the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Dec 2025
Mechanism of Storm Surge Induced by Low-Pressure Systems along the Northern Coast of Kyushu, Japan
Abstract. This study aims to clarify the mechanism of storm surge caused by low-pressure systems, which differ from typical storm surges associated with typhoons and have been prominently observed along the northern coast of Kyushu, Japan. An integrated approach was employed by combining the analysis of observational data with numerical simulations using an ocean circulation model. First, based on tide gauge and wind data collected at multiple locations along the Northern Coast of Kyushu, the temporal relationship between storm surge anomalies and wind variations was evaluated. A strong correlation was identified between the rotational component of wind direction and the amplitude of the anomalies. Subsequently, numerical simulations using realistic coastal topography successfully captured the characteristics of coastal wave propagation (i.e. coastal-trapped gravity waves) generated by wind rotation and coastal geometry, and accurately reproduced both the timing and amplitude of the storm surge anomalies in the gauge. Furthermore, numerical experiments using idealized topographic meshes and simplified wind conditions quantitatively assessed how the rotation period of wind and the presence or absence of topographic features affect the resonant amplification or suppression of storm surges. In particular, it was revealed that the Goto Islands contribute to the earlier formation and amplification of storm surges, while the Korean Peninsula suppresses wave amplitude by limiting the spatial extent of wave propagation. These findings contribute to a better understanding of storm surge mechanisms anywhere in the world, including regions with similar geographic settings, and provide valuable insights for enhancing the accuracy of anomaly prediction models and strengthening disaster prevention and mitigation strategies.
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Status: open (until 20 Feb 2026)
- RC1: 'Comment on egusphere-2025-5619', Anonymous Referee #1, 12 Jan 2026 reply
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RC2: 'Comment on egusphere-2025-5619', Anonymous Referee #2, 06 Feb 2026
reply
GENERAL COMMENTS
This is an interesting and relevant study of the mechanisms of storm surges induced by extratropical cyclones (characterised by almost spatially uniform wind displaying rotation over time, as opposed to more localized typhoons) along the northern coast of Kyushu, Japan. A strong element of this work is the combination of data analysis, complex numerical modelling and systematic idealized modelling. From their simulations, the authors compose a schematic illustration of the surge-generating mechanism (Fig.17), with a prominent role for rotating winds of a certain rotation period. The sensitivity with respect to some changes in geometry (including/excluding Goto Islands, Korean Peninsula) is further analysed.
Overall, the paper is well written, with a solid methodology and a convincing analysis and presentation of the results. However, before recommending publication, I need some clarification on (1) Ekman/Kelvin wave claims, (2) role of wind direction in power spectrum analysis, (3) role of spatial variations in atmospheric forcing, and (4) considerations behind the idealised model simulations.
These four major points are elaborated below. Please also consider my list of technical corrections, also touching upon phrasing and presentation style. I look forward to seeing a revised version of the manuscript.
Anonymous, 6 February 2026
SPECIFIC COMMENTS
- Claims on Ekman transport and Kelvin wave type of behaviour in this complex geometrical setting can be further substantiated by carrying out a simulation without Coriolis effect (f=0), or with smaller values of the Coriolis parameter f. However, the authors did not do this. In my opinion, such a more systematic approach is perfectly in line with the idealised model philosophy and would make the interpretation less speculative and thus more powerful. The same goes for the differences between theoretical values of Kelvin wave propagation speed and those obtained in the simulations here. To me, they seem too drastic to be attributed to bottom friction only. Additional simulations without bottom friction could help these interpretations. Please realize that – compared to classical Kelvin wave theory – these highly idealised simulations are still quite complex in terms of e.g. coastlines, water depth, wind forcing.
- I disagree with (or do not understand) the claim on capturing variations in wind direction, made in the text around Figure 8 in Section 3.2. Since only the absolute value of the power spectrum is presented (and not the temporal exchange between its real and imaginary components), it seems to deal exclusively with the magnitude of the wind vector. As a result, the analysis shown here effectively disregards wind direction, making it unable to detect wind rotations, let alone distinguish between fast and slow wind rotations (unless these rotations also display fluctuations in wind speed). Please clarify/adjust. Based on the above, I also disagree with the statements in L216-217, L230-231 and L235-236, at least how they would follow from the preceding analysis. An alternative way to phrase my point: what would the power spectrum (again, plotting only its absolute value) look like for the idealised rotating cases presented later on in Section 4.3?
- As I understand, the realistic topography simulations are forced with spatially varying wind and pressure forcing, whereas the idealised runs are forced with a spatially uniform wind field and no atmospheric pressure gradient. Is that a strong schematisation, i.e. how strong are the actual spatial variations in wind and pressure over the domain? Besides visual inspection of Figures 11 and 12, I would appreciate some discussion on this aspect in Section 5.
- Can you motivate the specific choices of including/excluding Goto Islands and including/excluding Korean Peninsula. Also, the potential effects of removing these geometric features on the atmospheric forcing (reduction of wind speed over land compared to over sea), not included, deserve some attention. Please comment.
TECHNICAL CORRECTIONS (ALSO COMMENTS ON PHRASING AND PRESENTATION)
L1 “which differ” --> “which differs” (mechanism is singular)
L13-15 Final sentence of abstract reads a bit awkward. My suggestion is to rephrase into: “These findings contribute to a better understanding of storm surge mechanisms, not only along the northern coast of Kyushu, but also in other regions around the world with similar geographic settings.”
FIGURE 1 looks nice and is helpful, but (1) land and water are both rather dark, (2) the thin red frame is hard to see on this dark background, and (3) readers not familiar with the Asian geography may welcomes a more zoomed-out version of the top figure showing a bigger surrounding region.
L68 “timing of tidal phases” --> “the timing of the storm surge with respect to high water”?
L84 “surge” --> “Surge” (start sentence with capital)
TABLES 1-3 surely contain all the relevant information, but I was struggling to extract it. Why not merge into a single table, listing all events in chronological order, and then adding for each of the three sites a set of columns displaying time/anomaly/ranking number. This would simplify and clarify matters at least to me, particularly the overview and comparison between the sites. Also, please include typhoon names to the table, particularly “Maysak” as it is explicitly mentioned in the text.
L112 “spring tide or high tide” sounds awkward to me, since spring tide may also mean extremely low water (so spring tide itself does not suggest flooding). Please rephrase.
L113 “overlapped” --> “coincided”?
L127 “z=u+iv” --> “u+iv”. I do not see the need of introducing the symbol z here, which is even confusing since z is usually a vertical coordinate (and in z_a and z_0 also used as such). My suggestion is to omit the “z=”-part as is actually also done later in the manuscript.
L133 I don’t understand what “the same physical schemes” means.
L136 Incorrect citation format; references to Yamashiro et al and Ide et al should be within brackets.
L144 I presume spin up from still water (no flow and zero elevation) at t=0?
L146 “allowing free inflow and outflow of sea level and velocity using open boundary conditions” makes no sense to me. What type of open boundary condition is applied: prescribing water level, prescribing flow velocities, or perhaps a non-reflective Riemann-type of boundary condition. Please explain.
L147 A sigma layering with only three layers sounds rather restrictive to me. Where did you put these? Please comment.
FIGURES 3-4 Scales or coordinates are missing so I cannot see how the domain sizes of the realistic and idealised simulations differ. Please add.
L155 Discrepancy in notation: C_b versus C_B
L155 What value of the representative height z_a is applied here: is that the vertical position of the lowest sigma layer?
L159 I do not understand the sentence “In the realistic topography simulations, we focused exclusively on storm surge anomalies.” Does this mean that tides were not at all included in the simulations, or that (as one would ideally expect) you subtracted tides from the water levels to obtain the storm surge anomalies. Please explain by clarifying the “focused exclusively on”.
L179 I guess the multiple cases with different rotation speeds under (3) are also spatially uniform? Please clarify and if all wind forcing patterns are spatially uniform, please state that upfront, rather than repeating it for each case. And how about the atmospheric pressure field? Is that also spatially uniform so the atmospheric pressure gradient vanishes?
FIGURE 5 does not show axis labels in the top plot. And the caption speaks of “x=0”, whereas the coordinate “x” is not mentioned in the axis label. Please adjust this. Finally, as a minor detail, with respect to what datum is “water depth” defined?
L187 (and caption of FIGURE 6) I get confused by the sentence “The storm surges shown here were computed without including astronomical tides, that is, they represent the deviations directly from non-tidal simulations.” I assume this refers to the red line (“simulated”) in the 2016 case only and not to the black lines in the top plot, which do not involve any simulations as they stem from subtracting the python-generated astronomical tide from the direct water level observations. Or am I missing the point here?
L199 “relatively lower compared to” --> “lower than in”
FIGURE 7 Please add a sentence to the caption explaining the meaning of the green boxes and that they are discussed later in Section 4.3
L206 Please extend the last sentence explaining how the discussion by Ide relates to the present study: “This point has been previously discussed by Ide et al. (2023), but ... remains unknown.”
FIGURE 8 is a clear figure, but it could perhaps be even more clear if the selected period is also shown in the left plots, e.g. using a dashed red line at T=11h (top), T=12h (middle) and T=11h again (bottom). Also, please spend a sentence in the caption explaining the meaning of the black boxes.
L251 Not sure what the added value of “extremely” is here.
L260 Although the header of section 4.2 may suggest another simulation, I guess that the simulation described here is the same as already in Section 4.1. Please clarify and perhaps consider rephrasing section headers.
L208 Please add the range of sqrt(gh)-values corresponding to this range of water depths. What I find is values from 12 – 18 m/s
L286 What do you mean with “the raised sea levels again” – levels as a verb?
L290 I think I prefer “The realistic topography simulation presented above”
L295 Awkward phrasing “in space and time”, since coastal geometry is not complex “in time”.
L307 I do not understand what you mean with “to organize”
L313 Grammar: “anomalies decreases” --> “anomaly decreases” or “anomalies decrease”
FIGURE 15, vertical axis label: why “tide” anomaly?
FIGURE 16: perhaps add in the caption that this case corresponds to the peak value of the base case in Figure 15.
L403 “the Goto Islands included case” --> “The case including the Goto Islands”
L418 I presume that the Goto Islands, as studied before, are included again in the simulations including/excluding Korean Peninsula?
FIGURE 20: perhaps use the term “flow velocity” to better distinguish from wind velocity that has been mostly discussed so far.
L433 Rossby deformation radius should be sqrt(gH)/|f|, i.e. involving the absolute value of the Coriolis parameter.
Citation: https://doi.org/10.5194/egusphere-2025-5619-RC2
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- 1
This is a generally well written paper on an interesting topic. Some clarification is needed on a few points
Section 2 lines 68-70 / Section 2.1.1 lines 81-82
Why is Hakata chosen as the representative site?
Section 2.1.1
I assume the reconstructed tide from pytides is used to de-tide the gauge data and provide the observed surge? Can this be made more explicit
Section 3.1 line 188
It is not clear why the tide has not been included – can you expand on this? I would think this is intended to only refer to the model simulation (the gauge observations must have had a tide removed?) but it uses the plural “storm surges” when there is only one event with a model simulation so this should be clarified.
Section 5 line 484
“The insights obtained in this study provide a physical basis for improving short-term storm surge prediction and risk assessment along the northern coast of Kyushu” – how is surge prediction currently done in this region? What improvements do you think could be made following this work?
Figures
Figures 5,13,14,16,20 rely on the reader being able to distinguish between red and green lines/points. These should use different colours that are more accessible to readers with colour-blindness, or use different shaped symbols to distinguish them.
Figures 8,11,12,18,20 – these use a “rainbow” colour map which is generally not recommend (eg https://towardsdatascience.com/why-the-rainbow-color-map-is-problematic-23293d0937d5/). I would suggest using a different colour scale in future
Technical/typos
Line 136
Either needs text inserted between “surrounding ocean areas” and the citations, or put the citations in brackets eg:
“… applied to the target and surrounding ocean areas (Yamashiro et al 2016, Ide et al 2013) …” or
“… applied to the target and surrounding ocean areas for example by Yamashiro et al (2016), Ide et al (2023) …”
Line 313
“surge anomalies” -> “surge anomaly”