Mechanism of Delayed Storm Surge in Straits: Seiche-Induced Oscillations Triggered by Typhoon Passage

Ozaki, Shinichiro; Ide, Yoshihiko; Yamashiro, Masaru

doi:https://doi.org/10.5194/egusphere-2024-3876

Preprints

https://doi.org/10.5194/egusphere-2024-3876

Preprints

06 Jan 2025

| 06 Jan 2025

Mechanism of Delayed Storm Surge in Straits: Seiche-Induced Oscillations Triggered by Typhoon Passage

Shinichiro Ozaki, Yoshihiko Ide, and Masaru Yamashiro

Abstract. A storm surge is a phenomenon in which the sea level rises significantly due to low-pressure systems, such as typhoons, accompanied by strong winds. Once storm surge-induced flooding occurs, it can rapidly inundate low-lying areas. Generally, the primary contributor to storm surge is wind set-up, where wind forces the sea towards the coast. As such, it is well-known that severe storm surges occur at typhoon's closest approach because of strong wind set-up. However, when Maysak (2020) struck the northern coast of Kyushu Island (NCKI), located on the south side of the Tsushima Strait, the sea level rose and flooding occurred approximately half a day after the typhoon had passed. At NCKI, both atmospheric pressure and wind had already weakened at the time of the flooding. Thus, the storm surge could not be explained by wind set-up or the inverted barometer effect. We examined storm surge observations for typhoons that impacted NCKI over the past 20 years and revealed a tendency for two peaks in storm surge when typhoons passed through the western channel of the strait. The second peak was identified as the maximum storm surge height, occurring approximately 10 hours after the typhoon had passed. The first peak occurred when the typhoon was closest to NCKI, coinciding with the time of minimum atmospheric pressure. This was attributed to the sea level rise caused by the inverted barometer effect. After the first peak, oscillations with a period of approximately 10 hours were observed, resulting in the second peak. NCKI, located along the Tsushima Strait, is subject to the geographical characteristics of the strait, which likely caused the oscillations leading to the maximum storm surge. To identify the oscillations that occurred after the typhoon’s passage, a continuous wavelet transform was applied to the results of storm surge simulations for time-frequency analysis. As a result, it was found that two types of seiches in a two-dimensional spatial domain of the strait (5-hour and 10-hour periods) occurred after the typhoon's passage. These seiches were triggered by the release of potential energy as external forces weakened following the typhoon's transit through the strait. Furthermore, the seiches were observed to occur approximately two hours earlier when the external force was wind, compared to when it was atmospheric pressure. This is because the time variation of atmospheric pressure drop is slower than that of wind direction. In this study, we identified the occurrence of anomalous storm surges caused by typhoons passing through a strait under specific conditions and conducted a detailed investigation of their generation mechanisms, and demonstrated storm surges can occur even after a typhoon has passed and improved understanding of storm surge characteristics in straits.

Received: 09 Dec 2024 – Discussion started: 06 Jan 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Shinichiro Ozaki, Yoshihiko Ide, and Masaru Yamashiro

Status: closed

RC1:
'Comment on egusphere-2024-3876', Anonymous Referee #1, 30 Mar 2025
This manuscript deals with storm surge characteristics around the northern coast of Kyushu Island in Japan. The topic is interesting, and it should be published. However, the manuscript needs to be improved before publication.

Major points
The governing equations by Eqs.(1)-(4) use the Cartesian coordinate, but the numerical results use unstructured grids. It should be consistent with the model.

The discussion about the oscillation modes between Korea and Kyushu in 3.3.2 is quite interesting and important. However, the analysis is qualitative and needs to improve more quantitatively.

English writing needs to be improved before acceptance.

Minor points
A storm surge is a sea surface anomaly from the astronomical tide. Therefore, the expression of s is incorrect. It should be reworded as storm surge or storm surge height.

There are many small subsections, mainly in section 2. There are also small paragraphs consisting of a few sentences. The size of sections, subsections, and paragraphs should be carefully considered following the standard manner of academic writing.
Citation: https://doi.org/10.5194/egusphere-2024-3876-RC1
- AC1:
  'Reply on RC1', Shinichiro Ozaki, 07 Apr 2025
  Response to Reviewer Comments
  We would like to thank the reviewer for the insightful comments and constructive suggestions. Below, we detail our responses to each point raised.
  Major Points
  Governing Equations and Coordinate Consistency
  
  Reviewer comment:
  
  “The governing equations by Eqs.(1)-(4) use the Cartesian coordinate, but the numerical results use unstructured grids. It should be consistent with the model.”
  
  Response:
  
  Thank you for pointing this out. The governing equations is written in a cartesian coordinate system, which remains valid when using unstructured grids in the finite volume framework. FVCOM utilizes the finite volume method and can be applied using either Cartesian (xy) coordinates or geographic coordinates (latitude and longitude). For a detailed description of the discretization procedures, please refer to Chen et al (2003).
  
  Quantitative Analysis of Oscillation Modes
  
  Reviewer comment:
  
  “The discussion about the oscillation modes between Korea and Kyushu in 3.3.2 is quite interesting and important. However, the analysis is qualitative and needs to improve more quantitatively.”
  
  Response:
  
  In the revised manuscript, we have added a new subsection in Section 3.3.1 to include a quantitative discussion of the amplitudes associated with each oscillation mode. Moreover, we would greatly appreciate your further insight into additional methods or metrics that could enhance the quantitative aspect of our analysis.
  
  Improvement of English Writing
  
  Reviewer comment:
  
  “English writing needs to be improved before acceptance.”
  
  Response:
  
  We have carefully revised the manuscript for grammatical accuracy and clarity. The manuscript has been edited by a native English speaker to enhance the overall quality of the language.
  
  Minor Points
  Expression of "s" as Storm Surge
  
  Reviewer comment:
  
  “A storm surge is a sea surface anomaly from the astronomical tide. Therefore, the expression of s is incorrect. It should be reworded as storm surge or storm surge height.”
  
  Response:
  
  We agree with the reviewer’s comment. The expression “s” has been revised throughout the manuscript to “storm surge” or “storm surge height” as appropriate.
  
  Organization of Sections and Paragraphs
  
  Reviewer comment:
  
  “There are many small subsections, mainly in section 2. There are also small paragraphs consisting of a few sentences. The size of sections, subsections, and paragraphs should be carefully considered following the standard manner of academic writing.”
  
  Response:
  
  We appreciate the suggestion regarding the structure of the manuscript. In the revised version, we have reorganized Section 2 by consolidating smaller subsections and expanding brief paragraphs where necessary.
  
  We hope that the revisions adequately address the reviewer’s concerns and improve the overall quality of the manuscript. Thank you again for your valuable feedback.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3876-AC1
RC2:
'Comment on egusphere-2024-3876', Anonymous Referee #2, 09 Jun 2025

The manuscript reported mechanism of delayed surges in straits: seiche-induced oscillation triggered by typhoon passage. The issue is acceptable and method is along with scientific ways. But the manuscript does not reach the standard form to be published in the journal.

Specially, proofreading is necessary by native English speakers. Details will be found below.
Abstract: The study's novelty is ambiguous. Also, what the authors did and what they found should be clarified.
Introduction

What are structural and non-structural measures for mitigating storm surge damage?
What are the references for the theory of low air pressure? Why don't you think both wind and pressure coincidently work?
In 40, what is wind setup height? On the head, the definition of wind setup should be given for better understanding.
In 40, "Regions that have historically suffered significant storm surge damage are often bays facing the ocean, characterized by geographical conditions that amplify wind setup" -> What do you mean? All bays face the ocean, suffering from storm surges.
In 45, the authors should explicitly point out where 100 cm surges occurred.
In Introduction, what is the novelty? What is the originality?
In 2.1, normally, we don't know what are 1,898 tracks and where is Hakata tidal station. So I suggest adding 1,898 tracks, 599 tracks, and Hakata. You can remove one of the figures among Figs. 1, 2, and 3. Or you can make new figure put them on a figure.
In 80, what is RDMDB? Full name is first given.
In 115, did you consider wind speed limits for the wind drag coefficient? If you don't consider it, what is the reason? If you consider it, what happens in your results?
I suggest editing Section 2 Data and Methodology. For instance, data, models, and simulation experiments. In particular, the description of simulation experiments is strongly suggested to look over the study.
Section 3 Results and discussion is unreadable because of too separated paragraphs in each section. The title of 3.3.1 might be better if it was like the effect of the superposition of oscillations on surges. My suggestion is that you can rename the titles of subsections with like impacts of something, not tools. It will be more readable.
Why don't you absolutely separate the two sections of "Results " and "Discussion"?
The reviewer cannot find discussions referring to other literature or comparing them to other works.

Citation: https://doi.org/10.5194/egusphere-2024-3876-RC2
- AC2:
  'Reply on RC2', Shinichiro Ozaki, 01 Jul 2025
  Response to Reviewer Comments We would like to thank the reviewer for the insightful comments and constructive suggestions. Below, we detail our responses to each point raised.
  
  <comments>
  The manuscript reported mechanism of delayed surges in straits: seiche-induced oscillation triggered by typhoon passage. The issue is acceptable and method is along with scientific ways. But the manuscript does not reach the standard form to be published in the journal.
  Specially, proofreading is necessary by native English speakers. Details will be found below.
  
  Abstract: The study's novelty is ambiguous. Also, what the authors did and what they found should be clarified.
  
  Thank you for your valuable comment. We have revised the Abstract to clearly describe the novelty, methodology, and key findings of the study. Specifically, we now highlight that this study is the first to identify the mechanism of delayed storm surges caused by westward-curving typhoons (WCTS-type) in strait regions, using wavelet analysis to extract 10- and 5-hour oscillations. These were shown to be natural modes of the Tsushima Strait. Furthermore, we conducted numerical and idealized experiments to confirm that these oscillations amplify and delay storm surges. The revised Abstract now emphasizes the role of resonance effects and provides a broader implication for other straits with similar geographic and meteorological conditions.
  
  What are structural and non-structural measures for mitigating storm surge damage?
  
  To address your point, we have revised the manuscript to clarify the distinction between structural and non-structural measures. Specifically, we now state that structural measures include storm surge barriers (e.g., Esteban et al., 2014), while non-structural measures include real-time forecasting systems (e.g., Igarashi et al., 2021). We have also emphasized that a comprehensive understanding of storm surge behavior is critical for designing and implementing effective mitigation strategies. The revised sentence can be found in Section 2 of the manuscript.
  
  What are the references for the theory of low air pressure? Why don't you think both wind and pressure coincidently work?
  
  To clarify the theoretical basis for the effect of low air pressure on sea level, we have now cited Wunsch and Stammer (1997), who provide evidence for the inverse barometer effect in the context of large-scale sea level variations.
  Regarding the interaction of wind and pressure, we fully agree that both forces act simultaneously. However, our results indicate that their respective contributions to storm surge height are approximately linear and additive under the conditions examined. To explicitly demonstrate this, we added a new subsection entitled "Contributions of Pressure and Wind to Storm Surge Anomalies", in which we compare the storm surge responses from pressure-only, wind-only, and combined-forcing simulations. This comparison confirms that the combined effect closely matches the sum of the individual effects, supporting the assumption of linearity in this context.
  
  In 40, what is wind setup height? On the head, the definition of wind setup should be given for better understanding.
  
  Thank you for pointing this out. To improve clarity, we have added a definition of wind set-up in the revised manuscript. Specifically, we now state that:
  “wind setup refers to the rise in sea level due to strong winds forcing seawater shoreward (e.g., Walton and Dean (2009)). This occurs when coastal winds drive surface water toward the shore, causing accumulation in shallow regions and elevating the local sea level.”
  
  In 40, "Regions that have historically suffered significant storm surge damage are often bays facing the ocean, characterized by geographical conditions that amplify wind setup" -> What do you mean? All bays face the ocean, suffering from storm surges.
  
  Thank you for your insightful comment. We agree that the original sentence was ambiguous. To clarify our point, we have revised the text and added a new schematic figure titled “Schematic figure of typhoon positions and wind directions during its passage through the WCTS: (a) when the typhoon is located over the Tsushima Strait; (b) when the typhoon is located over the Sea of Japan.”
  This figure illustrates how wind direction and coastline orientation interact during typhoon passage, showing that certain bays—such as those facing directly toward the prevailing wind direction—experience stronger wind set-up effects due to geographic alignment. We have also revised the explanation in the text to reflect this more clearly.
  
  In 45, the authors should explicitly point out where 100 cm surges occurred.
  
  Thank you for your comment. To address this point, we have revised the manuscript to explicitly indicate the location where the 100 cm storm surge occurred. Specifically, we added the location of Gwangyang—the affected area—on the figure titled “Bathymetry of Tsushima Strait with points of tidal stations around the strait. The right side of the figure shows an enlarged view of Hakata Bay, and the points indicate the locations of tidal and weather stations.”
  
  In Introduction, what is the novelty? What is the originality?
  
  Thank you for this important comment. To clarify the novelty and originality of our study, we have revised the Introduction to explicitly state our research objective and unique approach.
  In 2.1, normally, we don't know what are 1,898 tracks and where is Hakata tidal station. So I suggest adding 1,898 tracks, 599 tracks, and Hakata. You can remove one of the figures among Figs. 1, 2, and 3. Or you can make new figure put them on a figure.
  
  Thank you for your helpful suggestion. In response, we have added a new subsection entitled “Selected Typhoons” to clarify the process of typhoon selection. Additionally, we created a new figure titled “Tracks of selected typhoons in this study”, which shows the full set of 1,898 typhoon tracks, the subset of 599 westward-curving tracks, and highlights the location of Hakata tidal station.
  This figure provides a comprehensive overview of the selection process and the geographical context, which we believe improves the reader's understanding of the study domain and methodology.
  
  In 80, what is RDMDB? Full name is first given.
  
  To address this point, we have revised the manuscript to spell out the full name of the abbreviation “RDMDB” as Regional Delayed Mode Data Base upon its first appearance.
  
  In 115, did you consider wind speed limits for the wind drag coefficient? If you don't consider it, what is the reason? If you consider it, what happens in your results?
  
  Thank you for your comment. In this study, we adopted the wind drag coefficient formulation proposed by Large and Pond (1981), which is also used in Niimi et al. (2022), a validated reference for storm surge simulations in this region.
  Although some studies suggest that the drag coefficient saturates at high wind speeds (e.g., at a maximum value of approximately 0.0025), we did not impose such a cap. The corresponding wind speed at which Cs reaches 0.0025 in our formulation is approximately 30.9 m/s. Since the maximum wind speeds in our target typhoons are around 35 m/s, we consider that applying a cap would have only a limited effect on the calculated storm surges. Therefore, the omission of a wind speed limit does not significantly affect our conclusions.
  
  I suggest editing Section 2 Data and Methodology. For instance, data, models, and simulation experiments. In particular, the description of simulation experiments is strongly suggested to look over the study.
  
  Thank you for your valuable suggestion. In response, we have significantly revised the structure of the manuscript to clarify the methodology. Specifically, we reorganized the relevant content into separate sections for observational data, numerical model setup, and simulation experiments.
  
  Section 3 Results and discussion is unreadable because of too separated paragraphs in each section. The title of 3.3.1 might be better if it was like the effect of the superposition of oscillations on surges. My suggestion is that you can rename the titles of subsections with like impacts of something, not tools. It will be more readable. Why don't you absolutely separate the two sections of "Results " and "Discussion"? The reviewer cannot find discussions referring to other literature or comparing them to other works.
  
  We appreciate your insightful feedback regarding the organization of the results and discussion sections. Following your suggestion, we have completely restructured this part of the manuscript. The results are now divided into two main sections:
  
  Numerical Simulation, with impact-oriented subsections such as:
  Model Setup
  
  Validation
  
  Contributions of Pressure and Wind to Storm Surge Anomalies (This specifically addresses your suggestion for impact-oriented titles)
  
  Spectral Analysis using Wavelet Transform, with revised titles like:
  Continuous Wavelet Transform
  
  Dominant Oscillation Modes Identified by CWT (These new titles are also crafted to be more descriptive of the scientific findings rather than simply the tools used.)
  
  In addition, we created a dedicated Discussion section to interpret the physical mechanisms behind the delayed storm surges. This section also includes comparisons with previous studies in the strait regions. We believe this separation significantly improves the clarity and logical flow of the manuscript, and more effectively communicates the study's scientific contributions.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3876-AC2

Status: closed

RC1:
'Comment on egusphere-2024-3876', Anonymous Referee #1, 30 Mar 2025
This manuscript deals with storm surge characteristics around the northern coast of Kyushu Island in Japan. The topic is interesting, and it should be published. However, the manuscript needs to be improved before publication.

Major points
The governing equations by Eqs.(1)-(4) use the Cartesian coordinate, but the numerical results use unstructured grids. It should be consistent with the model.

The discussion about the oscillation modes between Korea and Kyushu in 3.3.2 is quite interesting and important. However, the analysis is qualitative and needs to improve more quantitatively.

English writing needs to be improved before acceptance.

Minor points
A storm surge is a sea surface anomaly from the astronomical tide. Therefore, the expression of s is incorrect. It should be reworded as storm surge or storm surge height.

There are many small subsections, mainly in section 2. There are also small paragraphs consisting of a few sentences. The size of sections, subsections, and paragraphs should be carefully considered following the standard manner of academic writing.
Citation: https://doi.org/10.5194/egusphere-2024-3876-RC1
- AC1:
  'Reply on RC1', Shinichiro Ozaki, 07 Apr 2025
  Response to Reviewer Comments
  We would like to thank the reviewer for the insightful comments and constructive suggestions. Below, we detail our responses to each point raised.
  Major Points
  Governing Equations and Coordinate Consistency
  
  Reviewer comment:
  
  “The governing equations by Eqs.(1)-(4) use the Cartesian coordinate, but the numerical results use unstructured grids. It should be consistent with the model.”
  
  Response:
  
  Thank you for pointing this out. The governing equations is written in a cartesian coordinate system, which remains valid when using unstructured grids in the finite volume framework. FVCOM utilizes the finite volume method and can be applied using either Cartesian (xy) coordinates or geographic coordinates (latitude and longitude). For a detailed description of the discretization procedures, please refer to Chen et al (2003).
  
  Quantitative Analysis of Oscillation Modes
  
  Reviewer comment:
  
  “The discussion about the oscillation modes between Korea and Kyushu in 3.3.2 is quite interesting and important. However, the analysis is qualitative and needs to improve more quantitatively.”
  
  Response:
  
  In the revised manuscript, we have added a new subsection in Section 3.3.1 to include a quantitative discussion of the amplitudes associated with each oscillation mode. Moreover, we would greatly appreciate your further insight into additional methods or metrics that could enhance the quantitative aspect of our analysis.
  
  Improvement of English Writing
  
  Reviewer comment:
  
  “English writing needs to be improved before acceptance.”
  
  Response:
  
  We have carefully revised the manuscript for grammatical accuracy and clarity. The manuscript has been edited by a native English speaker to enhance the overall quality of the language.
  
  Minor Points
  Expression of "s" as Storm Surge
  
  Reviewer comment:
  
  “A storm surge is a sea surface anomaly from the astronomical tide. Therefore, the expression of s is incorrect. It should be reworded as storm surge or storm surge height.”
  
  Response:
  
  We agree with the reviewer’s comment. The expression “s” has been revised throughout the manuscript to “storm surge” or “storm surge height” as appropriate.
  
  Organization of Sections and Paragraphs
  
  Reviewer comment:
  
  “There are many small subsections, mainly in section 2. There are also small paragraphs consisting of a few sentences. The size of sections, subsections, and paragraphs should be carefully considered following the standard manner of academic writing.”
  
  Response:
  
  We appreciate the suggestion regarding the structure of the manuscript. In the revised version, we have reorganized Section 2 by consolidating smaller subsections and expanding brief paragraphs where necessary.
  
  We hope that the revisions adequately address the reviewer’s concerns and improve the overall quality of the manuscript. Thank you again for your valuable feedback.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3876-AC1
RC2:
'Comment on egusphere-2024-3876', Anonymous Referee #2, 09 Jun 2025

The manuscript reported mechanism of delayed surges in straits: seiche-induced oscillation triggered by typhoon passage. The issue is acceptable and method is along with scientific ways. But the manuscript does not reach the standard form to be published in the journal.

Specially, proofreading is necessary by native English speakers. Details will be found below.
Abstract: The study's novelty is ambiguous. Also, what the authors did and what they found should be clarified.
Introduction

What are structural and non-structural measures for mitigating storm surge damage?
What are the references for the theory of low air pressure? Why don't you think both wind and pressure coincidently work?
In 40, what is wind setup height? On the head, the definition of wind setup should be given for better understanding.
In 40, "Regions that have historically suffered significant storm surge damage are often bays facing the ocean, characterized by geographical conditions that amplify wind setup" -> What do you mean? All bays face the ocean, suffering from storm surges.
In 45, the authors should explicitly point out where 100 cm surges occurred.
In Introduction, what is the novelty? What is the originality?
In 2.1, normally, we don't know what are 1,898 tracks and where is Hakata tidal station. So I suggest adding 1,898 tracks, 599 tracks, and Hakata. You can remove one of the figures among Figs. 1, 2, and 3. Or you can make new figure put them on a figure.
In 80, what is RDMDB? Full name is first given.
In 115, did you consider wind speed limits for the wind drag coefficient? If you don't consider it, what is the reason? If you consider it, what happens in your results?
I suggest editing Section 2 Data and Methodology. For instance, data, models, and simulation experiments. In particular, the description of simulation experiments is strongly suggested to look over the study.
Section 3 Results and discussion is unreadable because of too separated paragraphs in each section. The title of 3.3.1 might be better if it was like the effect of the superposition of oscillations on surges. My suggestion is that you can rename the titles of subsections with like impacts of something, not tools. It will be more readable.
Why don't you absolutely separate the two sections of "Results " and "Discussion"?
The reviewer cannot find discussions referring to other literature or comparing them to other works.

Citation: https://doi.org/10.5194/egusphere-2024-3876-RC2
- AC2:
  'Reply on RC2', Shinichiro Ozaki, 01 Jul 2025
  Response to Reviewer Comments We would like to thank the reviewer for the insightful comments and constructive suggestions. Below, we detail our responses to each point raised.
  
  <comments>
  The manuscript reported mechanism of delayed surges in straits: seiche-induced oscillation triggered by typhoon passage. The issue is acceptable and method is along with scientific ways. But the manuscript does not reach the standard form to be published in the journal.
  Specially, proofreading is necessary by native English speakers. Details will be found below.
  
  Abstract: The study's novelty is ambiguous. Also, what the authors did and what they found should be clarified.
  
  Thank you for your valuable comment. We have revised the Abstract to clearly describe the novelty, methodology, and key findings of the study. Specifically, we now highlight that this study is the first to identify the mechanism of delayed storm surges caused by westward-curving typhoons (WCTS-type) in strait regions, using wavelet analysis to extract 10- and 5-hour oscillations. These were shown to be natural modes of the Tsushima Strait. Furthermore, we conducted numerical and idealized experiments to confirm that these oscillations amplify and delay storm surges. The revised Abstract now emphasizes the role of resonance effects and provides a broader implication for other straits with similar geographic and meteorological conditions.
  
  What are structural and non-structural measures for mitigating storm surge damage?
  
  To address your point, we have revised the manuscript to clarify the distinction between structural and non-structural measures. Specifically, we now state that structural measures include storm surge barriers (e.g., Esteban et al., 2014), while non-structural measures include real-time forecasting systems (e.g., Igarashi et al., 2021). We have also emphasized that a comprehensive understanding of storm surge behavior is critical for designing and implementing effective mitigation strategies. The revised sentence can be found in Section 2 of the manuscript.
  
  What are the references for the theory of low air pressure? Why don't you think both wind and pressure coincidently work?
  
  To clarify the theoretical basis for the effect of low air pressure on sea level, we have now cited Wunsch and Stammer (1997), who provide evidence for the inverse barometer effect in the context of large-scale sea level variations.
  Regarding the interaction of wind and pressure, we fully agree that both forces act simultaneously. However, our results indicate that their respective contributions to storm surge height are approximately linear and additive under the conditions examined. To explicitly demonstrate this, we added a new subsection entitled "Contributions of Pressure and Wind to Storm Surge Anomalies", in which we compare the storm surge responses from pressure-only, wind-only, and combined-forcing simulations. This comparison confirms that the combined effect closely matches the sum of the individual effects, supporting the assumption of linearity in this context.
  
  In 40, what is wind setup height? On the head, the definition of wind setup should be given for better understanding.
  
  Thank you for pointing this out. To improve clarity, we have added a definition of wind set-up in the revised manuscript. Specifically, we now state that:
  “wind setup refers to the rise in sea level due to strong winds forcing seawater shoreward (e.g., Walton and Dean (2009)). This occurs when coastal winds drive surface water toward the shore, causing accumulation in shallow regions and elevating the local sea level.”
  
  In 40, "Regions that have historically suffered significant storm surge damage are often bays facing the ocean, characterized by geographical conditions that amplify wind setup" -> What do you mean? All bays face the ocean, suffering from storm surges.
  
  Thank you for your insightful comment. We agree that the original sentence was ambiguous. To clarify our point, we have revised the text and added a new schematic figure titled “Schematic figure of typhoon positions and wind directions during its passage through the WCTS: (a) when the typhoon is located over the Tsushima Strait; (b) when the typhoon is located over the Sea of Japan.”
  This figure illustrates how wind direction and coastline orientation interact during typhoon passage, showing that certain bays—such as those facing directly toward the prevailing wind direction—experience stronger wind set-up effects due to geographic alignment. We have also revised the explanation in the text to reflect this more clearly.
  
  In 45, the authors should explicitly point out where 100 cm surges occurred.
  
  Thank you for your comment. To address this point, we have revised the manuscript to explicitly indicate the location where the 100 cm storm surge occurred. Specifically, we added the location of Gwangyang—the affected area—on the figure titled “Bathymetry of Tsushima Strait with points of tidal stations around the strait. The right side of the figure shows an enlarged view of Hakata Bay, and the points indicate the locations of tidal and weather stations.”
  
  In Introduction, what is the novelty? What is the originality?
  
  Thank you for this important comment. To clarify the novelty and originality of our study, we have revised the Introduction to explicitly state our research objective and unique approach.
  In 2.1, normally, we don't know what are 1,898 tracks and where is Hakata tidal station. So I suggest adding 1,898 tracks, 599 tracks, and Hakata. You can remove one of the figures among Figs. 1, 2, and 3. Or you can make new figure put them on a figure.
  
  Thank you for your helpful suggestion. In response, we have added a new subsection entitled “Selected Typhoons” to clarify the process of typhoon selection. Additionally, we created a new figure titled “Tracks of selected typhoons in this study”, which shows the full set of 1,898 typhoon tracks, the subset of 599 westward-curving tracks, and highlights the location of Hakata tidal station.
  This figure provides a comprehensive overview of the selection process and the geographical context, which we believe improves the reader's understanding of the study domain and methodology.
  
  In 80, what is RDMDB? Full name is first given.
  
  To address this point, we have revised the manuscript to spell out the full name of the abbreviation “RDMDB” as Regional Delayed Mode Data Base upon its first appearance.
  
  In 115, did you consider wind speed limits for the wind drag coefficient? If you don't consider it, what is the reason? If you consider it, what happens in your results?
  
  Thank you for your comment. In this study, we adopted the wind drag coefficient formulation proposed by Large and Pond (1981), which is also used in Niimi et al. (2022), a validated reference for storm surge simulations in this region.
  Although some studies suggest that the drag coefficient saturates at high wind speeds (e.g., at a maximum value of approximately 0.0025), we did not impose such a cap. The corresponding wind speed at which Cs reaches 0.0025 in our formulation is approximately 30.9 m/s. Since the maximum wind speeds in our target typhoons are around 35 m/s, we consider that applying a cap would have only a limited effect on the calculated storm surges. Therefore, the omission of a wind speed limit does not significantly affect our conclusions.
  
  I suggest editing Section 2 Data and Methodology. For instance, data, models, and simulation experiments. In particular, the description of simulation experiments is strongly suggested to look over the study.
  
  Thank you for your valuable suggestion. In response, we have significantly revised the structure of the manuscript to clarify the methodology. Specifically, we reorganized the relevant content into separate sections for observational data, numerical model setup, and simulation experiments.
  
  Section 3 Results and discussion is unreadable because of too separated paragraphs in each section. The title of 3.3.1 might be better if it was like the effect of the superposition of oscillations on surges. My suggestion is that you can rename the titles of subsections with like impacts of something, not tools. It will be more readable. Why don't you absolutely separate the two sections of "Results " and "Discussion"? The reviewer cannot find discussions referring to other literature or comparing them to other works.
  
  We appreciate your insightful feedback regarding the organization of the results and discussion sections. Following your suggestion, we have completely restructured this part of the manuscript. The results are now divided into two main sections:
  
  Numerical Simulation, with impact-oriented subsections such as:
  Model Setup
  
  Validation
  
  Contributions of Pressure and Wind to Storm Surge Anomalies (This specifically addresses your suggestion for impact-oriented titles)
  
  Spectral Analysis using Wavelet Transform, with revised titles like:
  Continuous Wavelet Transform
  
  Dominant Oscillation Modes Identified by CWT (These new titles are also crafted to be more descriptive of the scientific findings rather than simply the tools used.)
  
  In addition, we created a dedicated Discussion section to interpret the physical mechanisms behind the delayed storm surges. This section also includes comparisons with previous studies in the strait regions. We believe this separation significantly improves the clarity and logical flow of the manuscript, and more effectively communicates the study's scientific contributions.
  
  Citation: https://doi.org/10.5194/egusphere-2024-3876-AC2

Shinichiro Ozaki, Yoshihiko Ide, and Masaru Yamashiro

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Cumulative views and downloads (calculated since 06 Jan 2025)

Month	HTML	PDF	XML	Total
Jan 2025	64	19	4	87
Feb 2025	20	4	1	25
Mar 2025	22	6	1	29
Apr 2025	34	6	3	43
May 2025	23	9	3	35
Jun 2025	30	11	5	46
Jul 2025	38	10	2	50
Aug 2025	69	9	0	78
Sep 2025	88	2	0	90

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Short summary

Typhoons struck the northern coast of Kyushu Island, which faces the Tsushima Strait, the sea level rises approximately half a day after the typhoon passed. We elucidated mechanisms behind this event in the strait. Results showed that two types of seiches were triggered by potential energy release after the typhoon's passage. This study demonstrates that storm surges can occur even after a typhoon has passed and enhances understanding of storm surge characteristics in straits.


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