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 -
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 -
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
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