日本における梅雨季の降水量変動の復元

Abstract

The purpose of this study is to clarify the rainfall variations of the Baiu, a rainy season in Japan, in the historical time and the Little Ice Age in particular. In order to reconstruct the rainfall variations in the historical time, old documents and diaries were used. The process of reconstruction was as follows: 1) Regional division based on the rainfall data for June, July and June plus July in the instrumental time was carried out using the Varimax rotated principal component analysis. With this method, Japan could be divided into eight regions for June and six regions for July and June plus July. 2) Unrotated principal component analysis was applied to the rainfall data in each region. Then the first principal component scores of each region were defined as ‘Rainfall Variation Index (RVI)’. 3) In order to reconstruct the RVIs in the historical time, regression equations were made using monthly number of days with precipitation more than 1 mm as independent variable for each region. With these equations, RVIs in the historical time were estimated. Comparison of RVI between in the instrumental time and in the Little Ice Age, whose boundary is set around 1900, shows that there was much rainfall in the Kanto district, East Japan, in the latter period. Moreover, it was known that there were regional differences in RVI through the ages. Finally, the authors pointed out that it was necessary to confirm further the reconstructed results with abundant material and to compare to other reconstruction by different kinds of proxy data.