the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jun 2024
Sea Surface Temperature over the Bay of Bengal: A key driver for South Asian Summer Monsoon rainfall during past 31 kiloyears
Abstract. Warmer Sea Surface Temperature (SST) in the Bay of Bengal (BoB) is crucial for driving deep atmospheric convection, facilitating low-level south-westerly winds, and enhancing moisture transport, thereby intensifying South Asian Summer Monsoon (SASM) rainfall over South Asia. However, the specific impact of BoB SST on SASM rainfall during the Glacial-Interglacial periods remains poorly understood. In this study, we reconstructed SST and evaporation versus rainfall variability over the past 31 kiloyears by simultaneously analyzing the carbonate clumped isotopes and stable oxygen isotopic composition of surface-dwelling planktic foraminifera Globigerinoides ruber from the Central West BoB (CWBoB), a key moisture source region. Additionally, cloud cover index was inferred from the abundance ratio of planktic foraminifera Globigerina bulloides to Neogloboquadrina dutertrei. Our SST reconstruction reveals an 8 °C variability over the past 31 kyr, coinciding with shifts in the G. bulloides to N. dutertrei ratio during the Last Glacial period and deglaciation, suggesting SST regulation by variable cloud cover. The increase in SST from the Early Holocene is attributed to CO2 radiative forcing. The stable oxygen isotope of seawater δ18Osw strongly aligns with a proxy record of SASM wind intensity, indicating that changes in wind patterns drive the variable evaporation versus rainfall dynamics over CWBoB. Furthermore, we examined the temporal variation in SASM continental runoff and rainfall to the Northern BoB (NBoB) by assessing changes in δ18Osw (∆18Osw), a proxy for Sea Surface Salinity (ΔSSS), between the NBoB and CWBoB. Our analysis revealed a significant relationship between SASM rainfall and SST in the CWBoB, indicating a sensitivity of 0.9±0.1 psu drop in ΔSSS across the NBoB per 1 °C rise in SST. These findings enhance our understanding of the relationship between CWBoB SST and SASM rainfall, highlighting the intricate dynamics of monsoon variability and paving the way for improved predictability of SASM rainfall patterns.
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RC1: 'Comment on egusphere-2024-1566', Anonymous Referee #1, 29 Jul 2024
This manuscript examines the influence of Sea Surface Temperature (SST) in the Bay of Bengal (BoB) on the South Asian Summer Monsoon (SASM) rainfall over the past 31 ka. It is done by analyzing various proxy records, including carbonate clumped isotopes and stable oxygen isotopic composition of surface-dwelling planktic foraminifera Globigerinoides ruber from the Central West BoB (CWBoB). This work is intriguing as the multiple proxy records provide valuable insights in enhancing our understanding of the dynamics of SASM rainfall. I have the following suggestions and questions.
- To verify the importance of CWBoB as a moisture source for continental rainfall in the modern-day context (lines 92-95), a figure illustrating moisture transport using observational data would be helpful.
- Since δ¹⁸O is a mixed signal influenced by temperature, isotopic composition of the water source, ocean pH, and other factors, how could the conclusion of “similarity in sea surface temperature and salinity variation across the spatial domain” (lines 289-292) be drawn?
- As different proxies are used at different sediment cores to reconstruct temperature, to what extent do the uncertainties associated with these different proxies influence the final calculation of δ¹⁸Osw? More discussion should be added.
- In Section 3.2, cloud cover reconstruction, which is important and challenging, is addressed. While the authors have made an attempt, I remain unconvinced due to the numerous assumptions and large uncertainties involved. I would suggest to first examine the relationship between cloud cover and sea surface temperature (SST) using observational data.
- Temperature is a crucial factor in shaping the moisture balance. This aspect should be discussed in Sect. 3.3.
- The uncertainty of Age-Depth model and related discussion should be added in Sect. 2.2.
- Could you clarify what are “late hemispheres” and “early hemispheres” (lines 179-180)?
- Please provide the exact duration for drying at 50 ℃ (line 192).
- Authors are encouraged to report their clumped data using the I-CDES system (doi.org/10.1029/2020GC009588), which allows for the comparison of clumped measurements from different laboratories and at various reaction temperatures (line 212).
- The equation of temperature estimation used should be provided in Sect. 2.4.
- When reporting temperatures, please ensure that the number of decimals is consistent between temperature values and their uncertainties throughout the manuscript.
Some technical suggestions:
Lines 81-82, 157: “15°19’36” N, 84°54’03” E” -> “15°19′36″ N, 84°54′03″E”; “2986m” -> “2986 m”.
Line 114: for radiocarbon, full name first and then abbreviation.
Line 146: Is 126-KL a site name? Please keep it consistent elsewhere in the manuscript (legend in Fig. 1a is 126KL, without dash in between; Line 147 is KL-126).
Line 157: Repeated information.
Line 197: please check “porapaq-Q”.
Line 214: “ETH1” -> “ETH-1”; “ETH3” -> “ETH-3”. Please check the original paper: doi.org/10.1029/2017GC007385.
Citation: https://doi.org/10.5194/egusphere-2024-1566-RC1 -
AC1: 'Reply on RC1', Prosenjit Ghosh, 10 Sep 2024
Dear Referee,
Thanks for your constructive feedback on our manuscript. We truly appreciate your time and effort in reviewing it. Your feedback has been extremely helpful in refining our work. We have addressed your comments in detail, and the point-by-point responses are included in the attached document.
Best regards,
Prosenjit Ghosh, on behalf of AC
-
RC2: 'Comment on egusphere-2024-1566', Anonymous Referee #2, 31 Jul 2024
Review on “Sea Surface Temperature over the Bay of Bengal: A key driver for South Asian Summer Monsoon rainfall during past 31 kiloyears” by Sakthivel et al.,
This topic is of scientific relevance, identification paleo-hydroclimates and a key driving forcing of South Asian Summer Monsoon from MIS 3 by using clumped isotopes and stable isotope records of Central West Bay of Bengal (CWBoB). I would suggest to reject this MS because the authors have some big problems on their data and interpretations. To sum up, these data cannot support the main points of the MS. Here I draw my comments as follows.
- In the Results and Discussion 3.2, the authors mentioned that the relationship existing between cloud cover and the depth of Chl a maxima, as well as the influence of Chl a on planktic foraminiferal abundance (Line 382-384). Cloud cover index was inferred from the abundance ratio of planktic foraminifera Globigerina bulloides to Neogloboquadrina dutertrei. The authors further suggest that internal feedback processes involving cloud cover index serve as significant factors in SST modulation in the BoB. However, the authors met some fundamental problems. Salinity, nutrient level, prey abundance, turbidity and illumination also affect their diversity, abundance and distribution locally. Besides, depth habitats of cold water dweller bulloides and N. dutertrei live at thermocline. Thus, the relationship between Chl a and planktic foraminifera cannot be applicable to all species. The authors ignore the important component - ocean where planktic foraminifera live. The abundance ratio of planktic foraminifera G. bulloides to N. dutertrei should be reflected changes in ocean hydrography, not only for cloud cover itself. In the MS, cloud cover index and related interpretation are incorrect.
- The authors calculated relationship between SST and ice core CO2 concentration/solar insolation at 30o These R-sq values explain 54% and 8% variability of internal and external earth system forcing, respectively (Line 354-355). However, the authors had big problem on statistics. A correlation between two variables cannot be interpreted as they mentioned. These data should be performed by factor analysis, EOF or principal component analysis. Moreover, in the Results and Discussion 3.1, 19 samples were analyzed for reconstruction of past SST variations. The authors calculated an average SST at a specific time window. It makes no sense that an average temperature with 1 STD was calculated by only 3-4 data at the time window.
- Line 180-183: The authors mentioned that a strong coherence of δ18O variability in ruber was observed in multiple sites adjacent to our core location (Rashid et al., 2011; Govil and Divakar Naidu, 2011; Clemens et al., 2021) confirming the proposed age-depth model. A good age model is dependent on how precise of radiocarbon dates, not for similar variations of adjacent cores. Some discrepancies can be found among these stable oxygen isotope records (Figure S3).
- Introduction, method and results of the MS are jumbled up. Core information and results of age model have too many redundancies. The information can be found in several paragraphs.
- Discussions with multiple SST records are unclear.
- Paragraph of 2. Materials and Methods is too long.
- Technical aspects: Figure 1: surface currents cannot be excluded.
- Others: Abbreviations: Bay of Bengal (BoB), sea surface temperature (SST), kilo years (kys)
Citation: https://doi.org/10.5194/egusphere-2024-1566-RC2 - AC2: 'Reply on RC2', Prosenjit Ghosh, 10 Sep 2024
Status: closed
-
RC1: 'Comment on egusphere-2024-1566', Anonymous Referee #1, 29 Jul 2024
This manuscript examines the influence of Sea Surface Temperature (SST) in the Bay of Bengal (BoB) on the South Asian Summer Monsoon (SASM) rainfall over the past 31 ka. It is done by analyzing various proxy records, including carbonate clumped isotopes and stable oxygen isotopic composition of surface-dwelling planktic foraminifera Globigerinoides ruber from the Central West BoB (CWBoB). This work is intriguing as the multiple proxy records provide valuable insights in enhancing our understanding of the dynamics of SASM rainfall. I have the following suggestions and questions.
- To verify the importance of CWBoB as a moisture source for continental rainfall in the modern-day context (lines 92-95), a figure illustrating moisture transport using observational data would be helpful.
- Since δ¹⁸O is a mixed signal influenced by temperature, isotopic composition of the water source, ocean pH, and other factors, how could the conclusion of “similarity in sea surface temperature and salinity variation across the spatial domain” (lines 289-292) be drawn?
- As different proxies are used at different sediment cores to reconstruct temperature, to what extent do the uncertainties associated with these different proxies influence the final calculation of δ¹⁸Osw? More discussion should be added.
- In Section 3.2, cloud cover reconstruction, which is important and challenging, is addressed. While the authors have made an attempt, I remain unconvinced due to the numerous assumptions and large uncertainties involved. I would suggest to first examine the relationship between cloud cover and sea surface temperature (SST) using observational data.
- Temperature is a crucial factor in shaping the moisture balance. This aspect should be discussed in Sect. 3.3.
- The uncertainty of Age-Depth model and related discussion should be added in Sect. 2.2.
- Could you clarify what are “late hemispheres” and “early hemispheres” (lines 179-180)?
- Please provide the exact duration for drying at 50 ℃ (line 192).
- Authors are encouraged to report their clumped data using the I-CDES system (doi.org/10.1029/2020GC009588), which allows for the comparison of clumped measurements from different laboratories and at various reaction temperatures (line 212).
- The equation of temperature estimation used should be provided in Sect. 2.4.
- When reporting temperatures, please ensure that the number of decimals is consistent between temperature values and their uncertainties throughout the manuscript.
Some technical suggestions:
Lines 81-82, 157: “15°19’36” N, 84°54’03” E” -> “15°19′36″ N, 84°54′03″E”; “2986m” -> “2986 m”.
Line 114: for radiocarbon, full name first and then abbreviation.
Line 146: Is 126-KL a site name? Please keep it consistent elsewhere in the manuscript (legend in Fig. 1a is 126KL, without dash in between; Line 147 is KL-126).
Line 157: Repeated information.
Line 197: please check “porapaq-Q”.
Line 214: “ETH1” -> “ETH-1”; “ETH3” -> “ETH-3”. Please check the original paper: doi.org/10.1029/2017GC007385.
Citation: https://doi.org/10.5194/egusphere-2024-1566-RC1 -
AC1: 'Reply on RC1', Prosenjit Ghosh, 10 Sep 2024
Dear Referee,
Thanks for your constructive feedback on our manuscript. We truly appreciate your time and effort in reviewing it. Your feedback has been extremely helpful in refining our work. We have addressed your comments in detail, and the point-by-point responses are included in the attached document.
Best regards,
Prosenjit Ghosh, on behalf of AC
-
RC2: 'Comment on egusphere-2024-1566', Anonymous Referee #2, 31 Jul 2024
Review on “Sea Surface Temperature over the Bay of Bengal: A key driver for South Asian Summer Monsoon rainfall during past 31 kiloyears” by Sakthivel et al.,
This topic is of scientific relevance, identification paleo-hydroclimates and a key driving forcing of South Asian Summer Monsoon from MIS 3 by using clumped isotopes and stable isotope records of Central West Bay of Bengal (CWBoB). I would suggest to reject this MS because the authors have some big problems on their data and interpretations. To sum up, these data cannot support the main points of the MS. Here I draw my comments as follows.
- In the Results and Discussion 3.2, the authors mentioned that the relationship existing between cloud cover and the depth of Chl a maxima, as well as the influence of Chl a on planktic foraminiferal abundance (Line 382-384). Cloud cover index was inferred from the abundance ratio of planktic foraminifera Globigerina bulloides to Neogloboquadrina dutertrei. The authors further suggest that internal feedback processes involving cloud cover index serve as significant factors in SST modulation in the BoB. However, the authors met some fundamental problems. Salinity, nutrient level, prey abundance, turbidity and illumination also affect their diversity, abundance and distribution locally. Besides, depth habitats of cold water dweller bulloides and N. dutertrei live at thermocline. Thus, the relationship between Chl a and planktic foraminifera cannot be applicable to all species. The authors ignore the important component - ocean where planktic foraminifera live. The abundance ratio of planktic foraminifera G. bulloides to N. dutertrei should be reflected changes in ocean hydrography, not only for cloud cover itself. In the MS, cloud cover index and related interpretation are incorrect.
- The authors calculated relationship between SST and ice core CO2 concentration/solar insolation at 30o These R-sq values explain 54% and 8% variability of internal and external earth system forcing, respectively (Line 354-355). However, the authors had big problem on statistics. A correlation between two variables cannot be interpreted as they mentioned. These data should be performed by factor analysis, EOF or principal component analysis. Moreover, in the Results and Discussion 3.1, 19 samples were analyzed for reconstruction of past SST variations. The authors calculated an average SST at a specific time window. It makes no sense that an average temperature with 1 STD was calculated by only 3-4 data at the time window.
- Line 180-183: The authors mentioned that a strong coherence of δ18O variability in ruber was observed in multiple sites adjacent to our core location (Rashid et al., 2011; Govil and Divakar Naidu, 2011; Clemens et al., 2021) confirming the proposed age-depth model. A good age model is dependent on how precise of radiocarbon dates, not for similar variations of adjacent cores. Some discrepancies can be found among these stable oxygen isotope records (Figure S3).
- Introduction, method and results of the MS are jumbled up. Core information and results of age model have too many redundancies. The information can be found in several paragraphs.
- Discussions with multiple SST records are unclear.
- Paragraph of 2. Materials and Methods is too long.
- Technical aspects: Figure 1: surface currents cannot be excluded.
- Others: Abbreviations: Bay of Bengal (BoB), sea surface temperature (SST), kilo years (kys)
Citation: https://doi.org/10.5194/egusphere-2024-1566-RC2 - AC2: 'Reply on RC2', Prosenjit Ghosh, 10 Sep 2024
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