The cause of the seasonal variation in the oxygen isotopic composition of precipitation along the western U.S. coast

This study seeks to find the primary influence on the seasonal cycle of the oxygen isotopic composition of precipitation (δ¹⁸O_p) along the western U.S. coast. Observed long-term mean seasonal variations of δ¹⁸O_p from 16 different stations along the west coast are presented. The most robust features in the observations are high values in the summer and a drop in δ¹⁸O_p during the winter, a feature observed at many mid-latitude locations. The Isotope-incorporated Global Spectral Model (IsoGSM) also simulates this wintertime drop in δ¹⁸O_p along the west coast of the U.S. To better understand the cause of this seasonal variation, sensitivity experiments are performed with IsoGSM where individual oxygen isotope fractionation processes are turned off. These simulations reveal that the primary control on the seasonal variations is equilibrium oxygen isotopic fractionation during vapor condensation. There is almost no influence of the temperature dependence of equilibrium fractionation on the seasonal δ¹⁸O_p cycle for both ocean evaporation and vapor condensation. Additional experiments (including tagging simulations) are performed to better understand why Rayleigh distillation causes the seasonal variation in δ¹⁸O_p. Tagging results suggest no strong influence of seasonal moisture source variations on the seasonal δ¹⁸O_p cycle, though the seasonal cycle of column-integrated water vapor δ¹⁸O values is in phase with the monthly shifts in moisture sources (in IsoGSM). The tagging simulations did however reveal that vertical oxygen isotope gradients and variations in condensation height cause the seasonal cycle in δ¹⁸O_p. This results from seasonal changes in the polar jet, and subsequent changes to divergence and vertical velocities, which affects the uplift of moisture. These findings suggest that δ¹⁸O_p in the western U.S. is a tracer of condensation height on seasonal timescales. The large influence of condensation height on δ¹⁸O_p seasonality complicates interpretations of interannual climate proxy records based on isotopes in precipitation as the seasonality is likely not static.