[A-HW23] Isotope Hydrology 2023

[AHW23-06] Variability of stable isotope ratios of precipitation and the origin of water vapor at Ishigaki Island, Japan

*Tatsuki Otake¹, Kimpei Ichiyanagi², Masahiro Tanoue³ (1.Graduate School of Science and Technology, Kumamoto University, 2.Faculty of Advanced Science and Technology, Kumamoto University, 3.Meteorological Research Institute, Japan Meteorological Agency)

Keywords:Ishigaki Island, stable isotope ratios of precipitation, d-excess, water vapor origin, IsoGSM(Isotopes-incorporated Global Spectral Model)

There have been many observational studies on stable isotope ratios and d-excess of precipitation in Japan. However, only a few studies have been conducted on the subtropical islands such as Ishigaki Island. Therefore, this study reveals the variability of stable isotope ratios of precipitation at Ishigaki Island from January 2013 to March 2017 and estimated the origin of water vapor by using backward trajectory analysis. Stable isotope ratios and d-excess of precipitation showed seasonal variation, low in summer and high in winter. In addition, stable isotope ratios of typhoon precipitation in this study tend to be lower than those of normal precipitation, same as the result from previous studies. Because precipitable water, evaporation and water vapor flux were possible factors controlling stable isotope ratios of precipitation, the IsoGSM (Isotopes-incorporated Global Spectral Model) was used in this study. Seasonal average of whole observation period shows seasonal differences that precipitable water is high in summer and low in winter, evaporation is low in summer and high in winter, water vapor flux is high in spring and summer and low in fall and winter. These differences may have influenced the stable isotope ratios of precipitation. As a result of the estimating the origin of water vapor, the Pacific Ocean (75%) and the Eurasian continent (68%) were main source areas of precipitation in summer and in winter, respectively. In some cases, water vapor was originated from the Indian Ocean in summer, the Philippine Sea, the Sea of Japan and the East China Sea in winter. Furthermore, the estimated d-excess of evaporated water on the sea surface and the simulated d-excess of evaporated water and precipitable water by the IsoGSM was compared with the observed values. The low d-excess in summer precipitation was found even vapor source was not from the Pacific Ocean. However, the high d-excess in winter precipitation was found when water vapor originated from the Continent and moves southward across the East China Sea. The high d-excess was also found when water vapor originated from the Sea of Japan, however, it was not so high originated from the Philippine Sea.