The 2023 Major Baltic Inflow Event Observed by Surface Water and Ocean Topography (SWOT) and Nadir Altimetry
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript provides a valuable analysis of the 2023 Major Baltic Inflow (MBI) event using SWOT altimetry, combined with traditional nadir altimetry and tide gauge data. The methodology is sound, and the results contribute significantly to understanding MBI dynamics in the Baltic Sea. However, several points are recommended to revised to enhance the quality of the manuscript:
1. The observation and prediction of ocean dynamic phenomena have been extensively investigated from diverse perspectives. In this context, recent studies focusing on correcting numerically forecasted sea surface dynamic fields using adversarial temporal convolutional networks are particularly noteworthy and warrant a comprehensive review.
2. Figure 1 could be enhanced by employing distinct shapes or symbols to differentiate tide gauges, thereby improving intuitiveness for readers.
3. The English text in Figure 3 does not align with the accompanying description.
4. It is recommended to avoid including additional English text on the image to maintain clarity and consistency.
5. Incorporating quantitative analysis is recommended to further substantiate the conclusions and enhance the robustness of the findings.
6. The discussion of discrepancies between SWOT data and model simulations could be expanded to include a more in-depth analysis of potential causes, such as model limitations, and to provide actionable suggestions for improvement.
Author Response
- The observation and prediction of ocean dynamic phenomena have been extensively investigated from diverse perspectives. In this context, recent studies focusing on correcting numerically forecasted sea surface dynamic fields using adversarial temporal convolutional networks are particularly noteworthy and warrant a comprehensive review.
Thank you for pointing out this very promising technique for the integration of remote sensing data into numerical models. We have identified two papers by Ren et al. (2025) and Kosana et al. (2022) that use the proposed technique in the geosciences. However, our present paper focuses on the measurements of the novel SWOT mission. Accordingly, the improvement of numerical models by machine learning is beyond the scope of this paper and not discussed further.
- Figure 1 could be enhanced by employing distinct shapes or symbols to differentiate tide gauges, thereby improving intuitiveness for readers.
We have included Table 2 at line 190-192 containing a list of the tide gauges used for the three Danish Straits and across the Darss Sill and the Arkona Sea.to sort the tide gauge names and the straits to improve the readability of the paper.
- The English text in Figure 3 does not align with the accompanying description.
The legend in the Figure 3 itself as well as the figure caption have been revised.
- It is recommended to avoid including additional English text on the image to maintain clarity and consistency.
We have reorganized and shortened the legend in Figure 7 for clarity and consistency.
- Incorporating quantitative analysis is recommended to further substantiate the conclusions and enhance the robustness of the findings.
The paper investigates a rare transient event using a novel remote sensing technique, that is operated for the very first time on a satellite. For the revised version of the paper, we have additionally introduced the metric of relative explained variance as a measure of the agreement of the new observations from SWOT and also legacy data (from tide gauges and nadir altimetry) with the numerical simulations.
- The discussion of discrepancies between SWOT data and model simulations could be expanded to include a more in-depth analysis of potential causes, such as model limitations, and to provide actionable suggestions for improvement.
Currently, BSH-HBMnoko is already able to explain 80% of SWOT data variance. Besides the continued refinement of model processes and resolution an effective approach to improving this agreement might be to assimilate additional data into the model, that have a direct relation to the sea surface height (e.g., tide gauge data, ocean bottom pressure recorders, or altimetry data).
We have included the following text at the very end of the paper: The temporal sampling rate is not adequate for the process under consideration, but the SWOT observations together with high-rate nadir altimetry can significantly improve the existing in-situ monitoring and open up the prospect of improving the model predictions by assimilating this novel data.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe study analyses a Major Baltic Inflow (MBI) based on sea-level data from the Surface Water and Ocean Topography (SWOT) and high-rate multi-mission nadir altimetry. Additionally, the authors analyse numerical simulations and tide gauge data.
This technical note shows the added value of SWOT data to study MBI events. It should be published after some modifications as detailed in the comments below.
In l.65-66 You refer that: “The last very strong MBI event was observed in 2014, the last moderate MBI in 2016” but this study is focused in a moderate MBI event that occurred in 2023. Please clarify the text.
l.162 and l.186 Specify the criteria used to identify outliers.
l.196 Please specify the “additional parameters” mentioned. Additionally, also specify the frequency of atmospheric forcing and of river run-off data?
l.198 Why are you using monthly climatological temperature and salinity values to analyze a transient event? Do you think this may affect the simulation outcome?
l.238-239 replace “the south-north sea level differences” by “the positive south-north sea level differences”
Please, discuss the two phrases in l.227-229 and l. 246-248, considering the power spectrum shown in Fig 7b.
l.227-229 “The higher frequency signal clearly distinguishable throughout the time series is related mainly to the half-daily tides in the Kattegat.”
l.246-248 “The 1-2 days signal visible in the time series is most probably related to Baltic seiches which have periods from 23 to 27 hours in this region [16,35,36].”
l.274 replace “und” by “and”
l.307 “… there might be an offset of about 5 cm.” Please, explain why you think there might be an offset.
l.332 Remove word “mission”?
l.351 Replace “discussed” by “showed”
In vertical axis units of Fig 7b verify, if you are missing .Hz-1
l.429-430 Specify the differences that you mean by the expression “long wavelength differences”
Add publication year to reference 6
Turn bold publication year of references 23 and 24
The publication year of reference 26 is shown twice!
Author Response
Dear Reviewer 2,
we would like to thank you for your most valuable suggestions that led to an improvement of the paper. We have been able to incorporate most of the suggestions provided. Please find below a point-by-point response to your comments and concerns.
Best regards, Saskia Esselborn
The study analyses a Major Baltic Inflow (MBI) based on sea-level data from the Surface Water and Ocean Topography (SWOT) and high-rate multi-mission nadir altimetry. Additionally, the authors analyse numerical simulations and tide gauge data.
This technical note shows the added value of SWOT data to study MBI events. It should be published after some modifications as detailed in the comments below.
In l.65-66 You refer that: “The last very strong MBI event was observed in 2014, the last moderate MBI in 2016” but this study is focused in a moderate MBI event that occurred in 2023. Please clarify the text.
We have changed the text accordingly to: The last very strong MBI event was observed in 2014, the most recent moderate MBI events in 2016 and 2023.
l.162 and l.186 Specify the criteria used to identify outliers.
In general, the tide gauge data provided by Copernicus are already quality checked and only the Gedser data contains apparent outliers. For Gedser, we only used data flagged as good quality, and eliminated all epochs that differed from the median value by more than 10 cm for 2-hour windows.
For the high resolution nadir altimeter data, we eliminated all SLi and SLA data exceeding ±2 m. In a next step, we eliminated all data that differed by more than 20 cm from the median within 3.5 km (Saral, Sentinel-3A/B, Sentinel-6 MF) and 5 km (all other missions) windows.
We have added this information to the corresponding subsections at lines 169-170 and lines 196-198.
l.196 Please specify the “additional parameters” mentioned. Additionally, also specify the frequency of atmospheric forcing and of river run-off data?
The additional parameters are: air humidity, cloud cover and 2 m-air temperature. Sea Surface Temperature originating from AVHRR & Sentinel-3 is assimilated every 12 hours. The model includes also hourly updated surge values at the open model boundary in the northern North Sea and in the English Channel.
The atmospheric forcing originates from the regional ICON model (13 km mesh size) which provides hourly output. River discharge data are sampled daily and are replaced by long-term mean values when missing.
We have included additional information to the relevant subsection, lines 209-213. However, as we only use the output of the model, we avoid going into too much detail and refer the reader to the literature.
l.198 Why are you using monthly climatological temperature and salinity values to analyze a transient event? Do you think this may affect the simulation outcome?
Monthly temperature and salinity data are only used for the open boundaries in the northern North Sea (61°N) and in the western English Channel (4°W). The response of the Baltic Sea temperature and salinity to changes in the Northeast Atlantic is expected to be on rather on multiannual to decadal time scales (Meier and Kauker, 2003). A the BSH-HBMnoku model is used operationally for water level forecasts and warnings for the German coasts, we are confident that it is capable of simulating transient events such as MBIs in this area.
We have clarified the relevant text on the open boundaries in lines 213-216 which now reads: At the open boundaries in the northern North Sea (61°N) and in the English Channel (4°W) monthly climatological temperature and salinity values, 19 partial ocean tides, and wind surge from BSH’s North East Atlantic model are prescribed [10].
l.238-239 replace “the south-north sea level differences” by “the positive south-north sea level differences”
We have changed the wording accordingly.
Please, discuss the two phrases in l.227-229 and l. 246-248, considering the power spectrum shown in Fig 7b.
l.227-229 “The higher frequency signal clearly distinguishable throughout the time series is related mainly to the half-daily tides in the Kattegat.”
l.246-248 “The 1-2 days signal visible in the time series is most probably related to Baltic seiches which have periods from 23 to 27 hours in this region [16,35,36].”
We have moved this sentences to subsection 4, lines 445 to 449.
l.274 replace “und” by “and”
We have changed the word accordingly.
l.307 “… there might be an offset of about 5 cm.” Please, explain why you think there might be an offset.
We have changed the wording: On 30 December, the sea level patterns observed by SWOT in the Little Belt area are similar to those simulated, but about 5 cm higher. A notable exception is the region just south of the Little Belt, where SWOT observed lower sea levels (~5 cm) than simulated (cf. Figure 5f).
l.332 Remove word “mission”?
We have changed the wording accordingly.
l.351 Replace “discussed” by “showed”
We have changed the word accordingly.
In vertical axis units of Fig 7b verify, if you are missing .Hz-1
We have corrected the units of Figure 7b.
l.429-430 Specify the differences that you mean by the expression “long wavelength differences”
We have changed the wording to: However, there are differences between the two on spatial scales of 50 to 200 km, which appear to be related to model deficiencies on daily to sub-daily time scales.
Add publication year to reference 6
We have eliminated this reference from the list.
Turn bold publication year of references 23 and 24
We have changed the format accordingly.
The publication year of reference 26 is shown twice!
The publication year is part of the title of reference 26.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for AuthorsManuscript:
The 2023 Major Baltic Inflow event observed by SWOT altimetry
Submitted by:
Saskia Esselborn, Tilo Schöne, Henryk Dobslaw and Roman Sulzbach
The structure of the manuscript is well-thought-out and appropriate. The introduction presents an analysis of the problem based on the available literature. The topic is original, and the authors' contribution consists of the comparative studies they conducted.
The research methodology and its implementation are correct. I ask the authors to clarify which vertical datum or height reference system was used for the height values. Was it the Mean Sea Level (MSL) or the currently valid Baltic Sea Chart Datum 2000 (BSCD2000)? Was it height reference system such as Kronstadt, Aarhus, or Kronstadt?
The research results are presented graphically. The figures are clear and properly prepared. The table on page 5 should have its font color adjusted.
The conclusions, based on the conducted research, are clear and evident. The number of cited publications is sufficient to understand the problem. The references are correctly formatted.
Author Response
Dear Reviewer 3,
we would like to thank you for your most valuable suggestions that led to an improvement of the paper. We have been able to incorporate most of the suggestions provided. Please find below a point-by-point response to your comments and concerns.
Best regards, Saskia Esselborn
Manuscript:
The 2023 Major Baltic Inflow event observed by SWOT altimetry
Submitted by:
Saskia Esselborn, Tilo Schöne, Henryk Dobslaw and Roman Sulzbach
The structure of the manuscript is well-thought-out and appropriate. The introduction presents an analysis of the problem based on the available literature. The topic is original, and the authors' contribution consists of the comparative studies they conducted.
The research methodology and its implementation are correct.
I ask the authors to clarify which vertical datum or height reference system was used for the height values. Was it the Mean Sea Level (MSL) or the currently valid Baltic Sea Chart Datum 2000 (BSCD2000)? Was it height reference system such as Kronstadt, Aarhus, or Kronstadt?
The vertical datum of the tide gauges is BSCD200. The Copernicus metadata point to BSCD2000 for the Swedish tide gauges and to DVR90 for the Danish tide gauges. In contrast to the metadata within the Copernicus data, the German tide gauges are referenced to DHHN2016 (personal communication with the data provider (authority) WSA Stralsund). For all practical purposes, however, DVR90 and DHHN2016 coincide with BSCD2000 (https://digitale-bibliothek.bsh.de/viewer/metadata/184272/21/).
For SWOT KaRIn and nadir altimetry missions we use the instantaneous sea level (SLi). The SLi is referenced to the equipotential surface defined by the difference between the mean sea surface derived from the long-term multi-mission altimetry and the mean ocean dynamic topography for the period 1993 to 2012 (https://doi.org/10.22541/essoar.170158328.85804859/v1; https://doi.org/10.3390/rs15112910). This means that there may be an offset to the tide gauge and the BSH-HBMnoku data, both of which are referenced to the datum of the Normaal Amsterdams Peil (NAP). In addition, each altimetry mission may have a mission-specific bias that can be estimated globally or regionally. Here, we use the mean difference to the BSH-HBMnoku model in December 2023 as a common reference for the SWOT KaRIn and the nadir altimetry missions.
The BSH-HBMnoku is referenced to an equipotential surface that should coincide with the official German quasigeoid referenced to DHHN2016. As a consequence, the references of BSH-HBMnoku and the tide gauges coincide. However, it is quite common that numerical ocean circulation models do not adequately capture the mean ocean dynamic topography, which can lead to regional offsets to the tide gauge and the radar altimetry data.
We have included extra information on the height systems at lines 129-131 and lines 224-232 and revised the text about insonsitencies in tide gauge datum (now lines 424-429).
We note, however, that our paper is primarily focused on the opportunities of the SWOT mission and not so much on details of the height systems adopted by the different national authorities. Since both DVR90 and DHHN2016 conincide with BSCD2000, we just refer to this system for the tide gauge data and omit any further details.
The research results are presented graphically. The figures are clear and properly prepared. The table on page 5 should have its font color adjusted.
We adjusted the color font in Table 1and extended the table caption.
The conclusions, based on the conducted research, are clear and evident. The number of cited publications is sufficient to understand the problem. The references are correctly formatted.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript has been largely improved. But I still have issues regarding literature review and formulation which require further improvement.
(a) In the response letter, the authors claimed to have identified two closely related papers but did not include a review of them in the manuscript. After reviewing the two papers, I believe they are highly relevant to the topic discussed in the submitted manuscript and could provide valuable insights or motivations for enhancing the current research. Therefore, I suggest adding one or two sentences to briefly review these papers, thereby making the related literature more comprehensive and well-rounded.
(b) Equations (1)-(3) do not qualify as proper equations. If the authors intend to convey their ideas using equations, they should first define the notations clearly and then structure them into professionally formatted equations. The use of terms like "orbit height" within equations or formulae is highly unprofessional and inconsistent with even high school-level academic standards. Such formulations should be revised to align with conventional mathematical and scientific practices.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
