High-Resolution Dataset Reveals Complex Relationship Between Storm Tracks and Oxygen Isotopes in Precipitation Across the U.S. Pacific Northwest

Abstract:

Storm tracks play a key role in controlling spatiotemporal patterns of precipitation in the U.S. Pacific Northwest and are already shifting due to climate change. Different approaches to quantifying and examining these tracks are important for interpreting pre-instrumental climate proxy records. Here, we combine a long-term and spatially dense network of precipitation δ¹⁸O measurements with back-trajectory analysis to examine the relationship between the oxygen isotope composition of precipitation and storm tracks. Our analyses focus on back trajectories developed in HYSPLIT associated with the weeks of highest-δ¹⁸O and lowest-δ¹⁸O precipitation at each of our five sites over ten years, and we perform cluster analyses to identify primary seasonal storm tracks and other frequently-traveled paths across given site–season–precipitation isotope combinations. Our results suggest that storms spend more time over land during the warm season, which aligns with the shift between fast-moving synoptic-scale systems during the cool season and convective precipitation during the warm season. Additionally, distance traveled over the 72 hours leading to precipitation decreases with increasing site elevation, highlighting the impact of orographic forcing on accelerating saturation and precipitation. Seasonal mean trajectories and cluster analyses show that isotopically light precipitation is often produced by more zonal paths originating in higher latitudes that occasionally spend more time over the continental interior than the heavy tracks, while isotopically heavy precipitation more frequently originates in southerly regions and follows more meridional paths, indicating that moisture source and temperature both influence precipitation isotopes. The cluster analyses, which rely on information from weekly precipitation samples, provide additional detail and information beyond what is possible when using monthly averaged data. Our findings demonstrate that topography, coastal proximity, and the amount effect all must be carefully considered when interpreting stable isotope records, and region-specific studies like this one are critical to help resolve these complexities in order to improve the interpretation of paleoclimate proxy records.