Weakened Isotope Altitude Gradient in the Central Asian Water Tower: Role of Topography and Local Circulation

Abstract The altitude effect (AE) of stable isotopes in meteoric water ( δ 18 O and δ 2 H), that is, the depletion of water isotopes with increasing altitude, is an important theoretical assumption of isotope‐based paleoaltimetry. However, this assumption has recently been challenged, as many in situ observations fail to consistently demonstrate the expected negative correlation between altitude and isotope values. Here we used 1,255 records of surface water isotopes to investigate AE and inverse altitude effect (IAE) and their mechanisms in arid Central Asia. The results show that isotope altitude gradients across Central Asia are weaker than the global average. Comparisons of the gradients for both the mountain‐basin system and mountain system reveal that the windward and leeward slopes of the westerlies consistently exhibit opposite gradients: AE on the windward side and IAE on the leeward. The observed IAE on the leeward slope across all basins is influenced by topography and local circulation. The orientation of mountain ranges perpendicular to large‐scale westerly circulation blocks eastward transport of westerly moisture, and the resulting longer moisture pathways weaken AE. Stronger local circulation and sub‐cloud evaporation processes enrich water isotopes in the leeward mountain regions, diminishing AE and even leading to the emergence of IAE. Our results highlight the impact of local circulation on water isotopes during different uplift phases when using stable hydrogen and oxygen isotopes to reconstruct paleoelevation.