The Northeast Cold Vortex (NECV) is an important weather system in the middle and high latitudes in East Asia. Its time scale is synoptic, yet the frequent activities of NECV have significant "climate effect" which influences not only the monthly temperature in the lower troposphere in Northeast China but also the Meiyu rainfall in East Asia. On the basis of ERA-40 reanalysis data provided by ECMWF, the "climate effect" of NECV and its relationship with Meiyu in East Asia are studied. It is shown that there is significant correlation between NECV during the Meiyu period and rainfall amount: strong NECV corresponds to more Meiyu rainfall and weak NECV corresponds to less rainfall. In strong NECV years, the dry and cold air from the north is led to the south by NECV, converges with the lower-level warm and wet southwesterly on the north verge of Meiyu region, thus forms an unstable stratification of "upper dryness and lower wetness" . Triggered by ascending motion, the Meiyu rainfall amount is more than usual. It is on the contrary in weak NECV years. The anomalous SST in north Pacific in the previ-ous year may be a factor that results in the anomalous NECV at Meiyu period. The land-sea thermal contrast in summer facilitates NECV, while that in winter inhibits NECV. All of the above provide a meaningful result for the short-term climate prediction of NECV and Meiyu.
HE JinHaiWU ZhiWeiJIANG ZhiHongMIAO ChunShengHAN GuiRong
Although Meiyu rainfall has its in-phase spatial variability over the Changjiang-Huaihe River Valley (CHRV) in most years, it is distributed in some years like a seesaw to the north and south of the Changjiang River, when the precipitation tends to be nearly normal throughout the valley, which would inevitably increase difficulties of making short-term prediction of the rainfall. For this reason, EOF analysis is made on 15 related stations’ precipitation from June to July during 1951─2004, revealing that the EOF2 mode shows largely a north-south seesaw-like pattern, and thereby classifying Meiyu patterns into two types: "northern drought and southern flood (NDSF)" and "northern flood and southern drought (NFSD)". Afterwards, the authors investigated ocean-atmospheric characteristics when these two anomalous types occured using the NCEP reanalysis (version 1) and the extended reconstructed SSTs (version 2). The results show that in the NDSF years, the low-level frontal area and moisture convergence center lie more southward, accompanied by weaker subtropical summer mon- soon over East Asia, with the western Pacific subtropical high and 200 hPa South Asia High being more southward. Both the Northern and Southern Hemisphere Annular Modes are stronger than normal in preceding February; SST is higher off China during boreal winter and spring and the opposite happens in the NFSD years. Also, this seesaw-form Meiyu rainfall distribution might be affected to some degree by the previous ENSO event.
