Precipitation isotope responses to monsoonal and westerly moisture transport across High Mountain Asia

Precipitation stable isotopes integrate information on moisture source regions, atmospheric transport, and rainout processes, yet their interpretation over the Himalayan region is complicated by the interaction of monsoonal and westerlies systems. Here we investigate the controls on precipitation δ¹⁸O variability at North Pakistan (NPK) and Nyalam on the southern Tibetan Plateau (NLM) using two-year daily-based isotope datasets. By combining isotope observations with Lagrangian moisture trajectories, outgoing longwave radiation (OLR), and mid-tropospheric dynamical diagnostics, we identify the key drivers of isotopic variation in these high-elevation regions.

Although both sites receive Indian Ocean moisture during the Indian Summer Monsoon, their isotope signatures diverge systematically due to distinct transport pathways and convective environments. Trajectory analysis shows that NPK is primarily influenced by the Arabian Sea branch, whereas NLM is dominated by Bay of Bengal moisture. Consistent with this separation, OLR patterns reveal persistently stronger and more spatially extensive convection along the Bay of Bengal pathway, while convection along the Arabian Sea route remains comparatively weaker. These contrasting convective regimes exert a first-order control on upstream rainout efficiency, resulting in systematically more depleted δ¹⁸O at NLM and relatively enriched values at NPK. Event-scale analysis further elucidates the governing mechanisms. At NPK, both depleted and enriched isotope extremes occur mainly during the non-monsoon season, indicating that moisture source alone cannot explain the variability. Depleted events are consistently associated with deep mid-tropospheric westerly troughs, characterized by strong synoptic-scale ascent and progressive upstream rainout, as quantified by a robust negative relationship between integrated rainfall and δ¹⁸O. In contrast, enriched events occur under dynamically weak and spatially disorganized ascent, limiting isotopic fractionation despite long-range transport. At NLM, isotopic depletion arises from two distinct processes. During the monsoon season, strong convective rainout along the Bay of Bengal pathway dominates δ¹⁸O variability. During the non-monsoon season, episodic depletion is linked to westerly trough-induced large-scale ascent, similar to NPK. However, unlike NPK, these troughs do not exert a significant control on seasonal isotope variability at NLM, instead acting as modulators of extreme events.

Overall, the results demonstrate that precipitation isotopes across High Mountain Asia reflect fundamentally different dynamical and convective histories along the two Indian Ocean moisture branches, emphasizing their direct relevance for interpreting Himalayan paleoclimate records at the interface of the Indian Summer Monsoon and midlatitude westerlies.