Characteristics of Modern Monsoon in China

Monsoon refers to the wind system in which the prevailing wind direction changes significantly with the seasons. Generally speaking, the stable prevailing wind direction between winter and summer is 120 ~ 180. Monsoon develops in many parts of the world, but in different areas, the forming factors of monsoon are different and can be summarized into three types.

Figure 1 1-25 East Asian monsoon formation map (revised according to An Zhisheng 2000)

First, due to the influence of land and sea, the thermal difference between land and sea changes with the seasons. In winter, the mainland is cold and the ocean is warm, and the wind blows from the mainland to the ocean. In summer, the mainland is hot and the ocean is cold, and the wind blows from the ocean to the mainland. For example, the East Asian monsoon is caused by this reason (Figure 1 1-25).

Second, influenced by large-scale planetary circulation, planetary wind belts are formed, which are basically zonal, such as tropical easterly belts and westerly belts at middle and high latitudes. Between winter and summer, these planetary winds have obvious north-south displacement and great changes in intensity. In the region where two planetary wind belts alternate, the planetary circulation shifts seasonally, and the prevailing winds tend to approach the opposite direction, which is called planetary monsoon. The planetary monsoon is most obvious in the low latitude area from 30 to S30 north latitude. The South Asian monsoon falls into this category.

Thirdly, influenced by the plateau topography, there is also a seasonal thermal difference between the huge and towering Qinghai-Tibet Plateau and the surrounding free atmosphere, which will inevitably produce a phenomenon similar to monsoon. In winter, the plateau is used as the cold source, and a cold high pressure forms in the lower part of the plateau, where an anticyclonic circulation prevails, and a north-northeast wind prevails in the southeast, which is consistent with the winter wind in East Asia. In summer, the plateau is the heat source, and a thermal depression is formed in the lower layer, and cyclone circulation prevails. Southwest wind appears on the east side of the plateau, which strengthens the southwest wind in summer. So the Qinghai-Tibet Plateau strengthened the East Asian monsoon.

According to research, there are several obvious monsoon climate zones in the world, namely northern Australia, the Pacific Northwest, the Arctic Ocean and North America, among which West Africa, East Africa, South Asia, Southeast Asia and East Asia are obvious monsoon climate zones. East Asia and South Asia are the most famous monsoon climate regions in the world.

2. Characteristics of modern monsoon in China.

China, located in the East Asian monsoon and South Asian monsoon regions, is one of the most remarkable monsoon climate regions in the world. This is because East Asia is located in the east of Eurasia, the largest land in the world, and the Pacific Ocean, the largest ocean in the world, is in the east. This makes a huge pressure difference between them in summer and winter, forming a significant monsoon. East Asian monsoon region is southeast wind in summer and northwest wind in winter. Therefore, it is warm and rainy in summer and cold and dry in winter.

In China, due to the influence of the East Asian monsoon, the monsoon phenomenon in eastern China is obvious, and the prevailing wind direction changes with the seasons. Northwest airflow prevails in winter; In summer, southeast airflow prevails in the east and southwest airflow prevails in the southwest. Therefore, winter is cold and dry, and summer is hot, humid and rainy, especially heavy rain. In the tropical and eastern regions of China, there are obvious dry seasons and rainy seasons, but the time is slightly sooner or later, and there is a law of advancing from south to north. The rainy season in South China, the rainy season in Jianghuai and the rainy season in North China and Northeast China all belong to the summer monsoon rainfall.