Geoscience Reference
In-Depth Information
Figure 11.34 Mean onset date of
the winter monsoon (i.e. retreat of the
summer monsoon) over South and
East Asia.
Sources : After Tao Shi-yan and Chen
Longxun. Reproduced by permission of
Professor Tao Shi-yan and the Chinese
Geographical Society, and John Wiley &
Sons, Inc.
for Bay of Bengal cyclones and it is these disturbances,
rather than the onshore northeasterly monsoon, that
cause the October/November maximum of rainfall in
southeast India (e.g. Madras, Figure 11.20).
During October, the westerly jet re-establishes itself
south of the Tibetan Plateau, often within a few days,
and cool season conditions are restored over most of
South and East Asia.
J F M A M J J A S O N D Year
78 91110853 3 6 77 86
The zonal westerlies retreat northward over China
in May to June and the westerly flow becomes concen-
trated north of the Tibetan Plateau. The equatorial
westerlies spread across Southeast Asia from the Indian
Ocean, giving a warm, humid airmass at least 3000 m
deep. However, the summer monsoon over southern
China is apparently influenced less by the westerly flow
over India than by southerly airflow over Indonesia near
100°E. In addition, contrary to earlier views, the Pacific
is only a moisture source when tropical southeasterlies
extend westward to affect the east coast.
The Maiyu 'front' involves both the monsoon
trough and the East Asian-West Pacific polar front, with
weak disturbances moving eastward along the Yangtze
valley and occasional cold fronts from the northwest.
Its location shifts northward in three stages, from south
of the Yangtze River in early May to north of the river
by the end of the month and into northern China in
mid-July (see Figure 11.24), where it remains until late
September.
The surface airflow over China in summer is
southerly (Table 11.2) and the upper winds are weak,
with only a diffuse easterly current over southern
China. According to traditional views, the monsoon
current reaches northern China by July. The annual
rainfall regime shows a distinct summer maximum with,
D EAST ASIAN AND AUSTRALIAN
SUMMER MONSOONS
China has no equivalent to India's hot, pre-monsoon
season. The low-level, northeasterly winter monsoon
(reinforced by subsiding air from the upper wester-
lies) persists in north China, and even in the south it
begins to be replaced by maritime tropical air only in
April to May. Thus at Guangzhou (Canton), mean
temperatures rise from only 17°C in March to 27°C
in May, some 6°C lower than the mean values over
northern India.
Westerly depressions are most frequent over China
in spring (see Figure 11.22). They form more readily
over Central Asia in this season as the continental anti-
cyclone begins to weaken; also, many develop in the jet
stream confluence zone in the lee of the plateau. The
average number crossing China per month during 1921
to 1931 was as follows:
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