The Isotopic Composition of Rainfall on a Subtropical Mountainous Island

Tropical islands are some of the most biodiverse and vulnerable places on Earth. Water resources help maintain the delicate balance on which tropical island ecosystems and populations depend. Hydrogen and oxygen isotope analysis is a powerful tool for studying the water cycle on tropical islands, although the scarcity of long-term and high-frequency data makes interpretation challenging. A new dataset is presented based on a weekly collection of the H and O isotopic composition of rainfall on the island of Oʻahu, Hawaiʻi, beginning in July 2019 and still ongoing. During this time, different weather conditions have affected the island, each producing precipitation with different isotopic ratios: precipitation from upper-level lows was found to have the lowest isotopic ratios, while trade wind showers had the highest. These data also show some differences between the windward and leeward sides of the island, the latter being associated with higher rainfall isotope ratios due to increased rain evaporation. As an example of such differences, the amount effect is not observed at all sites. The measured deuterium excess shows a pronounced seasonal cycle, which is attributed to different origins of the air masses responsible for rainfall in the winter and summer months. The local meteoric water line is then determined and compared with similar lines for Oʻahu and other Hawaiian islands. Finally, a comparison is made with data collected on Hawaiʻi Island over a longer period of time, and it is shown that the isotopic composition of precipitation exhibits significant interannual variability.