CIrculation systems over China


CIRCULATION SYSTEMS OVER CHINA

Introduction:
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)

Monsoon:
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