Influence of ENSO on moisture provenance and stable isotope dynamics during the North American Monsoon

The North American Monsoon (NAM) moisture sources have long been debated. This study explores moisture sources and precipitation isotope variability across the North American Monsoon (NAM) region during La Niña (2022) and El Niño (2023). By integrating isotope data with meteorological conditions, air mass back trajectories, and atmospheric profiles at Ciudad Obregón, Ciudad Juárez, and Albuquerque, we identified clear differences in moisture provenance and isotopic response between years. During La Niña, inland and Gulf of Mexico-derived moisture played a larger role northeast of the Sierra Madre Occidental. In contrast, El Niño conditions favored westerly, Gulf of California, and nearby Pacific Ocean sources. The isotopic values reflected these changes, particularly in d-excess, which differentiated coastal and continental influences. Coastal sites were more sensitive to radiation and convective factors, whereas inland sites showed stronger control by air mass transport history. Multivariate and Random Forest analyses confirmed that isotopic variability was more predictable at inland locations, suggesting greater sensitivity to large-scale atmospheric dynamics. Our results highlight the dynamic interplay between oceanic and continental moisture pathways in shaping NAM precipitation. They also demonstrate how ENSO phases alter the dominant controls on precipitation isotopes, offering new insight into hydroclimatic variability in this critical monsoonal region.