Essay/Term paper: Circulation systems over china

Essay, term paper, research paper:  World History

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 ¾
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.