the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Oct 2024
The satellite chlorophyll signature of Lagrangian eddy trapping varies regionally and seasonally within a subtropical gyre
Abstract. Vertical motions of mesoscale ocean eddies modulate the resource environment, productivity, and phytoplankton biomass in the ocean's subtropical gyres. The horizontal circulations can trap or disperse the eddy-driven chlorophyll anomalies, which can be observed from space. From two decades of satellite remote sensing observations in the North Pacific Subtropical Gyre (NPSG), we compared the chlorophyll anomalies within "leaky" eddy boundaries identified using an Eulerian Sea Level Anomaly (SLA) method, and within strictly coherent "trapping" bounds derived from Lagrangian particle simulations. On average, NPSG Lagrangian coherent vortices maintain stronger chlorophyll anomalies than Eulerian SLA eddies due to the limitation of lateral dilution. This is observed in both cyclones and anticyclones. However, there is variability in the biological signature of trapping by sub-region and season. Eddy trapping of positive chlorophyll anomalies is most significant in the southern regions of the NPSG, counter to expectations from the latitudinal trend of the nonlinearity parameter. We found weak relationships between eddy age and the magnitude of surface chlorophyll anomalies in most observations of long-lived Lagrangian coherent vortices with the strongest exception in wintertime anticyclones in the Lee of the Hawaiian Islands. These results challenge the assumption that Eulerian-identified mesoscale eddy boundaries are coherent and suggest that Lagrangian trapping, combined with regional and seasonal factors, shapes the chlorophyll concentrations of subtropical mesoscale eddies.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3211', Anonymous Referee #1, 18 Dec 2024
The paper investigates the role of mesoscale ocean eddies in shaping chlorophyll-a distributions within the North Pacific Subtropical Gyre (NPSG) by comparing Eulerian and Lagrangian methods for eddy identification. Using two decades of satellite observations, the authors demonstrate that Lagrangian Coherent Vortices (RCLVs) maintain stronger chlorophyll anomalies compared to Eulerian-identified eddies due to limited lateral dilution. The study reveals significant regional and seasonal variability, highlighting distinct patterns in chlorophyll anomalies among northern, southeastern, and Hawaiian Lee Eddies. This research challenges traditional assumptions about mesoscale eddy trapping and provides valuable insights into their biogeochemical impacts.
The paper presents a well-rounded introduction enriched with relevant references, offering a concise and clear overview of the state of the art. That being said, I suggest the authors reconsider whether it is necessary to split the introduction into three different sections. I believe it would work better as a single section without subsections.
The limitations of each method are clearly stated and well-argued. The results are well-scoped, the figures are of high quality, and future steps are clearly defined. Some results are supported by illustrative sketches, which enhance clarity and help readers visualize complex processes. The discussion is extensive, well-founded, enriched with relevant references, and solid in its argumentation.
My recommendation is acceptance if the minor revisions are successfully addressed to ensure that the present results are confirmed to be robust against error analysis (see Major comments).
Minor comments:
- In the abstract, every key concept is briefly explained; however, non-expert readers may struggle to understand what the authors mention in lines 10–11 unless a brief explanation of the nonlinearity parameter is provided.
- In the abstract, line 13: I suggest the authors explicitly state the finding. Is it a positive or negative relationship? The answer seems to be in lines 322–323. Please avoid leaving open questions or ambiguity about your results in the abstract.
- Line 128: Which figure are the authors referring to? Please clarify.
- Lines 380–381: I suggest the authors properly frame the results to the study region in these lines. While the findings are suggestive of broader applications, the authors cannot extend these conclusions globally without demonstration.
- Figure 2: The caption could benefit from more details about the zoomed-in view shown in the right-hand side panel.
- Figure 6: Increase the grid resolution to improve the readability of values in the composite subplots.
- Figures B6, B7, and B9: The percentages are difficult to read due to overlapping text. Adjust the layout to improve clarity.
Major comments:
- There is an important aspect that the authors have omitted in their work and that requires attention. To increase the robustness of the results and the significance of the observed distinct patterns, I suggest the authors include an error analysis to better assess the magnitude of the differences discussed. On many occasions, the differences presented are very small, yet they are claimed to be significantly distinct. Since the values discussed here are not straightforward or intuitive for routine daily use, readers need background information about the errors to properly interpret and contextualize the differences being reported. Please provide information on this. The current version lacks on any information about error analysis.
- I believe the authors should expand the conclusions section. It should be self-contained and comprehensible when read in isolation. In its current form, it omits many relevant results, limitations, and future perspectives. Please enrich the conclusions with more details (drawn from the discussion section) to provide a comprehensive closure to the study.
Citation: https://doi.org/10.5194/egusphere-2024-3211-RC1 -
RC2: 'Reply on RC1', Anonymous Referee #1, 27 Dec 2024
Dear Authors and Editor,
I would like to clarify a point in my comments. When I wrote:
"My recommendation is acceptance if the minor revisions are successfully addressed to ensure that the present results are confirmed to be robust against error analysis (see Major comments)."
I actually meant to say: "... if the major revisions are successfully addressed ..."
I believe it is clear this was a mistake, given that I raised Major comments, but I wanted to explicitly clarify it just in case.
Citation: https://doi.org/10.5194/egusphere-2024-3211-RC2
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RC3: 'Comment on egusphere-2024-3211', Anonymous Referee #2, 15 Jan 2025
This paper investigates the chlorophyll-a signature of ocean eddies in a subtropical gyre using combined eulerian/lagrangian eddy atlases and analyses based on conditional Probability Density Distributions (PDD) of climatological chlo-a anomalies, both applied on satellite data. It is clearly written and presented in an intelligible and didactic manner that made the reading very enjoyable. I also appreciated the honesty and integrity of the presentation. It introduces a well-thought and powerful methodology (that would allow exploring further these biophysical interactions in many different oceanographic settings) containing plenty of (useful for reproducibility) details. Finally, it reports several novel and synthetically-presented results that exemplify the complexity of detecting ocean eddies and their biological responses (yet, common patterns emerge), which reveal the sensitivity of the results to the methodology used, which (importantly) show that the non-linearity parameter is not predictive of Lagrangian coherency, and which typify ocean eddies in terms of their dynamics and associated chlo-a signatures. While the results were already reasonably well discussed, I suggested some developments below. All in all, I recommend a minor revision to address the points raised here.
Major points
- definition of the studied region (sect. 2 lines 90-97): I understand the rationale (and usefulness when studying such complex coupled mechanisms) of the restricted region of study chosen by the authors. Yet, I wonder if a slightly larger domain (in both latitude and longitude, or essentially in longitude considering the predominant zonal eddy trajectories) would have allowed the authors to track eddies for longer time, hence providing more statistics and potentially slightly affecting the overall results (relying on PDDs). Could you please comment on this? The other reason for which I’d be curious to see similar analyses performed over a larger domain would be to investigate how such bio-physical coupled dynamics would evolve depending on the oceanic provinces considered. The ‘domain choice’ was essentially based on “physical oceanography” arguments; conversely, it could have been made using “biological arguments” and/or following “biogeochemical provinces” (see for instance Reygondeau et al. 2013 GBC). Even without considering such level of details, one could split, at first order, the North Pacific basin into two regions: an “eutrophic” domain (e.g. coastal and equatorial upwellings as well as shelf areas) and an “oligotrophic” domain (e.g. the rest, excluding the very high latitudes). Previous studies that analysed the interplay between eddies and chloro-a (primarily in major upwelling systems, but also considering the link with the nearby oligotrophic gyres, such as Rossi et al. 2008 GRL, Rossi et al. 2009 NPG, Gruber et al. 2011 NatGeo, Hernandez-Carrasco et al. 2014 DSR) discussed several coupled mechanisms that hold over different oceanic settings. Previous work also tried to contrast and reconcile both positive (enhancement) and negative (depletion) effects that have been documented in the literature; such perspective at basin-scale would surely bring new insights. Indeed, a larger studied region would have allowed the authors to expand their regional/seasonal analyses (sect. 3.2, which I found very interesting) to larger portion of the ocean surface, including several oceanic biomes and potentially supporting distinct coupled mechanisms. I suggest to add a paragraph of discussion (with refs) in sect. 4 and a perspective (sect. 5) along these lines.
- Parameters of the algorithm parameters (“32 days” time-scale): I understood that the youngest eddies contained in the original Lagrangian Atlas were 32 days old, and that this has been modified here using new Lagrangian particle experiments to allow tracking eddies sooner, i.e. from when they are 8-day old onward (sect. 2.1.2). I wonder however how the physical and geometrical connections are ensured for a given eddy when associating earlier images (from 8-days to 32 days, deduced from one set of particle trajectories) with later images (32 days and older, deduced from another set of particle trajectories, if I understood well Fig. B2). Lagrangian models are sensitive to initial conditions, so would the results be the same if the atlas would have been computed upfront from a single set of trajectories covering all affordable eddy’s lifetimes from 8-day onward? Would this affect global statistics of eddy lifetimes?
- The authors generated a Eulerian atlas by imposing a minimum lifespan of 32 days to match the original version of the Lagrangian Atlas and then perform a neat cross-comparison. They used daily input SLA data but that were first reduced to an 8-d frequency to match other input data, which make sense to me. It is worth noting that the daily gridded CMEMS products have already been numerically interpolated in space and time, and that the mean frequency of altimeters revisiting the same oceanic location is of the order of a week or more. A short discussion point could be added to emphasize the fact that upstream numerical treatments applied on input data may affect downstream physical analyses such as transport estimates and eddy detection (Capet et al. 2014 GRL). Similar biases could arise from the missing data in chlo-a maps; this has been already acknowledged in Appendix C but I suggest to emphasize it in the main text.
- Another methodological interrogation concerns the parameter called “eddy disappearance”, set on 3 days here. Would other values affect substantially the overall eddy statistics? I do not understand well how this parameter could (or not) affect statistically the number and ages of detected eddies. One could consider adding basic statistics to explore the sensitivity of the Eulerian atlas to a few values of the “eddy disappearance” parameters. Could you please comment on this? More generally, it would be useful for readers to show simple statistics (to be reported in Appendix C for instance) derived from the total numbers of detected eddies (by eddy type) and their lifetime in the different atlases (e.g. eulerian, lagrangian v1, lagrangian v2). I wonder for instance if the min/mean/max ages are statistically similar when comparing Eulerian and Lagrangian approaches.
- The temporal resolution of most input data (“8 days” frequency) would prevent the authors to analyse fast biological dynamics (such as fast-reacting but short-lived blooms of pico-/nano- phytoplankton, sub-daily photo-acclimatation, intense grazing events, viral shunts, etc…), despite the fact that such processes have been documented in the ocean. Physically talking, it may also be missing rapid (sub-weekly) changes of coherency (or trapping ability) of the detected eddies, as the dynamics of some eddies may evolve fast due to wind-events and/or eddies merging, etc… (see for instance Fig. 4 of Froyland et al. 2015 Chaos). I suggest to add a few lines of discussion in a potential new sect. “BGC discussion” (see below) and in sect. 4.3, respectively.
- Somewhere in sect. 4, I suggest to refer to and discuss the results of Hernandez-Carrasco et al. 2018 SR. The main point that seems relevant to discuss here is the fact that hydrodynamics may enhance/deplete phytoplankton through active mechanisms (e.g. affecting vertical dynamics so that it would fuel the euphotic zone with new nutrients, or conversely deplete surface layers, then affecting plankton growth rates) but it could also just re-organise spatially (e.g. passive horizontal aggregation, vertical deepening/shoaling of chlo-a patches without growth, etc…) the surrounding chloro-a standing stocks.
- In sect. 4, I suggest to discuss the fact that ocean colour data sensed by satellites might be missing a non-negligible part of the biological responses in these oligotrophic gyres (especially in these eddies in which the pycnocline deepens) as phytoplankton maximum are sometimes organized a deep chloro-a maximum. I suggest the authors to slightly more develop this point (already written in Appendix A) and transfer it to the main Discussion sect. 4 (introducing a new “BGC” subsection?).
- In sect. 4.2, I suggest to add a discussion point on a possible reason that could be invoked to explain why the biological signatures of Hawaiian Lee eddies differ quite substantially from the rest of the gyre: it consists in the biogeochemical enrichments (of macro and/or micro-nutrients) and/or biological seeding (e.g. bringing new coastal species into open-ocean waters) of the depleted open-ocean waters when they flow over the (narrow) continental shelves and at a close proximity of the islands’ coastlines. This effect to-date has been essentially studied in the Southern Ocean (the lee of the Crozet archipelago for instance, where macro-nutrients are in excess) but it is possible that the Hawaiian archipelago would also release some elements/plankton community in the water column that affect downstream responses. This would fit nicely in the new suggested BGC discussion subsection.
Minor points
- The three first sentences of Sect. 1.3 need to be supported by one (or more) references. Otherwise they should be rewritten.
- p. 6 line 128: refer to Fig. B1
- p. 22, suggest harmonizing the wording (‘backward-in-time”)
Refs cited:
https://doi.org/10.1038/s41598-018-26857-9
https://doi.org/10.1029/2008GL033610
https://doi.org/10.1002/gbc.20089
https://doi.org/10.5194/npg-16-557-2009
https://doi.org/10.1038/ngeo1273
https://doi.org/10.1016/j.dsr.2013.09.003
https://doi.org/10.1063/1.4927830
https://doi.org/10.1002/2014GL061770
Citation: https://doi.org/10.5194/egusphere-2024-3211-RC3
Data sets
North Pacific Subtropical Gyre RCLV Atlas (version 2) Alexandra E. Jones-Kellett https://doi.org/10.5281/zenodo.10849221
Model code and software
RCLVAtlas Alexandra E. Jones-Kellett https://github.com/lexi-jones/RCLVatlas
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