The isotopic composition of rainfall on a subtropical mountainous island

Tropical islands are among the most biodiverse and vulnerable places on the planet. Water resources are one of the key players in the equilibrium on which tropical island ecosystems and populations depend. Stable water isotopes have proven to be powerful tools for studying the hydrometeorology of tropical islands, although the scarcity of long-term and high-frequency data often makes data interpretation challenging. Here, we present a new dataset consisting of weekly rainfall data collected over a two-year period beginning in July 2019 at five different locations on the island of Oʻahu. By analyzing the weather conditions during the collection period, we determine the isotopic composition associated with different synoptic systems, such as cold fronts and Kona lows, that affect the Hawaiian Islands. The data also show significant differences between the isotopic composition of rainfall on the windward and leeward sides of the island due to differences in rain evaporation rates. Using trajectory analysis, we also determine the different origins of air masses throughout the year and use this to explain the differences in deuterium excess observed in rainfall. Finally, we discuss the interannual variability of rain isotopes and its large-scale drivers.