Marine heatwaves in the Arabian Sea

Chatterjee, Abhisek; Anil, Gouri; Shenoy, Lakshmi R.

doi:https://doi.org/10.5194/os-18-639-2022

Articles | Volume 18, issue 3

https://doi.org/10.5194/os-18-639-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/os-18-639-2022

© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 18, issue 3

Research article

| Highlight paper

|

10 May 2022

Research article | Highlight paper |

| 10 May 2022

Marine heatwaves in the Arabian Sea

Abhisek Chatterjee, Gouri Anil, and Lakshmi R. Shenoy

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Final revised paper (published on 10 May 2022)
Preprint (discussion started on 11 Jun 2021)

Interactive discussion

Status: closed

RC1:
'Comment on os-2021-52', Anonymous Referee #1, 11 Sep 2021

The comment was uploaded in the form of a supplement: https://os.copernicus.org/preprints/os-2021-52/os-2021-52-RC1-supplement.pdf

Citation: https://doi.org/10.5194/os-2021-52-RC1
- AC1: 'Reply on RC1', Dr. Abhisek Chatterjee, 09 Jan 2022
  
  The comment was uploaded in the form of a supplement: https://os.copernicus.org/preprints/os-2021-52/os-2021-52-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/os-2021-52-AC1
RC2:
'Comment on os-2021-52', Anonymous Referee #2, 13 Oct 2021

This article examines the history of marine heatwaves (MWHs) in the Arabian Sea over approximately the past 40 years. The authors are motivated by the fact that few studies of MHWs in the Arabian Sea have been carried out, yet they have significant impacts on marine ecosystems and fisheries. The paper is well organized, interesting and informative. It may be worthy of publication subject to revision, especially with regard to the first three points below.

1) I don't get a sense of what a marine heat wave looks like in the Arabian Sea. Statistics are presented in terms of trends, correlations, and time series. For the three stated metrics (frequency, duration, amplitude), please make a table that compares these for the period before and after 2000. Also, provide a description of one exemplary heat wave after 2000 (spatial structure, time history, etc).

2) The heat balance analysis is misleading because it is only an analysis of MWHs when they last an entire season, which is rare (only 2016 according to Figure 2). It is an analysis of the background conditions on which MWHs develop, but the MHW itself may arise through different processes that are not same as occur in a seasonal mean heat balance. This point needs to be clearly articulated.

3) Line 109-110. The model has been validated for other purposes than the study of MHWs. Please quantify the level of agreement between observed and modeled SSTs in Figures 7 and 8. Also, how sensitive are results to use of different flux forcing products?

Other:

*Abstract, line 10. "The Indian Ocean received almost no attention..." This statement is not correct. The term "marine heatwave" was coined to describe the Ningaloo Nino that occurred off Western Australia in 2011. More appropriately, the Arabian Sea has not received much attention.

*Related to the comment above, somewhere in the discussion in lines 34-52, Ningaloo Ninos, and the mechanisms responsible for them, should be mentioned (Feng et al, 2013).

*Line 177. "...coincide with the El-Niño year or the year next to the El-Niño year..." This is confusing. El Nino events typically span two calendar years, e.g. 2015-16. So it is not clear what El Nino year (singular) means. Likewise, the second year of an El Nino is part of the same El Nino.

*Lines 260-62. It needs to be mentioned here that the IOBM is often forced by ENSO via the atmospheric bridge, so that the results for the IOBM automatically contain ENSO effects.

*Lines 270-74. Same point as immediately above: the IOD is often forced remotely by ENSO.

*Figure 1. Say over what period the trends are computed.

*Figure 9. Identify in some way (different color?) the track of the storm in panel (a) that is highlighted in panel (b).

*There is need for editing. For example, "makes this regions remained unexplained" (line 331), "wracked" (line 376), "stapling" (Figure 1 and 6 captions) are incorrect words or grammar.

Reference

Feng, M., M. J. McPhaden, S.-P. Xie, and J. Hafner, 2013: La Niña forces unprecedented Leeuwin Current warming in 2011. Nature Sci. Repts., 3, 1277, doi 10.1038/srep01277.

Citation: https://doi.org/10.5194/os-2021-52-RC2
- AC2: 'Reply on RC2', Dr. Abhisek Chatterjee, 09 Jan 2022
  
  The comment was uploaded in the form of a supplement: https://os.copernicus.org/preprints/os-2021-52/os-2021-52-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/os-2021-52-AC2
- AC3: 'Reply on RC2', Dr. Abhisek Chatterjee, 09 Jan 2022
  
  The comment was uploaded in the form of a supplement: https://os.copernicus.org/preprints/os-2021-52/os-2021-52-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/os-2021-52-AC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Dr. Abhisek Chatterjee on behalf of the Authors (19 Jan 2022) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (04 Feb 2022) by Neil Wells

RR by Anonymous Referee #1 (16 Feb 2022)

Suggestions for revision or reasons for rejection

Line-based comments (tracked changes document)
L32/33: Replace ‘These extreme warm SST conditions were’ with ‘The term “marine heatwave” was’
L38: Replace ‘it’s’ with ‘their’ as MHWs is plural here
L46: replace ‘responsible’ with ‘to be a major driver’ as other can also play a role for Ningaloo Nino
L60: delete ‘Unfortunately’ at beginning of sentence
L71: delete ‘also’
L133-150: I think the additional information provided with respect to the physical processes in the two seasons is generally useful but might be slightly too detailed out of context at this point where no results have been shown yet. I would instead add explanatory sentences regarding the circulation when the actual MHW trend pattern in the different seasons is described. For example add a short discussion at the end of the paragraph finishing at L172.
L143: modify ‘…via advected by the coastal currents…’ e.g ; ‘Intrusions of freshwater via the coastal current’ or ‘freshwater advected by the coastal current’
L164: This still leaves 40% for other seasons which is a non-negligible amount.
L182: ‘..turned much prolonged…’ is still not correct use of language. Why not just say ‘the duration of heatwaves has increased in the recent decade’?
L192-193: add that El Nino and +IOBM alter the background state to warmer temperatures thus potentially explaining these exceptions. This may be a personal style but as a reader I find this kind of information at this point very helpful.
L206/207: Delete ‘Interestingly’ and rephrase if necessary. As mentioned in the first round of revision “Per MHW definition this is causation and should not be presented as a novelty”. It is fine to state the connection as the authors draw the correct conclusion in the next sentence that this increase in MHWs is likely driven by an increase in mean SST.
L225ff: change to “The use of a fixed climatological baseline in this (and other) studies inherently means that a rapid warming in the recent decade shifted the mean SST 226 towards the heatwave threshold”
L242: you mean 2016 was the second largest or strongest event? ‘most’ does not make sense here
L326: Consider changing the section title to something like: ‘Dynamical mechanisms inferred from a case study’
L344: Do you imply that the onset of summer monsoon clouds reduce the shortwave radiation and thus contribute to decreasing temperatures/ a termination of the MHW? If so, add that explanation.
L400: ‘The warming peaks in late April to early May’ which plot are you referring and are you only talking about the model now in this paragraph?
L406: delete ‘was’
L405ff: So does the model exactly reproduce the cyclone? I.e. is it included in the forcing as discrete event? It would maybe be helpful to state in the methods which temporal resolution the atmospheric forcing of the model has. Do you think the MHW contributed to the generation of the cyclone? I Somewhere here it might be helpful to mention the absolute temperatures and conditions that are needed for a cyclone. Also is this the peak season for cyclones as the temperatures reach their maximum in June?
L411ff: Is it really the cyclone or the shift into the summer monsoon season and an associated change in winds/circulation? I guess both.
L419: adjust to the fact that you are doing a case study now instead of a general budget
L458/459: So this suggest that generally the onset of summer monsoon winds is a mechanism that terminates MHWs in the pre-monsoon region? I think it is an interesting and important discussion if the strong seasonality associated with the monsoon is a limiting factor for the duration of MHWs in each season?
L469: In terms of cyclogenesis, would it make sense to come up with a new metric such as an X amount of days above a certain absolute temperature?

Figure 8
Are these snapshots of SST or weekly averages? This should be stated in the caption
Figure 10
Would it make sense to also show the modeled climatology or threshold?
Figure 15
There is a (b) label in the figure but not panel (a)

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RR by Anonymous Referee #2 (08 Mar 2022)

ED: Publish subject to minor revisions (review by editor) (22 Mar 2022) by Neil Wells

AR by Dr. Abhisek Chatterjee on behalf of the Authors (28 Mar 2022) Author's response Author's tracked changes Manuscript

ED: Publish as is (04 Apr 2022) by Neil Wells

Short summary

Marine heatwaves refer to discrete, prolonged warm ocean conditions known to cause severe destruction in marine ecosystems. We find that coastal waters off the west coast of India have experienced a rapid multifold increase in heatwave days since the early 80s. This resulted in more frequent and longer marine heatwave events in the last decade. We show that the rapid warming in the Arabian Sea in the last decade is the primary cause of the observed enhanced heatwave events in this basin.