Synoptic and Mesoscale Mechanisms Drive Winter Precipitation δ18O/δ2H in South-Central Alaska

Measurements of oxygen and hydrogen stable isotopes in precipitation (delta O-18(P) and delta H-2(P)) provide a valuable tool for understanding modern hydrological processes and the empirical foundation for interpreting paleoisotope archives. However, long-term data sets of modern delta O-18(P) and delta H-2(P) in southern Alaska are entirely absent, thus limiting our insight and application of regionally defined climate-isotope relationships in this proxy-rich region. We present and utilize a 13-year-long record of event-based delta O-18(P) and delta H-2(P) data from Anchorage, Alaska (2005-2018, n = 332), to determine the mechanisms controlling precipitation isotopes. Local surface air temperature explains similar to 30% of variability in the delta O-18(P) data with a temperature-delta O-18 slope of 0.31 %/degrees C, indicating that delta O-18(P) archives may not be suitable paleo-thermometers in this region. Instead, back-trajectory modeling reveals how winter delta O-18(P)/delta H-2(P) reflects synoptic and mesoscale processes in atmospheric circulation that drive changes in the passage of air masses with different moisture sources, transport, and rainout histories. Specifically, meridional systems-with either northerly flow from the Arctic or southerly flow from the Gulf of Alaska-have relatively low delta O-18(P)/delta H-2(P) due to progressive cooling and removal of precipitation as it condenses with altitude over Alaska's southern mountain ranges. To the contrary, zonally derived moisture from either the North Pacific and/or Bering Sea retains relatively high delta O-18(P)/delta H-2(P) values. These new data contribute a better understanding of the modern Alaska water isotope cycle and provide an empirical basis for interpreting paleoisotope archives in context of regional atmospheric circulation.