Why is it rainy in rainy season?

(1) The basic reason for the early Meiyu is that the northward jump of the western Pacific subtropical high occurred early, which made the main rainfall belt in the eastern half of China suddenly move from the south of Nanling to the middle and lower reaches of the Yangtze River. Meiyu like 197 1 started on May 26th, more than half a month earlier than normal meiyu. The turning point of the weather this year began in the tropics. In late May, three typhoons occurred on the tropical ocean surface of the northwest Pacific Ocean. These typhoons occurred on the south side of the subtropical high and moved northwest. This activity is like the subtropical high being pushed northward, which makes the high rise northward. During this period of change, the high-altitude westerly jet moved northward, and the plum rains in the middle and lower reaches of the Yangtze River suddenly began. It should be noted that the early northward jump of subtropical high is not entirely caused by the activity of hot typhoon, and other reasons can also lead to its early northward jump. As long as there is a stable situation that the subtropical high moves to the coast of South China in advance, it will cause "early plum rain". (2) Late Meiyu: The southern branch of westerly jet is strong and stable in early summer in some years. In June, a strong westerly wind was maintained in the middle troposphere from the Yangtze River valley in China to the Pacific Northwest in southern Japan. Its existence has restrained the northward uplift of the subtropical high in the northwest Pacific Ocean, making it impossible to establish the rain belt in the middle and lower reaches of the Yangtze River for a long time. In this way, the rainy season came late. For example, 1982, the rainy season in the middle and lower reaches of the Yangtze River only started on July 9, which is the case. Before June 20th this year, westerly jet was southerly and the subtropical high was southerly. Around the 20th, the subtropical ridge of the northwest Pacific Ocean extending along the southeast coast of China was once strengthened and carried northward to the coast of South China. Three or five days of rainy weather appeared in the middle and lower reaches of the Yangtze River. However, this change is not a seasonal turning point. The main body of the subtropical high in the northwest Pacific has not moved northward, especially the westerly wind in the south branch still stays near 30 degrees north latitude. Under the influence of westerly jet, the cold air moved southward, and the subtropical high quickly withdrew southward. It was not until early and late July that the westerly jet over East Asia suddenly retreated northward, causing the subtropical high over the eastern coast of China and Japan to jump northward obviously, and the Meiyu began to come. (3) Extra-long plum rain: The main reason for the extra-long plum rain is that "plum blossom" is particularly late. For example, the plum rain in 1954 didn't end until the end of July and the beginning of August. What is the reason why the plum rain can't end for a long time? This year, there were many anomalies in the characteristics of atmospheric motion in the whole northern hemisphere. From the perspective of Asia and the Pacific Northwest, there are two main aspects: First, during June-July of 1954, the subtropical high in the Pacific Northwest is much southward than normal. Especially in July, the ridge line of subtropical high in the middle troposphere along the East Asian coastal area is basically along the north latitude 18 degrees, about 800 kilometers south than normal. It is very rare that the average position for a whole month deviates so much from the normal year. After the "light summer", the southeast monsoon prevailed in the middle and lower reaches of the Yangtze River under the control of subtropical high, and the weather was sunny and hot. However, due to this deviation, the warm and humid southwest monsoon in the lower troposphere replaced the southeast monsoon in July this year, which provided important conditions for Meiyu front to stay in the Yangtze River basin. Second, during June and July this year, a strong high pressure stayed over East Siberia for a long time. Like a mountain, it "blocked" the passage of the westerly belt at high altitude and divided westerly jet into two branches. -One branch passes through its north, the other branch spreads eastward from its south, and the south branch is still stable. As mentioned earlier, the jet stream will move to 40-50 degrees north latitude during the rainy season, but the south branch jet stream in July this year still extends to the Japanese archipelago along the Jianghuai Basin. On this rapids, there is a trough of low pressure moving from west to east. Whenever a trough of low pressure passes by, it will merge with the southwest airflow in the northwest of subtropical high, causing heavy rain. Until the beginning of August, this "blocking" high suddenly disappeared, the jet moved northward, and the subtropical high also jumped northward, and the meiyu ended. Although 1954 is the most typical of the above two characteristics, they are both representatives of the year of long meiyu. Most years in which the rainy season ends in the second half of July have this characteristic. This is because cold air has not usually invaded the middle and lower reaches of the Yangtze River after entering the "Little Summer". Only when there is a strong high pressure "blocking" over Siberia, the jet stream splits into two parts, and one part passes through the high pressure south much more south than normal, can cold air affect the middle and lower reaches of the Yangtze River. Similarly, by that time, in the year when the subtropical high in the northwest Pacific is obviously southward, the rain belt can only stay in the Yangtze River basin. Once this high pressure strengthens the control of the middle and lower reaches of the Yangtze River, it will be a hot summer day. (4) "Short plum" and "empty plum": "Short plum" and "empty plum" are usually the result of disharmony between subtropical high and cold air activities in the north. When the cold air activity is frequent, the subtropical high is located in the south, and the cold and warm air meet in Nanling Mountains. When the subtropical high jumps northward to the coast of South China, usually at the beginning of the rainy season in the middle and lower reaches of the Yangtze River, the high-altitude westerly jet quickly jumps northward again, and the cold air activity suddenly weakens. It makes the subtropical high jump northward again soon and controls the Yangtze River basin. In this way, the plum rain is short-lived or does not appear. But in some years, "empty plum" is the result of abnormal atmospheric movement. The subtropical high in the northwest Pacific is particularly strong. It moved northward to control the middle and lower reaches of the Yangtze River much earlier than normal, and it stayed in this area for a long time. In this "empty plum" year, serious droughts often occur. (5) "Inverted Huangmei": The appearance of "Inverted Huangmei" is mainly caused by the repeated seasonal changes of atmospheric movement over East Asia from early summer to midsummer. Specifically, it is mainly because the high-altitude westerly jet and the western Pacific subtropical high retreated southward for a period of time after the second northward jump. It seems that the two armies confronted each other, and after the front was advanced, their positions were not consolidated and they had to retreat temporarily. In this case, the main rainfall belt in eastern China has moved from Huanghuai Basin to the middle and lower reaches of the Yangtze River. After a period of time, the high-altitude westerly jet and subtropical high really moved northward and stabilized, and the rain belt moved northward from then on, ending "Huangmei". (6) The causes of abnormal meiyu are explained from the aspects of westerly jet and subtropical high in the northwest Pacific. However, it must be pointed out that the occurrence of meiyu anomaly is not an isolated weather and climate phenomenon in the middle and lower reaches of the Yangtze River. In some severe years, it is also associated with the abnormal atmospheric activities in the whole northern hemisphere and even the whole world. The global atmospheric movement anomaly is closely related to the abnormal changes of seawater temperature (such as El Nino and La Nina), global volcanic activity and sunspot activity.