Pre-1906 Extension of Precipitation Data for Chichi-Jima in the Ogasawara (Bonin) Islands Based on the Analysis of Historical Documents

Abstract

In the Western North Pacific, there is little meteorological data available for the 19th century. We found historical documents at Chichi-jima in the Ogasawara (Bonin) Islands that report routine meteorological observations and weather conditions before 1906. In 1906, Japan Meteorological Agency began observations at Ohmura in the northwestern part of the island. The Home Ministry in Japan conducted the observations until 1906 at Ohgiura, the central part of Chichi-jima, which is 3 km away from Ohmura. We analyzed precipitation data at Ohgiura and Ohmura from 1975 to 2020 and found that the former is systematically greater than the latter. Therefore, we corrected pre-1906 precipitation at Ohgiura to ensure correlation with post-1906 precipitation at Ohmura. No discontinuous changes in the data were found around 1906. A previous study reported that annual precipitation at Ohmura for 1907–1944 was higher than that after World War II. We found that in some years from 1882 to 1906, the precipitation was less than that for 1907-1944, which agrees with the reported higher atmospheric pressure around Chichi-jima before 1906. Our study suggests that the trend of decreasing precipitation at Chichi-jima in the 20th century is not monotonous but a part of longer-scale temporal variations.