Dominant influence of the humidity in the moisture source region on the 17O-excess in precipitation on a subtropical island

A high-precision analysis of the triple oxygen isotope composition of water provides a new tracer denoted as O-17-excess. Early theoretical and experimental studies suggest that changes in the O-17-excess in precipitation can be interpreted as variations in the relative humidity in moisture source regions for precipitation. However, subsequent studies on O-17-excess in the precipitation in polar and dry regions suggest the importance of additional fractionation, such as snow formation and raindrop re-evaporation processes, which diminish the relative humidity information in oceanic source regions. To date, whether humidity in the moisture source region can be quantified based on observations of O-17-excess in precipitation has not been proven. Here, we show a two-year record of the O-17-excess in precipitation on a maritime island in the East Asian monsoon region. The normalized relative humidity in the source region was reconstructed from O-17-excess data using a simple evaporation model. We demonstrate that the reconstructed relative humidity is quantitatively consistent with observations in the oceanic moisture source region. This result suggests that the O-17-excess in precipitation on the subtropical island is determined largely by diffusional fractionation during evaporation in the ocean. We also test the impact of a possibly different O-17-O-18 slope suggested by a recent study on the O-17-excess definition. This different definition results in a minor impact on the data from tropical and subtropical regions but a significant change in the data from polar regions. These results suggest that the O-17-excess in precipitation in tropical and subtropical regions is a unique quantitative tracer for the relative humidity in oceanic moisture source regions and thus will be a useful tracer and proxy for hydrological and paleoclimate studies. (C) 2019 Elsevier B.V. All rights reserved.