What is the relationship between summer precipitation in the eastern coastal areas of China and the north-south movement of subtropical high ridge? Please explain in detail.

Subtropical high is an important system for transporting water vapor to Chinese mainland. The northern side of the western Pacific subtropical high is the intersection of warm and humid air along the subtropical high to the north and cold air at mid-latitude to the south, which often forms a wide range of rainy weather and is an important precipitation belt in Chinese mainland. Therefore, the north-south movement of the precipitation belt in China is consistent with the seasonal activity of the western Pacific subtropical high, and the precipitation belt is usually located at 5 ~ 8 latitudes north of the subtropical ridge. From February to April every year, when the subtropical ridge line is stable between north latitude 18 ~ 20 degrees, there is continuous low temperature and rainy weather in South China. In June, the ridge line of the subtropical high jumped northward over 20 degrees north latitude and stabilized between 20 degrees and 25 degrees north latitude. The precipitation belt is located in the lower reaches of the Yangtze River and Japan, which is the beginning of the rainy season. By July, the subtropical ridge jumped northward again, and the rainfall belt moved from the Yangtze River basin to the Huanghuai basin. Meiyu in the middle and lower reaches of the Yangtze River ended and began to be controlled by the western Pacific subtropical high, and the weather became hot and rainy. If the subtropical high is strong and controlled for a long time, it will cause serious drought. From the end of July to the beginning of August, the ridge line of high pressure further crossed 30 degrees north latitude, and the rain belt also moved northward to North China and Northeast China. At the beginning of September, the ridge line of high pressure began to rebound to the south, and the rain belt also moved from north to south.

The above situation is only the general law of the influence of the subtropical high activity in the western Pacific on the weather in China. In fact, the seasonal movement of the subtropical high in the north and south is often abnormal, which leads to drought in some areas and abnormal weather in others. For example, 1956, the first northward jump of the western Pacific ridge was earlier, and the second northward jump was later. This year, the plum rains were very abundant, and there was too much rain in the middle and lower reaches of the Yangtze River. During the period of 1954, the subtropical high was stable at 20 ~ 25 degrees north latitude for a long time, and the plum rains in the Yangtze River basin lasted for two months, resulting in a rare flood in the Jianghuai area for decades. 1958 The first northward jump of subtropical ridge line was late, and the second northward jump was early, which resulted in the empty plum blossoms in this year and caused the drought.