the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Sep 2025
Drivers of the δ18O Changes in Indian Summer Monsoon Precipitation between the Last Glacial Maximum and Pre-industrial Period
Abstract. In this study, we investigate the changes in water isotope ratios in the Indian summer monsoon precipitation (δ18Oprecip) during the Last Glacial Maximum (LGM, ~21 ka Before Present) compared to the pre-industrial (PI) period, and the mechanisms driving these changes, using a general circulation model with water isotope and novel water vapor source-tagging capabilities.
During the LGM, the model simulates a substantial reduction (15 %) in monsoon precipitation over the Indian subcontinent, consistent with proxy records. This drying in LGM is associated with reduced atmospheric water vapor, a thermodynamic response to cooling, while the westerly circulation, a dynamics response, is strengthened over parts of the subcontinent. Additionally, zonal temperature gradients between a relatively less-cooled tropical Western Pacific Ocean and Indian subcontinent lead to anomalous subsidence over the Indian region, enhancing the drying. Water vapor source tagging shows that while the four dominant moisture sources for the monsoon (South Indian Ocean, Arabian Sea, Indian land recycling, and Central Indian Ocean) remained the same, their contributions were reduced during the LGM. The δ18Oprecip values over the Indian monsoon region are enriched by approximately 1 ‰ in the LGM simulation, and we find that this enrichment was not driven by the local amount effect. A decomposition analysis shows that the enrichment was primarily caused by reduced contributions from distant, isotopically depleted water vapor sources and secondarily by reduced rainout during moisture transport from the Indian Ocean.
These findings have important implications for paleoclimate reconstructions, suggesting that δ18O records from the Indian region could be indicators of broad-scale atmospheric circulation rather than being direct proxies for local precipitation amount.
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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.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2611 KB) - Metadata XML
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Supplement
(3570 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-4468', Anonymous Referee #1, 19 Oct 2025
- AC1: 'Reply on RC1', Thejna Tharammal, 12 Dec 2025
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RC2: 'Comment on egusphere-2025-4468', Anonymous Referee #2, 22 Oct 2025
This manuscript uses isotope-enabled CESM with water-vapor source tagging to compare Indian Summer Monsoon precipitation and δ18O between the Last Glacial Maximum and pre-industrial climates. The authors find monsoon drying at LGM and enrichment of precipitation d18O, and convincingly show that the enrichment cannot be explained by the local amount effect but instead arises primarily from reduced relative contributions from distant, isotopically depleted sources and weaker upstream rainout along transport pathways. The modeling framework and decomposition are solid, and the results are clearly presented. I recommend minor revision.
Minor comments:
1. It is interesting to see that some monsoon indices show the Indian monsoon is strengthened in the LGM, while other indices show the opposite. Since the paper is run with prescribed SST/sea ice, do similar results appear in the coupled run of iCESM?
2. It is also interesting to see that the changes in Pacific moisture contribution are very important in regulating the positive precip d18O anomaly in India. Why is that? Is it because less moisture comes from the Pacific due to the strengthened westerlies in the subtropics? The authors can add a discussion for this. Also, how to understand that precipitation is less in India, but the condensation effect is negative in Figure 8c?
3. After the authors draw conclusions that Indian precip d18O in the LGM cannot be explained by the amount effect but by moisture source changes, how should we interpret speleothem/marine sediment d18O changes in the Indian monsoon region in the LGM?
Equation (1): Some letters in the subscripts of P are not subscribed.
Line 251: P J et al. 2020: Please write the complete last name
Equation (3): Please write the equation in the equation mode.
Lines 327-337: Though the averaged precip d18O has a large bias, it seems that precip d18O in most places of India is close to the observations.
Figure 2c: “Source contribution” is kind of misleading here. I thought it was the contribution due to moisture source changes (like you defined in Line 293). Please consider changing it to other words.
Lines 439-442: What is the region of the “weakening of westerley”? It is not clear to me. The strengthening of the westerly is obvious.
Line 467: missing right parentheses
Line 485: It seems that the vertical advection term can also explain some drying of western India.
Line 623: Speleothem -> speleothem
Citation: https://doi.org/10.5194/egusphere-2025-4468-RC2 - AC2: 'Reply on RC2', Thejna Tharammal, 12 Dec 2025
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-4468', Anonymous Referee #1, 19 Oct 2025
This study examines changes in the isotopic composition of Indian summer monsoon precipitation during the Last Glacial Maximum (LGM) relative to the pre-industrial period, using an isotope-enabled general circulation model with vapor source tagging. The authors found that the LGM simulation shows 15% less monsoon rainfall, mainly due to thermodynamic drying from lower atmospheric moisture and enhanced subsidence over India. With the water vapor source tagging method, they further found that while primary moisture sources remain the same, their contributions weaken, producing δ¹⁸O enrichment. This enrichment stems mainly from reduced input of isotopically depleted vapor rather than the local amount effect. The results suggest that δ¹⁸O in Indian monsoon records reflects large-scale circulation changes rather than local precipitation intensity. The paper is very well organized and clearly described. I would like to recommend an acceptation for publication after some minor improvements.
- For Fig. 4, it would be better to also mark the research domain as in Fig. 3.
- One of the main points of this study is to explain the reason of the reduced rainfall during the LGM compared to the pre-industrial period. While the manuscript has only 8 figures in the main text, I suggest to change one Figure (Fig. S8 or S9) from the supplements into the main text to show the corresponding changes in temperature and circulation.
Citation: https://doi.org/10.5194/egusphere-2025-4468-RC1 - AC1: 'Reply on RC1', Thejna Tharammal, 12 Dec 2025
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RC2: 'Comment on egusphere-2025-4468', Anonymous Referee #2, 22 Oct 2025
This manuscript uses isotope-enabled CESM with water-vapor source tagging to compare Indian Summer Monsoon precipitation and δ18O between the Last Glacial Maximum and pre-industrial climates. The authors find monsoon drying at LGM and enrichment of precipitation d18O, and convincingly show that the enrichment cannot be explained by the local amount effect but instead arises primarily from reduced relative contributions from distant, isotopically depleted sources and weaker upstream rainout along transport pathways. The modeling framework and decomposition are solid, and the results are clearly presented. I recommend minor revision.
Minor comments:
1. It is interesting to see that some monsoon indices show the Indian monsoon is strengthened in the LGM, while other indices show the opposite. Since the paper is run with prescribed SST/sea ice, do similar results appear in the coupled run of iCESM?
2. It is also interesting to see that the changes in Pacific moisture contribution are very important in regulating the positive precip d18O anomaly in India. Why is that? Is it because less moisture comes from the Pacific due to the strengthened westerlies in the subtropics? The authors can add a discussion for this. Also, how to understand that precipitation is less in India, but the condensation effect is negative in Figure 8c?
3. After the authors draw conclusions that Indian precip d18O in the LGM cannot be explained by the amount effect but by moisture source changes, how should we interpret speleothem/marine sediment d18O changes in the Indian monsoon region in the LGM?
Equation (1): Some letters in the subscripts of P are not subscribed.
Line 251: P J et al. 2020: Please write the complete last name
Equation (3): Please write the equation in the equation mode.
Lines 327-337: Though the averaged precip d18O has a large bias, it seems that precip d18O in most places of India is close to the observations.
Figure 2c: “Source contribution” is kind of misleading here. I thought it was the contribution due to moisture source changes (like you defined in Line 293). Please consider changing it to other words.
Lines 439-442: What is the region of the “weakening of westerley”? It is not clear to me. The strengthening of the westerly is obvious.
Line 467: missing right parentheses
Line 485: It seems that the vertical advection term can also explain some drying of western India.
Line 623: Speleothem -> speleothem
Citation: https://doi.org/10.5194/egusphere-2025-4468-RC2 - AC2: 'Reply on RC2', Thejna Tharammal, 12 Dec 2025
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Govindasamy Bala
Jesse Nusbaumer
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2611 KB) - Metadata XML
-
Supplement
(3570 KB) - BibTeX
- EndNote
- Final revised paper
This study examines changes in the isotopic composition of Indian summer monsoon precipitation during the Last Glacial Maximum (LGM) relative to the pre-industrial period, using an isotope-enabled general circulation model with vapor source tagging. The authors found that the LGM simulation shows 15% less monsoon rainfall, mainly due to thermodynamic drying from lower atmospheric moisture and enhanced subsidence over India. With the water vapor source tagging method, they further found that while primary moisture sources remain the same, their contributions weaken, producing δ¹⁸O enrichment. This enrichment stems mainly from reduced input of isotopically depleted vapor rather than the local amount effect. The results suggest that δ¹⁸O in Indian monsoon records reflects large-scale circulation changes rather than local precipitation intensity. The paper is very well organized and clearly described. I would like to recommend an acceptation for publication after some minor improvements.