CIRCULATION SYSTEMS OVER CHINA
The Earth's atmosphere is in continuous motion: movement which is attempting to balance the constant differences in pressure and temperature between different parts of the globe. It is this motion which carries water from the ocean to the continents to provide precipitation and moves heat energy from the tropical regions toward the poles, warming the high latitudes. It is this circulation which plays a basic part in maintaining a steady state in the atmosphere and generating the climatic zones which characterise different parts of the earth.
China, from its latitudinal location, mostly belongs to the mid-latitudes, with a small part to the low latitudes. It is located at south of Siberia and the north of the tropical Pacific. At this distinctive location, the country is affected by the alternate seasonal expansion and contraction of the polar continental highs and tropical maritime air masses, along with the seasonal shifts of the overhead sun. These changes in the pressure systems over Asia generate the unique Asian monsoon circulation which prevails over China throughout the year.
Surface Pressure Field and Winds:
For any fluid to initiate movements, pressure gradient must exists. Therefore, for a close understanding of the circulation system that operates over China, we should start from discussing the seasonal pressure distribution at sea-level over the Asia-Pacific region, which is the driving force for the air movements in China. Most clearly to be seen, the largest difference in the atmospheric pressure occurs between winter and summer, whereby January and July can be considered as representative months.
In January, a typical cold anticyclone with central pressure above 1,040 hectopascals (hPa) developed over mid-Siberia and Mongolia (Mongolian High); while a strongly established cyclone over the north-western Pacific Ocean (Aleutian Low). Since both pressure systems practically lie in the same latitude of 50° to 55°N, a steep pressure gradient occurs which produces strong and persistent north-westerlies over Northeast China.
A third pressure system which affects China, although limited only to south-eastern China, is the equatorial Low over Australia and New Guinea. The vast territory of East China lies in the middle of the path along which the Mongolian cold air tries to rush southward into the Equatorial Low. Northerly and north-easterly flows prevail over the eastern half of China.
As a typical feature, the Mongolian High is only a rather shallow pressure system. It disappears at the 500-hPa level. West China which has a higher elevation, therefore feels little of its influence; Yunnan highlands are even predominated by south-westerlies during most of the winter.
The pressure pattern at sea-level during summer differs completely from winter conditions. In July, a strong cyclone is located over the north-western Indian-Pakistan subcontinent, with central pressure below 1,000 hPa. Although it covers an extensive area that the circulation around it affects almost all of the continental Asia, the pressure field shows a relatively weak gradient so that for China only a moderate variation of pressure is experienced.
An extensive subtropical high with pressures exceeding 1,025 hPa is situated in the western North Pacific to the east of the China coast. Because of these two intense pressure systems, the surface wind distribution over China in the summer season is characterised by southerlies in the eastern parts and easterlies over the Northwest.
In contrast with the Mongolian High in winter, the heat low in July is quite thick. In 500-hPa level, the low pressure cell still exists, which is about the highest level it could attain. Even the surface winds over the Tibet Plateau in West China are governed by the heat low during the summer season.
During winter as a whole, January experiences the strongest anticyclonic pressure field; whereas in summer, the circulation over China is predominated by the heat low centred at the Indian-Pakistan region. Long term records indicates that the period from June to September comprises the summer pattern, typified by July. The period from October to May comprises the winter circulation pattern, typified by January. (Zhang, 1992)
We can conclude that the prevailing winds over most parts of China are from north, north-west and north-east in winter, whilst in summer, they follow a persistent southern direction which varies from south-westerlies to south-easterlies. This marked seasonal variation in wind direction (over 120°) is often defined as 'monsoon', which results from the seasonal variation of the thermal structure of the underlying surfaces and involves different air masses, producing noticeable effects on the weather and climate of the areas concerned.
Chinese meteorologists often define monsoon as an alternation of two kinds of air-flows with different properties: prevailing winds direction differ largely in winter and summer; since winter and summer monsoons originated in different regions, there are substantial differences in their air-mass properties; and finally, they are accompanied by various weather phenomena, thus bringing a great diversity of seasons. (Manfred, 1988)
The monsoon index , which expresses the relative strength of the alternating wind directions, is often applied as a indicator for the characteristic of the change of wind direction. For the monsoon near the surface, the area of maximum monsoon indices is found south of the Nanling Mountains at the Guangdong and Fujian coastal region. A minimum index is found over Sichuan and eastern Yunnan, but the indices increase again further west to another maximum over southern Tibet. This implies that the minimum over Sichuan and Yunnan represents a boundary area between two monsoons. Eastern parts of the area belong to the East Asian monsoon, which is well established in both summer and winter, although winter monsoon is stronger; monsoon precipitation is associated with the polar front. The parts west of the boundary are affected by the Indian monsoon, which is most noticeable in summer and rains fall mostly within the area of the summer monsoon air.
Apart from the directional variation of the monsoon, another distinctive property is the different nature of the summer and winter monsoons which is governed by their origin. Due to its origin from mid-Siberia and Mongolia, the winter monsoon can be characterised by cold and dry air masses (cP). As for every air masses, the character is gradually averaged out with increasing distance from their origin. This implies that the dry-cold character of the winter monsoon are weakened from North to South China, and that over the southern parts warmer and moister air masses of an oceanic origin may even take over the climatic condition in winter. However, due to the advancing speed of the winter monsoon, its thermal effect is still very noticeable even to the southernmost of China. Representing a typical phenomenon of the winter monsoon, cold waves migrate far southward throughout China and finally even invade Hainan Island. In summer, warm and moist air masses of a tropical origin (mT) prevail. They 'invade' China although their nature is gradually weakened as they are going further into the continent. The effects of the summer monsoon are negligible over West and North-west China where geographical and topographical conditions prevent the invasion of the moist and warm summer-monsoonal air.
The different nature between winter and summer monsoon air masses also leads to a clear seasonal difference in precipitation. As a general rule, winter represents a dry, summer a wet period. In summer, the front of the advancing equatorial air masses provides most of the monsoonal precipitation, while the interior air masses lead to less rainfall and fine weather which last a few consecutive days. The northward advance of the front of the equatorial air masses may 'catch-up' the retreating polar air masses in the first half of June in the middle and lower parts of the Yangtze to constitute extensive rainfall called "plum rains" (Mei-yu), which is associated with very hot and damp air, massive low cloud and depressing weather.
Aside from the seasonal occurrence of monsoons, there are other periodic circulation systems which affect the climate of China. Although there are a number of them, I am intended to discuss only some of them in this section, namely the upper westerly troughs in the westerlies, the extra-tropical cyclones and anticyclones and typhoons.
Except for summer, China comes mostly under the influence of westerlies, which are divided by the Tibet Plateau and flow over China as 'northern westerlies' and 'southern westerlies'. Often come along with these westerlies are troughs and ridges of pressure systems which are transported from west to east, and some of them are accompanied by cyclones and anticyclones on the earth surface. The northern branch of westerlies which carry the majority of the troughs move to the east through Xinjiang and Inner Mongolia into Northeast China, then into the North China Sea. While advancing to the east, the troughs located at the southern part of the waves would affect Gansu, Inner Mongolia, Ningxia and North China. The second branch of the westerlies come from the south of Tibet Plateau originated from the Mediterranean Sea and North Africa. These westerlies enter China and bring moist air to southern China.
Besides the Mongolia High that we have discussed, China is also influenced by a high frequency of cyclones and anticyclones. The cyclones in China are extra-tropical cyclones and some of them are related to the westerlies discussed. Most of them occur in spring and pass through China in a west-east direction. Anticyclones in China are more evenly distributed over the seasons. Their source regions are mostly Siberia and the Mongolian Plateau, and they often travel through China in a east and south-eastward direction. The extra-tropical cyclones and anticyclones bring a variety of weather to China, from rainfall to snow, and from warm, cloudy to cold, clear weather.
Typhoons represent an important weather system in China. They are associated with gales and torrential rain in South, East and North China. Especially for the typhoon rainfall, which accounts for more than 50% of the annual total in the coastal areas of Zhejiang, Fujian and Guangdong Provinces, is of extreme importance to China's main agricultural regions. The typhoon season is in the period from June to November, with high concentration from July to October when the formation criterion prevail. All of the typhoon originated either from North Pacific Ocean east of Taiwan and the Philippines; and the South China Sea, they generally move in a east-west direction in the Pacific and some of them may recurve to the north-east as they approach to the coast.
The climate of China is principally determined by the monsoonal nature of the area. Nevertheless, we should not forget that China's climate is also affected by other occasional disturbances that vary from season to season. Moreover, the monsoonal nature is gradually weaken from its point of origin 3/4 the air mass source region. It is therefore debatable whether or not China as a whole experiences a monsoon climate. In general, Chinese climatologist often regard Xinjiang, the central and western part of North Qaidam Basin, western part of the Tibet Plateau, northern part of Inner Mongolia as under non-monsoonal continental climate type, and the rest of the vast territory is under circulation-determined monsoon type climate.
JOHN J. H. & JOHN E. O. (1993), Climatology: An Atmospheric Science, Macmillan Publishing Company: New York.
MANFRED D. & PENG G. (1988), The Climate of China, Springer-Verlag: Berlin Heidelberg.
ZHANG J. & LIN Z. (1992), Climate of China, John Wiley & Sons, Inc. & Shanghai Scientific and Technical Publishers: Shanghai.