Relative Sea Level Trends for the Coastal Areas of Peninsular and East Malaysia Based on Remote and In Situ Observations

Round 1

Reviewer 1 Report

Line 60: Please revise the sentence that includes "...monitor RSL monitoring..."

Figure 1 needs improvements. It is impossibe to read/understand some of the inscriptions.

Line 190: remove "to" after w.r.t.

Line 536: "timer" should read "time"

Line 580: "positon" should read "position"

Line 581: Shouldn't it be 1.96 sigma instead of 1 sigma?

Page 19, 4th line: I would like to see in supp material, an extended description on the parts of the position time series that were excluded. As such information will allow a repetition of the analysis (and confrontation if needed).

Figure 11: all information printed over land is hard/impossible to read. 

Line 746: A mismatch of 2~3 mm/yr seems to be quite significant, provided it represents one order of magnitude larger than the SD obtained in both methods/timeseries. The authors' comment in this line minimizes/omits this disparity and can be misleading.

Author Response

Thank you for your review and the useful comments.

Line 60: Please revise the sentence that includes "...monitor RSL monitoring..."

Ok. Revised

Figure 1 needs improvements. It is impossibe to read/understand some of the inscriptions.

Figure 1 is high resolution, and should be readable when viewed separately. We also have increased the size of the figure in the manuscript, and the small inscriptions should be readable.

Line 190: remove "to" after w.r.t.

Ok. Revised

Line 536: "timer" should read "time"

Ok. Revised

Line 580: "positon" should read "position"

Ok. Revised

Line 581: Shouldn't it be 1.96 sigma instead of 1 sigma?

Yes, thanks for spotting as should be indeed 1.96!

Page 19, 4th line: I would like to see in supp material, an extended description on the parts of the position time series that were excluded. As such information will allow a repetition of the analysis (and confrontation if needed).

Ok, we now provide this information (Table A2) in the supplementary material. We there also have added additional information on the position jump estimation and an overview (Table A1) of all the estimated position jumps and station connection ties. This should allow a smooth repetition of the GPS time series analysis.

Figure 11: all information printed over land is hard/impossible to read.

We have opted to split Figure 11 into two separate figures (11 and 12). Hence the 2 plots can be printed bigger, and we have also ensured we now use the highest resolution for both plots. At the same time we have re-ordered the 3 plots of Figure 12 (now Figure 13) to make each of the individual plots clearer to the readers.

Line 746: A mismatch of 2~3 mm/yr seems to be quite significant, provided it represents one order of magnitude larger than the SD obtained in both methods/timeseries. The authors' comment in this line minimizes/omits this disparity and can be misleading.

We actually consider the discrepancy in between the RSL and ASL (SALT) minus VLM (GNSS) quite significant for East-Malaysia, as we do not observe these differences for Peninsular Malaysia. We also have tried to investigate possible causes further down in this section. Hence we have more confidence in the RSL results obtained from ASL minus VLM for East-Malaysia. We have rephrased this paragraph to ensure other readers understand we do not intend to minimize or omit this disparity.

Reviewer 2 Report

This paper discussed sea level trends around Malaysia using remote and in situ records. The topic of this paper fits within the stated scope of Remote Sensing and their results seem reasonable. However, several aspects could be further improved in order to having it published in this journal. Therefore, my recommendation is major revision. The main questions I encountered when reading the manuscript are as follows:

 

Major technical concerns:

The authors only use T/P-Jason series for sea level trends, which in not enough. I suggest that the authors can use gridded multi-satellite sea levels (Taburet et al., 2019). This product is originated from all accessible satellite altimeter records.

 

Reference:

Taburet, G.; Sanchez-Roman, A.; Ballarotta, M.; Pujol, M.-I.; Legeais, J.-F.; Fournier, F.; Faugere, Y.; Dibarboure, G. DUACS DT2018: 25 years of reprocessed sea level altimetry products. Ocean Sci. 2019, 15, 1207–1224.

 

Major editorial concerns:

1、The abstract and introduction are not well-written. The authors do not clearly indicate the motivation of their work. I believe that there are numerous studies on sea level trends in the Southeast Asia. What’s the problem or defects of previous researches? What’s the novelty of this paper (compared to previous studies)? Also, too many abbreviations in the abstract, please revise.

2、This paper is not well-organized. A single section is needed to describe the data (tide gauges, satellite altimeters, GNSS records) used in detail. The next section can describe the methods to process data. Table 1 can provide more information, such as time range, the percentage of missing values, as well as their ID in PSMSL/DSMM. If the methodology is too complex, please simplify it and show the details in the Appendix.

 

Minor concerns:

1. L1: what’s the type of this paper? Why Article, Review, Communication are displayed simultaneously?

2. L29: what is ‘SE Asia”? Southeast Asia?

3. L83: change the section title to ‘study area’

4. It is suggested that not use abbreviations in the Figure captions.

5. Some figures are not clear (e.g. Figure 3), please revise.

6. Multi-satellite altimeters (not only T/P-Jason series) should be used and their tracks should be displayed in Figure 4.

7. Too many abbreviations in the main text, please reduce them.

8. Some paragraphs are anomalously short (e.g. L703-704), please revise.

9. The type setting of this paper is in a mess, please revise.

10. L795: What is the meaning of ‘East Malaysia.’ in the start of the sentence?

11. Language can be further improved.

Author Response

Comments and Suggestions for Authors

This paper discussed sea level trends around Malaysia using remote and in situ records. The topic of this paper fits within the stated scope of Remote Sensing and their results seem reasonable. However, several aspects could be further improved in order to having it published in this journal. Therefore, my recommendation is major revision. The main questions I encountered when reading the manuscript are as follows:

Thank you for your review and the useful comments.

Major technical concerns:

The authors only use T/P-Jason series for sea level trends, which in not enough. I suggest that the authors can use gridded multi-satellite sea levels (Taburet et al., 2019). This product is originated from all accessible satellite altimeter records.

Reference:

Taburet, G.; Sanchez-Roman, A.; Ballarotta, M.; Pujol, M.-I.; Legeais, J.-F.; Fournier, F.; Faugere, Y.; Dibarboure, G. DUACS DT2018: 25 years of reprocessed sea level altimetry products. Ocean Sci. 2019, 15, 1207–1224.

We thank the reviewer for his suggestion, but we have published our previous papers (the latest in Remote Sensing on Phuket by Naeije et al, 2022) using the same technique and RADS altimeter database (which is also co-maintained by one of the co-authors).  Also there is an already recently published companion paper (Remote Sens. 2022, 14(23), 6179; https://doi.org/10.3390/rs14236179) where we use the same technique to compute the mean sea-level (MSL) from satellite altimetry.

Though the work (DUACS) as suggested is appreciated by us and serves many applications we intentionally choose to not use a ready-made gridded product. There are a few pitfalls using (somebody else’s) interpolated products: you have no control over the editing, no control over the accuracy, and the data density is not the same over time which can introduce artifacts. Basically, using gridded products you adopt interpolation twice, one time to form the grids, and one more time to translate from the grid cells to the tide gauge position. It is not bad, but also not the best you can do in this respect. We have done similar analyses before making our own gridded products, which is not hard, also from RADS data, but we came to the conclusion that we could do a better job by using ‘original' (raw) data directly in the vicinity of the tide gauge (as described in our paper). It is also quite clear that we want the best data there is, which is data from the reference missions. These have been thoroughly calibrated and validated and they seamlessly connect to each other over time. For data accuracy and data sampling characteristic consistency over time we choose only data over the same (reference) tracks. We do not add any other altimeter sources because that would introduce biases between the different missions and changes in data density (sampling characteristic) over time.

And yes of course there is a price you pay in spatial resolution and the data averaged location can be as far as 150 km away from the tide gauge, but at least for that point it is really the best you can do (we average only the points that are not more than 20 seconds (a pass) apart, so no actual interpolation over larger areas. We think this outweighs having more data in the vicinity that have (unknown) biases and can be of less accuracy.

It is also clear that we apply the same routine for all our data sources, be it altimetry, tide gauge or GNSS measurements; we start off with the raw data and have full control over the data, have knowledge about the shortcomings, the accuracy, and the quality and do NOT depend (too much) on others’ analyses and products: they normally do not come with either a good description or thorough error analysis, and they are fixed, meaning you cannot make changes if you want to exclude a certain processing step. So yes we also do not use PSMSL ready-made tide gauge products but we analyse the raw data obtained from the local agencies, etc. These are all well thought through decisions.

Therefore for this manuscript we refrain from using gridded multi-satellite sea levels but for future work we will investigate if the use of such a product can be beneficial for our study areas in Southeast Asia.

Major editorial concerns:

1、The abstract and introduction are not well-written. The authors do not clearly indicate the motivation of their work. I believe that there are numerous studies on sea level trends in the Southeast Asia. What’s the problem or defects of previous researches? What’s the novelty of this paper (compared to previous studies)? Also, too many abbreviations in the abstract, please revise.

With regards to the abbreviations in the abstract, we have significantly reduced them, and now only the terms ASL, VLM and GNSS are used. It is however still necessary to use more abbreviations in the main text as it would otherwise unnecessarily lengthen the manuscript if we would each time need to spell them out completely.

We also have rewritten the abstract and introduction to better reflect the motivation of our work: estimate relative sea-level changes for entire Malaysia, using the same method/technique to analyse the tide-gauge satellite altimetry and GNSS data and compare 2 independent methods (tide gauge and satellite altimetry minus vertical motion from GNSS, the latter as a first for Malaysia) whereby we also differentiate between the vertical land motion regime before and after the 2004 Sumatra-Andaman earthquake.  All these ingredients were already in the abstract, we have modified it so it will now hopefully be clearer to the readers.  

2、This paper is not well-organized. A single section is needed to describe the data (tide gauges, satellite altimeters, GNSS records) used in detail. The next section can describe the methods to process data. Table 1 can provide more information, such as time range, the percentage of missing values, as well as their ID in PSMSL/DSMM. If the methodology is too complex, please simplify it and show the details in the Appendix.

We assume that the “paper is not well-organized” remark is about that we have opted to use a single section to describe both the in-situ and remote geodetic data and their processing methods. This was a well elaborated choice since we ultimately want to produce relative sea-level rise estimates as the main result and thus regard the (combined) individual data sets and applied methods as input for that. We thought that splitting this into two different sections would lead to unnecessary repetitions and clarifications especially for non-specialist readers and wanted to keep each data and processing technique tidy within a single data & method (sub) section. None of the other 3 reviewers also had an issue with this and we have seen other published paper on this topic use the same approach. We think this is mainly a personal choice/opinion which everyone (both authors and reviewers) are entitled to. We did rename the section title to ‘Data and Methodology’.

We do now provide more information in Table 1 on the TG data.

Minor concerns:

1. L1: what’s the type of this paper? Why Article, Review, Communication are displayed simultaneously?

We sorry as we did not remove the other options.  It should be a scientific article of course.

2. L29: what is ‘SE Asia”? Southeast Asia?

That abbreviation was indeed not explained in the abstract, and it was indeed Southeast Asia. We not use the term SE Asia in the manuscript.

3. L83: change the section title to ‘study area’

Ok. Revised.

4. It is suggested that not use abbreviations in the Figure captions.

We have removed the abbreviations SALT, TG, ASL, RSL and VLM from all the Figure captions. We do keep GPS/GNSS as they can be considered as common knowledge.

5. Some figures are not clear (e.g. Figure 3), please revise.

Some of the figures unfortunately did not come out clearly in the PDF version of the manuscript. In the Word version of the manuscript they are clearer. Nonetheless we have revised and reordered them (e.g. below each other instead of next each other) so they could be enlarged.

6. Multi-satellite altimeters (not only T/P-Jason series) should be used and their tracks should be displayed in Figure 4.

Please refer to our rebuttal to the major technical concern you raised above. We prefer to only use T/P-Jason series for this publication.

7. Too many abbreviations in the main text, please reduce them.

We have removed abbreviations is both the abstract and the figures. The main purpose of abbreviations and acronyms are to save space and to avoid distracting the reader if they appear two or more times in the text. We like to adhere to this principle, but we have included a list of abbreviations in the manuscript. We also like to promote some of the abbreviations to become more part of the common knowledge. There unfortunately also is no other way than to deal with abbreviations for the many tide-gauge and GNSS stations, but they are needed in the main text to identify and mention issues/findings with them.

8. Some paragraphs are anomalously short (e.g. L703-704), please revise.

We checked the entire manuscript and ensured there are no (too) short paragraphs left by combining sentences into the same paragraph were needed.

9. The type setting of this paper is in a mess, please revise.

We agree the type setting of the PDF version of the manuscript is not ideal, but the PDF is mainly intended for reviewing the content of the paper, not the final published format of a manuscript. In the Word version it looks better, but final improvements will surely be made, both by us and MDPI when the manuscript is accepted. This involves checking all figure (sizes) and ensuring we get a good flow of the paper, with complete tables and figures/captions filling each page. Our apologies for any inconvenience during the review reading.

10. L795: What is the meaning of ‘East Malaysia.’ in the start of the sentence?

Somehow the start of the sentence was missing. It should have read: “Relative sea-level (RSL) trends have been estimated from TG data at 21 locations in Peninsular and East Malaysia”. Corrected.

11. Language can be further improved.

We have had the revised manuscript proofread by a native English speaker. We hope the English will now be good enough.

Reviewer 3 Report

Well written and organized study. I do have one comment related to the error analyses of GPS vertical motions. Authors should elaborate in more detail, accurate estimates related to fitting discontinuities due to earthquake events and tectonically stable periods. Uncertainties due to earthquakes are not given (or disregarded). Consequently, estimates of total vertical motions look quite unrealistically small.

Comments related to technical quality of presentations:

1/ Table 1 could provide also time period of tide-gauge data

2/ Increase Figs. 3 , so they are readable.

3/ Move Table 2 down so legend and full table is on one page.

4/ Increase Figs. 5 , especially legend.

5/ Figs. 6, 7 and 12, improve the quality.

5/ Many tables exceed margins.

Authors could consider to publish this study in the special issue: "Geodesy of Earth Monitoring System" of Remote Sensing 

Author Response

Comments and Suggestions for Authors

Well written and organized study. I do have one comment related to the error analyses of GPS vertical motions. Authors should elaborate in more detail, accurate estimates related to fitting discontinuities due to earthquake events and tectonically stable periods. Uncertainties due to earthquakes are not given (or disregarded). Consequently, estimates of total vertical motions look quite unrealistically small.

Thanks for your comments, and you are absolutely correct that these uncertainties due to jumps in the time series as a result of earthquake events should not be ignored.  However, for all far field GPS data (500-850 km away from the 2004 Mw 9.2 and other major earthquake epicenters) there are no (detectable) discontinuities in the vertical position time series due to these earthquake events. The only discontinuities in the vertical position time series came from GNSS equipment (antenna type and/or height) and monumentation changes.  These have been carefully estimated and removed from the vertical time series. So we end up with a clean continuous vertical position time series, where we estimate (split) linear vertical motion trends from. If we would just use the statistics from the linear regression, we would end up (due to the time span and amount of positions) with even lower uncertainties. Therefore we used the described method by Simons et al. (2007) to get a better grip on the trend uncertainties.  We have also used this method for the TG and SALT time series.  We actually explain this already in the manuscript, but we have modified the text in section 3.3.3 to ensure the readers understand that the uncertainties were reliable estimated and are indeed not too small. 

Comments related to technical quality of presentations:

1/ Table 1 could provide also time period of tide-gauge data

This was also suggested by Reviewer 2, and we have now provided more information on the tide-gauge data there.

2/ Increase Figs. 3 , so they are readable.

Ok, we have now placed the 2 plots below each other and made them bigger

3/ Move Table 2 down so legend and full table is on one page.

Ok, we have done so, and also checked that all figures and other tables are not crossing pages

4/ Increase Figs. 5 , especially legend.

Ok, as with Fig, 3 we have placed the 2 plots below each other and made them bigger

5/ Figs. 6, 7 and 12, improve the quality.

Ok. Done. Also taking into account comments Reviewer 4

5/ Many tables exceed margins.

We have reduced the size of the wide tables, and they should no longer exceed the margins. Also the final type setting by MDPI will probably ensure the table margins are nowhere exceeded.

Authors could consider to publish this study in the special issue: "Geodesy of Earth Monitoring System" of Remote Sensing

That is a good suggestion, and we will consider this as long as the special issues does not significantly delay the final publishing of the article.

Reviewer 4 Report

In this paper, the author's relative sea-level trends across the entire of Malaysia are investigated. The paper deals with the data (1984-2019) from tide-gauges (TG) analysed (RSL), along with a subset (1994-2021) of nearby Malaysian GNSS stations (VLM). At the same time also the absolute sea-level (ASL) changes at these locations (1992-2021) were estimated from satellite altimetry data. The authors wrote, that as a first for Malaysia, the combination ASL minus VLM was used to validate RSL from TG. The authors processed and analyzed a significant volume of data. The obtained results are generally interesting to researchers working in this field. Are you sure there are no publications and studies on this topic?  In my opinion, the introduction, and conclusions have not revealed the novelty of this research with respect to other publications. There is a shortage of references (only 30 papers). What's new the present study is contributing?  First, for Malaysia, the combination ASL minus VLM was used to validate RSL from TG, but in my opinion, the publications are is are on a similar topic. Maybe a new accuracy and/or robustness level was achieved? Maybe something else? Please, clarify this aspect. The abstract does not have a main goal.
Pieces of Advice and Questions:
Many figures are too low-resolution to make sense.
Figures 6 and 7 have no coordinates, please correct them.
What do you think is the reason and influence of the crustal movements in this area?
How did you validate the study results? And What is the limitation of this study?
The methodology is not well-explained/documented.

Author Response

Comments and Suggestions for Authors

In this paper, the author's relative sea-level trends across the entire of Malaysia are investigated. The paper deals with the data (1984-2019) from tide-gauges (TG) analysed (RSL), along with a subset (1994-2021) of nearby Malaysian GNSS stations (VLM). At the same time also the absolute sea-level (ASL) changes at these locations (1992-2021) were estimated from satellite altimetry data. The authors wrote, that as a first for Malaysia, the combination ASL minus VLM was used to validate RSL from TG. The authors processed and analyzed a significant volume of data. The obtained results are generally interesting to researchers working in this field. Are you sure there are no publications and studies on this topic?  In my opinion, the introduction, and conclusions have not revealed the novelty of this research with respect to other publications. There is a shortage of references (only 30 papers). What's new the present study is contributing?  First, for Malaysia, the combination ASL minus VLM was used to validate RSL from TG, but in my opinion, the publications are is are on a similar topic. Maybe a new accuracy and/or robustness level was achieved? Maybe something else? Please, clarify this aspect. The abstract does not have a main goal.

Thanks for your review and useful comments. Although still sparse, there are indeed some other publications on this topic available, be it that they have been published by members of the same research team (and universities) as in this article. Most of the past work however focusses on absolute sea-level (ASL) change studies and use RSL from TG and VLM from GPS to correct the RSL into ASL. So this article can still be considered as a first for Peninsular and East Malaysia that fully focusses on RSL and robustly used the combination ASL minus VLM to validate relative sea-level rise at all tide-gauge location over a 25+ year common data period. We hereby get excellent results for Peninsular Malaysia and try to better understand why the RSL comparisons for East Malaysia may not be as good.

 We do now discuss these other publications in the introduction, and compare our results with the most recent paper in the conclusions.  As a result 7 additional references have now been included. The abstract and introduction should now better reveal the goal and novelty of our research (focus on RSL, longer data time spans, split trend estimates for Peninsular Malaysia to take the trend change after 2005 into account).

Pieces of Advice and Questions:

Many figures are too low-resolution to make sense.

This was also a comment by the other reviewers. The problem was more in the size of the figures than their resolution. We have enlarged these figures were necessary.

Figures 6 and 7 have no coordinates, please correct them.

Ok. We have included them, but they available options to plot them are limited in Google Earth.

What do you think is the reason and influence of the crustal movements in this area?

The crustal movements themselves will have no influence, as by definition they are only horizontal. But you probably refer to the deformation of the crustal plates near their boundaries. The reason (for Peninsular Malaysia) is of course the convergence of the India/Australia and the Sundaland plate. The megathrust seismic cycle is causing vertical deformation that switches between tectonic uplift and subsidence depending on the phase of the seismic cycle. Before 2005, we were in the inter-seismic phase (deformation build up and (for Peninsular Malaysia and Thailand) tectonic uplift. Since then we are in the post-seismic phase, which is causing post-seismic tectonic subsidence that will eventually flatten out and then tectonic uplift will again slowly occur. This explained and taken (as a first for Malaysia) into account in our work as most geodetic data is from both before and after 2005, and therefore a single trend estimation to compare RSL will not give the best result. 

How did you validate the study results? And What is the limitation of this study?

We validate RSL using two independent methods at each tide-gauge location, and we find a very good agreement (with 1 mm/yr) especially for Peninsular Malaysia. There are not many other publication on RSL in Malaysia, and they also make use of a different data time span and use a single trend estimation. They most useful publication to compare with is that of Din et al. (2019) which is now also included in the manuscript and referred to in the results discussion (Chapter 4.0).

The limitation of this study is that we only estimated RSL at tide-gauge stations, as we otherwise could not validate the results from both methods.  So this limits the spatial distribution from the RSL estimates and we opted not to try estimate RSL at other coastal regions (with GNSS stations present) making use of only SALT minus VLM.  Another limitation is that we do not have detailed information on the monumentation of TG and nearby GNSS stations. Hence we cannot determine if the VLM recorded by a GNSS stations is also applicable for the TG station in that location. Most of the results however suggest that this is the case, but co-location of TG and GNSS stations would ensure that the VLM is the same.

The methodology is not well-explained/documented.

This comment unfortunately is a bit too general to take into account. We tried our best to explain and document all the methodology very well, and we have used a very similar approach that was explained in detail in Simons et al. (2019) and Naeije et al. (2022) for a similar study in the south of Thailand. We refer to these and other papers for more details, as we did not want to duplicate the methodology description in this article and saturate non-specialist readers with the (same details). We did include some additional information in Table 1 (requested by reviewers 3 and 4) and on the data excluded in the GPS time series (requested by reviewer 1) as well on the position jump estimation in the supplementary materials.

Round 2

Reviewer 2 Report

This paper can be accepted.