特集 日本の小氷期 １８世紀の日本における冬の気候復元

Abstract

In order to reconstruct the winter climate in Japan in the 18th century, the authors tried to gather weather records of old diaries written in that period. The winter climate in Japan is influenced by the east high-west low pressure pattern and its severity is highly correlated with the frequency of the pressure pattern on a seasonally or monthly time scale. The weather distribution pattern under the west high-east low pressure type is characterized by the bad weather in Japan Sea Coast and fine one in Pacific Coast.By the use of the character of the weather distribution pattern under the winter pressure, the frequencies of the typical winter days are interpreted from the weather distribution diagram made for the research period on a daily base from 1st November of the former year to the 31 st March of the year. The stations of the diaries gathered for the study are Hirosaki, Takada, Kanazawa, Sabae, Tottori and Hagi as Japan Sea Coast, Hachinohe, Morioka, Nikko, Kofu, Ise, Kyoto, Ikeda, Tsuyama and Usuki as inland or Pacific Coast.The results obtained from the study are shown in Figure 1 as an annual number of the frequency of winter pressure days. On the same time scale, the freezing dates and the records of unusual weather in winter (cold or mild) are composed in the figure. The relationsbetween the frequency and other historical documents are investigated.The secular changes of severity in winter were well coincided with the other weather proxies. The cold winter years can be found in the decades in the 1700s-1710s, the former 1730s, the 1750s and the former 1780s. The warmer winter years were in the decades in the 1720s, the 1740s, the 1770s, and the 1790s. Unusual severe winters recorded in the periods are not always identical to the frequency of the winter pressure days, because of its temporal or local effects.