The Asian Monsoon

The name monsoon is derived from the Arabic word mausim, which means season, referring to large-scale seasonal reversals of the wind regime. The Asiatic seasonal wind reversal is notable for its vast extent and the penetration of its influence beyond tropical latitudes (Figure 11.14). However, such seasonal shifts of the surface winds occur in many regions that are not traditionally considered as monsoonal. Although there is an overlap between these traditional regions and those experiencing over 60 per cent frequency of winds from the prevailing octant, it is obvious that a variety of unconnected mechanisms can lead to seasonal wind shifts. Nor is it possible to establish a simple relationship between seasonality of rainfall (Figure

11.15) and seasonal wind shift. Areas traditionally designated as 'monsoonal' include some of the tropical and near-tropical regions experiencing a summer rainfall maximum and most of those having a double rainfall maximum. It is clear that a combination of criteria is necessary for an adequate definition of monsoon areas.

In summer, the equatorial trough and the subtropical anticyclones are everywhere displaced northward in response to the distribution of solar heating of the earth, and in South Asia this movement is magnified by the effects of the landmass. However, the attractive simplicity of the traditional explanation, which envisages a monsoonal 'sea breeze' directed towards a summer thermal low pressure over the continent, provides an inadequate basis for understanding the workings of the system. The Asiatic monsoon regime is a consequence

Figure 11.15 The annual distribution of tropical rainfall. The shaded areas refer to periods during which more than 75 per cent of the mean annual rainfall occurs. Areas with less than 250 mm y-1 (10 in y-1) are classed as deserts, and the unshaded areas are those needing at least seven months to accumulate 75 per cent of the annual rainfall and are thus considered to exhibit no seasonal maximum.

Figure 11.15 The annual distribution of tropical rainfall. The shaded areas refer to periods during which more than 75 per cent of the mean annual rainfall occurs. Areas with less than 250 mm y-1 (10 in y-1) are classed as deserts, and the unshaded areas are those needing at least seven months to accumulate 75 per cent of the annual rainfall and are thus considered to exhibit no seasonal maximum.

Source: After Ramage (1971), copyright © Academic Press. Reproduced by permission.

Meridional Circulation Over India

Figure 11.16 Schematic representation of the meridional circulation over India at 90°E at five characteristic periods of the year: winter monsoon (December to February); approach of the monsoon season (April); the active summer monsoon (June to August); a break in the summer monsoon (June to August); and the retreat of the summer monsoon (September). Easterly (JE) and westerly (JW) jet streams are shown at sizes depending on their strength; the arrows mark the positions of the overhead sun; and zones of maximum precipitation are indicated.

Source: After Webster (1987a). Copyright © 1987. Reproduced by permission of John Wiley & Sons, Inc.

of the interaction of planetary and regional factors, both at the surface and in the upper troposphere. It is convenient to look at each season in turn; Figure 11.16 shows the generalized meridional circulation at 90°E over India and the Indian Ocean in winter (December to February), spring (April) and autumn (September), and those associated with active and break periods during the June to August summer monsoon (p. 287).

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  • fiori
    What is the 'Asiatic winter Monsoon'?
    2 months ago

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