circulation systems over china

CIRCULATION SYSTEMS OVER CHINAIntroduction: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
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.Temporary Disturbances: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.Conclusion: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.References: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.