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 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 ? 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.
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