Application and Comparison of Satellite-Derived Sea Surface Temperature Gradients to Identify Seasonal and Interannual Variability off the California Coast: Preliminary Results and Future Perspectives

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this article, the author compares the calculation of Sea Surface Temperature (SST) gradients derived from the MUR and OSTIA datasets in the offshore California coastal region, exploring the potential sources of discrepancies. This study offers valuable insights for future research on utilizing SST data to analyze upwelling zone dynamics. However, the manuscript has three main limitations:

1. The figures lack clarity and visual appeal, with some appearing improperly cropped.
2. The error analysis should incorporate multi-year statistical results rather than relying on a single observation.
3. In Section 4.4 (Spectrum and Coherence), the analysis should be extended to the entire study area using specific parameters instead of focusing on individual points or latitudes.

Given these issues, I conclude that the article requires major revisions. Specific recommendations for improvement include the following:

 

1. Line 21-23. The abstract does not sufficiently summarize the key findings. For instance: 1. While the study mentions examining "issues of both cloud cover and the collocation of the MUR SST to the OSTIA SST grid product," it fails to report the specific results of these analyses (e.g., the magnitude of discrepancies or their spatial patterns). The actual differences between the two datasets (e.g., mean gradient variations, regional biases) should be explicitly stated to highlight the study’s contributions.
2. Consider expanding the keyword list to improve discoverability.
3. The manuscript contains several citations that do not conform to standard formatting guidelines.
4. Line 46-49 ‘They also had correlations of 0.97 and 0.98 when directly compared with MUR SST gradients with those derived from Saildrones’ and ‘Correlations of 0.81 between the MUR and Saildrone derived gradients indicate that MUR was able to derive SST gradients along the coastal region of the California Coast.’ Both sentences suffer from unclear expression and insufficient explanation. Substantial revisions are necessary to enhance comprehension.
5. Lines 81–82: " One goal will be to examine whether the application of SST gradients leads to similar analysis using winds" Does "using winds" here refer to wind data? Did you use data from two datasets?
6. Line 90. Can linear trend analysis effectively capture decadal-scale changes?
7. The introduction currently lacks conceptual coherence. The opening paragraphs introduce certain research focuses, but later sections shift to different priorities without proper justification. This creates confusion about the study's core objectives.
8. The mathematical formulations in the current manuscript would benefit from improved visual clarity and typographic consistency.
9. Line 158. ‘2Xgrid distance’？
10. Line 170 ‘2002-20223’？
11. Line 166-168 ‘This issue was addressed because, although re-gridded at a lower resolution (5 km), the MUR collocated data for seasonal means showed decreased gradients compared to OSTIA.’ Do you have any cited references?
12. Could the authors clarify whether alternative interpolation methods (e.g., spline interpolation, linear interpolation, or other approaches) were tested during the analysis?
13. Line 174. Could the authors explain the scientific basis for defining winter as JFM rather than the standard DJF period?
14. The numerical precision in Figure 1 appears unnecessarily high, with values displayed to four significant digits. The color scheme in Figures 3-4 requires clarification regarding white-colored regions.
15. Regarding Figure 7 and the statement in Line 270: "Lower correlations exist along the coast indicative of differences in the area of coastal upwelling," the following observations and questions arise:1. The graph actually shows lower correlations in the offshore area, with values decreasing further offshore. This appears inconsistent with the claim about coastal regions. 2. Comparing Figure 5 reveals that as the difference between the two datasets approaches zero, the correlation also nears zero, suggesting fundamental inconsistencies in their variability. 3. the correlation analysis should include confidence testing, which is currently lacking in the presentation.
16. The content in Lines 273–381 has already been covered in the previous analysis and is therefore omitted here.
17. Why does Figure 8 not include data for all four seasons? Furthermore, confidence testing should be performed to validate the results.
18. Section 4.1 (Seasonal Signal) lacks substantive analytical content and requires significant supplementation. Similarly, Section 4.2 (Trend Analysis) fails to provide substantial original analysis, largely duplicating previous content. Furthermore, neither section includes meaningful discussion.
19. Line 342-345. ‘There are several reasons this could be the cause which includes issues such as the MUR 1 km gradients being derived after collocating with 5km OSTIA using a nearest neighbor approach. The nearest neighbor would mask out variability that is associated with smaller scale frontal features.’ Could you provide any relevant analytical results to support this finding? Additionally, are there any direct visual representations (e.g., figures or images) available?
20. The text states (Lines 361-362): 'Using a nearest neighbor approach, unlike an average, or interpolation could lead to missing high resolution frontal features in the MUR SST data.' What are the rationale for persisting with this approach given these drawbacks?
21. While Line 399 indicates 'Visually, it is difficult to determine differences,' the analysis would benefit from including actual quantitative differences between the two cases.
22. Please clarify which MUR SST dataset was subtracted from OSTIA in Figure 10d.
23. The relatively small differences between red lines in Figure 11(a)-(d) demonstrate that both the NN and averaging methods have limited influence on SST gradients, which contradicts the pronounced impact suggested in the discussion.
24. Line 454 ‘3°4N？
25. A thorough reorganization and rewriting of the Conclusion section is recommended to strengthen the manuscript's final presentation.

Author Response

Reply to Review REPORT 1

 

We thank the Reviewer for providing useful comments for improving the manuscript. Please find below our point-by-point replies.

In this article, the author compares the calculation of Sea Surface Temperature (SST) gradients derived from the MUR and OSTIA datasets in the offshore California coastal region, exploring the potential sources of discrepancies. This study offers valuable insights for future research on utilizing SST data to analyze upwelling zone dynamics. However, the manuscript has three main limitations:

1. The figures lack clarity and visual appeal, with some appearing improperly cropped.

Thanks, figures have been edited

 

2. The error analysis should incorporate multi-year statistical results rather than relying on a single observation.
   
   

Thanks. We already have an error time series over a ~20 years span and clearly shows seasonal variability.Figure 12 gives an exemplary time series of errors associated with the MUR data. It also includes error analysis maps on two given dates.

 

3. In Section 4.4 (Spectrum and Coherence), the analysis should be extended to the entire study area using specific parameters instead of focusing on individual points or latitudes.
   
   

Spectral analysis is performed over several transects to account for different conditions (e.g. cloud cover and upwelling intensity) . The proposed approach to provide single spectra for each transect is the best way to investigate the SST variability across scales (see also Ciani et al. 2020, where spectra are presented for small sub-basins to investigate SST variability, https://www.mdpi.com/2072-4292/12/10/1601 ). The locations were chosen as exemplary of a near coast and offshore coordinates. The overall rationale was to examine the spectra associated with areas of possible upwelling.

Given these issues, I conclude that the article requires major revisions. Specific recommendations for improvement include the following:

 

1. Line 21-23. The abstract does not sufficiently summarize the key findings. For instance: 1. While the study mentions examining "issues of both cloud cover and the collocation of the MUR SST to the OSTIA SST grid product," it fails to report the specific results of these analyses (e.g., the magnitude of discrepancies or their spatial patterns). The actual differences between the two datasets (e.g., mean gradient variations, regional biases) should be explicitly stated to highlight the study’s contributions.
   
   

Some key information (then discussed in the manuscript) are now provided in the abstract. We provided details on the SST datasets resolutions and the maximum gradient intensities observed in the study area

 

2. Consider expanding the keyword list to improve discoverability.

Done

3. The manuscript contains several citations that do not conform to standard formatting guidelines.

 

References in the manuscript have been inserted in the “[x]” format (as generally done in RS journal). Some details in the References section have been updated

 

4. Line 46-49 ‘They also had correlations of 0.97 and 0.98 when directly compared with MUR SST gradients with those derived from Saildrones’ and ‘Correlations of 0.81 between the MUR and Saildrone derived gradients indicate that MUR was able to derive SST gradients along the coastal region of the California Coast.’ Both sentences suffer from unclear expression and insufficient explanation. Substantial revisions are necessary to enhance comprehension.

This has been clarified

 

5. Lines 81–82: " One goal will be to examine whether the application of SST gradients leads to similar analysis using winds" Does "using winds" here refer to wind data? Did you use data from two datasets?

The sentence has been removed, in order not to create confusion. A wind analysis was not performed, actually.

 

6. Line 90. Can linear trend analysis effectively capture decadal-scale changes?
   Thanks for this comment. We specified that we wish to investigate long-term changes and not specifically decadal variability

 

7. The introduction currently lacks conceptual coherence. The opening paragraphs introduce certain research focuses, but later sections shift to different priorities without proper justification. This creates confusion about the study's core objectives.

We disagree with the reviewer on this point. We feel that our analysis provides guidance for users of the OSTIA and MUR SST products in the study of upwelling systems.  There are differences between the products and we examine the reasons for those differences.

 

8. The mathematical formulations in the current manuscript would benefit from improved visual clarity and typographic consistency.
9. Line 158. ‘2Xgrid distance’？

Changed to 2 multipled by the grid spacing.

10. Line 170 ‘2002-20223’？

Thanks, Done

 

11. Line 166-168 ‘This issue was addressed because, although re-gridded at a lower resolution (5 km), the MUR collocated data for seasonal means showed decreased gradients compared to OSTIA.’ Do you have any cited references?

Thank you for pointing this out. This is referring back to the seasonal maps. The following statement was added:

This is in reference to the derived seasonal means for MUR and OSTIA.

 

12. Could the authors clarify whether alternative interpolation methods (e.g., spline interpolation, linear interpolation, or other approaches) were tested during the analysis?

Two approaches were tested, deriving the mean value and the nearest neighbor. This is clarified in figure  10 where a comparison is made. The goal was to show that two regridding approaches did not significantly lead to large differences in Figure 10. The issues of downscaling/upscaling is something that will require further work in future research.

 

13. Line 174. Could the authors explain the scientific basis for defining winter as JFM rather than the standard DJF period?

The following statement was added:

The definition of the seasons should be considered standard and based on using the entire months for that particular season.  However, the JFM period was alsoadopted by Fiedler et al. 2019 (DOI: 10.1002/qj.3644)

 

14. The numerical precision in Figure 1 appears unnecessarily high, with values displayed to four significant digits. The color scheme in Figures 3-4 requires clarification regarding white-colored regions.

We chose the precision based on how gradients change spatially. We believe the color scheme chosen illustrates the spatial variability of the gradients, especially in the coastal regions. The same issue was true for figures 3 and 4. The color scheme chosen illustrates the changes in the seasonal variability, capturing well the coastal changes.

 

15. Regarding Figure 7 and the statement in Line 270: "Lower correlations exist along the coast indicative of differences in the area of coastal upwelling," the following observations and questions arise:1. The graph actually shows lower correlations in the offshore area, with values decreasing further offshore. This appears inconsistent with the claim about coastal regions. 2. Comparing Figure 5 reveals that as the difference between the two datasets approaches zero, the correlation also nears zero, suggesting fundamental inconsistencies in their variability. 3. the correlation analysis should include confidence testing, which is currently lacking in the presentation.
    
    

Parts of the paragraph were rewritten:

Figure 7 shows the correlation between SST                gradients from the OSTIA and MUR data sets for the four different seasons. Significant differences in correlations exist depending on the season. Compared to the other seasons, winter shows a band of low correlation along the coast. Spring shows increasing correlations, but lower correlations still exist along the coast. Summer shows high correlation in the California Current System (CCS) region offshore from the coast, and lower in the coastal band. Lower correlations exist along the coast indicative of differences in the area of coastal upwelling. Fall shows the overall highest correlations extending offshore several 100 kilometers covering the coastal regions and also the region associated with the CCS.

 

16. The content in Lines 273–381 has already been covered in the previous analysis and is therefore omitted here.

Thank you! We would prefer to leave those statements there as they summarize the results. They lead to the summary in table 1 and further discussion about the possible causes of the differences.

 

17. Why does Figure 8 not include data for all four seasons? Furthermore, confidence testing should be performed to validate the results.

Figure 8 (trends) does include all the data of the timeseries. Upwelling is mainly dependent on surface winds and is expected to occur in every season. Although there might be periods of the year where the upwelling signal is stronger, here we want to assess the trends over a ~20 years time scale including all available data.

    Figure 8 shows the application of a linear trend to the MUR and OSTIA SST data, over the full time period from 2002-2023.

In terms of confidence testing other papers are referenced that compare MUR gradients with in-situ measurements. Many papers have been published comparing SST with in-situ data, but not gradients over seasonal time scales. The Saildrone paper does compare directly gradients and is referenced. It shows that significant gradients can be derived from satellites.

 

18. Section 4.1 (Seasonal Signal) lacks substantive analytical content and requires significant supplementation. Similarly, Section 4.2 (Trend Analysis) fails to provide substantial original analysis, largely duplicating previous content. Furthermore, neither section includes meaningful discussion.

Thank you for the important feedback. We feel that this work does include “substantial original analysis”.  To the best of our knowledge there is no previous work comparing the seasonal gradients derived from MUR and OSTIA.  There is no mention in the work that the intent is to show for the first time the seasonal cycle of coastal upwelling. In fact references are cited of previous work on the identification of the seasonal cycle of coastal upwelling as well as trends in the upwelling. We feel the main emphasis of the work here is to identify satellite derived SST products that could potentially be used for identifying gradients and possible fronts associated with coastal upwelling. It is intended to elucidate future research.  Thus we believe there is new work presented which also sets up even more new research for the future.  

 

19. Line 342-345. ‘There are several reasons this could be the cause which includes issues such as the MUR 1 km gradients being derived after collocating with 5km OSTIA using a nearest neighbor approach. The nearest neighbor would mask out variability that is associated with smaller scale frontal features.’ Could you provide any relevant analytical results to support this finding? Additionally, are there any direct visual representations (e.g., figures or images) available?

Figure 10 shows the comparison we did between taking the MUR/OSTIA difference using the mean for the co-location and the nearest neighbor. The differences are pretty much nonexistent. One of the primary conclusions of the paper is that future work does need to be done on this issue. The primary conclusion in the work is that most likely the differences can most likely be due also to cloud cover. This is covered in section 4.3.

 

20. The text states (Lines 361-362): 'Using a nearest neighbor approach, unlike an average, or interpolation could lead to missing high resolution frontal features in the MUR SST data.' What are the rationale for persisting with this approach given these drawbacks?

 

The rationale is stated in the text as because of the issue of computer processing time. We now have 20+ years of high resolution 1km MUR and 5km OSTIA data. Running a bilinear interpolation or even the mean would take several hours.

 

21. While Line 399 indicates 'Visually, it is difficult to determine differences,' the analysis would benefit from including actual quantitative differences between the two cases.

 

Just added that over a majority of the region differences varied less that 0.003 degrees/km.

 

22. Please clarify which MUR SST dataset was subtracted from OSTIA in Figure 10d.

Thank you

Done added “nearest neighbor”

 

23. The relatively small differences between red lines in Figure 11(a)-(d) demonstrate that both the NN and averaging methods have limited influence on SST gradients, which contradicts the pronounced impact suggested in the discussion.
24. Line 454 ‘3°4N？

Thanks for reporting this. That was corrected.

 

25. A thorough reorganization and rewriting of the Conclusion section is recommended to strengthen the manuscript's final presentation.

 

We are confident the conclusions are effective in summarizing the highlights of the study and identifying future research lines.

 

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Ocean fronts have significant impacts on various fields such as fisheries, military operations, and marine environmental protection. The gradients of sea surface temperature (SST) at ocean fronts are exceptionally large, making SST gradients a powerful monitoring tool for the formation and evolution of ocean fronts. In this manuscript, the authors systematically compared the gradient characteristics of MUR and OSTIA SST products along the California coast, revealing differences in seasonal/interannual variability and the influence of cloud cover, while proposing future research priorities. The study holds practical value.

However, after reviewing the full manuscript, the following questions require clarification from the authors:

1. Page 1, Line 28. There is a typographical error in the "Keywords" section. Please review and revise.
2. Page 2, Lines 46-47. The author states in the text: "They also had correlations of 0.97 and 0.98 when directly compared with MUR SST gradients with those derived from Saildrones." Please verify whether this statement is correct.
3. Page 2, Line 55. The author states in the text: "Upwelling areas are also characterized by the surface features the coastal transport leads to (i.e., fronts, eddies)." Please check and confirm whether this statement needs revision.
4. Page 2, Line 87. Here and in subsequent sections, abbreviations such as OSTIA, MUR, GHRSST, and RMSD are repeatedly redefined.
5. Page 2, Lines 92-93. Punctuation is missing here.
6. Page 3, Line 127. Redundant abbreviation definition: "Advanced Very High Resolution Radiometer (AVHRR)."
7. Page 4, Line 156. Equations (1-3) are not formatted correctly.
8. Page 4, Lines 159-160. The author states in the text: "Once the SSTgrad was derived it was used to derive trends in the gradients." Please check whether punctuation is missing here.
9. Page 4, Line 170. The author states in the text: "...from 2002-20223." Please check and correct the date error.
10. It is recommended that the latitude and longitude labels on the figures be placed in the blank area outside the frame for better clarity.
11. Page 8, Line 243. The author states in the text: "...cloud free pl pixels." Please check and correct this expression.
12. Page 12, Line 325. The author states in the text: "...than MUR. ." Please check and correct this expression.
13. Page 13, Line 375. The author mentions in the text: "The dt parameter in the MUR data indicates the number of hours to the first cloud-free MODIS pixel." Please explain in detail what this statement means. Why is it "the number of hours to the first cloud-free MODIS pixel" rather than the number of hours for cloud-free pixels at each pixel location?
14. Page 17, Line 498. The author claims the data covers 2002-2023, but why does the x-axis maximum in Figure 12 extend to 2025?
15. Page 18, Line 512. It is recommended to modify Table 2 into a three-line table format for the three columns to improve clarity.
16. Page 18, Line 515. The author states: "To further analyze the difference between datasets trends in SST gradients we examine the..." Is there a missing comma after "gradients"?
17. The labels in Figures 13, 17, and 18 are difficult to read.
18. Page 20, Line 558. The author states in the text: "...the MUR and SST..." Please check whether this expression is correct.
19. Page 22, Line 613, as well as Figures 19 and 20, all display the year 2024. This is inconsistent with the timeframe mentioned earlier. Please check whether this is an error.

20. Higher spatial resolution better reflects the detailed variations in SST. Could the authors quantitatively explain the difference in resolution and cloud coverage contributing to OSTIA SST gradients being larger than MUR SST gradients? Have the authors considered the precision differences between OSTIA and MUR SST measurements?

Comments on the Quality of English Language

None.

Author Response

 

Reply to Review REPORT 2

 

We thank the Reviewer for providing useful comments for improving the manuscript. Please find below our point-by-point replies.

 

Ocean fronts have significant impacts on various fields such as fisheries, military operations, and marine environmental protection. The gradients of sea surface temperature (SST) at ocean fronts are exceptionally large, making SST gradients a powerful monitoring tool for the formation and evolution of ocean fronts. In this manuscript, the authors systematically compared the gradient characteristics of MUR and OSTIA SST products along the California coast, revealing differences in seasonal/interannual variability and the influence of cloud cover, while proposing future research priorities. The study holds practical value.

However, after reviewing the full manuscript, the following questions require clarification from the authors:

1. Page 1, Line 28. There is a typographical error in the "Keywords" section. Please review and revise.

Thanks. That was corrected

 

2. Page 2, Lines 46-47. The author states in the text: "They also had correlations of 0.97 and 0.98 when directly compared with MUR SST gradients with those derived from Saildrones." Please verify whether this statement is correct.

Thanks. The statement has been corrected, also providing the correct references

 

3. Page 2, Line 55. The author states in the text: "Upwelling areas are also characterized by the surface features the coastal transport leads to (i.e., fronts, eddies)." Please check and confirm whether this statement needs revision.

Thanks. This has been corrected

 

4. Page 2, Line 87. Here and in subsequent sections, abbreviations such as OSTIA, MUR, GHRSST, and RMSD are repeatedly redefined.
   Thanks, That was corrected

 

5. Page 2, Lines 92-93. Punctuation is missing here.
   Thanks, That was corrected

 

 

6. Page 3, Line 127. Redundant abbreviation definition: "Advanced Very High Resolution Radiometer (AVHRR)."

Thanks, That was corrected

 

7. Page 4, Line 156. Equations (1-3) are not formatted correctly.
   Thanks, That was corrected

 

8. Page 4, Lines 159-160. The author states in the text: "Once the SSTgrad was derived it was used to derive trends in the gradients." Please check whether punctuation is missing here.
   Thanks, That was corrected

 

9. Page 4, Line 170. The author states in the text: "...from 2002-20223." Please check and correct the date error.
   Thanks, That was corrected

 

10. It is recommended that the latitude and longitude labels on the figures be placed in the blank area outside the frame for better clarity.
    This is consistent with figures already published in previous papers in the Remote Sensing Journal (e.g. https://doi.org/10.3390/rs15020484)
    
    
11. Page 8, Line 243. The author states in the text: "...cloud free pl pixels." Please check and correct this expression.
    Thanks, that was corrected

 

12. Page 12, Line 325. The author states in the text: "...than MUR. ." Please check and correct this expression.
    Thanks, that was corrected

 

13. Page 13, Line 375. The author mentions in the text: "The dt parameter in the MUR data indicates the number of hours to the first cloud-free MODIS pixel." Please explain in detail what this statement means. Why is it "the number of hours to the first cloud-free MODIS pixel" rather than the number of hours for cloud-free pixels at each pixel location?
    
    

The manuscript has been edited as follows:

The dt_1km_data variable contains the number of hours between the analysis time of the MUR file and the most recent MODIS 1 km L2P datum within 0.01 degrees of the grid point. MODIS cloud-free data are only ingested if they are available within the +/- 2 days of the analysis time of the MUR file, so the dt variable has a range of -50 to 50 hours. 

 

 

14. Page 17, Line 498. The author claims the data covers 2002-2023, but why does the x-axis maximum in Figure 12 extend to 2025?
    The figure 12 was corrected

 

15. Page 18, Line 512. It is recommended to modify Table 2 into a three-line table format for the three columns to improve clarity.

Thanks, This has been modified as below:

 

 

	MEAN	Nearest NeighbourN
Mean OSTIA (°C)	0.013	0.013
Mean MUR (°C)	0.016	0.016
RMS OSTIA (°C)	0.014	0.014
RMS MUR (°C)	0.014	0.014
Bias  (°C)	0.002	0.002
Correlation	0.63	0.61

 

16. Page 18, Line 515. The author states: "To further analyze the difference between datasets trends in SST gradients we examine the..." Is there a missing comma after "gradients"?

Thanks, this has been corrected

 

17. The labels in Figures 13, 17, and 18 are difficult to read.

Thanks, that was corrected

 

18. Page 20, Line 558. The author states in the text: "...the MUR and SST..." Please check whether this expression is correct.

Thanks that was corrected

 

19. Page 22, Line 613, as well as Figures 19 and 20, all display the year 2024. This is inconsistent with the timeframe mentioned earlier. Please check whether this is an error.

Thanks, that was corrected

20. Higher spatial resolution better reflects the detailed variations in SST. Could the authors quantitatively explain the difference in resolution and cloud coverage contributing to OSTIA SST gradients being larger than MUR SST gradients? Have the authors considered the precision differences between OSTIA and MUR SST measurements?

 The OSTIA L4 SST analysis uses geostationary observations, yielding  a larger number of input IR SSTs (e.g. from the SEVIRI and GOES sensors) compared to MUR, which  only ingests SSTs from polar-orbiters.

 

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript “Application and comparison of Satellite Derived Sea Surface Temperature Gradients to identify seasonal and interannual  variability off the California Coast: preliminary results and future perspectives“  revealed some interesting results. However,some comments need to be clarified.

• The authors should review previous studies and add the references. There are lots of studies concerning the SST gradient or ocean fronts. The authors should clarify the difference between the gradient and ocean fronts. Why not compare the ocean front?
• To compare two products, Why not merging the two products to do the comparison?
• In figure 8, are the trend of SST gradient different from the SST trend?

In the abstract, some expressions should be modified. And some grammars should be modified. “Additionally both products show an increasing trend of SST gradients near the coast. However differences exist between the two products. Issues of both cloud cover and the collocation of the MUR SST to the OSTIA SST grid 22 product were examined. Additionally,“

The figures/Tables should be modified. They are too coarse to be read.

Many grammar errors or typos exist in the manuscripts.

P4. ranging from 2002-20223

Comments on the Quality of English Language

“Additionally both products show an increasing trend of SST gradients near the coast. However differences exist between the two products. Issues of both cloud cover and the collocation of the MUR SST to the OSTIA SST grid 22 product were examined. Additionally,“

Author Response

Reply to Review REPORT 3

 

We thank the Reviewer for providing useful comments for improving the manuscript. Please find below our point-by-point replies.

The manuscript “Application and comparison of Satellite Derived Sea Surface Temperature Gradients to identify seasonal and interannual  variability off the California Coast: preliminary results and future perspectives“  revealed some interesting results. However,some comments need to be clarified.

• The authors should review previous studies and add the references. There are lots of studies concerning the SST gradient or ocean fronts. The authors should clarify the difference between the gradient and ocean fronts. Why not compare the ocean front?

SST gradients are more widespread than actual fronts, enabling to capture the 2D SST variability, so the comparison among products is broader. Additionally, the focus of the paper was on the comparison of SST gradients from the two products, which to the best of our knowledge is new.

 

• To compare two products, Why not merging the two products to do the comparison?

 

The analysis in the paper actually does merge them in the calculation of biases and calculation of differences in gradient intensities.  The merging was done by nearest neighbor matching of MUR data to the OSTIA grid.

• In figure 8, are the trend of SST gradient different from the SST trend?
  The scope of this work is to detect trends on SST spatial gradients, rather than SST itself

 

In the abstract, some expressions should be modified. And some grammars should be modified. “Additionally both products show an increasing trend of SST gradients near the coast. However differences exist between the two products. Issues of both cloud cover and the collocation of the MUR SST to the OSTIA SST grid 22 product were examined. Additionally,“

Thanks, we reworded the abstract as below:

 

However differences in gradient intensities exist between the two products (maximum gradient intensities were around 0.11 and 0.06 °C/km for OSTIA and MUR, respectively). The potential contribution of both cloud cover and the collocation of the MUR SST to the OSTIA SST grid product to these differences were examined.

 

The figures/Tables should be modified. They are too coarse to be read.

Many grammar errors or typos exist in the manuscripts.

Thanks, figures and tables were edited

P4. ranging from 2002-20223

That was corrected

 

 

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors 1. The correlation coefficient in Figure 7 needs to be subjected to a significance test. 2. The linear trend in Figure 8 also requires significance testing.

Author Response

 

Reviewer 1: “1. The correlation coefficient in Figure 7 needs to be subjected to a significance test.

 

We added the following sentence:

Figure 7 indicated that the highest correlations were on the order of 0.5. A confidence test for 6000 independent samples would indicate statistical significance with a p-value of 0.05 or 95 percent confidence. Thus, over a majority of the area close to the coast (red), statistically significant correlations exist between MUR and OSTIA.

 

 

The linear trend in Figure 8 also requires significance testing.”

 

Using the IDL linfit routine we examined the statistical significance for the trends observed in Figure 8. The linfit routine in IDL allows one to derive a SIGMA value which is the uncertainty in the trend. Close to the coast uncertainty values in the trends were on the order 0.000015 degrees C/km/day while trends were on the order of: .0002.  Thus, uncertainties were considerably lower than the magnitude of the trends, indicative of the statistical significance of the trends. Additionally, the idl linfit routine has a probability value which indicates the overall the probability that of the fit being representative. Thus, values close to the coast, based on these tests, could all be identified as representative of the trends in the MUR and OSTIA data.   

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you very much for the author’s response to my suggestions. However, as the changes are not highlighted in the paper, it is quite difficult to confirm the modifications. Additionally, regarding my previous question, I would like to revisit it.

 

1. Higher spatial resolution better reflects the detailed variations in SST. Could the authors quantitatively explain the difference in resolution and cloud coverage contributing to OSTIA SST gradients being larger than MUR SST gradients? Have the authors considered the precision differences between OSTIA and MUR SST measurements?

Author Response

Reviewer 2:  Higher spatial resolution better reflects the detailed variations in SST. Could the authors quantitatively explain the difference in resolution and cloud coverage contributing to OSTIA SST gradients being larger than MUR SST gradients? Have the authors considered the precision differences between OSTIA and MUR SST measurements?

 

Perhaps we need to add the following sentence:

A major point in the paper is that more work needs to be done to clearly understand the differences. The authors have identified two possible differences between the data sets: 1) Clouds 2) interpolation of MUR to the lower resolution OSTIA grid. Figure 5 quantifies the differences in gradients between the two data sets. Clearly, along the coast for the summertime, gradients in  OSTIA show higher gradients than MUR with maxima reaching 0.015 Degrees C/km.  

The reviewer makes the point about the precision differences between OSTIA and MUR. As both use similar inputs, except for Geostationary satellites incorporated in OSTIA, differences should not be related to issues of precision differences between the satellites.  Although in the manuscript we do not describe in detail the processing of the MUR and OSTIA products references are given. MUR uses a wavelet approach which preserves the collocation of SST value. Thus this is another reason why explanations for the differences indentified must be understood in the context of such issues as clouds and collocation of MUR to the lower resolution grid.  We thank thank the reviewers for the comments and hope this clarifies the issues.  

Reviewer 3 Report

Comments and Suggestions for Authors

The authors do not address the comments in detail. The improvement is very limited. The authors lack telling the added value in the study since they did not review the previous studies. 

Can the authors tell whether the difference exists if the gradients or ocean fronts are used for the study?  

Can the author display the difference between the trend of SST gradient and the SST trend?

Author Response

Reviewer 3: The authors do not address the comments in detail. The improvement is very limited. The authors lack telling the added value in the study since they did not review the previous studies. 

Can the authors tell whether the difference exists if the gradients or ocean fronts are used for the study?  

Can the author display the difference between the trend of SST gradient and the SST trend?

The paper is focused on SST gradients because that is the new contribution. Although there has been work examining SSTs and trends,  the issue of gradients has not been addressed in detail.  A goal of the paper is, because of the importance,  to initiate evaluation and usefulness of satellite derived SST products in one of the world’s major upwelling zones. Previous studies on SST gradients were reviewed(see below). Additionally, studies were cited related to possible trends in upwelling and thus impacts on SST gradients.  In the paper itself we did not review of derivation of fronts from gradients, etc. There is extensive research on the issue of the derivation of fronts. The rationale for not reviewing that literature is that it wasn’t the focus of the paper. We wanted to focus on gradients and the differences between MUR and OSTIA GHRSST level 4 products. We felt this was a new subject that needs to be explored.

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2. Vazquez-Cuervo, J.; Gomez-Valdes, J.; Bouali, M.; Miranda, L.E.; Van der Stocken, T.; Tang, W.; Gentemann, C. Using Saildrones to Validate Satellite-Derived Sea Surface Salinity and Sea Surface Temperature along the California/Baja Remote Sens. 2019, 11, 1964. https://doi.org/10.3390/rs11171964
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