Influence of Western Pacific Subtropical High on Weather in China

The influence of the western Pacific subtropical high on the weather in China is very important, especially in the summer. On the one hand, this influence is manifested in the western Pacific subtropical high itself; On the other hand, it is also reflected in the interaction between the western Pacific subtropical high and its surrounding weather system. In the area controlled by the western Pacific high, there is a strong subsidence inversion, which makes it difficult for water vapor in the lower layer to form clouds and cause rainfall, which leads to the stable clear sky in Wan Li, and a large-scale drought may occur after a long time.

Subtropical high is an important system for transporting water vapor to Chinese mainland. Although the water vapor source of precipitation in China mainly depends on the southwest airflow transported by the Indian Ocean, the location, intensity and activity of the Pacific subtropical high are not only related to the water vapor transport of the southwest airflow, but also affect the water vapor transported from the Pacific Ocean to the mainland by the southeast monsoon on its south side. At the same time, the northern side of the western Pacific subtropical high is the intersection of warm and humid air along the subtropical high and cold air south of the mid-latitude, 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 subtropical high in the western Pacific, 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 high ridge line is stable between 18-20N, there will be continuous low temperature and rainy weather in South China. In June, the ridge line of subtropical high jumped more than 20° n to the north and stabilized at 20-25 N. The precipitation belt is located in the lower reaches of the Yangtze River and Japan, which is the beginning of the rainy season. Due to the influence of subtropical high every year, the speed of advancing northward is different, and the length of meiyu period and the time of entering and leaving Meiyu are also very different. Meiyu can appear in all periods from May to July. Meiyu in May is called early Meiyu, and Meiyu in June-July is called normal Meiyu. Generally, plum blossoms come in around mid-June and come out in early July. The average meiyu period is about 20 days. The main weather systems that cause continuous rainfall in Meiyu period are quasi-static front, shear line and southwest vortex. The continuous appearance, slow movement and stagnation of these systems in the middle and lower reaches of the Yangtze River can cause large-scale floods. 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 N, 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, causing drought in some areas; Abnormal weather caused by waterlogging in other areas. For example, 1956, the first northward jump of the western Pacific ridge was earlier, and the second northward jump was later. Plum rains are abundant this year, and there is too much rain in the middle and lower reaches of the Yangtze River. The subtropical high 1954 was stable between 20 and 25N for a long time, and the meiyu 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.