The global westerly circulation has a great influence on the weather and climate in China. Most of China's territory is in the middle latitudes. Although some areas in the south are at low latitudes, due to the southward movement of the mid-latitude westerly wind and the role of the Qinghai-Tibet Plateau, its influence scope has expanded southward, and it has also been deeply affected in South China at low latitudes.

Although the strong monsoon formed by the thermal difference between land and sea has seriously broken the planetary wind system on the east coast of Eurasia, the westerly circulation in the east of China has been unrecognizable, forming a typical monsoon circulation, but the influence of the monsoon is limited to the east and south of the Daxinganling-Yinshan-Helan-Bayankala-Gangdise line. The summer monsoon in the west and north of this line cannot be reached, and the spring breeze does not cross Yumen Pass, but is in the west. In the eastern monsoon region, although the monsoon is extremely strong and dominates the weather and climate in the east to a great extent, the influence of the west wind is still everywhere, and sometimes it is very obvious and prominent.

We can see from the satellite cloud images broadcast by the Central Meteorological Observatory for many years that the cloud systems in Central Asia, West Asia, North Africa and the Mediterranean always move eastward and enter China from the west. It is not difficult to see from the satellite cloud picture that common important weather systems such as cold front and extratropical cyclone have also moved from Europe, Siberia and Central Asia from west to east to China. Why? Westerly circulation! This is caused by westerly activity. In the Atlantic Ocean, the west wind landed from the west coast of Eurasia and headed east. In the process of eastward movement, it crosses the easterly wind from the north, forming fronts and cyclones, or the westerly wind fluctuates due to various factors in the process of propulsion, forming high-altitude long-wave troughs or short-wave troughs. These troughs further developed into fronts and cyclones. These weather systems, in the process of formation and evolution, are all cloud systems generated by their activities and move eastward with the west wind. So this complicated weather cloud map was formed.

These eastward moving weather systems or cloud systems, driven by hurricanes, all enter the eastern ocean of China, but due to the influence of Siberian cold high, they tend to deflect to the southeast, and their paths to the sea are different. If we pay attention to the weather map of China broadcast by the Central Meteorological Observatory, we can clearly see that the cold front and cyclone entering China from the mountain pass in western Xinjiang often enter the northwest from Xinjiang in different seasons or at different times, and their sea entry positions can enter the South China Sea, the East China Sea and the Yellow Sea from South China, East China, North China or Northeast China respectively, and enter the Korean Peninsula and the Sea of Japan from the northeast. On the one hand, it is related to the rise and fall of Siberian cold high and Javert subtropical high in the North Pacific Ocean. On the other hand, the role of the west wind can not be ignored. Driven by western countries, it always moves eastward into the sea, and the general direction of its eastward movement remains unchanged.

The activity of the west wind is not only closely related to the front and cyclone, but also has an impact on the activity path of the typhoon. According to meteorological observation data, in the process of westerly moving eastward, if the high-latitude long-wave trough formed by westerly winds is superimposed with the mid-latitude long-wave trough, the "* * * vibration" of the same trough will be generated, or the long-wave trough will be replaced, which will make the typhoon turn west or north, and its activity path will move west or north.

In addition, the strong westerly wind, blocked by the Qinghai-Tibet Plateau due to seasonal southward movement, is divided into north and south branches. Its southern branch enters China from South Asia and Southeast Asia, such as India, Bay of Bengal, Myanmar, etc., and its activity boundary can be pushed southward to 20 N, and it moves between 20 N and 30 N, which seriously affects the weather and climate in Yunnan, Guizhou, South China and even the south of the Yangtze River. The activity time is about the middle and late period of 10 every year to June of the following year. There is no trace of it except in midsummer, especially in winter and spring. In the process of its eastward movement, under the influence of various factors, it often fluctuates, forming a high-altitude short-wave trough. These low-pressure troughs are produced in the Indo-Myanmar region and often enter China from the Bay of Bengal, bringing precipitation or rainy weather to the Yunnan-Guizhou Plateau and the southern regions of Guangxi, Guangdong, Hunan and Jiangxi. If they overlap with the cold front formed by cold air from north to south, it will cause cold weather in the passing area. In winter and spring, this trough of low pressure often appears one after another and moves eastward continuously, resulting in long-term rainy weather in the passing area. If it is superimposed with the static front formed by the cold air from the north to the south, there will be long-term low-temperature weather in the south. If it is during the spring sowing and transplanting of rice, it will cause rice seedlings to rot, which is what farmers call "rotten seedling weather"

This activity of the south branch westerly wind can not only guide the cold air from the north to the south, but also guide the warm and humid air from the south to the north, thus causing more complicated weather changes. For example, in winter and spring, due to the activity of low pressure trough, the southwest warm current can be guided northward, and its influence can reach Yunnan, Guizhou, Guangdong and Guangxi, the middle and lower reaches of the Yangtze River and even Henan. The warm current glides over the cold air here, causing rainy weather in these places. If the southwest warm current intersects with the cold air from north to south between Kunming and Guiyang, it can form a quasi-static front (this is the famous "Kunming quasi-static front"). In winter, Kunming is as warm as spring, and Guiyang is cold and rainy, which is why it comes. The winter rain in Sichuan basin is also closely related to this.

After passing through the south side of the plateau, the south branch westerly wind can also meet the west wind passing through the north side of the plateau on the east side, forming a front or vortex (southwest vortex). These fronts and vortices often move to the southeast, affecting South China. Bring weather changes to these areas.

The westerly wind moving eastward can also cross the Qinghai-Tibet Plateau and reach the Sichuan Basin and the middle and lower reaches of the Yangtze River. When it crosses the plateau, it also fluctuates, forming a trough of low pressure-plateau trough or Qinghai-Tibet trough. On the weather satellite cloud picture broadcast by the Central Meteorological Observatory, we can see that the cloud system drifting from west to east over the Qinghai-Tibet Plateau is often the cloud system formed by the activity of this high-altitude trough. This kind of high-altitude trough can be formed in the western part of the plateau or over the plateau. It develops continuously in the process of moving eastward, which mainly affects the weather in the plateau itself, the Sichuan basin and the middle and lower reaches of the Yangtze River. When it passes through the above areas, it often brings heavy precipitation. If the cold air moves south, its rain area will move to the southeast, bringing precipitation to the vast areas in the south of the Yangtze River, and rainy weather will often form in the south of the Yangtze River. The activity of plateau trough will also guide warm air northward. At this time, if there is warm air going north, it will also bring blizzard weather to the plateau.

Thus, the influence of the west wind on the weather and climate in Chinese mainland is ubiquitous. The activities of fronts and cyclones, the most important weather systems that affect the weather in China, are directly or indirectly related to them. The formation of many climatic characteristics in various places-rainy winter and spring in the south, "quasi-static front in Kunming" in Yunnan-Guizhou Plateau, four seasons like spring in Kunming, cold and rainy winter in Guizhou, and night rain in Sichuan Basin, etc. Are closely related to the activity of the west wind. Therefore, when we study China's climate, we must not ignore the west wind. When we study the weather and climate in China, we must also study and analyze the activities of the west wind and its subtle relationship with the monsoon. Only in this way can we objectively understand the weather changes and climate conditions in various parts of China, accurately grasp these meteorological and climate resources, and develop and utilize them more effectively in economic design.