Tropical and Sub-tropical Monsoon Climate

The South China Sea strides the Northern and Southern Hemispheres from 23.5°N to 3°S, mostly in the tropical zone. Its southern boundary is in the equatorial tropics and northern boundary in the southern sub-tropical zone.The sun shines vertically two times each year. So it is with strong radiation and high air temperature there. The South China Sea is a very deep water mass. It goes to the Pacific Ocean in the east and back to the Europe-Asian Continent, the largest continent in the world, and the Qinghai-Tibet Plateau and the Himalayas, the highest in the world. Seas and land are not well distributed there. In the macro-situation of land, sea and air, strong monsoon comes into being. In winter, the cold air mass of high atmospheric pressure in the Asian Continent spreads to the seas. It forms the northeast monsoon going across the South China Sea, and through the equator, it becomes the northwest monsoon in the Southern Hemisphere (in summer). In summer, the southeast trade wind in the South Hemisphere (in winter) goes across the equator to become moist southwest monsoon through the South China Sea. Due to the disturbance of monsoon, there are no doldrums and northeast trade wind in the South China Sea. They are substituted by the monsoons. The northeast trade wind at the low latitudes of the Pacific Ocean can not go into the South China Sea due to the Philippine Islands. Therefore, the whole South China Sea is marine monsoon climate. It is southern sub-tropical monsoon climate, tropical monsoon climate and equatorial tropical monsoon climate from north to south. The air temperatures are different from north to south.

Air temperature increases progressively from north to south, which is rather obvious from southern sub-tropical zone to tropical zone. The progressive increase of air temperature is obvious in winter, not in summer. The differences between the tropical zone and the equatorial tropical zone are: (1) Dry and wet seasons are clear in the tropical zone (sub-tropical zone), but it is rainy all around the year in the equatorial tropical zone where there is no division of wet and dry seasons. Due to the adjustment of rainfalls, the air temperature in the equatorial tropical zone is lower than that in the south of the tropical zone. So, the hottest place (for example, Bangkok) in the South China Sea is not located at the equatorial tropical zone, but at the area from the south of the tropical zone to the north of the equatorial tropical zone.

Yearly precipitation is 1,200 to 2,000 mm in the southern sub-tropical zone. There are three rainy places: area around Haifeng to the south of the Lotus Mount; area around Yangjiang to the southeast of the Yuwu Mount; area around Dongxing to the south of the Shiwan Mountain. Rainy season is from April to September. It is rainy mostly in June and August. Generally speaking, it is more and more rainy from summer, spring, autumn to winter. Yearly precipitation is 1,000 to 4,000mm in the tropical zone, the highest of which is in the area to the south of the Round Cardamom Mount of Vietnam. It is 4,268mm at Kenong Cliff. It is no more than 2,500mm in the area of the South China Sea in China. The rainy area is Qionghai and Wanning cities to the east of Wuzhi Mount, Hainan Island. Rainy season is from April to November. June and September is two peaks. It is more and more rainy from autumn, summer, spring to winter, which is different from that in the southern sub-tropical zone. Precipitation is higher in summer than that in autumn at Beibu Gulf. The lowest precipitation is at the east of Qiongxi city, where average yearly precipitation is only 993mm and it changes distinctly every year.

Precipitation increases and rainy season becomes longer and longer from the Dongsha Islands to south. Rainy season is 6 months at the Dongsha Islands and the Xisha Islands. It is 7 months at the Nansha Islands. The 12 months are all rainy season at the equatorial tropic zone where yearly precipitation increases to 2,500 to 4,000 mm.

The climate of the South China Sea is influenced by the following weather systems: winter and summer monsoons, tropical cyclone, tropical convergence, subtropical high, low-pressure trough and so forth. Sea area of the South China Sea in the tropics of 12°-22°N, 112°-118°E is one of sources of the tropical cyclones. 80 percent of the tropical cyclones originate from the tropical convergence zone where the low eddy forms. In certain conditions, it can develop to become the tropical low pressure (force 6-7 wind around the center), the tropical storm (force 8-9), the heavy tropical storm (force 10-11) and the typhoon (force 12 and over). The tropical convergence moves to south in winter and to north in summer. It affects the time and space distributions of tropical cyclones.

Average 10.12 tropical cyclones affect the South China Sea every year (1947-1981), among which, over half are from the Pacific Ocean to the south of Philippines, and the remains from the South China Sea. The latter is called “local typhoon” by local fishermen. In the 33 years from 1949 to 1981, 221 tropical cyclones occur at the South China Sea, among which are 28 typhoons, 93 heavy tropical storms and tropical storms together. The South China Sea is influenced by the tropical cyclones in all round the year, but mainly during June to October, especially in August and September when 3 tropical cyclones sometimes generate each month. Typhoons and tropical cyclones often hit land at the costs of the Guangdong, Guanxi and Hainan provinces and the Vietnam coast. They disappear at the Chinese mainland or at sea. Tropical cyclones often bring hard rain or heavy rain (both are called typhoon rain).

The subtropical high of the Western Pacific often moves to south or north, spread to west and shrink in the east in the middle of a year. Sometimes, it spreads into the South China Sea and become the South China Sea high pressure. During April to July, the southwestern low-pressure trough often appears at the area from Yunan-Guizhou Plateau to the east of Qinghai-Tibet Plateau. It also has some influences to the climate of the South China Sea.

The South China Sea strides the Northern and Southern Hemispheres from 23.5°N to 3°S, mostly in the tropical zone. Its southern boundary is in the equatorial tropics and northern boundary in the southern sub-tropical zone.The sun shines vertically two times each year. So it is with strong radiation and high air temperature there. The South China Sea is a very deep water mass. It goes to the Pacific Ocean in the east and back to the Europe-Asian Continent, the largest continent in the world, and the Qinghai-Tibet Plateau and the Himalayas, the highest in the world. Seas and land are not well distributed there. In the macro-situation of land, sea and air, strong monsoon comes into being. In winter, the cold air mass of high atmospheric pressure in the Asian Continent spreads to the seas. It forms the northeast monsoon going across the South China Sea, and through the equator, it becomes the northwest monsoon in the Southern Hemisphere (in summer). In summer, the southeast trade wind in the South Hemisphere (in winter) goes across the equator to become moist southwest monsoon through the South China Sea. Due to the disturbance of monsoon, there are no doldrums and northeast trade wind in the South China Sea. They are substituted by the monsoons. The northeast trade wind at the low latitudes of the Pacific Ocean can not go into the South China Sea due to the Philippine Islands. Therefore, the whole South China Sea is marine monsoon climate. It is southern sub-tropical monsoon climate, tropical monsoon climate and equatorial tropical monsoon climate from north to south. The air temperatures are different from north to south.

Air temperature increases progressively from north to south, which is rather obvious from southern sub-tropical zone to tropical zone. The progressive increase of air temperature is obvious in winter, not in summer. The differences between the tropical zone and the equatorial tropical zone are: (1) Dry and wet seasons are clear in the tropical zone (sub-tropical zone), but it is rainy all around the year in the equatorial tropical zone where there is no division of wet and dry seasons. Due to the adjustment of rainfalls, the air temperature in the equatorial tropical zone is lower than that in the south of the tropical zone. So, the hottest place (for example, Bangkok) in the South China Sea is not located at the equatorial tropical zone, but at the area from the south of the tropical zone to the north of the equatorial tropical zone.

Yearly precipitation is 1,200 to 2,000 mm in the southern sub-tropical zone. There are three rainy places: area around Haifeng to the south of the Lotus Mount; area around Yangjiang to the southeast of the Yuwu Mount; area around Dongxing to the south of the Shiwan Mountain. Rainy season is from April to September. It is rainy mostly in June and August. Generally speaking, it is more and more rainy from summer, spring, autumn to winter. Yearly precipitation is 1,000 to 4,000mm in the tropical zone, the highest of which is in the area to the south of the Round Cardamom Mount of Vietnam. It is 4,268mm at Kenong Cliff. It is no more than 2,500mm in the area of the South China Sea in China. The rainy area is Qionghai and Wanning cities to the east of Wuzhi Mount, Hainan Island. Rainy season is from April to November. June and September is two peaks. It is more and more rainy from autumn, summer, spring to winter, which is different from that in the southern sub-tropical zone. Precipitation is higher in summer than that in autumn at Beibu Gulf. The lowest precipitation is at the east of Qiongxi city, where average yearly precipitation is only 993mm and it changes distinctly every year.

Precipitation increases and rainy season becomes longer and longer from the Dongsha Islands to south. Rainy season is 6 months at the Dongsha Islands and the Xisha Islands. It is 7 months at the Nansha Islands. The 12 months are all rainy season at the equatorial tropic zone where yearly precipitation increases to 2,500 to 4,000 mm.

The climate of the South China Sea is influenced by the following weather systems: winter and summer monsoons, tropical cyclone, tropical convergence, subtropical high, low-pressure trough and so forth. Sea area of the South China Sea in the tropics of 12°-22°N, 112°-118°E is one of sources of the tropical cyclones. 80 percent of the tropical cyclones originate from the tropical convergence zone where the low eddy forms. In certain conditions, it can develop to become the tropical low pressure (force 6-7 wind around the center), the tropical storm (force 8-9), the heavy tropical storm (force 10-11) and the typhoon (force 12 and over). The tropical convergence moves to south in winter and to north in summer. It affects the time and space distributions of tropical cyclones.

Average 10.12 tropical cyclones affect the South China Sea every year (1947-1981), among which, over half are from the Pacific Ocean to the south of Philippines, and the remains from the South China Sea. The latter is called “local typhoon” by local fishermen. In the 33 years from 1949 to 1981, 221 tropical cyclones occur at the South China Sea, among which are 28 typhoons, 93 heavy tropical storms and tropical storms together. The South China Sea is influenced by the tropical cyclones in all round the year, but mainly during June to October, especially in August and September when 3 tropical cyclones sometimes generate each month. Typhoons and tropical cyclones often hit land at the costs of the Guangdong, Guanxi and Hainan provinces and the Vietnam coast. They disappear at the Chinese mainland or at sea. Tropical cyclones often bring hard rain or heavy rain (both are called typhoon rain).

The subtropical high of the Western Pacific often moves to south or north, spread to west and shrink in the east in the middle of a year. Sometimes, it spreads into the South China Sea and become the South China Sea high pressure. During April to July, the southwestern low-pressure trough often appears at the area from Yunan-Guizhou Plateau to the east of Qinghai-Tibet Plateau. It also has some influences to the climate of the South China Sea.