Synoptic and mesoscale mechanisms drive winter precipitation δ¹⁸O/δ²H in South‐Central Alaska

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Measurements of oxygen and hydrogen stable isotopes in precipitation (δ¹⁸O_P and δ²H_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 δ¹⁸O_P and δ²H_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 δ¹⁸O_P and δ²H_P data from Anchorage, Alaska (2005–2018, <em>n</em> = 332), to determine the mechanisms controlling precipitation isotopes. Local surface air temperature explains ~30% of variability in the δ¹⁸O_P data with a temperature‐δ¹⁸O slope of 0.31 ‰/°C, indicating that δ¹⁸O_P archives may not be suitable paleo‐thermometers in this region. Instead, back‐trajectory modeling reveals how winter δ¹⁸O_P/δ²H_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 δ¹⁸O_P/δ²H_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 δ¹⁸O_P/δ²H_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.</p></div>