Synoptic weather events are known to strongly influence the isotope composition ofprecipitation in continental locations. In this study, we present hourly values of water vaporisotopologues (HDO and H218O) measured over a 30 day period in locally extreme weatherconditions, including Santa Ana winds and winter rainstorms, in San Diego, California,USA. We investigate how atmospheric and hydrological processes influence HDO andH218O using an isotope-enabled GCM model (IsoGSM). Combining measurements andIsoGSM simulation, we demonstrate that convective mixing of marine and continental airmasses are responsible for the isotopic variation of near-surface water vapor in this coastallocation. The isotopic variability is most pronounced during Santa Ana winds. The SantaAna winds represent a unique boundary layer condition in which atmospheric mixingbecomes the process that dominantly controls the changes in the isotopic compositionrelative 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 fromthat, to infer the location of the moisture origin that contributes to the atmospheric moisturecontent in southern California. The present study is unique because it combines large-scaleisotope GCM modeling with a robust and high-resolution isotope data set to disentangle thecontrol of atmospheric and hydrologic processes on the atmospheric humidity in anextratropical climate. Our results demonstrate the utility of using single-point, ground-basedisotope observations as a complementary resource to existing satellite isotopemeasurements for constraining isotope-enabled GCMs in future investigation ofatmospheric water cycle.

Speleothem oxygen isotope records over East Asia reveal apparently large and rapid paleoclimate changes over the last several hundred thousand years. However, what the isotopic variation actually represent in terms of the regional climate and circulation is debated. We present an answer that emerges from an analysis of the interannual variation in amount-weighted annual δ18O of precipitation over East Asia as simulated by an isotope-enabled model constrained by large-scale atmospheric reanalysis fields. 18O-enriched years have reduced summer seasonality both in terms of precipitation isotopes and in the large-scale circulation. Changes occur between June and October, where the δ18O of precipitation (δ18Op) transitions from the isotopically heavier winter to the lighter summer regime. For 18O-enriched years, this transition is less pronounced. Variations in precipitation amount alone are insufficient to explain the amount-weighted annual δ18Op between 18O-enriched and 18O-depleted years. Reduced summer seasonality is also expressed in the low-level monsoonal southerlies and upper-level westerlies; for the latter, the northward migration across the Tibetan Plateau in the summer is less pronounced. Our result thus implicates the westerlies across the plateau as the proximate cause of East Asian paleomonsoon changes, manifested as a modulation of its summer peak.

Speleothem oxygen isotope records over East Asia reveal apparently large and rapid paleoclimate changes over the last several hundred thousand years. However, what the isotopic variation actually represent in terms of the regional climate and circulation is debated. We present an answer that emerges from an analysis of the interannual variation in amount-weighted annual δ¹⁸O of precipitation over East Asia as simulated by an isotope-enabled model constrained by large-scale atmospheric reanalysis fields. ¹⁸O-enriched years have reduced summer seasonality both in terms of precipitation isotopes and in the large-scale circulation. Changes occur between June and October, where the δ¹⁸O of precipitation (δ¹⁸O_p) transitions from the isotopically heavier winter to the lighter summer regime. For ¹⁸O-enriched years, this transition is less pronounced. Variations in precipitation amount alone are insufficient to explain the amount-weighted annual δ¹⁸O_p between ¹⁸O-enriched and ¹⁸O-depleted years. Reduced summer seasonality is also expressed in the low-level monsoonal southerlies and upper-level westerlies; for the latter, the northward migration across the Tibetan Plateau in the summer is less pronounced. Our result thus implicates the westerlies across the plateau as the proximate cause of East Asian paleomonsoon changes, manifested as a modulation of its summer peak.

The isotopic composition of water vapour provides integrated perspectives on the hydrological histories of air masses and has been widely used for tracing physical processes in hydrological and climatic studies. Over the last two decades, the infrared laser spectroscopy technique has been used to measure the isotopic composition of water vapour near the Earths surface. Here, we have assembled a global database of high temporal resolution stable water vapour isotope ratios (δ18O and δD) observed using this measurement technique. As of March 2018, the database includes data collected at 35 sites in 15 Köppen climate zones from the years 2004 to 2017. The key variables in each dataset are hourly values of δ18O and δD in atmospheric water vapour. To support interpretation of the isotopologue data, synchronized time series of standard meteorological variables from in situ observations and ERA5 reanalyses are also provided. This database is intended to serve as a centralized platform allowing researchers to share their vapour isotope datasets, thus facilitating investigations that transcend disciplinary and geographic boundaries.