Sea Surface Salinity Seasonal Variability in the Tropics from Satellites, Gridded In Situ Products and Mooring Observations

Round 1

Reviewer 1 Report

Summary

This manuscript, “Sea Surface Salinity Seasonal Variability in the Tropics from Satellites, in situ compilations and Mooring Observations” by Bingham, Brodnitz and Yu, the authors use various gridded salinity products to produce seasonal statistics.   They compute seasonal harmonics including amplitude, phase, and variance and for each product, these values are compared against the in situ tropical moorings.   The paper is generally well written, well organized, and useful for a researcher considering which data set to use for some scientific study.   Therefore, I would recommend publishing this work but only after some substantial modifications.  

I have two major suggestions for improving the paper.   My primary objection is that the details of the different products are glossed over.   For example, what does “Daily” mean for the Aquarius gridded product (from Table 1)?   The satellite takes about 1 week to produce a global grid, so I suspect this is likely a running mean of some sort.   I recommend that the authors go into detail describing how each gridded product is formulated.   It would be much easier on the reader to have a paragraph explaining the details of how the raw satellite (i.e. L1) data ends up as a gridded product.    The reader should not be asked to track down each reference from the supplemental material.  

The second suggestion to improve the manuscript is that the authors should add a paragraph or two describing the vertical gradient between the SSS and in situ measurements.   The satellite products are fundamentally different than the in situ measurements used to validate the seasonal cycle, etc.   Satellite SSS is valid at ~1cm and the in situ observations are typically measured at ~5m.   Also, EN4 and SIO use these same and other in situ measurements and so are fundamentally different products than the satellite products. Adding the discussion of temporal and spatial sampling of the products will add clarity for the reader.          

Issues/Questions/Comments

~line 45 – important to say what level moorings/satellites measure and please add paragraph on what potential near-suface vertical salinity gradient may impact your results.

Line 55-56 – awkward sentence

Figure 1 – and all figures – Titles have weird spacing in them, please correct this

Line 74 – “up to 0.5 in the tropics, “  what units?

Line 87 – somewhere in the Data and Methods section please add comments on how the different products have different averaging periods and add details about how each product was formulated.

Table 1 – I suggest you add all the details of each product (including references) here instead of shipping it to the supplemental material

Line 96 – “closest grid node to each mooring site” – with your differences in grid size couldn’t the actual observation be relatively displaced by as much as 0.75^o longitude or latitude?

Figure 2 and all Figures – please mark a), b) c) etc. on the figures for clarity.

Line 130-131 – “The amplitude and phase of that annual cycle is relatively stable, except for 2015-2016.  That is likely associated with the El Nino event that occurred at that time”.   I don’t see that result very clearly.  Could you add a blow up of 2015-2016 with the annual cycle overlayed in the Supplemental Material to make your point?   Is this result due to the off-equatorial Rossby wave interrupting the normal annual signal?    

Line 143 – 146 – I read the phase plot (Figure 3) to show maximum in April-May (pink-orange) and the minimum in Oct-Nov (blue-green-green).   Am I reading it wrong?

Line 180 – “October” should be March and “December” should be July (again if I read the plot correctly).  

Figure 7 – Titles on the plot with product name would be helpful (redundant but helpful).

Figure 7 – comment/question   Gridded fields of satellite SSS typically smooth in some manner.  Is there any way to do Figure 7 for the L2 comparison?   I know it’s beyond the scope of this paper, but it might be nice to do this one figure and add it in the Supplemental Material if possible. 

Line 262-266 – when the satellite data are averaged don’t you lose the satellite footprint?  Since the gridding and time averaging details are not included in this paper, the points on line 265-266 are not well understood to the general reader.

Figure 10 – please comment on why you chose these four examples for plotting.

Thanks for the opportunity to see this nice work before it's published.

Author Response

Please see the attachment.

Reviewer 2 Report

The authors used seasonal harmonics to compare sea surface salinity data from satellites and in situ data and found general agreement between mooring observations and the satellite data and in situ products. This manuscript is well organized and useful conclusions are provided. I recommend this manuscript to be accepted subject to minor revisions.

Comments:

1. Because the temporal coverages of the mooring data and satellite observations are different, the authors should address if the harmonics analysis results of the mooring data are consistent for the whole period and for the SSS satellite period after 2010.
2. How is the climatology bias of satellite SSS compared with mooring data? Does it agree with previous studies and this harmonic analysis? Does it vary with seasons?

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 3 Report

Major comments:

It seems that the authors exactly studied annual cycle (both in result and discussion) of the SSS products. So the seasonal time scale (both in title and conclusion) is somehow misleading. The authors also mentioned “We show results for the annual harmonics only in this paper.” So I recommend they modified their presentation accordingly. Since the second harmonic (semiannual cycle) may also be significant especially in tropical regions, which is seldom discussed in this paper.

Since the authors have submitted a relative work to JGR, so it better to clarify the difference between two papers, especially scope and motivation.

In the abstract, the first three sentences are a little bit lengthy, two sentences could be enough for such motivation. Some important results (e.g., concluded in line 339-340) could be clearly stated in the abstract. Please add “root mean square” before the “RMS”.

The authors may modify their presentation for some sentences. For example, “[19] found that …” (line 68) may better be “Bingham and Lee (2017) found that ….[19]” or “It is found that …[19]”, so as to “[15,16] using sparse historic…” (line 70).

It seems that the amplitude is normalized. So authors should add this information in line 97, such as “amplitudes (normalized by….)”.

The locations of moorings might be wrong due to map in Figs 3b, 4b, 5b, 6b, since some of them locate in mainland other than oceans.

Figure 2d, the note above graph has different font/style from others.

Figure 4, they are “percent variance” in notes above graph but “fraction of variance” in figure caption.

Figure 7 and 8, they are rings in graphs but pies in notes.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 4 Report

The problem addressed in this paper is of great interest in the field of oceanic remote sensing. It deals with the analysis of the seasonal variability of sea surface salinity in the tropics from satellites, compilation of in situ data and mooring observations. The context, the objectives and the methodology developed are well presented. Despite many questions raised in the paper, the results and discussions were well conducted. Satellite observations of sea surface salinity over different seasons were assessed against other data, including using different forms of in situ data (the differences between the data were estimated via degree of agreement) . However, several questions remained unanswered, especially in several cases where the differences are significant. And the questions concerned the satellites (equipment, correction methods, processing algorithms, etc.). Overall the paper is well written with a logical structure. However, several questions and comments given below should be taken into account.

• It would be useful to reinforce the introduction by specifying the motivation and interest of the study in a practical and / or operational context. Also, it would be important to clearly specify the methodology adopted in the framework of the study with inputs and outputs through the different methods considered, with a precise positioning in relation to the open literature (this methodology could be presented in the form of a block diagram specifying the different stages of the process (input data, pre-processing, processing, extraction, evaluation, etc.). It would also be useful to end the introduction by clarifying the organization of the content of the paper.
• Same observation for the following sections, in particular the one dedicated to “data and methods”. It would be important to draw up a structure highlighting different types of data (satellite and in situ) with the characteristics of the sensors used (frequency, configurations, polarizations, etc.). And then to present the different methods that are either developed in the context of the paper and / or already used in the open literature.
• It would be useful to further specify the advantages and disadvantages of the overall methodology developed, particularly in terms of its implementation in a practical and / or operational context.
• Undoubtedly, it would be important to strengthen the conclusion and the perspectives offered to the work carried out and presented in this paper.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 4 Report