This paper had gone through one round of review when the editor sent it to me for a third opinion. The paper investigates sea-level variability and trends along the Norwegian coastlines from tide gauge observations and altimetry data. They also use hydrographic data to estimate the steric contribution to the observed sea-level changes. The revised paper does not have any serious flaws which would prevent its publication, the use of statistics is overall appropriate, and the presentation of the results meets the required standards. That said, I think that the paper could be strengthened. To this end, I have several questions of clarification as well as some corrections and suggestions to make, as outlined below.
Line 24. A hyphen is not required here. Generally, the term “sea level” only needs to be hyphenated when it is used as an adjective before a noun.
Line 25. I suggest: “Accurate estimation and attribution of…”
Lines 24-29. I think that the argument for this study could be made more convincingly in this first introductory paragraph by emphasizing the societal impacts of understanding local sea level. For example, the authors start the introduction by saying that “Sea level is considered a key indicator to monitor the earth’s energy imbalance and climate change”, but this point is not even relevant to the work presented here because it is global mean sea level that is a key climate indicator rather than local sea level. I would say that the motivation for this study is that changes in local sea level can considerably differ from global mean sea level, and hence understanding such local changes is crucial to the implementation of coastal adaptation plans.
Lines 52-53. Here I would add a clarification along the lines of “typical of the open ocean for distances to the coast of about 3 km”.
Line 65. You do not really assess the “sea-level budget”. I would suggest you reword this sentence.
Lines 104-105. Could you please specify here exactly which tidal corrections have been applied? This is important because, while tide gauges only sense the ocean tide and the ocean pole tide, altimeter data are also influenced by the solid earth tide, the load tide, the solid earth pole tide, etc.
Section 2.1. Altimetry measurements are influenced by GIA, albeit differently from tide gauge data. Yet there is no mention of any GIA corrections applied to the altimetry data, even though tide gauge records appear to have been corrected for GIA (both crustal uplift and gravity contributions).
Lines 145-147. “lesser extent”. In some regions, the gravity GIA contribution is larger than the VLM induced by GIA. Perhaps, you could clarify that in Norway VLM is the dominant GIA contribution.
Lines 168-176. Could you please specify the temporal resolution of the hydrographic data?
Line 195. While there is nothing wrong with this nonlinear regression model, the model can be easily written in linear form, which greatly simplifies the estimation of the regression coefficients and their uncertainty: z(t) = a + b*t + c*cos(wa*t) + d*sin(wa*t) + e*cos(was*t) + f*sin(was*t), where “wa” and “was” are the annual and semiannual angular frequencies. I am not suggesting the authors redo the analysis with the linear model, but I thought it was worth pointing this option out.
Line 203. “depend”.
Line 212. This equation appears three times in the manuscript. I would suggest you write this equation only once, possibly in the Appendix, number it, and then refer to it whenever you use it.
Line 268. As written, Equation (3) is incorrect. In particular, it is missing a minus sign. Assuming a standard definition for the haline contraction coefficient (beta = 1/rho * drho/dS), the equation as it stands implies that sea level increases as salinity increases, which is not correct. I assume that this is just a typo and that the halosteric calculations have been done properly, otherwise the authors need to adjust their results.
Lines 281-284. I do not understand what the authors mean here. Could you please clarify?
Line 395. The term used in Benveniste et al. (2020) is “fractional differences”.
Lines 395-399. It is not clear how the authors compute the FDs. In Benveniste et al. (2020), “tau” is defined as the trend of the time series of sea-level differences. That is, Benveniste et al. (2020) subtract the altimetry time series from the tide gauge time series, and then compute the trend of the residual time series. Here, the authors define “tau” as “the linear trend difference between altimetry and each tide gauge”, which seems to imply that “tau” is the difference of the two trends from the tide gauge and altimetry. If this is how you do it, what value do you use for the standard error (SE) in the FD equation? The whole point of defining “tau” as the trend of the residual time series as in Benveniste et al. (2020) is that the value of SE is straightforward to calculate. When you define “tau” as the difference of two trends, as opposed to the trend of sea-level differences, then you need to make assumptions about the dependence between SE_TGs and SE_altimetry. Please clarify.
Lines 399-400. From Fig. 8, the confidence intervals overlap significantly, even at Tregde, Måløy, and Bergen, so I am surprised that FDs are > 1 at those stations.
Section 5.2. This section is called “Steric contribution to the sea-level trend”. However, the authors do not really place the steric contribution in the context of the sea-level trends. They simply say that the steric trends lie within a particular range and then refer to Fig. 11. I would expect the authors to say what fraction of the sea-level trends is explained by the steric contribution, distinguishing between stations. This Section could benefit from a more focused discussion of the results.
Line 637 (Appendix A). Strictly speaking, this is the semi-variogram.
Lines 649-651. How do estimates of the degrees of freedom (dof) computed with this approach compare to estimates computed using the equation: dof * (1 – r)/(1 + r), where ‘r’ is the autocorrelation? Providing such a comparison would give the reader additional confidence in your uncertainty estimates.
Abstract. The authors write “we find that the sea-level annual cycle is more affected by variations in temperature than in salinity, and that both temperature and salinity give a comparable contribution to the sea-level change along the entire Norwegian coast”. I find this sentence a bit confusing. Seasonal variations are “sea-level changes”, so what do you mean by “contribution to sea-level change”? Could you please rewrite this important sentence to improve clarity? |