the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jul 2024
Control of spatio-temporal variability of ocean nutrients in the East Australian Current
Abstract. The East Australian Current (EAC), the South Pacific's southward flowing western boundary current, dominates the marine environment of the east coast of Australia. Upwelling of deep EAC nutrient rich water into the oligotrophic surface waters is very important for maintaining upper ocean productivity. However, the role of EAC dynamics in upper ocean nutrient variability and resulting productivity is poorly understood. In this study we use physical and biogeochemical data collected from 2012–2022 to improve understanding of the variability of the nutrients in the upper water column at ~27° S, a subtropical region strongly influenced by EAC.
The 10-year data set shows that there is a seasonal increase in nutrient concentrations in the upper water column (0–200 m) in the Austral spring (September–November) and autumn (March–May), and a minimum in winter (June–August). We also find that the nutrient concentrations in the upper water column are influenced by the position of the EAC jet. Two main modes of variability in the EAC’s position are identified: an inshore mode with jet flowing along the continental slope and; an offshore mode with the current core detached from the continental slope and flowing over the adjacent abyssal plain. The position of the EAC jet influences the location of upwelling of nutrient-rich water at depth (>200 m). For the EAC inshore mode, cooler, nutrient-rich waters are restricted to the area of the continental shelf and slope that is inshore of the EAC. The offshore mode exhibits a wider distribution of nutrient-rich waters over both the inshore shelf and slope and, offshore abyssal Tasman Sea. Our analysis highlights the important interactions between nutrient concentrations and distribution and the highly variable EAC, which has implications for primary production, fisheries, and the biological carbon pump.
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RC1: 'Comment on egusphere-2024-2265', Anonymous Referee #1, 22 Aug 2024
Jeffers et al. use a combination of physical and biogeochemical long-term data sets from moorings and CTD deployments to examine the role of seasonal cycles and East Australian Current flow patterns on the distribution of nutrients in the pelagic zone off the coast of subtropical Eastern Australia. The study is generally well-written and presented and provides a good use of the collected data. I just have some minor comments, as well as some places where I would like the authors to provide some clarifications. Hopefully these will make the revised version of the manuscript also easier to follow by the future readers.
I particularly liked the well-referenced introduction, clearly outlining our current understanding of the system both in terms of physical and biogeochemical oceanography. Between lines 49 and 64, the authors present various published hypotheses on the drivers behind the nutrient variability. I feel like this is exactly what the current study can help elucidate. However, I don’t think you make it that clear in the conclusion which of the previous hypotheses better fits with what your data showed, or if there is a different new finding. I just think a stronger ending without leaving so many open questions would benefit the paper.
In lines 68-71, you are presenting results from the current study in the Introduction section. It’s good to mention what you will be doing without revealing the findings.
Figure 1 could benefit from an inset zoomed-out map to show where exactly on the Australian coast the study area is located. I think you have space for this in the bottom-left corner.
Figure 1 legend – the triangles are yellow, no?
Section 2.2 You already have a section called “Methods” – I don’t think you can have another subsection with the same name
Line 110 – You give the citation for MLD calculation. Can you be more specific and explicitly state in the text what this means (e.g. density difference from surface)?
Figure 2 – Is this the full 10 years of data? Maybe make this clear to the reader? Also, for example, at 154.8, I see two lines in the CTD plot (green and yellow), but I only see green dots in the plot above. Does this mean no samples were collected from the CTD on one occasion? Please clarify. Furthermore, you need units on your axis labels.
Line 118 and Figure 3 – How come? Is the EAC mostly (a majority of the total time; or a majority of the time samples) in the inshore mode making the average being so close to the inshore plot?
Figure 4 – I find myself confused about what the stippling means. In this caption you say it indicates statistically significant departures from the average, but in the methods section you say that stippling shows areas of statistical significance.
Figure 5 could benefit from three labels that show what is shown on each horizontal row: autumn, winter, spring. Figure 5 caption should say “light grey lines”.
Line 178: “there are a few significant changes”
Figure 8: I think you need to explain to the reader what a violin plot is and think about if it’s really useful to show the data in this way. Even in the text, you only very briefly mention this figure but you don’t interpret it almost at all. Is there a significance of the flat base of some violins in subfigures b and c being higher than 0?
Line 204: “Interesting impact” is too subjective.
Line 205 and line 211: don’t these two statements contradict each other? Are the water properties changed or not between the different modes?
Line 207: “with of cooler”? something missing?
Lines 229 and further: while I appreciate putting your results in the context of other studies, I feel like you are drifting too far from your results and instead it turns into a literature review. Please base your discussion more on your findings.
Line 247: That is exactly what you should find out through a study like this one, no?
Line 256: data are needed
Final remarks: You have 10 years of data to work with. While probably not long enough to find statistically significant biogeochemical trends, should you not mention if you have seen any trends at all? Maybe a timeline plot, or a Hovmöller where you have longitude and time on your axes at a chosen depth?
Citation: https://doi.org/10.5194/egusphere-2024-2265-RC1 -
RC2: 'Comment on egusphere-2024-2265', Anonymous Referee #2, 23 Sep 2024
General comments:
This study uses 10-year transect data to investigate the seasonality of upper-ocean nutrients in the EAC region and its variability with the location of the EAC jet. A key finding is the inshore-offshore difference in the nutrient distribution; the position of the EAC jet determines the horizontal (longitudinal) extent of the near-surface nutrient-rich water. Similar patterns were found for the Kuroshio region in a recent study (Hayashida et al. 2023, JGR Oceans: Anomalous Nutricline Drives Marked Biogeochemical Contrasts During the Kuroshio Large Meander), so the existence of the inshore-offshore difference revealed in this study sounds reasonable. I have mainly three suggestions to improve the manuscript: 1) explanation of the mechanism for the inshore-offshore difference in the nutrient distribution; 2) more quantitative assessment of the inshore-offshore difference; and 3) addition of an introductory figure.
1) The near-shore nutrient-rich water west of the EAC jet has been attributed to upwelling (L272), but I am not fully convinced that it is the cause because it is not shown. Can this be shown using data below 200 m (L12 refers to >200 m, but the results presented are restricted to the upper 200 m)? Or is this attribution a speculation based on previous knowledge? In the case of the Kuroshio (Hayashida et al. 2023), it was attributed to the uplifting of nutricline that makes sub-surface nutrient-rich water accessible to the surface via winter convective mixing.
2) The findings about the inshore and offshore modes (L12-14) need to be supported by improved figures. The transect figures (Figures 4,6,7,9) show the inshore and offshore composites of nutrients but none of these show the corresponding locations of the EAC jet, which makes it difficult to understand the relationship between the EAC jet position and the nutrient distributions. Would it be possible to add the locations of the inshore and offshore EAC jets based on composites to these figures? Furthermore, while Table 2 is useful for understanding the probability distribution of inshore vs offshore modes, it would be also useful to produce time series of the longitudinal location of the EAC jet (similar to the Kuroshio axis time series such as Figure 2a of Hayashida et al. 2023). This will help better understand the temporal variations in the EAC jet and link with climate variability such as ENSO (L250).
3) Introduction contains a lot of useful background information on the jets that governs the EAC and extensions. Many numbers (latitudes) are mentioned, but it would be helpful to visualize these details. Figure 1 can be zoomed out to include these details with arrows and lines? It can also denote 154.2 E along the transect used for the jet position definition (L117).
Specific comments:
L20-21: Consider citing a reference relevant to this sentence.
L58: “coastal” is adjective. Suggest adding a word after it or use “coast” instead.
L84: “intensification zone”. It would be helpful to indicate this zone in an introductory figure.
Figure 1: The caption says “black triangles” but they are yellow in the figure.
L104: Suggest deleting “depth” at the end of the sentence.
L106: This reasoning only supports the claim for phosphate limitation. Any reference for nitrate limitation?
L107: This sentence is a bit awkward; silicate limitation could be due to large uptake of silicate by these diatoms. Suggest rephrasing something like “Siliceous diatoms dominate … that contributes to silicon limitation in this region”.
L112: Suggest adding a few words to briefly describe the density-based procedure.
L113: “grouped by season”. It is unclear which months are considered as which season. This could be indicated in Figure 2 caption. (*I noticed these are mentioned later in L131, but they should be mentioned here at first appearance).
L114: “qualitative”. I think this definition is rather quantitative. It seeks the longitude at which the southward velocity between 40-100 m is at maximum.
L116: “was between 40-100 m” should be “between 40-100 m was” because the former sounds like the strongest flow can be located beyond the 40-100 m range, but the method looks for the strongest flow within the 40-100 m range, correct?
L113: The definition of the inshore/offshore mode is a bit awkward and can be written more simply, something like “The EAC is considered as an inshore mode when the strongest southward flow between 40-100 m along the CTD transect is located west of 154.2E. Otherwise, it is considered an offshore mode”.
Figure 2: Markers (a,b) and lines (c,d) often overlap, which makes it difficult to visualize the density of the data coverage. Suggest fine-tuning the figure with the transparency parameter (“alpha” in Python) for markers and lines.
Figure 3 b and c: Since the rest of the analysis is based on the inshore and offshore composites, would it be more consistent and robust to show the composites in these panels too, instead of an example transect?
L144: The past tense “was” is used twice, whereas the rest of the section uses present tense. Suggest changing “was” to “is” for consistency.
Figure 4: The caption says “upper 200 m” but the y-axes are given as “pressure”. Also, the widths of the left panels are narrower than those of the left and right panels as they are squeezed by the presence of the colorbars. It is also hard to read the longitudes as they are close to each other (some possible solutions are to: reduce the number of decimals displayed, tilt the labels, or use fewer ticks/labels). The same goes for the subsequent figures displaying the vertical distributions.
Figure 5: It took me a while to figure out what the grey circles represent. It is a bit misleading to label panel A as autumn, but it shows for other seasons too (and the same goes for other panels). While I understand the benefit of showing all data in grey as background, these panels (A/C/E and B/D/F) are easily comparable without the grey circles because they are shown using the same x and y axes. Therefore, I would suggest removing the grey circles for potential confusion. Also, the caption refers to the isopycnal lines as “light grey”, but they look more like black. Suggest referring to these lines as “black dotted lines”.
Figure 6: Why does the colorbar for nitrate include negative values? Also, the colors range from light to dark for nitrate and phosphate, whereas it goes from dark to light for silicate, which makes the visual comparison counterintuitive. The same goes for Figure 9.
L155: Figure 8 is cited before Figure 7. In this case, the order of these figures should be switched.
L157-159: It is unclear whether these sentences are referring to Figure 8 or Figure 6.
L167 and L170: Missing the closing brackets for the figure citations.
L182: “was” should be “is” for consistency with the rest of the text in the section?
L184-L186: Suggest deleting this paragraph, because it is based on the results not shown and also because part of the results is already mentioned in L175-L177 and the range can be inferred from Figure 7.
Figure 8: It would be helpful to provide a description for violin plots, as I think not all Ocean Science readers are familiar with violin plots, which seems more complex than others like Taylor diagrams or box plots. At the least, please provide the reference where readers can obtain the necessary information to understand these plots.
L205: “the average properties of the water column” is unclear. Which properties (temperature?) and what does it mean by “average”?
L207: “we see evidence of upwelling” is unclear. Was such evidence shown in Results?
L210: “meaning that upwelling in the offshore mode results in higher …” requires information on the vertical location of the nutrient rich water, rather than the horizontal location mentioned in the previous sentence.
L249: I am not sure if it is ok to bring the not-shown-results into discussion. Is there a reason for not showing the results in the paper? I think it would be beneficial to show such time series comparison even though the lack of temporal coverage. If page limit is an issue, it can be included as supplementary information.
L272: “. M” should be “, m”
Citation: https://doi.org/10.5194/egusphere-2024-2265-RC2
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