Reconstruction of surface ocean freshening trend from oxygen isotope record in coral skletons

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Surface ocean freshening trend for last 100 years is reconstructed using massive, hermatypic coral skeletons, which distribute widely in tropical-subtropical region. Time-series data for oxygen isotope ratios of coral skeletons archived in NOAA Paleoclimate Program are used because oxygen isotope ratio in coral skeletal carbonates is controlled by water temperature and salinity. With global SST reanalysis dataset and the relationship between coral oxygen isotope ratio and water temperature determined from previous investigations, spatio-temporal distribution of salinity for tropical-subtropical Pacific are reconstructed. In the western Pacific ocean, long-term freshening trend is insignificant, whereas significant freshening trend is recognized at central to eastern Pacific region. According to these results, it is suggested that tropical climate is gradually shifted to the El Nino-like condition for last 100 years.