Monsoon Forcing on Precipitation Stable Isotopes in the Yarlung Tsangpo Grand Canyon: Evidence from Indian Summer Monsoon Dynamics
ObjectiveThe lower reaches of the Yarlung Tsangpo River, characterized by a significant topographical gradient, play a crucial role in transporting moisture from the Indian Ocean through the Indian monsoon to the Tibetan Plateau. This study investigates isotopic altitudinal effects on atmospheric pr...
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| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Editorial Department of Journal of Sichuan University (Engineering Science Edition)
2025-07-01
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| Series: | 工程科学与技术 |
| Subjects: | |
| Online Access: | http://jsuese.scu.edu.cn/thesisDetails#10.12454/j.jsuese.202300739 |
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| Summary: | ObjectiveThe lower reaches of the Yarlung Tsangpo River, characterized by a significant topographical gradient, play a crucial role in transporting moisture from the Indian Ocean through the Indian monsoon to the Tibetan Plateau. This study investigates isotopic altitudinal effects on atmospheric precipitation and the distribution characteristics of d-excess in this region. It compares the isotopic altitudinal gradients of atmospheric precipitation in the midstream and downstream regions of the Yarlung Tsangpo River to clarify the factors contributing to discernible differences. The study explores the complex dynamics of the Indian Ocean monsoon airflow as it traverses and circulates within this geographical expanse by carefully examining these disparities. This expanded investigation proves pivotal for advancing comprehension of the nuanced interactions between topography and meteorological phenomena, shedding light on the complex mechanisms influencing isotopic variations in precipitation and refining insights into broader climatic processes in the studied area.MethodsThe data source for this study included atmospheric precipitation and surface water samples collected through field surveys. The Karst Geological Resources and Environment Supervision and Testing Center of the Ministry of Land and Resources conducted sample testing using a liquid water isotope analyzer (L2130-i). Isotope results were expressed as the relative thousandth deviation value of the same isotope ratio <italic>R</italic><sub>sample</sub> and the international standard <italic>R</italic><sub>standard</sub> (V‒SMOW, Vienna standard average ocean water): <italic>δ</italic> = (<italic>R</italic><sub>sample</sub>/<italic>R</italic><sub>standard</sub>‒1) × 1 000. The primary research methodology employed in this study involved the simulation of isotopic altitudinal gradients using the Rayleigh distillation model. This model operated under the assumption of an idealized open system in which condensed rainwater promptly exited the atmospheric system, leading to variations in the stable isotopic content of hydrogen and oxygen in precipitation at different altitudes. The study aimed to capture the complex processes that contributed to observed isotopic differences in precipitation at various elevations utilizing the Rayleigh distillation model. This approach provided a theoretical framework for understanding the dynamics of isotopic fractionation in rainfall within the context of altitude, facilitating a comprehensive analysis of atmospheric conditions in the studied region. Building upon this theoretical framework, the study involved inputting actual isotopic initial values within the research area and simulating the altitudinal effects of precipitation isotopes under ideal conditions. The simulation assumed that for every 1 000 meters of elevation gain, 10% of the precipitation condensed, allowing for the verification of actual altitudinal gradient values within the study area. In addition, the backward trajectory model employed the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. This Lagrangian trajectory model analyzed atmospheric transport, providing insights into the composition of water vapor sources within the research area. The model served as a complementary tool to support the understanding of the transport processes of the Indian Ocean monsoon within the study area, enriching the interpretation of the complex dynamics that influenced isotopic variations in precipitation at different altitudes.Results and DiscussionsThe stable hydrogen and oxygen isotope analysis enabled the establishment of a local atmospheric precipitation line and isotopic altitudinal gradient. The equation representing the atmospheric precipitation line in the research area is <italic>δ</italic>(D) = 8.02<italic>δ</italic>(<sup>18</sup>O) + 13.21. The elevated d-excess in this region is attributed to the abrupt ascent of moist air encountering the topography upon entering the Tibetan Plateau, leading to a sudden decrease in temperature and relative humidity. Another contributing factor is the local vapor recycling within the large canyon, which contributes to the observed high d-excess. These findings provide valuable insights into the factors that influence isotopic variations in precipitation in the study area, highlighting the complex interplay between atmospheric dynamics and geographical features. Interpolation analysis was employed to discern the distribution pattern of d-excess in atmospheric precipitation within the study area. The findings revealed a distinctive low-high-low pattern of d-excess across the research region, with elevated d-excess observed in the Himalayan eastern tectonic zone. This geographical area exhibited a high d-excess, indicating unique atmospheric processes and moisture sources that contributed to the isotopic composition in this specific locale. Identifying such spatial variations in d-excess enhanced the understanding of regional atmospheric dynamics and the complex factors influencing isotopic characteristics in precipitation. Through regression analysis and validation using an ideal model, the altitudinal gradient in the Yarlung Tsangpo River Grand Canyon region was determined to be ‒1.43‰/km, while in the midstream, the altitudinal gradient was found to be 3.30‰/km. The primary drivers for this disparity were attributed to variations in topographical slopes and the influence of monsoons. The distinct altitudinal gradients were validated by the composition of airflow sources across different locations in the backward trajectory model. This analysis highlighted the significance of both topography and monsoonal patterns in shaping the isotopic characteristics of precipitation, providing valuable insights into the regional atmospheric dynamics within the Yarlung Tsangpo River basin.ConclusionsThrough an examination of atmospheric precipitation isotopes in the Yarlung Tsangpo River Grand Canyon region, this study,establishes the atmospheric precipitation line and the isotopic altitudinal gradient within the area. Compared to the midstream, the altitudinal gradient in this region is relatively modest and closely linked to variations in topography and water vapor composition. In addition, the research highlights the retention of Indian Ocean moisture within the Yarlung Tsangpo Grand Canyon region following its ingress into the Tibetan Plateau. |
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| ISSN: | 2096-3246 |