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Influence of synoptic weather events on the isotopic composition of atmospheric moisture in a coastal city of the western United States

Synoptic weather events are known to strongly influence the isotope composition of precipitation in continental locations. In this study, we present hourly values of water vapor isotopologues (HDO and H218O) measured over a 30 day period in locally extreme weather conditions, including Santa Ana winds and winter rainstorms, in San Diego, California, USA. We investigate how atmospheric and hydrological processes influence HDO and H218O using an isotope‐enabled GCM model (IsoGSM). Combining measurements and IsoGSM simulation, we demonstrate that convective mixing of marine and continental air masses are responsible for the isotopic variation of near‐surface water vapor in this coastal location. The isotopic variability is most pronounced during Santa Ana winds. The Santa Ana winds represent a unique boundary layer condition in which atmospheric mixing becomes the process that dominantly controls the changes in the isotopic composition relative to air humidity. We demonstrate that a two‐source mixing approach (Keeling plot) can reliably be used to estimate the isotopic composition of the source moisture, and from that, to infer the location of the moisture origin that contributes to the atmospheric moisture content in southern California. The present study is unique because it combines large‐scale isotope GCM modeling with a robust and high‐resolution isotope data set to disentangle the control of atmospheric and hydrologic processes on the atmospheric humidity in an extratropical climate. Our results demonstrate the utility of using single‐point, ground‐based isotope observations as a complementary resource to existing satellite isotope measurements for constraining isotope‐enabled GCMs in future investigation of atmospheric water cycle.