What Controls the Summer Climate in Japan?: A Reassessment of Tree-ring Oxygen Isotope Ratios

Recently, flood and drought disasters have been rapidly increasing due to global warming. In order to accurately predict changes in the water cycle, it is necessary to understand its multidecadal variability in nature through collaborations in the fields of meteorology and paleoclimatology. During the last decade, the summer climate in central Japan over the past 2600 years has been successfully reconstructed using the tree-ring cellulose oxygen isotope ratio, a proxy for summer relative humidity, in many wood samples obtained from living trees, old buildings, archaeological remains, and natural buried logs. Given that there are various periodicities of variations, including the multidecadal time scale, in the 2600-year time-series of tree-ring oxygen isotope ratios, it is worth investigating the meteorological and paleoclimatological characteristics of the time-series in detail. By analyzing modern observational data, it is found that summer relative humidity in central Japan is not only affected by regional air temperature, but is also controlled by sea surface temperature around Japan through changes in water vapor pressure. Moreover, during the past 2,000 years, relative humidity in central Japan reconstructed from the tree-ring oxygen isotope ratio has changed roughly in sync with the land surface temperature in Northern Hemisphere, and discrepancies between the two are entirely due to variations in sea surface temperature. In this way, it can be concluded that variations in the summer water cycle of central Japan are controlled explicitly by two factors, continental air temperature and oceanic water temperature, at every time scale including the multidecadal time scale. This means that collaborations in the fields of meteorology, oceanography, paleoclimatology, and paleoceanography are critical for accurately predicting climate change in the near future.