Moisture source temperatures and precipitation δ18O-temperature relationships across the United States

Understanding and modeling precipitation isotope (δ18O and δD) patterns for large regions of the globe requires quantifying processes governing continental‐scale climatology and hydrology. In this study, we have evaluated the extent to which inclusion of monthly moisture source temperature and moisture source locations in the Rayleigh distillation model aid in reproducing the relationships between monthly time series of precipitation isotope (δ18O) values and temperature across the contiguous United States. The steepest isotope‐temperature slopes (0.5‰–0.6‰ δ18O/°C) and greatest δ18O value correlations with temperature (r2 = 0.5–0.8), derived from 5 continuous years of the data from the United States Network for Isotopes in Precipitation, occurred in the high altitudes of the Rocky Mountains and the upper Great Lakes region. The isotope‐temperature slopes derived from the site‐specific time series were consistently lower along the coastal regions of the United States as were the coefficients of determination. The low coastal isotope‐temperature slopes are not easily explained by the simple Rayleigh model that uses condensation temperatures as the primary driver of precipitation isotopes. However, the Raleigh model equipped with moisture source temperatures defined by seasonal temperature oscillations and migrating moisture source locations provides a robust replication of the δ18O‐temperature slopes along coastal regions. These findings emphasize the importance of moisture sources conditions when interpreting palaeoclimate proxies (i.e., tree rings, ice cores, etc.), and argue that moisture source dynamics should always be included in models that use isotopes as diagnostic tools in testing hydrologic models.