Why does the rainy weather in the south of China make the north dry and sultry? What are the factors that affect it?

Especially in summer, the western Pacific subtropical high (hereinafter referred to as subtropical high) is the most important weather system affecting China. The subtropical high is a closed anticyclone, and the southerly wind blows from the northwest of the cyclone to the west, forming a warm ridge (warm because the wind blows from the south, which is warmer). Generally speaking, the north and south ends of the subtropical high are about 10~ 15 latitude apart, and the center of the subtropical high is dominated by dry and hot weather. However, in the northwest of subtropical high, because the southerly warm wind, the inland southerly cold air or the mid-latitude westerly wind (which is generally weak in eastern China, due to the role of the Qinghai-Tibet Plateau) meet, frontal rain is easy to form, and precipitation is also formed.

The place where precipitation is formed is also about 5~ 10 latitude from the subtropical high center.

In normal years:

At the end of April and the beginning of May, the ridge line of the subtropical high moved near 20 degrees north latitude. At this time, South China is in the Qingming period, that is, the pre-flood season in South China (also known as dragon boat water). At this time, in the north, especially in the North China Plain, the rainy season has not yet arrived, the temperature has risen, and there has been a "spring drought" weather.

From the end of May to June, the northern ridge line of the subtropical high reaches about 25 degrees north latitude. At this time, there is plum rain in Central China, and the rain in South China is relatively reduced (this year is special).

In July and August, the ridge line of subtropical high jumped to about 30 degrees north latitude. At this time, the Meiyu technology in Central China ended, and it was dry and sultry, resulting in "autumn drought" (autumn tiger). There are frequent floods and heavy rains in North China in summer. South China began to be affected by tropical cyclones (typhoons) and the southwest monsoon of the Indian Ocean, and precipitation increased. (The typhoon came early this year, which is also quite special. )

However, in the year of abnormal movement of subtropical high, due to the rapid movement of subtropical high, there may be local floods and local droughts. For example, if the ridge of subtropical high stays near 25 north latitude for a long time, serious floods will occur in the middle and lower reaches of the Yangtze River, which is more serious than that in the northeast of North China.