RECENT TREND OF FALLING TEMPERATURE IN JAPAN AND ITS DYNAMICAL CONSIDERATIONS

Abstract

In the previous papers (1968, 1969), the present writer concluded that the winter temperature in Japan still kept its increasing trend until the end of 1960's while in Europe and North America it had already ceased in 1940's. In this study, the subsequent change up to 1970 are examined with an important result that the winter temperature had begun to fall in Japan at the beginning of 1960's (Fig. 1) while for the summer temperature its decreasing trend had been found thus early in 1940's. A schematic model for these changes is shown in Fig. 3. Before the discussion on main subject, meaning of trend index adopted here and homo-genuity of the temperature records are closely checked. After these preliminary works, dis-tribution maps of the trend indices for winter and summer temperatures during the two periods 1941-60, 1960-70 are constructed (Fig. 4-7). For the climatological explanation of these distribution pattern, some dynamical considerations are given. The main results obtained in this study are as follows: 1) For the representation of the winter cold and summer heat in Japan, the writer adopts the mean temperatures of the coldest and warmest months respectively, because their secular changes are nearly parallel to those of the mean minimum and maximum temperatures of the respective months and the accumulated sum of monthly temperatures lower than critical levels during the whole winter or summer months. Hence they are likely to represent the total state of winter and summer temperatures in the respective years. In a study of temperature fluctuations, mean temperatures for a definite interval, say every five or ten years are often used. However those higher and lower means do not necessarily represent its rising and falling tendency respectively as clearly indicated in Fig. 2. That is, a general trend of temperature cannot be expressed merely by their mean values for the respective periods. Among indices available for us to express such a kind of phenomena, the writer selected the trend index devised by Dr. E. Suzuki as the most preferable one for the purpose of this study. With reference to this index, the writer commented in the previous paper on the recent heavy rainfall in Japan and hence it is omitted here. (See January number of this Journal in 1972, pp. 1-12) 2) Temperature rise is not only caused by the climatic change in a wide area, but in large cities also is highly controlled by the expansion of built-up areas together with the increase of population. Of course, urban influence on the increasing rate of temperature is not the same at different places. With respect to this subject, the writer obtained an important result in the former study (1968) that a relation between the increasing rate of temperature and population growth in Japan is not the same in large cities with their population over 500, 000 as compared with other smaller cities. On the other hand, in the sparsely populated areas such as high mountain tops, protruding ends of promontories, small islands etc., temperature rise due to an artificial heat might be negligible and therefore a true state of temperature variation can be distinctly found. For this reason, seventy two stations selected for this study are classified into the following seven groups by their geographical locations and population : A) Gigant cities with a population over 2, 000, 000 .(Tokyo, Osaka and Nagoya) B) Large cities with a population over 500, 000 (Sapporo, Sendai, Hiroshima and Fukuoka) C) Middle class cities with a population 300, 000_??_500, 000 (Kanazawa and other 12 cities) D) Small cities or towns with a population less than 50, 000 (28 places) E) Stations at the high mountain tops such as Ibukiyama (5) F) Stations at the protruding end of promontories (5) G) Stations at the small islands (4)