FURTHER STUDIES ON THE CLIMATIC BOUNDARY DIVIDING THE JAPAN SEA COAST AND THE PACIFIC SIDE OF JAPAN

Abstract

1. As is widely known, the most striking contrast of climate exists in Japan between the Japan Sea coast and the Pacific side of Japanese mainland. The boundary line dividing these two climatic provinces has been considered to coincide roughly with the water-sheds of the central mountain chains running form the Oo-u Mts, in the north, through central Japan, to the Chugoku Mts. in the west. However, this is not precisely correct and at many parts the influence of the Japan Sea invades far into the Pacific side crossing the central mountain ranges. In order to determine the exact position of this boundary line, these two climates are distiguished by the rainfall types. The Japan Sea type has three maxima including that in winter (Fig. 1.) while the other is characterized by the two maxima in summer and the primary minimum in winter. Japan Sea type thus defined develops along the inner side of the Japanese mainland from Tôhoku in the north to the western end of the Chugoku district. Put it is most distinct between Tsuruga and Aomori with the primary maximum in winter whereas the winter maximum is less than these of summer in the west of Tsuruga gradually decreasing westwards and finally disappears at the northern coast of Kyushu.
2. Using this criterion, climatic types are determined for the local stations at 1769 places. As is shown by Fig. 3., Japan Sea type is found here and there on the Pacific Side of the Tohoku district and we have fifteen places where the influence of the Japan Sea is invading or spreading into the Pacific slope spilling over the low mountain pass or blowing through the narrow gorges of the transverse valleys (Fig. 4. ) . In the Kantô, Chubu and Kinki districts, we can find such a feature at ten places (Fig. 5. ) and along the Himekawa and Jinzûgawa they reach about fifty kilometre's distance from their water-sheds. Most broad invasion is experienced in the lowland surrounding the Lake Biwa, the northern half of Shiga Prefecture is under the influence of the winter monsoon and the slender tongue-like arm of te cold air extends to Sekigahara through the narrow path at the foot of Mt. Ibuki.
We have some isolated small areas with distinct type of Japan Sea coast such as at Yôrô (Gifu Prefecture), Mandokoro (Shiga Pref.), Fukusumi (Kyoto Pref.), Toyosaka (Hiroshima Pref.) as shown in Fig. 3. and 4. All of these are at the relatively higher position facing directly to the northwesterly winds which are not disturbed by the uneven land surface bringing much amount of precipitation.
For the precise determination of the climatic boundary, present available data are not sufficient especially in the mountainland and other supplementary methods are needed for the further study.
Obtained results by the snow-survey may be partly utilized for this purpose in the snowy regions.
3. Japan Sea type once disppears at the northern part of Kyûshû and Iki Island, and it is found again at some localities of the Tsushima Islands and along the western coast of Kyûshû including the Goto, Amakusa and Koshiki Islands reaching to the southern ends of the Satsuma and Oosumi Peninsulas (Fig. 8.). Notwithstanding the winter maximum in these regions becomes much faint (Fig. 2.) and is statistically significant merely at the high level of risk, they still maintain some systemtic distribution not disturbed by other irregular causes and we may call this properly the Japan Sea type tendency. In such a way, this pattern is somewhat curious although it develops in far smaller scale and not so distinct and sharp as in the Japan Sea coast proper. This is explained by the transformation of air-mass from the Siberian continent due to the supply of he at and water vapour over the sea surface.