Geoscience Reference
In-Depth Information
spring (see
Figure 11.22
). They form more readily
over Central Asia during this season as the
continental anticyclone begins to weaken; also,
many develop in the jet stream confluence zone in
the lee of the plateau.
The zonal westerlies retreat northward over
China in May to June and the westerly flow
becomes concentrated north of the Tibetan
Plateau. The equatorial westerlies spread across
Southeast Asia from the Indian Ocean, giving
a warm, humid air mass at least 3000m 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
the rain falls during thunderstorms associated
with shallow lows, and the existence of the
ITCZ in this region is doubtful (see
Figure 11.1
).
The southerly winds, which predominate over
northern China in summer, are not necessarily
linked to the monsoon current further south.
Indeed, this idea is the result of incorrect inter-
pretation of streamline maps (of instantaneous
airflow direction) as ones showing air trajectories
(or the actual paths followed by air parcels). The
depiction of the monsoon over China in
Figure
11.24
is, in fact, based on a wet-bulb temperature
value of 24
C. Cyclonic activity in northern China
is attributable to the West Pacific Polar Front,
forming between cP air and much-modified mT
air (
Figure 11.35
).
In central and southern China, the three
summer months account for about 40-50 percent
of the annual average precipitation, with another
30 percent or so being received in spring. In
southeast China, there is a rainfall singularity
in the first half of July; a secondary minimum in
the profile seems to result from the westward
extension of the Pacific subtropical anticyclone
over the coast of China. A strong southeast Asia
monsoon (20-30
°
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 it 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
southwesterly (
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, for example, 64 percent
of the annual total occurring at Tianjin (Tientsin)
(39
°
E) is related to
higher SSTs in the western North Pacific, which
weaken the subtropical anticyclone and allow
more cyclonic circulations.
A similar pattern of rainfall maxima occurs
over the Korean peninsula and over southern and
central Japan (
Figure 11.36
), comprising two of
the six natural seasons that have been recognized
there. The main rains occur during the Bai-u
season of the southeast monsoon resulting from
waves, convergence zones and closed circulations
moving mainly in the tropical airstream around
the Pacific subtropical anticyclone, but partly
°
N, 110-145
°
°
N) in July and August. Nevertheless, much of
Table 11.2
Surface circulation over China
January
July
North China
60% of winds from W, NW and N
57% of winds from SE, S and SW
Southeast China
88% of winds from N, NE and E
56% of winds from SE, S and SW
