the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Sep 2025
Monsoons, plumes, and blooms: intraseasonal variability of subsurface primary productivity in the Bay of Bengal
Abstract. During the southwest monsoon, seasonal storms bring torrential rainfall to the South Asian subcontinent and the northern Indian Ocean. Dense cloud cover limits the amount of sunlight that reaches the ocean surface, and sediment-laden river runoff limits the depths to which light can penetrate. Changing light availability should affect phytoplankton primary productivity and its dependent biogeochemical processes, yet little is known about how subtropical weather is linked to ecosystem processes below the ocean’s surface. Here, using novel physical and bio-optical measurements from an array of free-drifting, autonomous systems in the Bay of Bengal, we show that the onset of cloudy conditions associated with 'active' monsoon conditions led to >50 % reduction in gross chlorophyll productivity (GCP) near the subsurface chlorophyll maximum (SCM) relative to sunny 'break' conditions. Optical backscatter measurements confirm chlorophyll fluorescence fluctuations correspond to biomass variability of a similar scale. Simultaneous bioacoustic measurements collected onboard the autonomous platforms suggest this intraseasonal variability in SCM chlorophyll and biomass generated a response in higher trophic levels. Long-term measurements from biogeochemical (BGC) Argo floats in the bay confirm the presence of intraseasonal oscillations in chlorophyll-a concentration with days-to-weeks variability in magnitude similar to the regional annual cycle in the region. Our findings demonstrate that intraseasonal subtropical air-sea variability modulates important regional biogeochemical ocean processes in the Northern Indian Ocean with implications for the Indian Ocean carbon cycle.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-4206', Anonymous Referee #1, 30 Oct 2025
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AC1: 'Reply on RC1', Tamara Schlosser, 04 Nov 2025
We thank the reviewer for their constructive feedback. We have carefully reviewed all comments and agree with the reviewer's suggestions for clarifying the text and figures. We will improve the manuscript accordingly.
Part of the comment related to the supplementary materials is missing so we request the reviewer clarify the following incomplete comment: “Firstly, it’s not clear why you use rather than ? A conversion factor would be involved, but the numbers would only be different by .”
Citation: https://doi.org/10.5194/egusphere-2025-4206-AC1 -
RC3: 'Reply on AC1', Anonymous Referee #1, 06 Nov 2025
Apologies, I wrote my review in a word document and the units rendered in the equation editor did not copy to the online platform, which I didn't notice. I have attached the word document and hope this clarifies the issue.
The specific sentence referred to was:
In the supplementary materials, you state that I is the vertical change in irradiance, with units \micro \watts \per \square \cm. Firstly, it's not clear why use use \micro \watts \per \square \cm rather than \watts \per \square \metre? A conversion factor would be involved, but the numbers would only be different by 10^-2.
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RC3: 'Reply on AC1', Anonymous Referee #1, 06 Nov 2025
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AC1: 'Reply on RC1', Tamara Schlosser, 04 Nov 2025
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RC2: 'Comment on egusphere-2025-4206', Anonymous Referee #2, 05 Nov 2025
This study provides valuable insights into how the active and break phases of the South Asian monsoon influence subsurface chlorophyll in the Bay of Bengal, while also highlighting diel variability in bio-optical properties. Overall, the manuscript is well written and presents a comprehensive dataset of physical and bio-optical observations. I have provided several major and minor comments below aimed at clarifying methodological details, improving figure presentation, and strengthening the interpretation of the results.
The field campaign deployed three DBASIS systems. Do all three systems include the same set of bio-optical measurements? If so, it is unclear why the authors present only the ChlF data from M3. The methods section should be clarified to explain this choice. If all systems collected comparable measurements, it would be more informative to present averaged results or to justify focusing solely on M3. The methods section is difficult to follow and does not clearly convey these details.
The observations were conducted during the break and active phases of the southwest monsoon. Please provide the corresponding atmospheric conditions (just values) for these periods in the Results section, for example, wind speed, net heat flux, and cloud cover. This would help readers better understand the background atmospheric conditions associated with each phase.
Chlorophyll concentration is primarily influenced by nutrient availability and light conditions. It is therefore unclear why the authors used an isopycnal criterion to identify the SCM instead of locating it directly from the maximum in the chlorophyll profile. The rationale for choosing the isopycnal based approach should be clarified and justified in the manuscript.
The authors calculate ITL depth as the depth where the temperature is ≥ 1oC from the surface value. What is the interpretation or treatment when the temperature difference is equal to1°C?
It is unclear why GCP is emphasized in this study. Since the field campaign directly measured bio-optical properties, the estimated GCP may carry considerable uncertainties. What would be the outcome if integrated ChlF were used instead of GCP? Does GCP provide additional information beyond what is captured by ChlF? The manuscript should include a justification for using GCP and clarify the added value it brings to the analysis.
The DBASIS array was deployed at the intersection of two eddies, but the presence of these eddies is not clearly visible in any of the figures. Additionally, the color bar used for chlorophyll is confusing and may hinder interpretation. Consider using a different color bar to improve clarity. Both chlorophyll and turbidity extend below 80 m depth on the initial date. Is this location near the coast? Please provide the actual depth at the measurement site. How do the authors determine that this signal is associated with a coastal plume rather than another source? Clarifying these points will help readers better interpret the observations.
At the SCM, ChlF was initially relatively low and then doubled after the drifting platform moved away from the plume. This reflects the ChlF conditions between two distinct regions. Please highlight this observation in the manuscript, as it helps illustrate the spatial variability in phytoplankton distribution. Since the observations cover two distinct regions - one influenced by a coastal plume and one without, it would be helpful to include a satellite chlorophyll-a map with bathymetry contours and the observation locations overlaid. This would allow readers to easily visualize and understand the spatial distinction between the plume and non-plume regions.
On July 17, low-salinity water was observed within the top 10 m, after which the MLD became shallower. Could this be due to a rain-induced surface layer? Is it plausible for rainfall to generate a low-salinity layer with 10 m thickness?
The manuscript mentions a diel cycle of the derived gross production and loss terms in the Discussion, but this is not clearly shown in the Results section. Please include evidence or plots in the Results to support this point. Additionally, it is unclear how the authors conclude that the SCM represents a maximum in biomass—this needs to be clearly demonstrated in the Results section.
Figure 7b shows clear diel variability, and I am curious how the authors identified vertical migration of zooplankton from this figure. Providing a brief explanation in the manuscript would be very useful for readers who are not familiar with this type of observation and help interpret the biological patterns more clearly.
Use ChlFSCM instead of SCM ChlF, similarly SCM PAR as PARSCM.
What is phase-locking of diel cycle with irradiance?
Figure 4c is a repetition of Figure 1d. Add ML, ILT to Figure 1d.
Why does Figure 2b start at 20 m depth, whereas Figure 1d shows data from the surface? Additionally, Figure 2b shows some black areas at the top—please clarify what these represent. Consistency or explanation of the depth ranges would help readers interpret the figures more clearly.
Check for typos in the manuscript, for example - 40% percent (“percent” is not required here)
Figure 1b has some unwanted axis labels (30)
I could not find the time derivatives of chlorophyll in the manuscript.
After the buoys moved away from the plume on the fourth day, surface PAR was also high, which may have contributed to the increased PAR at the SCM. It would be helpful if the authors could provide vertical profiles of PAR for each condition to better illustrate the light environment influencing the SCM.
Where was the GCP calculated - at SCM specifically, or over the entire water column? Please clarify this in the Methods or Results section to avoid confusion.
Add y-axis labels on Figures. For example Figure 4 d to f.
Please specify the height at which the buoy’s meteorological package measured the atmospheric parameters.
Citation: https://doi.org/10.5194/egusphere-2025-4206-RC2
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Overall, this is a well-written paper with solid analysis and provides some useful new insights into the role of intraseasonal oscillations in driving variability in chlorophyll fluorescence and gross chlorophyll productivity at the subsurface chlorophyll maximum. I have found no major issues with the analysis or interpretation, but I have highlighted some minor revisions below.
L35 – should “oscillate” be “oscillates”?
L99-101. “We note fitting a log-transformed curve…” – I don’t understand this sentence. You didn’t fit a log-transformed curve, you fitted an exponential curve. You also refer to supplementary materials, but I don’t see any discussion of the precision error being “not equally weighted” in these supplementary materials. Please clarify this sentence.
Supplementary material and units of I. In the supplementary materials, you state that I is the vertical change in irradiance, with units Firstly, it’s not clear why you use rather than ? A conversion factor would be involved, but the numbers would only be different by . Secondly, if this is a vertical change in irradiance then shouldn’t these units be divided by vertical distance (e.g., per metre)?
L134: “Corrected ChlF was small with the 9th percentile of 0.016 [micrograms per L]” – do you mean that the correction to ChlF was small? Figure S2 seems to support this, but actual values of ChlF were much larger (order 1).
L181 – Why is the SCM at a deeper/denser isopycnal in the BGC argo float data than the wirewalker observations?
L202: “deployed at the intersection of two eddies” – is this shown anywhere? How do you know this to be the case?
Fig. 2. I find it very hard to follow which day is which from the bottom panel to the top panel. I think this figure would be improved if panels (a) and (b) were switched, so that the daily-averaged values in panel (e) could be more easily linked to the raw values in (a).
In addition, Fig. 2 has numerous instances of poor formatting – in particular, the title text for several panels overlaps with the plotting area. The colorbar for panel (b) is also not labelled.
L221: “The time-derivative of SCM ChlF” – is this shown anywhere? Perhaps a better way to phrase this would be to say that PAR is maximum at this time and the SCM ChlF typically increases rapidly, as seen in Fig. 2c.
L239-244: For a while, I was confused by this paragraph as I thought subsurface PAR was PAR at -0 m (I know you refer to the latter as surface PAR in water, but hopefully you can see where the confusion comes from). I would suggest using SCM PAR rather than “subsurface”, especially here.
L256: “temperature <= 1°C the sea surface temperature” – insert “lower than” or similar.
L257: “density increase… corresponds to a temperature increase of 1 °C” – I think you mean decrease? Increasing temperature decreases density!
L261: “July 17.8” – typo?
L264-269: I’m not convinced by this argument, I think it needs more evidence. From what I can see, the SCM ChlF is not strongly affected by the shoaling of the MLD. There is almost no effect of the MLD shoaling from day 16-18 on SCM ChlF – this only increases after 20 July as the MLD increases in depth. Therefore, the final two sentences of this paragraph seem very speculative.
L315: “from southern BoB” –> “from the southern BoB”