The Far East monsoon regime is a consequence of the interaction of both planetary and regional factors. When the westerlies over the Eastern Hemisphere are weak in winter, the stational trough of the planetary wave, which is located normally in the neighborhood of the Japanese islands, tends to retreat in the Japan Sea side, producing the monsoonal climate, that is to say, the lower air temperatures in the south-western Japan and more snowy winter in the Hokuriku District. On the contrary, when the westerlies become stronger, the trough moves across the Japan Island to the Pacific side, resulting the non-monsoonal winter climate, warm winter in the southwestern Japan, less snowy in the Hokuriku District and more rainy in the Pacific coast. In summer, the trough, which is normally located farther west in the Japan Sea, moves westwards or eastwards, corresponding to the weaker or stronger westerlies, and produces hot-dry summers or cool-rainy summers in Japan. In other words, the winter and summer monsoons in the Far East change their vigor in contrast with the intensity of the prevailing westerlies throughout the year, and this is the reasons why the cold-snowy (warm-less snowy) winters correspond to hot-dry (cool-rainy) summers, and the curve of 11-year running means of the annual means of wind speeds at the summit of Mt. Fuji (3782m) changes opposite to those of the annual amplitude of air temperatures at various points in the central Japan.
Through the rhythm of the decade means of summer air temperatures in the north-eastern Japan and the running means of the amount of January precipitations on the Japan Sea coast, a significant trend of long-term change is noticable, which must be attributed to the north-southward displacement of the climatic zone. The northward shift or the southward retreat of the climatic zone generally appears in company with the weak or vigorous state of the prevailing westerlies.
Consequently, in the case of Japanese climate, the effect of the Far East monsoons on the winter climate, for instance, the lowering or rising air temperatures due to the change of the Far East monsoon, objects to the effect influenced by the north-southward displacement of climatic zone, while, in summer, both effects cooperate with each other.
However, in the estreme cases of the displacement of the climatic zone, the global effect exceeds the regional effect of the Far East monsoons, producing the extreme case of“Little Climatic Optimum” with warm-less snowy winters and hot-dry summers and the extreme case of“Little Ice Age”with cold-snowy winters and cool-rainy summers.
It is beyond question that the most favorable condition for the accumulation of snow and ice must be the extreme case of“Little Ice Age”distinguished with much snowfalls, lower air temperature in all seasons, and rainy summers associated by the reduced durations of sunshine. The climate in the first half of XV Century, for instance, is estimated to be the extreme case of“Little Ice Age”from the various records of decuments. In accord with the estimation, the ¹⁴C dating test on the wood which was discovered by Horie (1968) in the outwash deposit of Shirouma Kitamata (1620M) gives 520±80 years B. P. Howeve, the duration of extreme period of“Little Ice Age”is not so long as far as it concerns within historical times. The climate of (warm-winter)-(cool-summer), which is caused by the predominant westerlies and distinguished with the intensified activities of Lows in the surroundings of the Japan Island, is also favorable, to a certain exextent, for the accumulation of snow.