A SHORT DRY PERIOD OF MID-SUMMER IN JAPAN

Abstract

1. The abundant summer rainfall in the eastern part of Asia including Japan has been commonly explained by the southwesterly monsoon winds carrying with much moisture from the Pacific Ocean. However, this problem is not so simple because the rainy season in summer has a threefold structure instead of a single continuous period. Except the northern region, there exists in Japan the double or triple maxima of monthly amount of precipitation. Fig. 1. shows the three regimes of precipitation, A, B and C, classified as follows A: Pacific type with double maxima in summer. A1 Primary maximum in. September and the secondary one in June. A2 Inverted type of A1, primary maximum in June and the secondary one in September. B: Japan Sea type with triple maxima. Of them, the primary maximum appears in winter and the other two in summer (June and September). C: Northern type with single maximum in September. In both regions of type A and B, two maxima of summer appear in September and June separated by the July-August minimum forming a short dry period which is very important not only from the geographical but also from the economic points of view. 2. The dry period above-mentioned is considered to be a result of the special type of pressure pattern developing in the special type of pressure patterns developing in the mid-summer over the north Pacific Ocean. After the Bai-u front along which the cloudy and rainy weather persists for a long spell parse: away to the north, the north Pacific anticyclone extends westward to the Japanese Islands and its western part is called the Ogasawara High. Even in a normal year, the main part of Japan, especially its western half, is located underneath a huge anticyclone (Fig, 2.) and fine weather lasts for many days with scarce rainfall, bright sunshine and strong evaporation. Because of the prevalence and westward expansion of the Ogasawara High, northward movement of cyclones and typhoons from the southwestern seas are checked to turn their paths to the westward direction along the outer periphery of the high pressure area and sc the frequencies of cyclones become very scarce (Fig. 3.). This strengthens further the occurrence of clear sky and scanty rain during this period. In connection with this subject, summer rain in the Far East is very important for the theory of non -soon, and it has been commonly explained by the monsoon winds from the Pacific Ocean laden with abundant moisture. However, this is inconsistent with the threefold structure of summer rainy season in Japan. Recent upper air observations afford another interpretation for this subject. That is, water vapor for summer precipitation in the Far East is originally transfered by the subtropical airmass from the equatorial region inflowing as southwesterly winds along the southern coast of Asiatic continent. This airmass is very moist with high equivalent potential temperature and connective instability. With the westward expansion of the Ogasawara High, tropical easterly with less moisture becomes strong and interrupts northward invasions of the southwesterly winds. As a result, fine weather persists for a long period. In such a way, the monsoon does really exist if it means only the seasonal alternation of winds with opposite directions. Hence; summer monsoon in East Asia is not exactly the same as that with abundant rainfall in India and other regions of southeastern Asia. 3. Furthermore, in summer season, the temperature becomes much higher with less rainfall because of intense solar heating unhindered and unweakened by the thick layer of clouds. A striking negative correlation is found between the amount of monthly rainfall and average monthly temperature in summer. Numerical figures for Fig. 4. give an amount of August rainfall in 1939 expressed as a percentage of normal year and those for Fig. 5, the August temperature deviation in 1939 from the normal.