Winter Monsoon Season

Figure 11.36 (A) Seasonal variation of daily normals at Nagoya, southern Japan, suggesting six natural seasons. (B) Average ten-day precipitation amounts for a station in southern Japan, indicating in black the proportion of rainfall produced by typhoon circulations. The latter reaches a maximum during the Shurin season.

Sources: (A) From Maejima (1967); (B) after Saito (1959), from Trewartha (|98I).

during late July and August, giving a period of more settled sunny weather. The secondary precipitation maximum of the Shurin season during September and early October coincides with an eastward contraction of the Pacific subtropical anticyclone, allowing low-pressure systems and typhoons from the Pacific to swing north towards Japan. Although much of the Shurin rainfall is believed to be of typhoon origin (see Figure 11.36), some is undoubtedly associated with the southern sides of depressions moving along the southward-migrating Pacific polar front to the north (see Figures 11.22 and 11.35), because there is a marked tendency for the autumn rains to begin first in the north of Japan and to spread southwards. The manner in which the location of the western margin of the North Pacific subtropical high-pressure cell affects the climates of China and Japan is well illustrated by the changing seasonal trajectories of typhoon paths over East Asia (Figure 11.37). The northward and southward migrations of the cell zonal axis through 15° of latitude, the northwestern high-pressure cell extensions over eastern China and the Sea of Japan in August, and its southeastern contraction in October, are especially marked.

Northern Australia experiences a monsoon regime during the austral summer. Low-level westerlies develop in late December associated with a thermal low over northern Australia. Analogous to the vertical wind structure over Asia in July, there are easterlies in the

Japan Shurin Weather

Figure 11.37 Typhoon paths over East Asia during January to April, May to June, July to September and October to December related to the mean latitude of the central ridge axis line of the subtropical high-pressure cell at 500 mb over the western Pacific.

Sources: Compiled from various sources, including Lin (1982) and Tao (1984). Reproduced by permission of the Geographical Society of China.

Figure 11.37 Typhoon paths over East Asia during January to April, May to June, July to September and October to December related to the mean latitude of the central ridge axis line of the subtropical high-pressure cell at 500 mb over the western Pacific.

Sources: Compiled from various sources, including Lin (1982) and Tao (1984). Reproduced by permission of the Geographical Society of China.

upper troposphere. Various wind and rainfall criteria have been used to define monsoon onset. Based on the occurrence of (weighted) surface to 500-mb westerly winds, overlain by 300 to 100 mb easterlies at Darwin (12.5°S, 131°E), the main onset date is 28 December and retreat date 13 March. Despite an average duration of seventy-five days, monsoon conditions lasted only ten days in January 1961 and 1986 but 123 to 125 days in 1985 and 1974. Active phases with deep westerlies and rainfall each occur on just over half the days in a season, although there is little overlap between them. However, summer rainfall may also occur during deep easterlies associated with tropical squall lines and tropical cyclones. Active monsoon conditions typically persist for four to fourteen days, with breaks lasting about twenty to forty days.

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