The Madden-Julian Oscillation (MJO) plays a regulatory role on rainfall in China, studying the impact of MJO on autumn rainfall in West China has important significance for improving intraseasonal prediction ability of autumn rainfall in West China. Anomaly composite analyses have been conducted to investigate the impact of MJO on autumn rainfall in West China and the circulation background fields corresponding characteristics by using the real-time multivariate MJO index from Australian Meteorological Bureau and the daily rainfall of gauge stations in China and the daily NCEP reanalysis data from September to October during 1979-2012. The results show that autumn rainfall patterns change from enhanced to suppressed in West China when the main MJO convective body propagating from west to east. When the main MJO convective body is situated over the central and western Indian Ocean (at phase 1~2), autumn rainfall in West China is above normal level. When the main MJO convective body is situated over the eastern Indian Ocean and central and western Pacific Ocean (at phase 3~8), autumn rainfall in West China is below normal level. The maximum positive and negative rainfall anomalies are found when the main MJO convective body is situated over the central and eastern Indian Ocean and western Pacific Ocean, exactly at phase 2 and phase 7. At the same time, analyzing the corresponding anomalies in the upper air circulation field and the middle-lower flow field, water vapor transportation and vertical motion at various MJO phases. At phase1 and phase 2, we find that the circulation feature is two trough-one ridge pattern or west low-east high pattern over China, the activity of cold air in West China is frequent at this point, and water vapor from Bay of Bengal, South China Sea and Western Pacific Ocean transports to West China is strong, which will helps dry-cold air meets with warm-wet air in West China, the water vapor convergence and upward motion are significant, such environment is in favor of rainfall enhancement. In contrast, at phase 7, we find that China is under the control of the westerly long wave high pressure ridge, and water vapor from the three ocean areas transports to West China is weak, the water vapor divergence and downward motion are strong, such environment is in favor of rainfall suppression.
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