China has no equivalent to India's hot, pre-monsoon season. The low-level, northeasterly winter monsoon (reinforced by subsiding air from the upper westerlies) 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 anticyclone 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:
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.
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 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,
Table 11.2 Surface circulation over China.
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
Source: Matsumoto (1985). Reproduced by permission, University of Tokyo.
for example, 64 per cent of the annual total occurring at Tianjin (Tientsin) (39°N) in July and August. Nevertheless, much of 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 interpretation 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 to 50 per cent of the annual average precipitation, with another 30 per cent 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 similar pattern of rainfall maxima occurs 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 originating partly in a southwesterly stream that is the extension of the monsoon circulation of South Asia (Figure 11.23). The southeast circulation is displaced westward from Japan by a zonal expansion of the subtropical anticyclone
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