Regarding the cause of monsoon, some people think that it is caused by the thermal difference between land and sea and the seasonal variation of this difference; Others think it is caused by the seasonal displacement of the planetary wind belt. Some climatologists in China believe that monsoon phenomenon is a comprehensive phenomenon under the influence of three factors, namely, the thermal action of land and sea distribution, the seasonal change of atmospheric circulation and the specific topography. For example, the Indian monsoon is particularly remarkable and stable because the seasonal changes of monsoon and planetary wind belt caused by the distribution of land and sea are just the same. In winter, India is in the south of the cold high pressure and in the position of the trade wind zone, so the northeast monsoon in India is particularly stable; In summer, India is in the south of the thermal depression and where the southwest wind passes when the equatorial trough moves northward. Therefore, the southwest monsoon is particularly stable.

Asia is located in the east of Asia-Europe continent, the largest continent on the earth, and is closely adjacent to the Pacific Ocean and the Indian Ocean, as well as the seasonal variation of planetary circulation in the upper air and the topographic effect of the Qinghai-Tibet Plateau, thus forming a unique monsoon circulation system in Asia. The thermal difference between sea and land in winter and summer causes the seasonal change of the atmospheric activity center between sea and land (that is, the high and low pressure activity center), which makes the monsoon wind direction change regularly.

In winter, the radiation on the Asian land cools rapidly, forming a dry and cold Siberian high (also known as Mongolian high) and a cold and wet Aleutian low in the North Pacific Ocean. Both of them are very developed, especially the former is very powerful and controls almost all the Asian continent. It is the center of continental anticyclone and the source of dry and cold polar continental air mass, and also the source of continental monsoon in winter. Because the subtropical high and the continental high are connected together in winter, they are combined and powerful, and become the dominant climate in the Asian continent in winter and half a year. The high-pressure dry and cold air flows outward, and the northwest wind prevails in the Pacific coast and the northeast wind in the Indian Ocean coast in winter. This is the winter wind in East Asia and South Asia, which has the characteristics of sunny, cold and dry winter high-pressure weather. The winter wind is a dry land wind, which is generally not easy to precipitation, but it can form a blizzard when it blows to the northwest wind of Japan. The warm sea metamorphic polar continental air mass invading the south of the Yangtze River can form cloudy and warm weather in winter and spring or rainy weather. The Qinghai-Tibet Plateau blocks the westerly airflow in winter and divides it into two branches, forming the circulation situation of the northern ridge and the southern trough. Because of the existence of high pressure ridge in the north of the plateau, it is very beneficial to the southward movement of winter monsoon, which strengthens the influence of winter monsoon and causes the cold climate in winter in China. The low pressure trough in the south of the plateau has a great influence on the weather and climate in southern China because of the large amount of warm and humid air flow in front of the trough. Due to the blocking of the plateau, the southern Xinjiang and Hexi areas on the north side of the plateau are dry and cold in winter; On the south side, there is little cold air activity in India and Myanmar, and it is dry and warm in winter. The Qinghai-Tibet Plateau is a cold source in winter, which will strengthen the downward flow in the vicinity of the plateau, thus strengthening the winter monsoon circulation.

In summer, the Asian continent is controlled by the thermal low pressure, the center of which is in northwest India, southern Iran and Arabia, and it is integrated with the equatorial low pressure. At this time, the Hawaiian high extends westward and advances northward, with the strongest force and the largest scope, thus forming the southeast monsoon blowing from the western edge of the North Pacific high to the eastern part of Asia. This is the summer monsoon in East Asia, which is a warm and humid air flow originating from tropical ocean air mass, and has the greatest influence on the Asian continent in summer. In southern Asia, at this time, due to the northward movement of the planetary wind belt, the equatorial depression moved to the northern hemisphere, and the southeast trade winds in the southern hemisphere turned into southwest winds after crossing the equator. This is the summer monsoon in South Asia. When the southwest monsoon reached the Bay of Bengal and then advanced northward, it was blocked by the Qinghai-Tibet Plateau and divided into two branches: one along the Himalayas to the west, thus maintaining the thermal depression in northwest India; The other branch runs along the mountains and flows to China, expanding the influence of southwest monsoon on China. In summer, the Qinghai-Tibet Plateau is a heat source for the surrounding free atmosphere, which will inevitably strengthen the monsoon depression in the vicinity of the plateau, thus playing a role in strengthening the summer monsoon