Factors controlling isotopic composition of precipitation on Okinawa Island, Japan: Implications for paleoclimate reconstruction in the East Asian Monsoon region

Stable isotope ratios of oxygen (δ18O) and hydrogen (δD) in precipitation collected every week during 2008–2011, Okinawa, Japan, were analyzed. The data provide new isotope data for an oceanic island located at the eastern end of the East Asian subtropical monsoon region. The monthly averaged isotope ratio shows clear seasonal variations: lower values in summer and higher values in winter. A multiple regression model shows that the relation between the δ18O value, temperature, and precipitation amount is quantitatively similar to that found in southern China region. To elucidate factors underlying isotope seasonality, we analyzed observation results using a dataset from an isotope-incorporated Atmospheric General Circulation Model. Our analyses revealed that the East Asian Monsoon causes strong seasonal differences of moisture sources for precipitation and the water budget in the island. In winter, moisture is recharged substantially in the East China Sea because a dry air mass from the continent flows over a warm ocean current: the Kuroshio Current. The precipitation is the first condensation from the air mass, resulting in higher isotope values of precipitation. In summer, lower δ18O of precipitation results from the convergence of water vapor whose isotope ratios are depleted because of large amount of condensation from the vapor source. The vapor source of such summer precipitation with low δ18O values is expected to be a low latitude ocean: the Western Pacific Warm Pool. Overall, the results suggest that the seasonality of the isotope ratio is controlled by the strength of distillation from moisture source to the precipitation site. These results provide a basis for interpretation of the past climate record preserved in stalagmites and tree rings in this region, and underline the need for the perspective of large-scale hydrological change for climate reconstruction.