Precipitation climatology at Iwoto Island, Ogasawara (Bonin) Islands, Japan

Abstract With permission from the Ministry of Defense, Japan, we analyzed daily precipitation and related meteorological variables observed on Iwoto Island in the Ogasawara (Bonin) Islands from January 1971 to December 2024. Monthly precipitation records for Iwoto Island in the Global Historical Climatology Network (GHCN) version 4 are nearly identical with those for Chichijima Island (approximately 270 km to the north) during the overlapping period of 1951–1960 in GHCN version 2, the latter being duplicates. Iwoto Island exhibits a distinct precipitation peak in July–August, reflecting the strong influence of the Western North Pacific Monsoon, whereas Chichijima Island is more affected by mid-latitude weather systems. Although previous studies have projected decreasing precipitation in parts of the subtropics including the Ogasawara Islands, observed annual precipitation across monitored islands in the study area shows a shift from a decreasing to an increasing trend around the year 2000. To investigate the relationship between annual precipitation variability on Iwoto Island and El Niño–Southern Oscillation (ENSO), we divided the analysis period into two stages: Stage 1 (1971–1997) and Stage 2 (1998–2024), and conducted equivalent analyses for Chichijima and Minami-torishima Islands. ENSO-related precipitation variability intensified on Iwoto Island during Stage 2, accompanied by higher mean precipitation amount than in Stage 1. In contrast, ENSO significantly influenced precipitation on Minami-torishima Island during Stage 1 but weakened in Stage 2, suggesting a westward shift of the ENSO-related precipitation systems. No statistically significant relationship between ENSO and precipitation was found for Chichijima Island.