Long-term Change in the Hydroclimatic Environment during the 20th Century on Chichi-jima in the Ogasawara (Bonin) Islands

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We examined the long-term change in the hydroclimatic environment during the 20th century on Chichi-jima in the Ogasawara Islands using monthly climatic data observed at the Chichi-jima Observatory over 78 years: 1907-1943, 1951-1959, and 1969-2000. The mean annual temperature for 1907-1943 was about O.4-0.6°C lower than that for 1951-1959 and 1969-2000. The mean annual temperature during the analyzed period increased by O.75°C 100yr-1. The average annual precipitation for 1969-2000 was about 20% less than that for 1907-1943. The water balance and the compensated potential evaporation calculated using the method of Thornthwaite demonstrated that the mean annual amounts of the water deficit (WD) in 1969-2000 were greater than those in 1907-1943. The dry months in summer, when the potential evaporation exceeded precipitation, persisted longer during 1969-2000 than during 1907-1943. These results suggest that the hydroclimatic environment during the 20th century tended to be drier on Chichi-jima in the Ogasawara (Bonin) Islands. The long-term change in the hydroclimatic environment also caused the summer dry season to be much longer and drier.
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