the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 07 Jan 2025
Moisture sources and dynamics over southeastern Tibetan Plateau reflected in dual water vapor isotopes
Abstract. The Tibetan Plateau (TP) serves as a water tower for major rivers in Asia, and mountain valleys in southeastern TP are key channels for moisture entering the TP. Water resources on the TP are experiencing spatially opposite changes due to climate change, and understanding the sources and dynamics of atmospheric moisture is vital. To investigate the role of ocean surface evaporation, continental air mass intrusion, and rain-vapor interaction, we present a three-year daily time series of near-surface water vapor isotope compositions (δ18O and d-excess) from the South-East TP station. We find that apparent negative correlations between d-excess and relative humidity over the Indian Ocean mainly reflect their similar seasonality. When analyzed for different seasons, the correlation is insignificant or only explains a marginal fraction of variance. Therefore, caution is required when interpreting the d-excess as a conservative tracer of ocean surface evaporation. Instead, local and upstream specific humidity is the main factor determining non-monsoon season d-excess variability due to the intrusion of cold and dry air from upper levels. During the summer monsoon season, d-excess and δ18O mainly reflect the effect of raindrop evaporation on humidity during transport which decreases lower vapor δ18O but increases d-excess values. These findings provide new insights into the significance of using water isotopes to track moisture sources and dynamics over the TP with seasonally alternating circulation systems. Particularly, the findings for d-excess will improve the understanding of different moisture sources and guide the interpretation of d-excess derived from other water bodies and ice cores.
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RC1: 'Comment on egusphere-2024-3801', Anonymous Referee #1, 11 Feb 2025
Cai et al. investigate moisture sources and dynamics over the southeastern Tibetan Plateau (TP) using three-year near-surface water vapor isotopes (δ¹⁸O, d-excess) and back trajectory analysis. Their findings reveal that correlations between d-excess and oceanic evaporation conditions are driven by seasonal covariation rather than direct moisture sourcing. During the non-monsoon season, high d-excess reflects dry, cold air intrusions from the westerlies, while monsoon-season isotopes are shaped by raindrop evaporation during transport. The authors also challenge interpretations of TP ice core d-excess as proxies for oceanic humidity, emphasizing instead the role of local moisture recycling, air mass mixing, and rain-vapor interactions. These insights refine understanding of TP hydroclimate drivers and caution against oversimplified linkages between terrestrial isotopes and remote oceanic processes. This is an very interesting research. However, the authors need to address several issues, including both scientific and English language aspects, before being considered for publication.
There is room for improvement in the English language of this article. Some sections need significant rephrasing or reorganizing, particularly Introduction. Paragraphs in Introduction section appears lacking logical connection, and even its sentences are not logically related. In the first paragraph, the authors point out that the TP’s water balance has undergone significant changes, such as a drying trend in the southeastern TP and wetting in the northern TP. However, they didn’t provide our current understanding of the mechanisms of this water imbalance. Stable isotope approach is just one of the techniques that can be used to understand this. What are other methods and what are the advantages of examining vapor over other methods? What is the importance of understanding water imbalance in TP or the urgency of studying vapor dynamics? This will be the motivation of this study. In the last sentence of the first paragraph, the authors introduce atmospheric water vapor and point out the important role of vapor dynamics in understanding water imbalance. Logically, the authors should start the second sentence with the research about water vapor. Instead, they talked about precipitation isotopes. I would suggest the authors to rewrite the Introduction with a focus on vapor and vapor isotopes.
Introduction is different from Abstract. There is no need to present your major findings in the Introduction. What you should present in this section is what you did with what approaches, and what are your objectives of this study.
Section 4.2 is also very difficult to follow, as the discussions in this section are not systematically presented. How Fig 7 supports the arguments is not clearly explained. The authors use composite analysis in the section. However, they did not include a brief introduction of this analysis in Data and Method section for the convenience of readers.
In lines 122-123, the sentence suggests that only one standard is used for the calibration of isotope results. The common practice is to use at least two in-house standards, which are normalized to VSMOW-SLAP scale. Please provide a brief description of how the calibration of water isotope results.
There are quite a few sentences that should be rephrased; they are either poorly presented or have grammatic errors. For details, please check the attached annotated file. This file includes more comments and suggestions.
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RC2: 'Comment on egusphere-2024-3801', Anonymous Referee #2, 22 Feb 2025
This paper studies moisture sources and dynamics over southeastern Tibetan Plateau using water vapor isotopes, especially in non-monsoon season. The findings offer valuable insights into the mechanisms at seasonal scale for this region and could provide an explanation for hydrological information recorded in paleo proxy isotopes. However, there are still many problems preventing the publication in current version. The structure of the paper is not well-organized and lack of logic. Especially, the discussion and results section needs to be carefully revised and organized. The English expression is not good and needs improvement.
Specific comments:
Abstract: lines 13-15: The study area in this paper is southeastern Tibetan Plateau, not the whole Tibetan Plateau. Please rewrite the abstract.
Introduction：This part is too long and distracted. It is hard to get why do you perform vapor isotopes analysis in southeastern Tibetan Plateau. There have been a lot of studies on moisture sources and dynamics in this region. What is your scientific question?
Line 60: why do you emphasize non-monsoon season. In fact, I don’t think it is a good idea to emphasize the non-monsoon season independently. I suggest the authors to emphasize the scientific question, and to introduce the important role of the non-monsoon season on resolving the scientific question.
Lines 318-325: this part should be results, not discussion.
Section 4.2, 4.3, 4.4: Most of these sections are results, not discussion. I strongly suggest the authors to reorganize the results and discussion. It is difficult to obtain information in current version.
Figure Legends: Ensure consistent formatting (e.g., font size, symbols, color schemes) across all figures to improve clarity and visual coherence.
Isotope Notation: Verify that "δ18O" is consistently formatted with proper superscripts (e.g., replace "d18O" or "delta18O" with "δ18O").
Citation: https://doi.org/10.5194/egusphere-2024-3801-RC2
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