Extensive Marine Heatwaves at the Sea Surface in the Northwestern Pacific Ocean in Summer 2021
Round 1
Reviewer 1 Report
Please see the attached file for more details.
Thanks.
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Author Response
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Reviewer 2 Report
Kuroda and Setou identified an intense marine heat wave (MHW) in the northwestern Pacific Ocean. This warm event is considered as the largest in the intensity and extent since 1982. The MHW develops around 10 July, peaks around 30 July-1 August, recede in August. The topic is very interesting to the community and the manuscript is well structured. The description and explanation are clear and reasonable. I would like to suggest a minor revision for this work.
General comments.
- Add the new references in the introduction part. The following study also reported an extreme event in 2020 in the northwestern Pacific Ocean. Please cite this study and also describe the difference between these two extreme events.
Hayashi, M., Shiogama, H., Emori, S., Ogura, T., and Hirota, N. (2021), The Northwestern Pacific Warming Record in August 2020 Occurred Under Anthropogenic Forcing, Geophysical Research Letters, 48(1), e2020GL090956.
- The core area of MHW seems expands to the entire Sea of Japan (Fig. 1d). However, the box in the Fig. 1a which is used to define the index does not include the Sea of Japan. Could the author explain this in the manuscript? If there was not specific reason, it might be better to extend the box westward until the continent.
- Is there any reason that the authors choose to use the ocean temperature at 20 m depth? If not, I would suggest at least using 50 m (slightly below the mixing-layer depth). The oceanic surface boundary layer is strange to me as an oceanographer. I would suggest using the mixing-layer depth. I also could understand if the authors have difficulties to provide the mixing-layer depth.
Minor comments;
- Line 13, were apparent --> were observed
- 1b, reset the colorbar from 0-200 days to probably 0-100 days. The largest days in the Fig. 1b does not exceed 80 or 100, in my eye.
- From Fig. 1c and 1d, the MHW starts from coastal area of Okhotsk (138E, 58N), then extrends to the south and east. The authors should describe this spatial-temporal variability which would help the readers to understand the process.
- 2a, it might be better to identify the extent of anomaly if the author could add the +2 std line for the red and blue lines (+2 std is okay, no need for the -2std).
- 2b, I would suggest adding the line for the composite mean of anomaly in year 1982-2018 (or the 2std of variability, or confidence interval). This could help the readers to understand the extreme departure of 2021 to the normal variability.
Author Response
Reply file is attached.
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper is very interesting because it explores the mechanisms responsible for the genesis of marine heatwaves (MHWs) that occurred in July-August 2021 at the sea surface over areas of the northwestern Pacific Ocean. The authors focus on a focal surface at the northwestern Pacific. The different heat fluxes across the ocean surface are carefully examined, as well as the meteorological conditions which prevailed during this transitory and quite exceptional phenomenon which led to the formation of MHWs.
These are therefore very recent phenomena that occurred barely more than a month ago, highlights both the responsiveness of the authors, quite remarkable, but also of the journal review process.
However, obviously the authors are not familiar with the long-period Rossby waves winding around the subtropical gyres, what are named Gyral Rossby Waves (GRWs): http://climatorealist.neowordpress.fr/long-period-gyral-waves/.
The focal region within 143°E–180° and 40°–50°N straddles the GRWs. The area of positive oceanic surface boundary layer anomalies in the focal area (Figure 6a) corresponds to the inner edge of the north Pacific subtropical gyre. To be understood, the feedback process from the ocean to the atmosphere that contributed to localized enhancement of MHWs (Lines 282-285) should involve GRWs around the north Pacific subtropical gyre.
According to the theory of GRWs, the efficiency of radiative forcing results from the imbalance between incoming and outgoing fluxes through the surface of the ocean. In the absence of resonance of GRWs the inflow is balanced mainly by evaporation. On the other hand, lowering of the thermocline and acceleration of the convergent radial current while the intensity of forcing is increasing makes that the GRW behaves as a heat sink because of downwelling. Half a period later, uprising of the thermocline and acceleration of the divergent radial flow while forcing intensity is decreasing makes the GRW returns the accumulated heat: it then acts as a heat source due to upwelling. The imbalance between incoming and outgoing fluxes is all the higher as the oscillation of the thermocline is larger, reinforcing the positive feedback acting on the speed of the polar current while reducing the temperature gradient between low and high latitudes of the gyre.
The present case is about fast transient phenomena completely unknown, and which would deserve an in-depth study involving GRWs to try to explain feedbacks, and dephasing between forcing and responses of the subsurface sea temperature. Such a work is hardly conceivable within the framework of the present study which must be published as soon as possible. But the authors should at least consider the interest of such deepening in the discussion and cite the relevant works appropriately.
Author Response
Reply file is attached.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
The exchange with the authors is rich and can open up new perspectives. They should not hesitate to ask me in their future research as they deem useful.
The addition made in the text is relevant.
