The Relationship Between East-west Movement of Subtropical High over Northwestern Pacific and Precipitation in Southwestern China

The Northwest Pacific subtropical high (abbreviated as subtropical high) is one of the large-scale circulation systems that affect the climate in East Asia. The seasonal north-south movement of the subtropical high has a very important effect on the position of summer rain belt in eastern China. Most of the related researches focus on the movement of the subtropical high in the north-south direction, while less attention is paid to the east-west displacement, especially the influence on the climate in Southwest China. Southwest China is located in the eastward part of the Qinghai-Xizang Plateau, with high altitude and low latitude. Though the influence of the subtropical high on the climate in Southwest China is not as direct as that in the eastern part of China, it is found that the change of subtropical high also plays a very important role by affecting the configuration of atmospheric circulation system. Therefore, it is of great significance to study the anomalous movement of the east-west position of the subtropical high and its influence on the climate in southwestern China in order to understand causes of the climate anomaly.To further understand the influence of the change of the subtropical high, a new index to measure the east-west position of the summer subtropical high is defined by the regionally averaged relative vorticity at 700 hPa in different regions based on the climatology circulation. Results show that the index can characterize the east-west displacement of the anticyclonic circulation of the subtropical high more objectively and qualitatively, and it can also reflect the linkage between the subtropical high and meridional circulation over East Asia. When the subtropical high is east (west), there is a negative-positive-negative (positive-negative-positive) meridional anomalous wave train over East Asia. Comparing with other indices, the new index can better reflect the seasonal movement of rain belt position in eastern China in summer and has a significant correlation with precipitation in Southwest China in June and July. When the subtropical high is west, there is less precipitation in western and southern Sichuan, and most in Guizhou in June, and there is more precipitation in northern and eastern Sichuan, northeast Guizhou, and less precipitation in central and northwest Yunnan in July. When the subtropical high is east, the precipitation distribution is almost the contrary. Further analysis also shows that the relationship between the subtropical high and SST is related to the active position of the subtropical high. The farther north the subtropical high is, the weaker the relationship between the subtropical high and the SST is.