Moisture sources and dynamics over southeastern Tibetan Plateau reflected in dual water vapor isotopes

Cai, Zhongyin; Li, Rong; Wang, Cheng; Mao, Qiukai; Tian, Lide

doi:https://doi.org/10.5194/egusphere-2024-3801

Preprints

https://doi.org/10.5194/egusphere-2024-3801

Preprints

07 Jan 2025

| 07 Jan 2025

Moisture sources and dynamics over southeastern Tibetan Plateau reflected in dual water vapor isotopes

Zhongyin Cai, Rong Li, Cheng Wang, Qiukai Mao, and Lide Tian

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.

Received: 03 Dec 2024 – Discussion started: 07 Jan 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Preprint (PDF, 7079 KB)

Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
Preprint (7079 KB)

Supplement (13108 KB)

Download & links

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

30 Sep 2025

Moisture sources and dynamics over the Southeast Tibetan Plateau reflected in dual water vapor isotopes

Zhongyin Cai, Rong Li, Cheng Wang, Qiukai Mao, and Lide Tian

Atmos. Chem. Phys., 25, 11633–11650, https://doi.org/10.5194/acp-25-11633-2025,https://doi.org/10.5194/acp-25-11633-2025, 2025

Short summary

Zhongyin Cai, Rong Li, Cheng Wang, Qiukai Mao, and Lide Tian

Interactive discussion

Status: closed

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.

Citation: https://doi.org/10.5194/egusphere-2024-3801-RC1
- AC1: 'Reply on RC1', Zhongyin Cai, 28 Mar 2025
  
  Please see the attached file for our point-by-point response.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3801-AC1
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
- AC2: 'Reply on RC2', Zhongyin Cai, 28 Mar 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3801/egusphere-2024-3801-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-3801-AC2

Interactive discussion

Status: closed

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.

Citation: https://doi.org/10.5194/egusphere-2024-3801-RC1
- AC1: 'Reply on RC1', Zhongyin Cai, 28 Mar 2025
  
  Please see the attached file for our point-by-point response.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3801-AC1
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
- AC2: 'Reply on RC2', Zhongyin Cai, 28 Mar 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3801/egusphere-2024-3801-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2024-3801-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Zhongyin Cai on behalf of the Authors (28 Mar 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (17 Apr 2025) by Franziska Aemisegger

RR by Anonymous Referee #1 (05 May 2025)

RR by Anonymous Referee #3 (26 May 2025)

Suggestions for revision or reasons for rejection

The authors have tried to revise the manuscript in response to the previous reviewers’ comments. However, in my understanding, the issues raised by the previous reviewers remain unresolved. Specifically, both Reviewer 1 and Reviewer 2 highlighted that the specific scientific question of this study remains unclear, and the structure of the manuscript lacks logic, particularly in the introduction section. Moreover, I find that some of the methods employed in the study are not reasonable, and key points of the conclusions are unconvincing. My detailed comments are as follows:

The authors believe that the significant negative correlation between d-excess and relative humidity of the Indian Ocean moisture source is due to the seasonal variation of relative humidity in the oceanic moisture source is opposite to that of excess deuterium. But there is no causal relationship between them. As analyzed by the authors, the moisture source for summer monsoon season is different with that for the non-monsoon season, i.e., the moisture is transported from the continents by westerlies during the non-monsoon season, while it comes from the Indican Ocean during the summer monsoon season. Given the presence of oceanic and continental moisture sources during the different seasons, it is inappropriate for the author to calculate the correlation between d-excess and relative humidity of the moisture source by combining data from the monsoon and non-monsoon periods. Additionally, the authors calculated the correlation between d-excess and relative humidity of the moisture source separately for the monsoon and non-monsoon periods, and found that compared to the annual data, the correlation weakened during either the monsoon or non-monsoon period. However, when calculating the correlation between d-excess and relative humidity of the moisture source, the authors did not account for the transport time of the moisture from the source to the study site, which is also inappropriate.

Furthermore, one of the key conclusions of this paper is that d-excess and δ¹⁸O during the monsoon season are influenced by the evaporation effect of raindrops. Although this is possible, the authors also need to consider other possible factors. For example, during the summer monsoon period, a strong rainout effect along the moisture transport pathway could also potentially account for the decrease in δ¹⁸O and the increase in d-excess.

Hide

RR by Anonymous Referee #4 (05 Jun 2025)

Suggestions for revision or reasons for rejection

Review of Cai et al ACP

Cai et al provide an analysis of a series of water vapor isotope measurements taken over three years from the South-East Tibetan Plateau Station to try to investigate the hydrological processes driving variations in water vapor isotope ratios in the region. They conclude that: a) d-excess may not be a reliable indicator of oceanic evaporation conditions, b) high d-excess values in the non-monsoon season are driven by dry air intrusions and westerly winds, and c) summer d18O and d-excess reflect “super-Rayleigh” processes and partial rain evaporation.

The analysis of the isotopic data seems sound, but the analysis seems incomplete and many of the conclusions are not novel. For example, several studies have investigated and questioned the preservation of d-excess in water vapor from the evaporative source, as the authors note in L. 71-74. This thread isn’t really followed up on in the current manuscript, and the results for this site would be stronger if they were put into better context with this existing literature.

In addition, many of these studies also used some sort of moisture footprint analysis such as Sodemann et al. (2008) but took an extra step to link evaporative sources to the hypotheses being tested. For example, the authors could analyze what fraction of the vapor would have arisen from within-boundary-layer evaporation over land, and correlated that with d-excess and d18O to support their claims regarding ET and recycling.

There are also a few aspects of the back trajectory setup and analysis that raise additional questions:
First, why were GDAS meteorological fields used for the back trajectories when ERA5 was used for the remainder of the analysis? In the introduction, the authors make the point that it is thought that mountain valleys in the SETP are important pathways for water vapor transport (L. 78-79), and it seems likely that to the extent these surface features are represented in either data product, ERA5 at a nominal 25km resolution is much more likely to capture topographic variations and their impact on wind fields and moisture transport than GDAS at ~100 km.

Second, how was vertical velocity calculated in these trajectories? I suppose from the description in section 2.4 that the model vertical velocity was used, but it would be good to specify. In addition, how were vertical heights used to launch trajectories chosen? I suspect that it may not change the conclusions too much, but it is also possible that potential contributions from more remote vapor sources are given too small a weight if the boundary layer height is substantially higher than 500 m, given there is so much weight near the surface.

Third, I am a bit unclear on what the statement “Unlike previous applications focused on identifying evaporative moisture sources from the Earth’s surface, this study emphasizes the contribution of air parcels themselves to SETP’s humidity.” (L. 181-183) means in the context of the Sodemann et al. (2008) tracer that has been applied here, as all studies using this or a similar tracer are inherently weighted by humidity contributions to the receptor site.

Finally, the authors indicate the role of the westerlies and dry, cold air intrusions as likely responsible for the non-monsoon period patterns of d18O and d-excess. However, can the authors rule out the importance of local evaporation of precipitation or surface/soil waters, which as described elsewhere in the manuscript and given the site’s elevation is likely to also have low d18O and high d-excess?

The above examples are ultimately why I assess the analysis to be incomplete or sparse given the previous work done on this topic that could be integrated better into the discussion of this work, as well as the interpretive power of the tools in this manuscript that are not used to their full potential in the current manuscript.

A few other minor comments:
1) could the authors explain a bit more why trajectory average values are used for meteorological variables here? For situations where condensation is important, for example, it may be the extreme value that matters the most (for example, the lowest f for a given air parcel is likely correlated with the smallest saturation vapor pressure experienced and therefore the coldest temperature)
2) The statement at L. 76-77 is contradicted by other parts of the manuscript, which give a series of possible explanations for this behavior (continental recycling, low Rayleigh f as in figure 3, etc.). There also have been a number of studies on precipitation d-excess and elevation in other regions (e.g., the Andes) that could be cited here for additional context

Hide

ED: Reconsider after major revisions (16 Jun 2025) by Franziska Aemisegger

AR by Zhongyin Cai on behalf of the Authors (30 Jun 2025) Author's response Author's tracked changes Manuscript

ED: Reconsider after major revisions (18 Jul 2025) by Franziska Aemisegger

AR by Zhongyin Cai on behalf of the Authors (14 Aug 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (15 Aug 2025) by Franziska Aemisegger

AR by Zhongyin Cai on behalf of the Authors (15 Aug 2025)

Journal article(s) based on this preprint

30 Sep 2025

Moisture sources and dynamics over the Southeast Tibetan Plateau reflected in dual water vapor isotopes

Zhongyin Cai, Rong Li, Cheng Wang, Qiukai Mao, and Lide Tian

Atmos. Chem. Phys., 25, 11633–11650, https://doi.org/10.5194/acp-25-11633-2025,https://doi.org/10.5194/acp-25-11633-2025, 2025

Short summary

Zhongyin Cai, Rong Li, Cheng Wang, Qiukai Mao, and Lide Tian

Supplement

https://doi.org/10.5194/egusphere-2024-3801-supplement

Zhongyin Cai, Rong Li, Cheng Wang, Qiukai Mao, and Lide Tian

Viewed

Total article views: 768 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	Supplement	BibTeX	EndNote
685	63	20	768	44	27	43

HTML: 685
PDF: 63
XML: 20
Total: 768
Supplement: 44
BibTeX: 27
EndNote: 43

Views and downloads (calculated since 07 Jan 2025)

Month	HTML	PDF	XML	Total
Jan 2025	70	13	3	86
Feb 2025	57	18	5	80
Mar 2025	35	11	3	49
Apr 2025	36	8	2	46
May 2025	25	3	1	29
Jun 2025	36	4	5	45
Jul 2025	27	2	0	29
Aug 2025	79	0	79
Sep 2025	320	4	1	325

Cumulative views and downloads (calculated since 07 Jan 2025)

Month	HTML	PDF	XML	Total
Jan 2025	70	13	3	86
Feb 2025	57	18	5	80
Mar 2025	35	11	3	49
Apr 2025	36	8	2	46
May 2025	25	3	1	29
Jun 2025	36	4	5	45
Jul 2025	27	2	0	29
Aug 2025	79	0	79
Sep 2025	320	4	1	325

Viewed (geographical distribution)

Total article views: 743 (including HTML, PDF, and XML) Thereof 743 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 30 Sep 2025

Download

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Preprint (7079 KB)
Metadata XML

Short summary

Local and upstream specific humidity is the main factor determining non-monsoon season d-excess variability over southeast Tibetan Plateau (TP) 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 which leads to lower vapor δ¹⁸O but higher d-excess values. These findings provide new insights into using water isotopes to track moisture sources and dynamics over the TP.


Total:	0
HTML:	0
PDF:	0
XML:	0