the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 Feb 2024
Signatures of Oxygen-Depleted Waters along the Sumatra-Java Coasts in the Southeastern Tropical Indian Ocean
Abstract. A prominent ocean region exhibiting depleted oxygen concentration is the northern Indian Ocean, whose projected deoxygenation trend in response to climate change requires a comprehensive understanding of the roles of ocean dynamics. We present newly compiled in situ data across platforms (e.g. cruises, Argo, buoy) in the Indonesian coasts of Sumatra and Java between 2010–2022. Combined with reanalysis products, our data detect oxygen-depleted waters attributed to the eastward advection of the northern Indian Ocean waters and monsoon-driven coastal upwelling. Oxygen limited zones (OLZs, DO < 60 μmol kg-1) occupy various depths off the Sumatra-Java coasts, in which dissolved oxygen (DO) reaches ~40 μmol kg-1 in northwest Sumatra. The eastward propagating Equatorial Counter Current plays a major role in delivering the oxygen-depleted waters during the boreal summer; similarly, the South Equatorial Counter Current in the winter monsoon and the Wyrtki Jet during the transition months. Coastal upwelling regulates DO variations via primary production and the respiration of organic matter at intermediate depths in southern Java as the upwelled waters being advected westward towards Sumatra in the summer monsoon. Indonesian Throughflow with enriched organic matter modifies the oxygenated-depleted waters at its outlets. We observe a trend towards deepened OLZ in western Sumatra, while positive Indian Ocean Dipole events (2006, 2012, 2015, 2019) lower DO in the thermocline depths of southern Java on the interannual timescale. Altogether, high-resolution observational biogeochemical data are key to advance our understanding of dynamical DO changes in the southeastern tropical Indian Ocean under the global deoxygenation trend.
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RC1: 'Comment on egusphere-2024-451', Anonymous Referee #1, 03 Jun 2024
Review Comments on the Manuscript ‘Signatures of Ocean Oxygen-Depleted Waters along the Sumatra-Java Coasts in the Southeastern Tropical Indian Ocean’ by Faisal Hamzah et al., submitted to EGUspere
In this paper, the authors study oxygen changes along the Java and Sumatra coasts using observations from multiple platforms, including Argo, Buoy, and Ship cruises. They supplement these observations with reanalysis products to assess oxygen variability along the coasts of Indonesia. Their analysis detects low oxygen waters along the Java-Sumatra coast, attributing their source to the North Indian Ocean. The major drivers for this export vary with the season and include the eastward-flowing equatorial undercurrent and Wyrtki Jets. The authors further discuss the role of upwelling and the Indian Ocean Dipole in driving the low oxygen zones along the Indonesian coast.
General Comments:
- The authors claim to discuss oxygen changes along the Indonesian coast. However, most of the introduction focuses on the Arabian Sea and Bay of Bengal. While these basins are crucial for the Indonesian coast, the introduction should focus more on the relevant processes in the study region.
- The manuscript, including the abstract, is poorly written and requires substantial revision to improve the quality of writing. Often, paragraphs contain unnecessary details and miss the most relevant information.
- The Materials and Methods section begins with an introduction to the study area. This content should be part of the main introduction of the manuscript.
- Figure 3a caption is incorrect. Since vertical sections are crucial for this study, modify these sections to ensure the main features are clearly visible.
- Figure 3 (all vertical sections): The eastward extension of the high salinity (34.75 PSS) contour, representing the intrusion of Arabian Sea water into the coastal regions of Indonesia, appears similar in all seasons. While this could be attributed to basin circulation in summer/annual mean, what explains this intrusion during winter when there is a reversal in the North Indian Ocean circulation?
- The conclusions in section 3.2 (lines 242-244) are premature. The role of horizontal advection is attributed without specific analysis, requiring a refined approach.
- Much of the results section includes information that is out of focus for the current study. For example, lines 258-261 discuss known seasonal spreading of oxygen-depleted water. Focus on explaining new findings in your study and compare or relate them to existing information.
- Section 4.2: When explaining the role of the IOD on Java-Sumatra upwelling and associated oxygen variability, the authors mix two different mechanisms. Upwelling along this coast is influenced by both Indian Ocean equatorial upwelling Kelvin waves and local easterly winds, which operate differently in terms of origins, characteristics, and impacts. Interpret processes associated with upwelling in this region with extreme care.
- The discussion section should focus more on interpreting the main findings and comparing them with similar existing studies. Therefore, this section needs to be revised.
As mentioned in my previous comments, the research question investigated in this study is relevant. However, the paper is poorly written, the scientific rigor is weak, and the results are not well presented. Therefore, I am not able to accept the paper and suggest rejection.
Citation: https://doi.org/10.5194/egusphere-2024-451-RC1 -
AC3: 'Reply on RC1', Faisal Hamzah, 31 Jul 2024
Dear Editor,
Here, we respond to comments from RC1. Our responses are written in Italic style.
Comment: In this paper, the authors study oxygen changes along the Java and Sumatra coasts using observations from multiple platforms, including Argo, Buoy, and Ship cruises. They supplement these observations with reanalysis products to assess oxygen variability along the coasts of Indonesia. Their analysis detects low oxygen waters along the Java-Sumatra coast, attributing their source to the North Indian Ocean. The major drivers for this export vary with the season and include the eastward-flowing equatorial undercurrent and Wyrtki Jets. The authors further discuss the role of upwelling and the Indian Ocean Dipole in driving the low oxygen zones along the Indonesian coast.
[Response]:
We appreciate the reviewer's constructive comments on improving our manuscript.
General Comments:
1. The authors claim to discuss oxygen changes along the Indonesian coast. However, most of the introduction focuses on the Arabian Sea and Bay of Bengal. While these basins are crucial for the Indonesian coast, the introduction should focus more on the relevant processes in the study region.[Response] :
We have revised the manuscript by adding information on relevant processes in the study region to the Introduction, originally placed under the Materials and Methods section.
2. The manuscript, including the abstract, is poorly written and requires substantial revision to improve the quality of writing. Often, paragraphs contain unnecessary details and miss the most relevant information.
[Response] :
We thank the reviewer for the suggestion and have made every effort to improve our manuscript, including removing unnecessary information.
3. The Materials and Methods section begins with an introduction to the study area. This content should be part of the main introduction of the manuscript.
[Response] :
The manuscript has been revised accordingly.
4. Figure 3a caption is incorrect. Since vertical sections are crucial for this study, modify these sections to ensure the main features are clearly visible.
[Response] :
We have modified the caption for Figure 3a, and the revised text now reads as “The area of study interest in the SETIO region adjacent to the coasts of Sumatra and Java. (a) Red (coastal), yellow (transition), and blue (SEC) lines are cross-sections in which DO concentrations are analyzed…”
5. Figure 3 (all vertical sections): The eastward extension of the high salinity (34.75 PSS) contour, representing the intrusion of Arabian Sea water into the coastal regions of Indonesia, appears similar in all seasons. While this could be attributed to basin circulation in summer/annual mean, what explains this intrusion during winter when there is a reversal in the North Indian Ocean circulation?
[Response]:
Our analysis of the WOA data (Figure 3) shows a clear intrusion of Arabian Sea water into the coastal regions of Indonesia during the winter season, mostly via the eastward Equatorial Counter Current. This current is located around the equator during this season and can extend to 6°S with decreasing magnitude. In the eastern tropical Indian Ocean, the South Java Current continues transferring the Arabian Sea water along the coasts of Sumatra and Java, with high intensity during the winter and weakening during the summer (Duan et al., 2016; Utari et al., 2019).
6. The conclusions in section 3.2 (lines 242-244) are premature. The role of horizontal advection is attributed without specific analysis, requiring a refined approach.
[Response] :
We have expanded section 3.2 to provide a clearer explanation of our analyses. In all, we identified the role of wind-driven horizontal ocean advection in transporting low-oxygenated western Indian Ocean waters to the SETIO region using dissolved oxygen and salinity data across depths in section 3.2. We made corroborating analyses in section 4.1 by assessing the relationship between zonal current velocity and salinity in the equatorial (80.5°E, 3°N-3°S) and at the Sunda Strait outlet (107°E, 7.75-9.5°S). Our in situ Argo data analyses also support the observed high salinity waters in southern Sunda Strait via reanalysis dataset.
7. Much of the results section includes information that is out of focus for the current study. For example, lines 258-261 discuss known seasonal spreading of oxygen-depleted water. Focus on explaining new findings in your study and compare or relate them to existing information.
[Response] :
We thank the reviewer for the constructive suggestion. In the revised text, we have attempted to explain more clearly analyses of our in situ record and the reanalysis data. It is well known that the oxygen minimum zone (OMZ) in the Indian Ocean is located in the northern part, while the eastern part receives less attention. Interestingly, low-oxygenated water is observed in the eastern boundary due to upwelling, similar to what is measured in the eastern Pacific and Atlantic Oceans. However, a signal of lower oxygen water is not found in the eastern Indian Ocean, despite the presence of seasonal upwelling events. Here, we attempted to diagnose the presence of low-oxygenated water in the SETIO as originating from a combination of sources from the northern Indian Ocean and seasonal upwelling.
In our original manuscript, we hypothesized that two types of low-oxygenated waters observed in the SETIO originate from the northern Indian Ocean, transported eastward by advection and influenced by seasonal upwelling during the southeast monsoon. We initially identified the seasonal characteristics of oxygen-depleted waters in the Arabian Sea and the Bay of Bengal and their potential spreading into the SETIO through physical processes. Since the OMZ core, where dissolved oxygen (DO) is less than 22 µmol kg-1, is not found in the SETIO, we focused on and discussed the oxygen-limited zone (OLZ), where DO is around 60 µmol kg⁻¹). The second process contributing to the present low oxygen water in our region is upwelling, which is observed by cruise campaigns.8. Section 4.2: When explaining the role of the IOD on Java-Sumatra upwelling and associated oxygen variability, the authors mix two different mechanisms. Upwelling along this coast is influenced by both Indian Ocean equatorial upwelling Kelvin waves and local easterly winds, which operate differently in terms of origins, characteristics, and impacts. Interpret processes associated with upwelling in this region with extreme care.
[Response] :
We thank the reviewer for suggesting a clear interpretation of section 4.2 regarding the upwelling along the coasts of Sumatra and Java affected by both Indian Ocean equatorial upwelling Kelvin waves and easterly winds. Our original manuscript explained upwelling processes associated with low oxygen waters in southern Java due to easterly winds based on the 2022 cruise observation.
In addition, we have added a brief introduction of the origins, characteristics, and impact of upwelling Kelvin waves associated with oxygen variability in the new version of our manuscript, as follows: "Indian Ocean equatorial upwelling Kelvin waves, generated by wind anomalies, travel eastward and propagate poleward along coastlines like the southern coast of Java (Iskandar et al., 2005). During the upwelling phase of Kelvin waves, the thermocline rises, bringing low-oxygen deep waters closer to the surface and decreasing dissolved oxygen levels in the sub-surface and surface layers. This upwelling also increases nutrient availability, boosting productivity and leading to further oxygen depletion due to the decomposition of organic matter, which can exacerbate hypoxic conditions.”
9. The discussion section should focus more on interpreting the main findings and comparing them with similar existing studies. Therefore, this section needs to be revised.
[Response] :
We thank the reviewer for the suggestion. We have revised the discussion section to provide more in-depth interpretations of the new findings we aim to convey through this publication. All revisions of the discussion section could be seen in the revised full paper draft.
10. As mentioned in my previous comments, the research question investigated in this study is relevant. However, the paper is poorly written, the scientific rigor is weak, and the results are not well presented. Therefore, I am not able to accept the paper and suggest rejection.
[Response]:
We have made an effort to revise this manuscript, addressing the reviewer's comments. We believe these revisions significantly enhance the manuscript's quality and effectively communicate the importance of our findings. We hope the revised manuscript is now suitable for publication, and there is an opportunity for this study to be published.
Reference:
- Duan, Y., Liu, L., Han, G. et al.: Anomalous behaviors of Wyrtki Jets in the equatorial Indian Ocean during 2013. Sci Rep 6, 29688. https://doi.org/10.1038/srep29688, 2016.
- Iskandar, I., Mardiansyah, W., Masumoto, Y., and T. Yamagata.: Intraseasonal Kelvin waves along the southern coast of Sumatra and Java, J. Geophys. Res., 110, C04013, doi:10.1029/2004JC002508, 2005.
- Utari, P. A., Setiabudidaya, D., Khakim, M. Y. N., and Iskandar, I.: Dynamics of the South Java Coastal Current revealed by RAMA observing network. Terr. Atmos. Ocean. Sci., 30, 235-245, doi: 10.3319/TAO.2018.12.14.01, 2019.
Citation: https://doi.org/10.5194/egusphere-2024-451-AC3
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RC2: 'Comment on egusphere-2024-451', Anonymous Referee #2, 25 Jun 2024
Review of “Signatures of Ocean Oxygen-Depleted Waters along the Sumatra-Java Coasts in the Southeastern Tropical Indian Ocean”
Deoxygenation is occurring globally. I applaud the authors for trying to address deoxygenation through the historical archival data sets from the Sumatra-Java Coasts, which remains least understood region within the Indian Ocean. I have the following suggestions that the authors might consider to improve their study:
- Abstract needs to clearly state new findings of this study.
- Introduction is over-focused on the Arabian Sea and Bay of Bengal. While some references are needed to put the things/circulation in perspective, authors should clearly state the objectives of this study and research gap. More info on the biogeochemistry of Sumatra-Java Coasts is needed.
- Only first four figures are cited in the results section, while remaining five occurs in the discussion. If the journal allows, either results or discussion should be merged or all the fig. should be explained in the results section, and results should only ne discussed in the discussion section.
- There are enough details on the surface oxygen values with an emphasis on primary productivity. This is well known and these details appear superficial. I suggest to focus more on the sub-surface DO explanation and explain the results through ocean processes.
- There are numerous typos, grammatical mistakes and unclear sentences (e.g., observ in line 332; 438-439). Authors should carefully revise the manuscript for such errors.
- Minimize the number of acronyms. Keep the one that are used > 10 times. Rather than calling Bay of Bengal as Bob, simply call it bay.
- Some of the recent papers should be cited appropriately. For example,
Liu, T., Qiu, Y., Lin, X., Ni, X., Wang, L., Li, H., & Jing, C. (2024). Dissolved oxygen recovery in the oxygen minimum zone of the Arabian Sea in recent decade as observed by BGC‐argo floats. Geophysical Research Letters, 51(12), e2024GL108841.
Citation: https://doi.org/10.5194/egusphere-2024-451-RC2 -
AC1: 'Reply on RC2', Faisal Hamzah, 30 Jul 2024
Dear Editor,
Here, we provide responses to every question from RC2. Our responses are written in Italic style.
1. Abstract needs to clearly state new findings of this study.
[Response]:
We have revised the abstract to clarify the new findings of this study.
2. Introduction is over-focused on the Arabian Sea and Bay of Bengal. While some references are needed to put the things/circulation in perspective, authors should clearly state the objectives of this study and research gap. More info on the biogeochemistry of Sumatra-Java Coasts is needed.
[Response]:
Thank you to the reviewer for the positive comments on improving our manuscript. Following your suggestions, we have added the objectives of this study area and identified the research gap. Additionally, information about the biogeochemistry of the Sumatra-Java coasts has also been included. The revised text now reads as
“The biogeochemistry of the Sumatra-Java coasts is influenced by several factors,…”
and
“Study of oxygen level along the coasts of Java Sumatra and Java in the southeast tropical Indian Ocean is crucial but not well understood. …”
3. Only first four figures are cited in the results section, while the remaining five occurs in the discussion. If the journal allows, either results or discussion should be merged or all the fig. should be explained in the results section, and results should only be discussed in the discussion section.
[Response]:
Thank you to the reviewer for the note. In the discussion section, all the figures shown support and strengthen the findings in the results section. For example, in Figure 3, we created transects at three different locations and identified the low oxygen levels and water mass characteristics in the study area. We then reinforced these findings with a discussion on the origin and mechanisms of the low oxygen levels, as seen in Figure 5.
4. There are enough details on the surface oxygen values with an emphasis on primary productivity. This is well known and these details appear superficial. I suggest to focus more on the sub-surface DO explanation and explain the results through ocean processes.
[Response]:
Thank you to the reviewer for the suggestion to improve our manuscript. In our original manuscript, the sub-surface DO explanation has been discussed in section 4.1.
According to the suggestions, we have added additional explanation related to sub-surface DO in section 4.2, how DO level along the coasts of Sumatra and Java is affected by vertical (upwelling and downwelling) and horizontal (South Java Current and South Java Undercurrent) advections. The revised version now is read as ” In addition to seasonal monsoon variations, the sub-surface oxygen levels along the southern coasts of Java and Sumatra are influenced by a complex interplay of vertical and horizontal advection processes and interannual phenomena….”
5. There are numerous typos, grammatical mistakes and unclear sentences (e.g., observed in line 332; 438-439). Authors should carefully revise the manuscript for such errors.
[Response]:
We appreciate the Reviewer’s positive comments on numerous typos, grammatical mistakes, and unclear sentences in the paper.
6. Minimize the number of acronyms. Keep the one that are used > 10 times. Rather than calling the Bay of Bengal Bob, simply call it Bay.
[Response]:
We have minimized the number of acronyms. However, we still use the term Bay of Bengal with the acronym BoB rather than Bay because researchers generally know and are familiar with it, as it is often used in international papers.
7. Some of the recent papers should be cited appropriately. For example, Liu, T., Qiu, Y., Lin, X., Ni, X., Wang, L., Li, H., & Jing, C. (2024). Dissolved oxygen recovery in the oxygen minimum zone of the Arabian Sea in recent decades as observed by BGC‐argo floats. Geophysical Research Letters, 51(12), e2024GL108841.
[Response]:
We have added some results from Liu et al. (2024) in the introduction part line (79-80): “Nevertheless, Liu et al (2024) observed the thickness of OMZ decreased by a 13 % and a dissolved oxygen recovery trend in the northeastern Arabian Sea, where oxygen concentration increased from 0.63 μM to 1.68 μM over decades”.
Citation: https://doi.org/10.5194/egusphere-2024-451-AC1
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RC3: 'Comment on egusphere-2024-451', Anonymous Referee #3, 10 Jul 2024
Comment on “Signatures of Ocean Oxygen-Depleted Waters along the Sumatra-Java Coasts in the Southeastern Tropical Indian Ocean” by Faisal Hamzah et al.
The study analyses the variability of low oxygen waters off Sumatra and Java, focusing on the role of ocean dynamics. The authors investigate the role of the equatorial zonal current system as a major supply path of low oxygen waters to the Indonesian coasts as well as seasonal upwelling and its relation to biogeochemical processes off Sumatra and Java. They also discuss the relationship between changes in DO concentrations and ENSO and IOD. The study is based on a wide range of data such as ship-based measurements, mooring, Argo, and Reanalysis data.
General comments:
Although the topic of the study is highly relevant, I have several major concerns that prevent me from recommending this manuscript for publication. My main concern is that the manuscript lacks clearly formulated scientific questions. Instead of focusing on one topic, the study deals with too many very complex issues (IOD, ENSO), which makes the discussion and the outcomes very vague. In addition, the paper is not well written, there are many inaccuracies and careless errors and most of the figures would need to be significantly improved.
Specific comments:
lines 95-107: The last part of the introduction, which describes the study, is not related to the previous description of the OMZs in the Arabian Sea and the Bay of Bengal. The introduction would also benefit from one or more clearly formulated scientific questions, which are stated at the end of the introduction together with the strategy how to answer these.
L116: “southeast monsoon” should read “southwest monsoon”
l 128-129: The connection of the SEC to the Agulhas current is not relevant for this study.
l173: Why is the newer climatology WOA23 not being used?
L186-187: It is not explained to the reader, why these specific stations (ST.04, 13, 14) are used for the study?
Chapter 3.1, 3.2, 3.3: The scientific question and the results of the individual chapters should be emphasised more clearly. Figures should be modified/improved to give exactly the information and results that the reader needs to understand the outcome (see below).
L287: Should it read “SECC” here instead of “ECC”? But the SECC does not exist during the summer monsoon.
Figures
Fig. 1: The figure caption does not explain where the DO data come from? Why do some of the isolines (132, 135 on the coast; 111, 114 at 3°S) look strange?
Fig. 2: The illustrations are too small and the text in the legend is hard to read. The profiles are difficult to distinguish. Would it make more sense to colour the profiles by region?
Fig. 3: The sections should have the same x-axis for a better comparison. The isolines for DO and salinity are not visible in the upper 200 m depth. Overall, the images are too small. The depths of 0-500m and 500-1500m should have a different scale so that the upper area is spread out.
Fig. 4: Figures are too small and too much information is given that is not relevant. To show the seasonal variation of the southward expansion of the OLZ and the OMZ, it would be maybe more helpful to show two meridional sections each for the annual mean, summer and winter from 20°S to 23°N in the upper 1000m.
Fig. 5: The figures are too small and the vector arrows are tiny. Most of the plots are not necessary, because they are not discussed in the text. The schematic circulation of the surface circulation during the winter monsoon is very helpful it should be also done for the summer monsoon, because the surface circulation is partly reversed. The caption is wrong: the South Equatorial Under Current should be the South Equatorial Counter Current. It is not clear to me what the legend in the bottom right of the picture shows.
Fig. 6: The relationship between salinity and velocity is difficult to see. It would be better to show the meridionally averaged salinity and velocity, respectively over time instead of the contour plot? In Fig. 6 c, d it is sufficient to show the upper 250 m depth. The map should be enlarged because the trajectories of the Argo float are hard to see.What does the black contour line in Fig. 6 c, d mean? The unit for the temperature is missing in the caption.
Fig. 8: In Fig. 8a, the colour scale should be adjusted so that the distribution of chlorophyll can be seen (maybe logarithmic scale).
Fig. 9: In Fig. 9 a,b the colour gradations of the DO are hard to see. Another colour scale or a few additional selected contour lines would be helpful.
Citation: https://doi.org/10.5194/egusphere-2024-451-RC3 -
AC2: 'Reply on RC3', Faisal Hamzah, 30 Jul 2024
Dear Editor,
Here, we respond to questions from RC3. Our responses are written in Italic style.
General comments:
Although the topic of the study is highly relevant, I have several major concerns that prevent me from recommending this manuscript for publication. My main concern is that the manuscript lacks clearly formulated scientific questions. Instead of focusing on one topic, the study deals with too many very complex issues (IOD, ENSO), which makes the discussion and the outcomes very vague. In addition, the paper is not well written; there are many inaccuracies and careless errors, and most of the figures would need to be significantly improved.[Response]:
We appreciate that the reviewer valued our study and have made efforts to improve our manuscript further.
Specific comments:
1. lines 95-107: The last part of the introduction, which describes the study, is not related to the previous description of the OMZs in the Arabian Sea and the Bay of Bengal. The introduction would also benefit from one or more clearly formulated scientific questions, which are stated at the end of the introduction together with the strategy how to answer these.[Response]:
We have changed the last part of the introduction with the sentence to describe the strategy to find the answer to the research question. The revised text now reads as follows: “The biogeochemistry of the Sumatra-Java coasts is influenced by several factors, including lateral currents, the ITF, ….”
2. line 116: “southeast monsoon” should read “southwest monsoon”
[Response]:
In this context, we have the right to make decisions regarding the southeast monsoon from June to November in Indonesia.
3. lines 128-129: The connection of the SEC to the Agulhas current is not relevant for this study.
[Response]:
We have deleted the sentence in Lines 128 - 129.
4. line 173: Why is the newer climatology WOA23 not being used?
[Response] :
We thank the reviewer for the suggestion. We have changed the WOA18 into the WOA23 for the climatology dataset in this study.
5. lines 186-187: It is not explained to the reader, why these specific stations (ST.04, 13, 14) are used for the study?
[Response]:
We have justified the selection of three observation stations (St. 04, St. 13, and St. 14) as they have continuous datasets and can represent the study areas. Additionally, these three stations are used for in-situ model validation.
The revised now reads as “We selected three stations observation stations that have continuous data and can represent the study areas (close to the coast in Sunda Strait, the west area of South Java, and the east area of South Java) (Figure 1). The three stations used in the in-situ model validation are St.04 (104.39°E, 6.54°S), St.13 (106.91°E, 8.12°S), and St.14 (107.07°E, 7.71°S), respectively”.6. Chapter 3.1, 3.2, 3.3: The scientific question and the results of the individual chapters should be emphasized more clearly. Figures should be modified/improved to give exactly the information and results that the reader needs to understand the outcome (see below).
[Response]:
We thank the reviewer for the suggestion. We have changed chapters 3.2 and 3.3 to give more clear information. For the figure, we make clear responses in “Figures comments” as below.
7. line 287: Should it read “SECC” here instead of “ECC”? But the SECC does not exist during the summer monsoon.
[Response]:
We thank the reviewer for the note. We did not revise the sentence because our study area is east of the Equator, so the eastward propagating Equatorial Counter Current (ECC) is stronger during the boreal summer.
Figure Comments
We provide supplement data to accompany clearly-understood figure comments (for Figures 1-6, 8-9).
8. Fig. 1: The figure caption does not explain where the DO data come from? Why do some of the isolines (132, 135 on the coast; 111, 114 at 3°S) look strange?
[Response]:
We have changed the caption for Fig. 1 as follows “Study area showing sampling stations during international cruise campaigns between 2010-2022 (color dots) and a RAMA buoy (red triangle). DO data from cruise observations measured in 2022 at St. 04, St. 13, and St. 14 in the 2022 cruise are used for the model validation. The contours represent the mark DO concentration (μmol kg-1) at a 600 m depth from WOA23 Annual Mean Data.
9. Fig. 2: The illustrations are too small and the text in the legend is hard to read. The profiles are difficult to distinguish. Would it make more sense to colour the profiles by region?
[Response]:
We have changed the font size for the legend in the revised illustrations (Figure 2) to improve readability. However, we could not make profiles of the relationships between potential temperature, salinity, and dissolved oxygen by region, as this would oppose the original purpose of our findings.
10. Fig. 3: The sections should have the same x-axis for a better comparison. The isolines for DO and salinity are not visible in the upper 200 m depth. Overall, the images are too small. The depths of 0-500m and 500-1500m should have a different scale so that the upper area is spread out.
[Response] :
We thank the reviewer for the suggestion. In this context, we want to highlight a finding from our research that depleted oxygen (OLZ) is found at intermediate depths of 200-1.200 m in the coastal section and at greater depths. For all transects, we already used the same x-axis. We still used our original Figure 3, since the OLZ is located below 200 m and DO concentration is higher in the upper 200 m. We also made different scales and times (Figure S1, see our supplement data), showing in the upper 150 m, DO concentration and salinity are slightly homogeneous.
11. Fig. 4: Figures are too small and too much information is given that is not relevant. To show the seasonal variation of the southward expansion of the OLZ and the OMZ, it would be maybe more helpful to show two meridional sections each for the annual mean, summer and winter from 20°S to 23°N in the upper 1000 m.
[Response]:
We appreciate the reviewer for suggesting improvements to our manuscript. We have created a new version of Figure 4 to show the seasonal variation of the southward expansion of the OLZ and the OMZ.
12. Fig. 5: The figures are too small and the vector arrows are tiny. Most of the plots are not necessary, because they are not discussed in the text. The schematic circulation of the surface circulation during the winter monsoon is very helpful it should be also done for the summer monsoon, because the surface circulation is partly reversed. The caption is wrong: the South Equatorial Under Current should be the South Equatorial Counter Current. It is not clear to me what the legend in the bottom right of the picture shows.
[Response]:
Thank you the reviewer for the suggestions. We have simplified the new version of our figure to only include data up to 100 meters depth during the winter monsoon and the summer monsoon. The revised figure also includes regional circulation patterns for both the boreal winter and the boreal summer.
13. Fig. 6: The relationship between salinity and velocity is difficult to see. It would be better to show the meridionally averaged salinity and velocity, respectively over time instead of the contour plot? In Fig. 6c, d it is sufficient to show the upper 250 m depth. The map should be enlarged because the trajectories of the Argo float are hard to see. What does the black contour line in Fig. 6c, d mean? The unit for the temperature is missing in the caption.
[Response] :
We have adjusted the meridionally averaged salinity and velocity over time in Figures 6a and 6b. Additionally, temperature and salinity data in the upper 250 meters are shown in Figures 6c and 6d. In the new version of our figure, the trajectory map of Argo #1901444 has been replotted. The black contour lines in panels 6c and 6d represent the isotherm at 27°C and the isohaline at 34.3 PSS.
14. Fig. 8: In Fig. 8a, the colour scale should be adjusted so that the distribution of chlorophyll can be seen (maybe logarithmic scale).
[Response]:
We thank the reviewer for the suggestions. We have adjusted the colour scale in the new version of our figure.
15. Fig. 9: In Fig. 9 a,b the colour gradations of the DO are hard to see. Another colour scale or a few additional selected contour lines would be helpful.
[Response] :
We thank the reviewer for the recommendations. We have adjusted the colour gradations of the DO in the new version of our figure.
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AC2: 'Reply on RC3', Faisal Hamzah, 30 Jul 2024
Status: closed
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RC1: 'Comment on egusphere-2024-451', Anonymous Referee #1, 03 Jun 2024
Review Comments on the Manuscript ‘Signatures of Ocean Oxygen-Depleted Waters along the Sumatra-Java Coasts in the Southeastern Tropical Indian Ocean’ by Faisal Hamzah et al., submitted to EGUspere
In this paper, the authors study oxygen changes along the Java and Sumatra coasts using observations from multiple platforms, including Argo, Buoy, and Ship cruises. They supplement these observations with reanalysis products to assess oxygen variability along the coasts of Indonesia. Their analysis detects low oxygen waters along the Java-Sumatra coast, attributing their source to the North Indian Ocean. The major drivers for this export vary with the season and include the eastward-flowing equatorial undercurrent and Wyrtki Jets. The authors further discuss the role of upwelling and the Indian Ocean Dipole in driving the low oxygen zones along the Indonesian coast.
General Comments:
- The authors claim to discuss oxygen changes along the Indonesian coast. However, most of the introduction focuses on the Arabian Sea and Bay of Bengal. While these basins are crucial for the Indonesian coast, the introduction should focus more on the relevant processes in the study region.
- The manuscript, including the abstract, is poorly written and requires substantial revision to improve the quality of writing. Often, paragraphs contain unnecessary details and miss the most relevant information.
- The Materials and Methods section begins with an introduction to the study area. This content should be part of the main introduction of the manuscript.
- Figure 3a caption is incorrect. Since vertical sections are crucial for this study, modify these sections to ensure the main features are clearly visible.
- Figure 3 (all vertical sections): The eastward extension of the high salinity (34.75 PSS) contour, representing the intrusion of Arabian Sea water into the coastal regions of Indonesia, appears similar in all seasons. While this could be attributed to basin circulation in summer/annual mean, what explains this intrusion during winter when there is a reversal in the North Indian Ocean circulation?
- The conclusions in section 3.2 (lines 242-244) are premature. The role of horizontal advection is attributed without specific analysis, requiring a refined approach.
- Much of the results section includes information that is out of focus for the current study. For example, lines 258-261 discuss known seasonal spreading of oxygen-depleted water. Focus on explaining new findings in your study and compare or relate them to existing information.
- Section 4.2: When explaining the role of the IOD on Java-Sumatra upwelling and associated oxygen variability, the authors mix two different mechanisms. Upwelling along this coast is influenced by both Indian Ocean equatorial upwelling Kelvin waves and local easterly winds, which operate differently in terms of origins, characteristics, and impacts. Interpret processes associated with upwelling in this region with extreme care.
- The discussion section should focus more on interpreting the main findings and comparing them with similar existing studies. Therefore, this section needs to be revised.
As mentioned in my previous comments, the research question investigated in this study is relevant. However, the paper is poorly written, the scientific rigor is weak, and the results are not well presented. Therefore, I am not able to accept the paper and suggest rejection.
Citation: https://doi.org/10.5194/egusphere-2024-451-RC1 -
AC3: 'Reply on RC1', Faisal Hamzah, 31 Jul 2024
Dear Editor,
Here, we respond to comments from RC1. Our responses are written in Italic style.
Comment: In this paper, the authors study oxygen changes along the Java and Sumatra coasts using observations from multiple platforms, including Argo, Buoy, and Ship cruises. They supplement these observations with reanalysis products to assess oxygen variability along the coasts of Indonesia. Their analysis detects low oxygen waters along the Java-Sumatra coast, attributing their source to the North Indian Ocean. The major drivers for this export vary with the season and include the eastward-flowing equatorial undercurrent and Wyrtki Jets. The authors further discuss the role of upwelling and the Indian Ocean Dipole in driving the low oxygen zones along the Indonesian coast.
[Response]:
We appreciate the reviewer's constructive comments on improving our manuscript.
General Comments:
1. The authors claim to discuss oxygen changes along the Indonesian coast. However, most of the introduction focuses on the Arabian Sea and Bay of Bengal. While these basins are crucial for the Indonesian coast, the introduction should focus more on the relevant processes in the study region.[Response] :
We have revised the manuscript by adding information on relevant processes in the study region to the Introduction, originally placed under the Materials and Methods section.
2. The manuscript, including the abstract, is poorly written and requires substantial revision to improve the quality of writing. Often, paragraphs contain unnecessary details and miss the most relevant information.
[Response] :
We thank the reviewer for the suggestion and have made every effort to improve our manuscript, including removing unnecessary information.
3. The Materials and Methods section begins with an introduction to the study area. This content should be part of the main introduction of the manuscript.
[Response] :
The manuscript has been revised accordingly.
4. Figure 3a caption is incorrect. Since vertical sections are crucial for this study, modify these sections to ensure the main features are clearly visible.
[Response] :
We have modified the caption for Figure 3a, and the revised text now reads as “The area of study interest in the SETIO region adjacent to the coasts of Sumatra and Java. (a) Red (coastal), yellow (transition), and blue (SEC) lines are cross-sections in which DO concentrations are analyzed…”
5. Figure 3 (all vertical sections): The eastward extension of the high salinity (34.75 PSS) contour, representing the intrusion of Arabian Sea water into the coastal regions of Indonesia, appears similar in all seasons. While this could be attributed to basin circulation in summer/annual mean, what explains this intrusion during winter when there is a reversal in the North Indian Ocean circulation?
[Response]:
Our analysis of the WOA data (Figure 3) shows a clear intrusion of Arabian Sea water into the coastal regions of Indonesia during the winter season, mostly via the eastward Equatorial Counter Current. This current is located around the equator during this season and can extend to 6°S with decreasing magnitude. In the eastern tropical Indian Ocean, the South Java Current continues transferring the Arabian Sea water along the coasts of Sumatra and Java, with high intensity during the winter and weakening during the summer (Duan et al., 2016; Utari et al., 2019).
6. The conclusions in section 3.2 (lines 242-244) are premature. The role of horizontal advection is attributed without specific analysis, requiring a refined approach.
[Response] :
We have expanded section 3.2 to provide a clearer explanation of our analyses. In all, we identified the role of wind-driven horizontal ocean advection in transporting low-oxygenated western Indian Ocean waters to the SETIO region using dissolved oxygen and salinity data across depths in section 3.2. We made corroborating analyses in section 4.1 by assessing the relationship between zonal current velocity and salinity in the equatorial (80.5°E, 3°N-3°S) and at the Sunda Strait outlet (107°E, 7.75-9.5°S). Our in situ Argo data analyses also support the observed high salinity waters in southern Sunda Strait via reanalysis dataset.
7. Much of the results section includes information that is out of focus for the current study. For example, lines 258-261 discuss known seasonal spreading of oxygen-depleted water. Focus on explaining new findings in your study and compare or relate them to existing information.
[Response] :
We thank the reviewer for the constructive suggestion. In the revised text, we have attempted to explain more clearly analyses of our in situ record and the reanalysis data. It is well known that the oxygen minimum zone (OMZ) in the Indian Ocean is located in the northern part, while the eastern part receives less attention. Interestingly, low-oxygenated water is observed in the eastern boundary due to upwelling, similar to what is measured in the eastern Pacific and Atlantic Oceans. However, a signal of lower oxygen water is not found in the eastern Indian Ocean, despite the presence of seasonal upwelling events. Here, we attempted to diagnose the presence of low-oxygenated water in the SETIO as originating from a combination of sources from the northern Indian Ocean and seasonal upwelling.
In our original manuscript, we hypothesized that two types of low-oxygenated waters observed in the SETIO originate from the northern Indian Ocean, transported eastward by advection and influenced by seasonal upwelling during the southeast monsoon. We initially identified the seasonal characteristics of oxygen-depleted waters in the Arabian Sea and the Bay of Bengal and their potential spreading into the SETIO through physical processes. Since the OMZ core, where dissolved oxygen (DO) is less than 22 µmol kg-1, is not found in the SETIO, we focused on and discussed the oxygen-limited zone (OLZ), where DO is around 60 µmol kg⁻¹). The second process contributing to the present low oxygen water in our region is upwelling, which is observed by cruise campaigns.8. Section 4.2: When explaining the role of the IOD on Java-Sumatra upwelling and associated oxygen variability, the authors mix two different mechanisms. Upwelling along this coast is influenced by both Indian Ocean equatorial upwelling Kelvin waves and local easterly winds, which operate differently in terms of origins, characteristics, and impacts. Interpret processes associated with upwelling in this region with extreme care.
[Response] :
We thank the reviewer for suggesting a clear interpretation of section 4.2 regarding the upwelling along the coasts of Sumatra and Java affected by both Indian Ocean equatorial upwelling Kelvin waves and easterly winds. Our original manuscript explained upwelling processes associated with low oxygen waters in southern Java due to easterly winds based on the 2022 cruise observation.
In addition, we have added a brief introduction of the origins, characteristics, and impact of upwelling Kelvin waves associated with oxygen variability in the new version of our manuscript, as follows: "Indian Ocean equatorial upwelling Kelvin waves, generated by wind anomalies, travel eastward and propagate poleward along coastlines like the southern coast of Java (Iskandar et al., 2005). During the upwelling phase of Kelvin waves, the thermocline rises, bringing low-oxygen deep waters closer to the surface and decreasing dissolved oxygen levels in the sub-surface and surface layers. This upwelling also increases nutrient availability, boosting productivity and leading to further oxygen depletion due to the decomposition of organic matter, which can exacerbate hypoxic conditions.”
9. The discussion section should focus more on interpreting the main findings and comparing them with similar existing studies. Therefore, this section needs to be revised.
[Response] :
We thank the reviewer for the suggestion. We have revised the discussion section to provide more in-depth interpretations of the new findings we aim to convey through this publication. All revisions of the discussion section could be seen in the revised full paper draft.
10. As mentioned in my previous comments, the research question investigated in this study is relevant. However, the paper is poorly written, the scientific rigor is weak, and the results are not well presented. Therefore, I am not able to accept the paper and suggest rejection.
[Response]:
We have made an effort to revise this manuscript, addressing the reviewer's comments. We believe these revisions significantly enhance the manuscript's quality and effectively communicate the importance of our findings. We hope the revised manuscript is now suitable for publication, and there is an opportunity for this study to be published.
Reference:
- Duan, Y., Liu, L., Han, G. et al.: Anomalous behaviors of Wyrtki Jets in the equatorial Indian Ocean during 2013. Sci Rep 6, 29688. https://doi.org/10.1038/srep29688, 2016.
- Iskandar, I., Mardiansyah, W., Masumoto, Y., and T. Yamagata.: Intraseasonal Kelvin waves along the southern coast of Sumatra and Java, J. Geophys. Res., 110, C04013, doi:10.1029/2004JC002508, 2005.
- Utari, P. A., Setiabudidaya, D., Khakim, M. Y. N., and Iskandar, I.: Dynamics of the South Java Coastal Current revealed by RAMA observing network. Terr. Atmos. Ocean. Sci., 30, 235-245, doi: 10.3319/TAO.2018.12.14.01, 2019.
Citation: https://doi.org/10.5194/egusphere-2024-451-AC3
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RC2: 'Comment on egusphere-2024-451', Anonymous Referee #2, 25 Jun 2024
Review of “Signatures of Ocean Oxygen-Depleted Waters along the Sumatra-Java Coasts in the Southeastern Tropical Indian Ocean”
Deoxygenation is occurring globally. I applaud the authors for trying to address deoxygenation through the historical archival data sets from the Sumatra-Java Coasts, which remains least understood region within the Indian Ocean. I have the following suggestions that the authors might consider to improve their study:
- Abstract needs to clearly state new findings of this study.
- Introduction is over-focused on the Arabian Sea and Bay of Bengal. While some references are needed to put the things/circulation in perspective, authors should clearly state the objectives of this study and research gap. More info on the biogeochemistry of Sumatra-Java Coasts is needed.
- Only first four figures are cited in the results section, while remaining five occurs in the discussion. If the journal allows, either results or discussion should be merged or all the fig. should be explained in the results section, and results should only ne discussed in the discussion section.
- There are enough details on the surface oxygen values with an emphasis on primary productivity. This is well known and these details appear superficial. I suggest to focus more on the sub-surface DO explanation and explain the results through ocean processes.
- There are numerous typos, grammatical mistakes and unclear sentences (e.g., observ in line 332; 438-439). Authors should carefully revise the manuscript for such errors.
- Minimize the number of acronyms. Keep the one that are used > 10 times. Rather than calling Bay of Bengal as Bob, simply call it bay.
- Some of the recent papers should be cited appropriately. For example,
Liu, T., Qiu, Y., Lin, X., Ni, X., Wang, L., Li, H., & Jing, C. (2024). Dissolved oxygen recovery in the oxygen minimum zone of the Arabian Sea in recent decade as observed by BGC‐argo floats. Geophysical Research Letters, 51(12), e2024GL108841.
Citation: https://doi.org/10.5194/egusphere-2024-451-RC2 -
AC1: 'Reply on RC2', Faisal Hamzah, 30 Jul 2024
Dear Editor,
Here, we provide responses to every question from RC2. Our responses are written in Italic style.
1. Abstract needs to clearly state new findings of this study.
[Response]:
We have revised the abstract to clarify the new findings of this study.
2. Introduction is over-focused on the Arabian Sea and Bay of Bengal. While some references are needed to put the things/circulation in perspective, authors should clearly state the objectives of this study and research gap. More info on the biogeochemistry of Sumatra-Java Coasts is needed.
[Response]:
Thank you to the reviewer for the positive comments on improving our manuscript. Following your suggestions, we have added the objectives of this study area and identified the research gap. Additionally, information about the biogeochemistry of the Sumatra-Java coasts has also been included. The revised text now reads as
“The biogeochemistry of the Sumatra-Java coasts is influenced by several factors,…”
and
“Study of oxygen level along the coasts of Java Sumatra and Java in the southeast tropical Indian Ocean is crucial but not well understood. …”
3. Only first four figures are cited in the results section, while the remaining five occurs in the discussion. If the journal allows, either results or discussion should be merged or all the fig. should be explained in the results section, and results should only be discussed in the discussion section.
[Response]:
Thank you to the reviewer for the note. In the discussion section, all the figures shown support and strengthen the findings in the results section. For example, in Figure 3, we created transects at three different locations and identified the low oxygen levels and water mass characteristics in the study area. We then reinforced these findings with a discussion on the origin and mechanisms of the low oxygen levels, as seen in Figure 5.
4. There are enough details on the surface oxygen values with an emphasis on primary productivity. This is well known and these details appear superficial. I suggest to focus more on the sub-surface DO explanation and explain the results through ocean processes.
[Response]:
Thank you to the reviewer for the suggestion to improve our manuscript. In our original manuscript, the sub-surface DO explanation has been discussed in section 4.1.
According to the suggestions, we have added additional explanation related to sub-surface DO in section 4.2, how DO level along the coasts of Sumatra and Java is affected by vertical (upwelling and downwelling) and horizontal (South Java Current and South Java Undercurrent) advections. The revised version now is read as ” In addition to seasonal monsoon variations, the sub-surface oxygen levels along the southern coasts of Java and Sumatra are influenced by a complex interplay of vertical and horizontal advection processes and interannual phenomena….”
5. There are numerous typos, grammatical mistakes and unclear sentences (e.g., observed in line 332; 438-439). Authors should carefully revise the manuscript for such errors.
[Response]:
We appreciate the Reviewer’s positive comments on numerous typos, grammatical mistakes, and unclear sentences in the paper.
6. Minimize the number of acronyms. Keep the one that are used > 10 times. Rather than calling the Bay of Bengal Bob, simply call it Bay.
[Response]:
We have minimized the number of acronyms. However, we still use the term Bay of Bengal with the acronym BoB rather than Bay because researchers generally know and are familiar with it, as it is often used in international papers.
7. Some of the recent papers should be cited appropriately. For example, Liu, T., Qiu, Y., Lin, X., Ni, X., Wang, L., Li, H., & Jing, C. (2024). Dissolved oxygen recovery in the oxygen minimum zone of the Arabian Sea in recent decades as observed by BGC‐argo floats. Geophysical Research Letters, 51(12), e2024GL108841.
[Response]:
We have added some results from Liu et al. (2024) in the introduction part line (79-80): “Nevertheless, Liu et al (2024) observed the thickness of OMZ decreased by a 13 % and a dissolved oxygen recovery trend in the northeastern Arabian Sea, where oxygen concentration increased from 0.63 μM to 1.68 μM over decades”.
Citation: https://doi.org/10.5194/egusphere-2024-451-AC1
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RC3: 'Comment on egusphere-2024-451', Anonymous Referee #3, 10 Jul 2024
Comment on “Signatures of Ocean Oxygen-Depleted Waters along the Sumatra-Java Coasts in the Southeastern Tropical Indian Ocean” by Faisal Hamzah et al.
The study analyses the variability of low oxygen waters off Sumatra and Java, focusing on the role of ocean dynamics. The authors investigate the role of the equatorial zonal current system as a major supply path of low oxygen waters to the Indonesian coasts as well as seasonal upwelling and its relation to biogeochemical processes off Sumatra and Java. They also discuss the relationship between changes in DO concentrations and ENSO and IOD. The study is based on a wide range of data such as ship-based measurements, mooring, Argo, and Reanalysis data.
General comments:
Although the topic of the study is highly relevant, I have several major concerns that prevent me from recommending this manuscript for publication. My main concern is that the manuscript lacks clearly formulated scientific questions. Instead of focusing on one topic, the study deals with too many very complex issues (IOD, ENSO), which makes the discussion and the outcomes very vague. In addition, the paper is not well written, there are many inaccuracies and careless errors and most of the figures would need to be significantly improved.
Specific comments:
lines 95-107: The last part of the introduction, which describes the study, is not related to the previous description of the OMZs in the Arabian Sea and the Bay of Bengal. The introduction would also benefit from one or more clearly formulated scientific questions, which are stated at the end of the introduction together with the strategy how to answer these.
L116: “southeast monsoon” should read “southwest monsoon”
l 128-129: The connection of the SEC to the Agulhas current is not relevant for this study.
l173: Why is the newer climatology WOA23 not being used?
L186-187: It is not explained to the reader, why these specific stations (ST.04, 13, 14) are used for the study?
Chapter 3.1, 3.2, 3.3: The scientific question and the results of the individual chapters should be emphasised more clearly. Figures should be modified/improved to give exactly the information and results that the reader needs to understand the outcome (see below).
L287: Should it read “SECC” here instead of “ECC”? But the SECC does not exist during the summer monsoon.
Figures
Fig. 1: The figure caption does not explain where the DO data come from? Why do some of the isolines (132, 135 on the coast; 111, 114 at 3°S) look strange?
Fig. 2: The illustrations are too small and the text in the legend is hard to read. The profiles are difficult to distinguish. Would it make more sense to colour the profiles by region?
Fig. 3: The sections should have the same x-axis for a better comparison. The isolines for DO and salinity are not visible in the upper 200 m depth. Overall, the images are too small. The depths of 0-500m and 500-1500m should have a different scale so that the upper area is spread out.
Fig. 4: Figures are too small and too much information is given that is not relevant. To show the seasonal variation of the southward expansion of the OLZ and the OMZ, it would be maybe more helpful to show two meridional sections each for the annual mean, summer and winter from 20°S to 23°N in the upper 1000m.
Fig. 5: The figures are too small and the vector arrows are tiny. Most of the plots are not necessary, because they are not discussed in the text. The schematic circulation of the surface circulation during the winter monsoon is very helpful it should be also done for the summer monsoon, because the surface circulation is partly reversed. The caption is wrong: the South Equatorial Under Current should be the South Equatorial Counter Current. It is not clear to me what the legend in the bottom right of the picture shows.
Fig. 6: The relationship between salinity and velocity is difficult to see. It would be better to show the meridionally averaged salinity and velocity, respectively over time instead of the contour plot? In Fig. 6 c, d it is sufficient to show the upper 250 m depth. The map should be enlarged because the trajectories of the Argo float are hard to see.What does the black contour line in Fig. 6 c, d mean? The unit for the temperature is missing in the caption.
Fig. 8: In Fig. 8a, the colour scale should be adjusted so that the distribution of chlorophyll can be seen (maybe logarithmic scale).
Fig. 9: In Fig. 9 a,b the colour gradations of the DO are hard to see. Another colour scale or a few additional selected contour lines would be helpful.
Citation: https://doi.org/10.5194/egusphere-2024-451-RC3 -
AC2: 'Reply on RC3', Faisal Hamzah, 30 Jul 2024
Dear Editor,
Here, we respond to questions from RC3. Our responses are written in Italic style.
General comments:
Although the topic of the study is highly relevant, I have several major concerns that prevent me from recommending this manuscript for publication. My main concern is that the manuscript lacks clearly formulated scientific questions. Instead of focusing on one topic, the study deals with too many very complex issues (IOD, ENSO), which makes the discussion and the outcomes very vague. In addition, the paper is not well written; there are many inaccuracies and careless errors, and most of the figures would need to be significantly improved.[Response]:
We appreciate that the reviewer valued our study and have made efforts to improve our manuscript further.
Specific comments:
1. lines 95-107: The last part of the introduction, which describes the study, is not related to the previous description of the OMZs in the Arabian Sea and the Bay of Bengal. The introduction would also benefit from one or more clearly formulated scientific questions, which are stated at the end of the introduction together with the strategy how to answer these.[Response]:
We have changed the last part of the introduction with the sentence to describe the strategy to find the answer to the research question. The revised text now reads as follows: “The biogeochemistry of the Sumatra-Java coasts is influenced by several factors, including lateral currents, the ITF, ….”
2. line 116: “southeast monsoon” should read “southwest monsoon”
[Response]:
In this context, we have the right to make decisions regarding the southeast monsoon from June to November in Indonesia.
3. lines 128-129: The connection of the SEC to the Agulhas current is not relevant for this study.
[Response]:
We have deleted the sentence in Lines 128 - 129.
4. line 173: Why is the newer climatology WOA23 not being used?
[Response] :
We thank the reviewer for the suggestion. We have changed the WOA18 into the WOA23 for the climatology dataset in this study.
5. lines 186-187: It is not explained to the reader, why these specific stations (ST.04, 13, 14) are used for the study?
[Response]:
We have justified the selection of three observation stations (St. 04, St. 13, and St. 14) as they have continuous datasets and can represent the study areas. Additionally, these three stations are used for in-situ model validation.
The revised now reads as “We selected three stations observation stations that have continuous data and can represent the study areas (close to the coast in Sunda Strait, the west area of South Java, and the east area of South Java) (Figure 1). The three stations used in the in-situ model validation are St.04 (104.39°E, 6.54°S), St.13 (106.91°E, 8.12°S), and St.14 (107.07°E, 7.71°S), respectively”.6. Chapter 3.1, 3.2, 3.3: The scientific question and the results of the individual chapters should be emphasized more clearly. Figures should be modified/improved to give exactly the information and results that the reader needs to understand the outcome (see below).
[Response]:
We thank the reviewer for the suggestion. We have changed chapters 3.2 and 3.3 to give more clear information. For the figure, we make clear responses in “Figures comments” as below.
7. line 287: Should it read “SECC” here instead of “ECC”? But the SECC does not exist during the summer monsoon.
[Response]:
We thank the reviewer for the note. We did not revise the sentence because our study area is east of the Equator, so the eastward propagating Equatorial Counter Current (ECC) is stronger during the boreal summer.
Figure Comments
We provide supplement data to accompany clearly-understood figure comments (for Figures 1-6, 8-9).
8. Fig. 1: The figure caption does not explain where the DO data come from? Why do some of the isolines (132, 135 on the coast; 111, 114 at 3°S) look strange?
[Response]:
We have changed the caption for Fig. 1 as follows “Study area showing sampling stations during international cruise campaigns between 2010-2022 (color dots) and a RAMA buoy (red triangle). DO data from cruise observations measured in 2022 at St. 04, St. 13, and St. 14 in the 2022 cruise are used for the model validation. The contours represent the mark DO concentration (μmol kg-1) at a 600 m depth from WOA23 Annual Mean Data.
9. Fig. 2: The illustrations are too small and the text in the legend is hard to read. The profiles are difficult to distinguish. Would it make more sense to colour the profiles by region?
[Response]:
We have changed the font size for the legend in the revised illustrations (Figure 2) to improve readability. However, we could not make profiles of the relationships between potential temperature, salinity, and dissolved oxygen by region, as this would oppose the original purpose of our findings.
10. Fig. 3: The sections should have the same x-axis for a better comparison. The isolines for DO and salinity are not visible in the upper 200 m depth. Overall, the images are too small. The depths of 0-500m and 500-1500m should have a different scale so that the upper area is spread out.
[Response] :
We thank the reviewer for the suggestion. In this context, we want to highlight a finding from our research that depleted oxygen (OLZ) is found at intermediate depths of 200-1.200 m in the coastal section and at greater depths. For all transects, we already used the same x-axis. We still used our original Figure 3, since the OLZ is located below 200 m and DO concentration is higher in the upper 200 m. We also made different scales and times (Figure S1, see our supplement data), showing in the upper 150 m, DO concentration and salinity are slightly homogeneous.
11. Fig. 4: Figures are too small and too much information is given that is not relevant. To show the seasonal variation of the southward expansion of the OLZ and the OMZ, it would be maybe more helpful to show two meridional sections each for the annual mean, summer and winter from 20°S to 23°N in the upper 1000 m.
[Response]:
We appreciate the reviewer for suggesting improvements to our manuscript. We have created a new version of Figure 4 to show the seasonal variation of the southward expansion of the OLZ and the OMZ.
12. Fig. 5: The figures are too small and the vector arrows are tiny. Most of the plots are not necessary, because they are not discussed in the text. The schematic circulation of the surface circulation during the winter monsoon is very helpful it should be also done for the summer monsoon, because the surface circulation is partly reversed. The caption is wrong: the South Equatorial Under Current should be the South Equatorial Counter Current. It is not clear to me what the legend in the bottom right of the picture shows.
[Response]:
Thank you the reviewer for the suggestions. We have simplified the new version of our figure to only include data up to 100 meters depth during the winter monsoon and the summer monsoon. The revised figure also includes regional circulation patterns for both the boreal winter and the boreal summer.
13. Fig. 6: The relationship between salinity and velocity is difficult to see. It would be better to show the meridionally averaged salinity and velocity, respectively over time instead of the contour plot? In Fig. 6c, d it is sufficient to show the upper 250 m depth. The map should be enlarged because the trajectories of the Argo float are hard to see. What does the black contour line in Fig. 6c, d mean? The unit for the temperature is missing in the caption.
[Response] :
We have adjusted the meridionally averaged salinity and velocity over time in Figures 6a and 6b. Additionally, temperature and salinity data in the upper 250 meters are shown in Figures 6c and 6d. In the new version of our figure, the trajectory map of Argo #1901444 has been replotted. The black contour lines in panels 6c and 6d represent the isotherm at 27°C and the isohaline at 34.3 PSS.
14. Fig. 8: In Fig. 8a, the colour scale should be adjusted so that the distribution of chlorophyll can be seen (maybe logarithmic scale).
[Response]:
We thank the reviewer for the suggestions. We have adjusted the colour scale in the new version of our figure.
15. Fig. 9: In Fig. 9 a,b the colour gradations of the DO are hard to see. Another colour scale or a few additional selected contour lines would be helpful.
[Response] :
We thank the reviewer for the recommendations. We have adjusted the colour gradations of the DO in the new version of our figure.
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AC2: 'Reply on RC3', Faisal Hamzah, 30 Jul 2024
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