Autumn sees the southward swing of the equatorial trough and the zone of maximum convection, which lies immediately to the north of the weakening easterly jet (see Figure 11.16). The breakup of the summer circulation systems is associated with the withdrawal of the monsoon rains, which is much less clearly defined than their onset (Figure 11.34). By October, the easterly trades of the Pacific affect the Bay of Bengal at the 500mb level and generate disturbances at their confluence with the equatorial westerlies. This is the major season

India Retreating Monsoon

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.

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