inducing the Jiangnan Meiyu to become strong. The further analyses on the sea surface temperature (SST) show that the SST anomalies around the Bering Sea may play a key role in the variation of the Huaihe Meiyu. Positive SST anomalies in this area commencing from the preceding winter (DJF) to the simultaneous Meiyu period (JJ) are usually accompanied with a strong Huaihe Meiyu event. As to the Jiangnan Meiyu, it is significantly related to the simultaneous SST anomalies around Philippines, which may in turn be associated with the preceding SST anomalies around the eastern equatorial Indian Ocean. 