Westerly-associated Inverse Altitude Effects in Near-Surface Water Vapor Isotopes Across Central Asian Mountain–basin Systems

The mountain-basin structures in Central Asia promote complex interactions between advected moisture and locally recycled moisture, leading to ongoing debates on the altitude or inverse altitude effects of water vapor isotopes in a westerlies-dominated arid environment. To quantify the altitude gradient of near-surface water vapor isotopes, we designed a vehicle-based observation campaign along four elevation transects in Central Asia during the summer of 2024, measuring the altitudinal variation in near-surface water vapor δ18O and examining the circulation mechanisms responsible for these gradients. The results show positive δ18O–altitude gradients along the Altai, Tianshan, and Kunlun transects. This pattern is most pronounced in the Altai and Tianshan transects, with overall gradients of approximately 1.7‰/km and 1.2‰/km, respectively, whereas the Kunlun transect shows a weaker positive gradient of about 0.4‰/km. In contrast, the Pamir transect exhibits a negative gradient, with substantial differences in the altitude gradients among geomorphic and catchment units. The relationships between near-surface meteorological variables and δ18O are strongly subregion-dependent and cannot, by themselves, explain the differences in altitude gradients among the transects. The back-trajectory analysis and the relationships between isotopes and specific humidity indicate that the Altai and Tianshan transects are more strongly influenced by stable and continuous westerly moisture transport, with weaker mixing from local or nearby moisture sources. These conditions favor the development of a pronounced inverse altitude effect in near-surface water vapor isotopes. By contrast, the Kunlun and Pamir transects are more strongly affected by topographic blocking, valley transitions, and local moisture inputs, which weaken or reorganize the westerly isotopic signal. Differences in underlying surface conditions may regulate δ18O through evapotranspiration and local moisture recycling, but their influence is largely confined to specific terrain units and is insufficient to alter the transect-scale circulation control. This study demonstrates that the inverse altitude effect of near-surface water vapor δ18O in Central Asia is primarily associated with stable westerly moisture transport, while also being modulated by local meteorology, topography, and land–atmosphere coupling processes. These findings provide modern process-based constraints for understanding anomalous altitudinal gradients in Central Asian precipitation, surface waters, and isotope-based proxies, and indicate that a stable depletion in water isotopes with increasing elevation should not be assumed in paleoaltimetric or paleohydrological interpretations within westerly-influenced regions.