the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Jan 2024
Importance of tides and winds in influencing the nonstationary behaviour of coastal currents in offshore Singapore
Abstract. Coastal currents significantly impact port activities, coastal landform morphodynamics and ecosystem functioning. It is therefore necessary to understand the physical characteristics and natural variability of these currents within coastal settings. Traditional methods such as harmonic analysis assume stationarity of tide-driven currents and thus may not be applicable to systems modulated by variable nontidal inputs and processes. Here we deployed eight tilt current meters at shallow (< 5 m) coral reef environments in southern Singapore. Tilt current meters were positioned around the reefs at the main compass bearings to analyse the spatiotemporal variability of coastal currents in the frequency domain for one year (March 2018 to March 2019). Tidal motions were the primary mechanism of current flow on reefs and account for between 14–45 % of total variance across all sites, with diurnal currents having either similar or greater proportion of energy than semidiurnal currents. In Singapore, the diurnal wind stress, characteristic of the land–sea breeze, strengthens during the monsoons, and its effect on currents was investigated using wavelet coherence. Findings suggest that currents and wind stress were highly correlated at the diurnal and subtidal frequencies during the monsoons with a varying time lag of up to 6 hours with respect to both the phase and the antiphase. We find that wind forcing was responsible for the observed seasonal variations in the diurnal K1 tidal constituent, its amplitude derived from short-term harmonic analysis. Given the importance of wind, we thus require longer time-series datasets to examine how atmospheric phenomena affect currents at greater time scales to improve predictions.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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Supplement
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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Supplement
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- EndNote
- Final revised paper
Journal article(s) based on this preprint
Coastal currents have wide implications for port activities, transport of sediments, and coral reef ecosystems; thus a deeper understanding of their characteristics is needed. We collected data on current velocities for a year using current meters at shallow waters in Singapore. The strength of the currents is primarily affected by tides and winds and generally increases during the monsoon seasons across various frequencies.
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-143', Anonymous Referee #1, 28 Apr 2024
This manuscript analyzed one-year observed tidal currents around two islands in offshore Singapore, using the short-term harmonic analysis, continuous wavelet transform and, magnitude-squared wavelet coherence. The results reveal the importance of tidal motions and monsoons in diurnal and subtidal periods. This manuscript is reasonably organized and the results can support the main conclusions. Please consider the below comments for improving the manuscript.
- The authors use the WRF model to downscale the winds to 10 km resolution, so wind data should be available for both Pulau Hantu and Kusu Island for comparison, may the authors update Figure 3 and have a look at if there is any difference in wind strength and directions, which are important for further exploring the correlation between wind and currents in next sections.
- May the authors provide the co-tidal and co-range charts in the vicinity of Singapore? Maybe it can help analyze the variation of the tidal amplitude and corresponding reasons.
Citation: https://doi.org/10.5194/egusphere-2024-143-RC1 - AC1: 'Reply on RC1', Jun Yu Puah, 17 Jul 2024
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RC2: 'Comment on egusphere-2024-143', Anonymous Referee #2, 19 May 2024
This manuscript presents a detailed study of the influence of tides and wind on tidal currents offshore Singapore, which was developed using a series of statistical methods. Careful analysis with solid data recorded (1 year) from fieldwork is presented, and the authors expose this detailed combination of statistical processes to assess the non-stationary behavior of coastal currents.Â
While I found the authors have done excellent work and the paper is well-written, I still suggest a moderate revision following the comments that I listed below:Â
- The authors need a map or more to show the directions of minor and major axes of the tidal current (e.g., tidal ellipse) at each TCM, the direction of wind to explain the conclusions. As a reader, I found it difficult to figure out the direction of wind stress, tidal currents, and the coastline.Â
- The wavelet analyses (CWT and WC) are really insightful in terms of the results. The non-stationary behaviors of coastal currents can be seen clearly. However, in my point of view, the authors should discuss this outcome as a "low-frequency signal". That is more meaningful as the authors also mentioned ENSO in their discussion. For low-frequency signals, as the wavelet indicates some period of 32 days (L266), I suggest the authors explore the linkage to the intraseasonal variability (30 to 90-day) in the tropical atmosphere (e.g., Madden-Julian Oscillation)
- Lines 275-280: The tidal currents are also distorted by the changes in bathymetry. Can the authors discuss the role of bathymetry more here or provide more information about bathymetry on this study site?Â
- I need clarification when the authors discuss the seasonal variation of the K1 amplitude of currents. The authors discuss that this might be due to the monsoonal winds. However, the results show that other major tidal constituents (O1, M2, and S2) are also large, but they did not present seasonal variation as K1 (Supp. Material). So why do coastal currents only follow K1? I think the authors need to discuss the interaction between tidal constituents in more detail here (e.g., in Van Maren and Gerritsen, 2012). The characteristics of the tide in the SCS are complicated as the oscillations of standing waves could produce resonance in a semi-closed or complex coastline water body. I suggest the authors put more analyses or explanations in place to make this point more solid.Â
Other minor comments:
- Fig.7: it is really hard to see the arrows on the graph. Also, a legend of a vertical color bar needed to be included.Â
- L175:Â need to make the unit consistent throughout the manuscript
- L284 and L288: The phase angles are 0-90 and 0-270, respectively, but why are they converted to a same time lag of up to 6 hours? Are they the same, or do we have a time lag and time lead here?Â
- Table 1: How much is the percentage of explained variance for PC2 at each site? As they kept PC2 in, they need to provide the percentage as they did for PC1.
- Table 1: Please check the header: "Direction of mean current speed"? Or just "Direction of mean current?"
- L193: As I commented above, the authors need a map to show the direction of the coastline and the currents.
- Fig 3: I suggest the authors show a graph of monthly averaged wind stress for the study site.Â
- L328-340:Â IÂ think this paragraph is better to be in the conclusion rather than in the discussion
Citation: https://doi.org/10.5194/egusphere-2024-143-RC2 - AC3: 'Reply on RC2', Jun Yu Puah, 17 Jul 2024
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RC3: 'Comment on egusphere-2024-143', Anonymous Referee #3, 26 Jun 2024
This manuscript presents a study of the spatiotemporal variability of coastal currents in shallow coral reef environments of southern Singapore. The authors employ a robust methodology, utilizing eight tilt current meters deployed over a one-year period to analyze current patterns in the frequency domain. The research makes several significant contributions to the field. It quantifies the relative importance of tidal motions in driving current flow, demonstrating that they account for 14-45% of total variance across sites. The finding that diurnal currents exhibit similar or greater energy proportions compared to semidiurnal currents is noteworthy and contributes to our understanding of local hydrodynamics. The authors apply wavelet coherence analysis to examine the relationship between wind stress and current. This approach reveals important correlations at diurnal and subtidal frequencies during monsoon periods, with observed time lags of up to 6 hours in both phase and antiphase.
Overall, this manuscript presents valuable findings that advance our understanding of coastal current dynamics in coral reef environments that have valuable implications for port activities, coastal landform morphodynamics, and ecosystem functioning. However, the manuscript can be improved further with another revision to better contextualize this study.
Specific comments:
- Include a regional map highlighting the area's importance (transit between Pacific and Indian Oceans; trade routes) and presenting regional winds and hydrodynamics.
- Lines 132-135: Provide more detail on the WRF model setup.
- Consider the location of each data collection point in the results analysis. For example, Kusu South, North, and East sites have different wind patterns throughout the year, which may explain the contrast in current patterns between Kusu South and North (line 254). Averaging wind stress over the four areas presented in Figure 6 may filter out their variabilities.
- Lines 234-237: Clarify how the percentages 26-45% and 14-36% were derived in Table 2.
Technical comments:
- Line 40: Clarify whether current ellipse parameters are estimated in this study or can be estimated in general.
- Line 236: Remove the duplicate mention of Hantu North.
- Figures 5 and 7: Confirm whether the Y-axis represents period or frequency.
Citation: https://doi.org/10.5194/egusphere-2024-143-RC3 - AC2: 'Reply on RC3', Jun Yu Puah, 17 Jul 2024
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-143', Anonymous Referee #1, 28 Apr 2024
This manuscript analyzed one-year observed tidal currents around two islands in offshore Singapore, using the short-term harmonic analysis, continuous wavelet transform and, magnitude-squared wavelet coherence. The results reveal the importance of tidal motions and monsoons in diurnal and subtidal periods. This manuscript is reasonably organized and the results can support the main conclusions. Please consider the below comments for improving the manuscript.
- The authors use the WRF model to downscale the winds to 10 km resolution, so wind data should be available for both Pulau Hantu and Kusu Island for comparison, may the authors update Figure 3 and have a look at if there is any difference in wind strength and directions, which are important for further exploring the correlation between wind and currents in next sections.
- May the authors provide the co-tidal and co-range charts in the vicinity of Singapore? Maybe it can help analyze the variation of the tidal amplitude and corresponding reasons.
Citation: https://doi.org/10.5194/egusphere-2024-143-RC1 - AC1: 'Reply on RC1', Jun Yu Puah, 17 Jul 2024
-
RC2: 'Comment on egusphere-2024-143', Anonymous Referee #2, 19 May 2024
This manuscript presents a detailed study of the influence of tides and wind on tidal currents offshore Singapore, which was developed using a series of statistical methods. Careful analysis with solid data recorded (1 year) from fieldwork is presented, and the authors expose this detailed combination of statistical processes to assess the non-stationary behavior of coastal currents.Â
While I found the authors have done excellent work and the paper is well-written, I still suggest a moderate revision following the comments that I listed below:Â
- The authors need a map or more to show the directions of minor and major axes of the tidal current (e.g., tidal ellipse) at each TCM, the direction of wind to explain the conclusions. As a reader, I found it difficult to figure out the direction of wind stress, tidal currents, and the coastline.Â
- The wavelet analyses (CWT and WC) are really insightful in terms of the results. The non-stationary behaviors of coastal currents can be seen clearly. However, in my point of view, the authors should discuss this outcome as a "low-frequency signal". That is more meaningful as the authors also mentioned ENSO in their discussion. For low-frequency signals, as the wavelet indicates some period of 32 days (L266), I suggest the authors explore the linkage to the intraseasonal variability (30 to 90-day) in the tropical atmosphere (e.g., Madden-Julian Oscillation)
- Lines 275-280: The tidal currents are also distorted by the changes in bathymetry. Can the authors discuss the role of bathymetry more here or provide more information about bathymetry on this study site?Â
- I need clarification when the authors discuss the seasonal variation of the K1 amplitude of currents. The authors discuss that this might be due to the monsoonal winds. However, the results show that other major tidal constituents (O1, M2, and S2) are also large, but they did not present seasonal variation as K1 (Supp. Material). So why do coastal currents only follow K1? I think the authors need to discuss the interaction between tidal constituents in more detail here (e.g., in Van Maren and Gerritsen, 2012). The characteristics of the tide in the SCS are complicated as the oscillations of standing waves could produce resonance in a semi-closed or complex coastline water body. I suggest the authors put more analyses or explanations in place to make this point more solid.Â
Other minor comments:
- Fig.7: it is really hard to see the arrows on the graph. Also, a legend of a vertical color bar needed to be included.Â
- L175:Â need to make the unit consistent throughout the manuscript
- L284 and L288: The phase angles are 0-90 and 0-270, respectively, but why are they converted to a same time lag of up to 6 hours? Are they the same, or do we have a time lag and time lead here?Â
- Table 1: How much is the percentage of explained variance for PC2 at each site? As they kept PC2 in, they need to provide the percentage as they did for PC1.
- Table 1: Please check the header: "Direction of mean current speed"? Or just "Direction of mean current?"
- L193: As I commented above, the authors need a map to show the direction of the coastline and the currents.
- Fig 3: I suggest the authors show a graph of monthly averaged wind stress for the study site.Â
- L328-340:Â IÂ think this paragraph is better to be in the conclusion rather than in the discussion
Citation: https://doi.org/10.5194/egusphere-2024-143-RC2 - AC3: 'Reply on RC2', Jun Yu Puah, 17 Jul 2024
-
RC3: 'Comment on egusphere-2024-143', Anonymous Referee #3, 26 Jun 2024
This manuscript presents a study of the spatiotemporal variability of coastal currents in shallow coral reef environments of southern Singapore. The authors employ a robust methodology, utilizing eight tilt current meters deployed over a one-year period to analyze current patterns in the frequency domain. The research makes several significant contributions to the field. It quantifies the relative importance of tidal motions in driving current flow, demonstrating that they account for 14-45% of total variance across sites. The finding that diurnal currents exhibit similar or greater energy proportions compared to semidiurnal currents is noteworthy and contributes to our understanding of local hydrodynamics. The authors apply wavelet coherence analysis to examine the relationship between wind stress and current. This approach reveals important correlations at diurnal and subtidal frequencies during monsoon periods, with observed time lags of up to 6 hours in both phase and antiphase.
Overall, this manuscript presents valuable findings that advance our understanding of coastal current dynamics in coral reef environments that have valuable implications for port activities, coastal landform morphodynamics, and ecosystem functioning. However, the manuscript can be improved further with another revision to better contextualize this study.
Specific comments:
- Include a regional map highlighting the area's importance (transit between Pacific and Indian Oceans; trade routes) and presenting regional winds and hydrodynamics.
- Lines 132-135: Provide more detail on the WRF model setup.
- Consider the location of each data collection point in the results analysis. For example, Kusu South, North, and East sites have different wind patterns throughout the year, which may explain the contrast in current patterns between Kusu South and North (line 254). Averaging wind stress over the four areas presented in Figure 6 may filter out their variabilities.
- Lines 234-237: Clarify how the percentages 26-45% and 14-36% were derived in Table 2.
Technical comments:
- Line 40: Clarify whether current ellipse parameters are estimated in this study or can be estimated in general.
- Line 236: Remove the duplicate mention of Hantu North.
- Figures 5 and 7: Confirm whether the Y-axis represents period or frequency.
Citation: https://doi.org/10.5194/egusphere-2024-143-RC3 - AC2: 'Reply on RC3', Jun Yu Puah, 17 Jul 2024
Peer review completion
Journal article(s) based on this preprint
Coastal currents have wide implications for port activities, transport of sediments, and coral reef ecosystems; thus a deeper understanding of their characteristics is needed. We collected data on current velocities for a year using current meters at shallow waters in Singapore. The strength of the currents is primarily affected by tides and winds and generally increases during the monsoon seasons across various frequencies.
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Ivan D. Haigh
David Lallemant
Kyle Morgan
Dongju Peng
Masashi Watanabe
Adam D. Switzer
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(3282 KB) - Metadata XML
-
Supplement
(1604 KB) - BibTeX
- EndNote
- Final revised paper