CLIMATIC VARIATIONS IN RECENT YEARS FROM THE VIEWPOINT OF THE SEASONAL TRANSITION OF PREVAILING PRESSURE PATTERN TYPES OVER EAST ASIA

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  • 東アジアにおける卓越気圧配置型の季節推移からみた近年の気候変動
  • ヒガシ アジア ニ オケル タクエツ キアツ ハイチガタ ノ キセツ スイイ

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Abstract

Seasonal transitions and Climatic variations from the viewpoint of the occurrence frequency of atmospheric pressure patterns over East Asia are discussed in the recent 45 years (1941-1985). The category of the pressure pattern types which was formed by Yoshino (1968) has been applied to many studies (ex.: Nomoto, 1975; Komabayashi and Nakamura, 1976; Hohgetsu, 1979; Hayashi, 1987). The pressure pattern calendar on the basis of this category was made by Yoshino and Fukuoka (1967), Yoshino and Kai (1974) and Yoshino and Yamakawa (1985).<br> The following is the classification of main pressure patterns over East Asia in this study.<br> I: West-high, east-low pattern or winter monsoon pattern.<br> II: Trough pattern.<br> II a: passing low over Hokkaido (the second biggest, northern island in Japan) or north of it.<br> II b: Passing low over the Sea of Japan.<br> lI c: Passing low along the Pacific coast.<br> lI d: Passing coupled lows across Japan.<br> III: Migratory anticyclone pattern.<br> IV: Stationary frontal pattern (The defined area of this category is shown in Fig. 11).<br> IVa: Stationary front over Honshu (the biggest, main island in Japan).<br> IVb: Stationary front along the Pacific coast of Japan.<br> V: South-high; north-low pattern or summer monsoon pattern.<br> VI: Typhoon pattern.<br> The main results in this study are as follows: 1) Six seasons and one specific period in Japan are recognized (Fig. 12) from the viewpoint of occurrence frequency over 30% during 1941-1985 of the main pressure pattern types: namely each index are Type I for the winter season, Type III for the spring and autumn seasons, Type V for the summer season, Type N for the Baiu season (early summer rainy season), Type (IV+II b, c, d+VI) for the Akisame season (early autumn rainy season) and. Type VI for the Typhoon period.<br> 2) The pentad (Nov. 2-6) including November 3, which is called the specific fine weather day, occupies the pretty much high frequency (51%) of migratory anticyclones. While in the pentad of April 11-15, the equivalently and considerably high frequency (52%) of migratory anticyclones appeares. (Fig. 3, Fig. 12).<br> 3) A singularity pentad of cyclonic or frontal pressure patterns accompanying the cloudy or rainy weather is June 25-29 (Fig. 7, Fig. 12), which was indicated by Maejima (1967, 1968).<br> 4) Around 1980, the Baiu fronts were more active over the Japanese Islands and were apt to cause heavy rain, as same as in the 1960's (Fig. 5).<br> 5) In the latest years (1971-1985), the autumn rainy season due to the fronts and cyclones became more active (Fig. 7, Fig. 12).<br> 6) The frequency of typhoons approaching to the Japanese Islands decreased during about the latest years, especially in autumn (Fig. 10, Fig. 12).<br> 7) In the latest years (1971-1985), comparing with the former years (1941-1970), the secular features of seasonal phenomena are noticed (Fig. 12); i, e, prevailing and longer winter, less frequent migratory anticyclones in the earlier spring, active Baiu fronts in the later period, less prevailing summer, more frequent autumnal fronts in the earlier and longer period, and less frequent migratory anticyclones in later autumn were often appeared. Probably, there is no causal relationship between these phenomena, but there are the correlations due to some causes of astronomy, upper air chemical content and circulation, sea or glacier.

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