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

Abstract

Measurements of oxygen and hydrogen stable isotopes in precipitation (δ18OP and δ2HP) provide a valuable tool for understanding modern hydrological processes and the empirical foundation for interpreting paleoisotope archives. However, long-term data sets of modern δ18OP and δ2HP 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 δ18OP and δ2HP data from Anchorage, Alaska (2005–2018, n = 332), to determine the mechanisms controlling precipitation isotopes. Local surface air temperature explains ~30% of variability in the δ18OP data with a temperature-δ18O slope of 0.31 ‰/°C, indicating that δ18OP archives may not be suitable paleo-thermometers in this region. Instead, back-trajectory modeling reveals how winter δ18OP/δ2HP 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 δ18OP/δ2HP 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 δ18OP/δ2HP 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.