Impacts of the Zonal Position of the East Asian Westerly Jet Core on Precipitation Distribution During Meiyu of China

The east-west location change of the East Asian westerly jet (EAWJ) at 200 hPa during Meiyu and the associated spatial distribution variation of precipitation in the middle-lower reaches of the Yangtze River (MLYR) are investigated by using the 40-yr NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) pentad mean reanalysis data and daily precipitation observation data from 1958 to 1997. The results show that there are two areas over which the 200-hPa EAWJ center appears most frequently during the Meiyu period: one is the western Pacific (WP) and the other is the East Asian continent (EAC). During the Meiyu period, the westerly jet over the EAC is weak,and the core of the westerly jet over the WP splits up with reduced intensity and disappears by the end of Meiyu. The changes in the location and intensity of the westerly jet are associated not only with the starting and ending dates of Meiyu, but also with the spatial distribution and intensity of precipitation in the MLYR. It is found that when the westerly jet core in the upper troposphere is located over the WP and is coupled with an 850-hPa southwesterly jet, heavy precipitation accompanied by strong convergence and plenty supply of water vapor, occurs in the lower reaches of the Yangtze River. If the 200-hPa westerly jet core is located over the EAC, and without an 850-hPa southwesterly jet, only weak precipitation occurs in the MLYR. Therefore, the longitudinal location of the EAWJ core plays an important role in determining the upper- to lower-level circulation structure and the spatial distribution of heavy precipitation in the MLYR during the Meiyu period.