Long-term Variation of Rainfall in the Far East

Abstract

Through the rhythmic change of the decade means of Dec., Jan. and Feb. air temperature in the middle and south-western part of Japan, a rising trend of winter air temperature with longer period is observed as shown in Fig. 1, in which it will be seen that the eight points of the decadal means of air temperatures change inversely with these of the pressure difference, Irkutsk (52°16'N, 104°19'E) - Nemuro 43°20'N, 145°35'E), which was taken as an index of the Far East monsoons strength, and, notwithstanding the inverse parallelism of the decade means, the secular rising trend of Japanese winter temperature is running parallel with that of the Far East monsoon strength.
The same relation can be seen in the case of rainfalls. In Fig. 2, 10-year running mean curve of January precipitations on the Japan Sea side indicates the same increasing trend with that of the Pacific side, though the 10-year running mean in itself change in parallel with opposite signs.
These facts lead us to the conclusion that the secular trend of the winter climate in Japan through centuries must be due to some longer period change in the general circulation other than the change in the Far East monsoons.
A comparison of the two mean December, January and February pressure charts averaged for the periods 1899/1900-1928/29 (n=30) and of 1929/30-1959/60 (n=27) is shown in Fig. 3. The pressure of the Siberian High increased from the first half period to the second period, while the Pacific Low decreased, and the position of the center of the Siberian High has shifted significantly to the northwest (about 3.5° northwards and 5.0° westwards) and that of the Pacific Low appears to have moved to the northwest too.
It is my opinion that the secular trends observed in the winter climate of Japan must be a reflection of the northwest-wards shift of the climatic zone which was seen in Fig. 3.
The summer climate of Japan in the past 80 years can be broadly divided into three epochs, basing on the facts which is seen in Fig. 4 and Table 3 (A) and (B).
(i) 1891-1910 (20 years) cool-rainy epoch
(ii) 1911-1950 (40 years) hot-dry epoch
(iii) 1951-1970 (20 years) cool-rainy epoch
By using all the northern hemisphere monthly mean chart on hand, the secular change of July pressure in the central part of the North Pacific High is investigated in Fig. 5, in which the area enclosed by 1026 mb isobar averaged for (ii) period is remarkably large compared with those before and after the period, and the center of the North Pacific High is situated moving to the northwest in comparison with those before and after the period. The patterns of the July pressure distribution in the neighborhood of the Japan Island are also investigated respectively for (i), (ii) and (iii) period. In Fig. 6, it will be seen that the North Pacific High bulges out strongly towards the south-western part of Japan in the period of (ii).
The geographical distributions of correlation coefficient of July air temperature with July monsoon strength, taking the pressure difference between Choshi (35°43'N, 140°51'E) and Asahikawa (43°46'N, 142°22'E) as an index of the monsoon intensity in the neighborhood of Japan, are shown for each period in Table 1, in which the region of maximum correlation coefficient goes northwards, or southwards, corresponding to the north-south oscillation of the center of the North Pacific High.
The July pressure difference between Bo-so Peninsula in Japan and the northeastern part of Korea or the Maritime Province in Siberia is taken as an index of the intensity of bulging of the North Pacific High and the correlation coefficient of the index with amount of July precipitation are computed at various places or district in the Far East.