The 200-hPa Wind Perturbation in the Subtropical Westerly over East Asia Related to Medium-Range Forecast of Summer Rainfall in China

The 200-hPa wind perturbation (WP) in the subtropical westerly over East Asia (SWEA) has seldom been examined in previous studies, especially in connection with forecast of the summer rainfall in China. Based on the daily NCEP/NCAR reanalysis data and precipitation observations in China from 1 June to 31 August of 1960–2015, this study first systematically analyzes the spatiotemporal distribution features of the 200-hPa WP in the SWEA on different scales, especially during the Meiyu season in the Yangtze–Huaihe region and during the rainy period in North China, by using spectral decomposition and period analysis. It is found that in the 56-yr mean fields, the 200-hPa WP in the SWEA is collocated with the East Asian subtropical jet (EASJ), with the centers of the two systems coincidentally overlapped. The WP filed in the subtropical westerly mainly comprises planetary- and synoptic-scale waves. The quasi-stationary planetary-scale wave seems to determine the shape and intensity of the WP in the SWEA, while the synoptic-scale wave is closely related to the local central intensity of the WP. The daily evolution of the 56-yr mean fields shows that, following the northward (southward) movement of SWEA from 1 June to 31 August, the planetary-scale WP in the SWEA becomes gradually weakened (intensified) whereas the synoptic-scale WP is slightly intensified (weakened). The results also reveal that during the Meiyu season in the Yangtze–Huaihe region, the WP in the SWEA moves northward slowly around 37°–39°N, demonstrating a quasi-biweekly oscillation in its geographic location, which is largely attributed to the strong (weak) planetary-scale (synoptic-scale) component. On the contrast, in the summer rainy season over North China, the WP in the SWEA further strives northward beyond 40°N, showing both quasi-biweekly and weekly oscillations in its position; meanwhile, the planetary-scale wave in the SWEA becomes weakened whereas the synoptic-scale wave is enhanced. These salient variational features of the WP in the SWEA and its scale-dependent components may be useful for the medium-range forecast of the rain belt migration in eastern China.