Characteristics of Precipitation at Haha-jima, Ogasawara (Bonin) Islands in Comparison with Those at Chichi-jima

Abstract

 Given the lack of studies on the characteristics of precipitation at Haha-jima, Ogasawara (Bonin) Islands, they are investigated using daily precipitation data from AMeDAS Haha-jima (August 30, 2007-December 31, 2020) and the Oki-mura Water Supply Branch of Ogasawara Village (January 1, 1978-December 31, 2017). A regression equation is produced to estimate daily precipitation at AMeDAS using data at Oki-mura for each month; then, they are applied to estimate daily precipitation at AMeDAS from January 1, 1978 to August 29, 2007. The monthly precipitation analysis for the period from January 1978 to December 2020 reveals that precipitation in February at Haha-jima decreased with a 5% significance level. In contrast, precipitation in September at Chichi-jima increased with a 5% significance level. A comparison between the histograms of daily precipitation on both islands in the former (1978-1999) and latter (2000-2020) periods reveals that in February 2000-2020, precipitation of 10-19 mm/day class on both islands decreased, along with a decrease of 20-29 mm/day class at Haha-jima. During the period September 2000-2020, the maximum daily precipitation on both islands surpassed previous records, and precipitation of 70-89 mm/day class increased on both islands. Most of the heavy daily precipitation occurred either during the approach or the passage of a tropical depression or a typhoon.