This manuscript addesses tidal propagation and subtidal circulation in a fjord-like strait. Authors adopted a modeling approach to estimate flow and salinity fields. The proximity to an amphidromic system induces fortnightly variations in sea level at both ends. Subtidal variations in exchange flows are attributed to fortnightly variations in the barotropic pressure gradient. 

I liked the topic and I enjoy readying the manuscript. I think the topic is appealing and of interest to OS readers. The manuscript have the potential to become a relevant contribution, but model output analysis and discussion is quite superficial in many instances (e.g. statements about mixing/stratification). Authors have a powerfull tool, which is ROMS, but my feeling is that they do not take all the advantage of it. 

The manuscript is mostly well organized, altough it is not terribly well written. Objectives could be better stated. The edition and selection of figures could be much improved. They sometimes fail to support rightly the story the authors want to tell. 

Overall, a substantial revision is needed before I can recommend publication. 

See specific comments below. 

Abstract

I rather focus the abstract on physics, processes, etc. more than the model implementation itself. (The same occurs elsewhere in the manuscript, mainly in introduction, discussion and conclusions.)

L1. 'describes the implementation'. I think this manuscript does more than that. 

L2. Not sure to call this 'estuary', as it is not semi-enclosed body of water. It wouldn't be more like a 'strait'? 

L9. 'Surprising'. I suggest to ommit this kind of valorative adjectives. (here and elsewhere)

L1 and L12. Rephrase "describes the implementation...". "We recommend that..." 

Introduction

L29. 'estaury'. Correct here and elsewhere (L30, L38,...) 

L30. Please indicate in which ways. 

L37-38. Please describe what are "normal conditions" are. Are those described in the next paragraph? What is "non-sill estuary circulation"?

L46. Perhaps would be convenient to show the location of the amphidromic point in a Figure.

L50. "To understand how these various forcing mechanisms affect mixing and circulation within and out of the estuary" This is the objective, am I right? Please state it clearly. The use of one numerical model or another is part of the methodology. 

L60-61. Is this also part of the objectives? General objective, perhaps? 

L50-66. I suggest the authors to reorganize this part of the introduction. General and specific objectives/aims should be clearly and logically stated. And then, describe how are they addressed (methods, numerical model). 

The Model

L83. I do not understand this. Does this mean that the daily input to ROMS is 1.7e+8/365 m3/day and 6.3e+7/365 m3/day? 

L85. Is it enough one day to spin-up the model? How do you determine that? 

I suggest the authors to move Section 3 to Section 2.1. After all, model calibration and validation is part of the set-up process. 

L98. Averages for the whole sampling period? Monthly? Please indicate the time span in which averaged are performed. Also in Figure 2 and 3. 

L101-102. Please explain briefly the 'modeling details'. 

Please consider to show one or two panels that show time series of observations confronted to model output. 

Last two paragraphs in section 3.2 may shed doubts if the model is correctly validated in terms of salinity. Should not salinity variations in the shelf and freshwater discharges time series be included as input in the model? 

Model results

L132-134. Move these lines to methods part. 

L135-. Too late, I think. Please mention this or move the paragraph above. 

L140. I think Results section should start here. 

L142. I guess qN and qN are cross-sectionally integrated. Please indicate. Are tidal variations in the sea level (elevations) also considered or only the mean cross-section? 

L146. How the one-hour lag compares with the tidal wave celerity?

L147. Not always. There are some intervals in which the volume flux is unidirectional both during ebb and flood. 

L159-161. Table 1. The cross-correlation between qS and qN has a 1 hour lag. If the barotropic pressure gradient is driving the volume flux, why there is no lag in the cross-correlation between qS and (hN-hS)? Any clue? Perhaps there is a baroclinic mode which is also driving the net volume flux in the estuary? 

L165. 'Speed' vS. Cross-sectionaly averaged velocity? 

L167. Assuming that there is no correlation between A and vS. 

L169. It would be nice to test the sensitivity of the fit to the locations of hS and hN. 

L170. I don't understand. Friction is always there. You mean when friction is included in Bernoulli equation? 

L175. This is quite usual in many estuaries. 

Figure 7. Define the daily averages <...> in the main text. is consistently lower in magnitud than . Why? I suggest to plot in the same panel qN, qS, hN, hS and their differences. It seems that and are slightly out of phase? 

Figure 8. Indicate that the horizontal axes is along-estuary. Label northern and southern sills (or simply S and N). Instead of "grid numbers", use km. 

L192-193. Denser water? What are the mixing rates? 

L194-196. Although it seems plausible, it would be good to provide mixing rates to support this statement. 

L201-202. Please clarify this. Shouldn't be a sign of this also in the salinity color map? 

L187-207. Overall, there are some unsupported statements here. In my opinion, authors' arguments on mixing, vertical water movements, effect of discharges, etc. do not seem to be supported by any data. Could the authors provide additional numerical evidences? (mixing rates, vertical and lateral velocities?) 

Figure 9. Is this figure really necessary? 

L212-214. Both time series can be corrected for the barometric effect. 

Figure 10. I guess panel a conveys the same information as correlations in Table 1. I suggest to remove panel a. (or substitute Table 1 by lagged cross-correlation plots)

L223-241. This is interesting, but I think it deserves to be much more elaborated. At which depths occur the inversions? Above or below the picnocline? More reliable/informative (but still simple) measurements for stratification could be Brunt-Vaisala frequency or potential energy anomaly. How they would compare with the turbulent kinetic energy? How much mixing induce then these inversions? Do you think this inversions could have a (baroclinic) influence on the net exchange flow? 

Discussion

L247. Please discuss the consequences of it. 

L253. As I mentioned above, it would be nice to see where the amphidromic point is. Also, I would expect that the tidal wave propagates faster around Faroe Islands than through the fjord. Is it so? If yes, there would be probably a superposition of both waves at a certain location near the south sill. Have you looked into this? 

L257-258. I think the authors analysis here is based on the Bernoulli equation. If I'm not wrong, the determination of gamma only indicates that friction is overall important, but not that friction occurs mainly in the southern sill. Could you elaborate more on that, please?

L258-260. It is always so, I guess. Not sure what the reader should conclude from this statement... 

L265. If the amphidromic point is located near Tórshavn, why the tidal wave enters the estuary from the north (L252)? Notice that this is about timing, not amplitudes.

L265-269. For me this is something that should have been mentioned in the introduction, not in the discussion section. This would have helped to better undertand the qN and qS variability. 

L276-285. A harmonic analysis of sea levels could shed some light on this. Probably a mutual non-linear interaction between M2 and S2 produces a fortnightly compound tide MSf, which is (almost) in phase with the spring-neap tidal cycle. 

L290. Again, computation of mixing rates and TKE along the estuary (or better computation of terms of the momentum) could support this. 

L311-317. I think this would be easy to check out by comparing (observed or modelled) elevations and currents at both sills. 

L332. Figure 12. Nice figure! 

L338. What is then the flushing time estimated from this? (The same in L341)

L344. There is no two layer circulation in the southern sill. However, the two layer circulation in the northern sill persists during both periods, altough it varies in magnitude. I think this is remarkable. It would be very interesting to comment a bit more on this. Is there any sign of this "fortnightly pumping" in the shelf, out of the estuary? Or inside the estuary, in its deep waters? Maybe because of this "fortnightly pumping", stagnant conditions are not observed during winter? How is the circulation during summer? Is this pumping effect also present when stagnant conditions near the bottom are observed? 

Are there other fjords or straits that show similar circulation patterns? BTW, please consider to put the study in a wider (global) context. Most references are local. 

L351-. Again I find mixing discussions somehow "loose"

Recommendations

Why not Conclusions instead of recommendations? I suggest to rewrite Section 6 to frame it properly as Conclusions. 

This study aims to explain residual and exchange flows in a fjord separating two major islands of the Faroer archipelago, North Atlantic ocean. The authors hypothesize that the residual flows are due to a fortnighly oscillation and the presented modelling results seem to support this hypothesis. This is a potentially interesting topic both to the scientific community and to local stakeholders and managers. The study is well written in terms of language, however the reasoning and methodology require several substantial decisions and revisions.

General

- not clear how geographical names are to be spelled - please provide phonetic translation; similar comments apply to maps (Fig. 1) where mesh indices are shown instead of coordinates in meters: please present information such that it is easily accessible to be memorised and interpreted by the average reader

- in the Introduction the study area is presented as an estuary or fjord, i.e. a land-ocean transition space, but obviously it is an ocean strait. Here a decent review on circulation in ocean straits is imperative, for example Danish straits and Bosporus (amongst others) have been studied well: Identify the knowledge that can be transferred from other straits to the local strait, identify the knowledge gaps and say how the gaps shall be closed using the methodology of this study.

- Model area, model validation: Why is model area so small? This creates several problems: As water level differences are substantial for the conlusions of the study, the model area should include both gauges shown in Fig. 1. Alternatively authors could validate the parent model against these gauges. Salinity validation reveals the model is too mixed - this hints at underestimated exchange flow/density driven circulation - here the area outside the sills could be crucial but it is excluded from the model which could be a serious dynamical flaw. Although tides are important the validation considers daily scale which does not make sense. Sub-tidal flows are usually a function of both overtides and density driven flows - it would make sense to start validation at the intra-tidal scale.

- Density inversion in a hydrostatic model - how is this possible?

Recommendation: Either increase the model area to allow for a decent validation on intra-tidal to sub-tidal scales or analyse outputs from the parent model. In case your model/observations have gaps that cannot be closed easily to support conclusions, use the scientific literature on ocean straits to hypothesize and discuss how exchange flow over the sills is generated here.

Details

ll15 "results from a model" from a model simulation

ll18 a sill is an elevation, maybe say depth is 4 m at the sill

ll27 how large?

ll37-38 why study winter conditions when for the Stakeholders (aqua farms etc) summer dynamics are more relevant?

ll62-66 Please provide a consistent description of the aims of the study, list the research questions

Section 3: Why not use the ADCP data to illustrate intra-tidal dynamics, validate simulated currents?

ll119-120 In estuaries per definition river flow affects the salinity field, and Fig. 4 shows that freshwater input is probably significant

Fig. 2a: what is the data basis for the red and blue colors - model or observations? Specify in caption.

Fig. 3: Consider illustration and validation on intra-tidal scales...consider showing these numbers in a table instead

ll247: "no hydrographic observations during zhe simulation period" - what about using climatological data?

ll291 please specify what is meant by highly non-linear flows

ll301 tidally-rectified currents: probably a tidal analysis of ADCP data and model results can be very helpful in this case

General comments

This manuscript reports about model simulations of the hydrodynamics in a narrow and deep sound situated between two islands of the Faroes. The sound is bounded by narrow and shallow sills at both sides. Tidal flow is complex due to the proximity of an amphidromic point for the M2 tide.

This study is highly site-specific, and for an article in general journal of “Ocean Science” it would be required to better motivate the study and to derive generalisations to other flow situations.

I am irritated about denoting the sound between the two islands as an estuary although you even write in the introduction that this is not an estuary in the classical sense. It would be better to call this here as a “narrow sound with strong freshwater run-off” or so. Here the major runoff (from a hydro power station) occurs at the open end of the sound and thus it can be expected that the general behaviour is much different than in an estuary where the freshwater run-off occurs at the closed end.

In fjords with sills, one of the major topic is the ventilation and renewal of the deep water. Since we have here a sound that is bounded by two narrow straits with sills, there is a large body of deep water residing near the bottom. To my opinion, it is a major limitation of this study that this topic is not discussed. What are the deep water renewal processes? Is it tides or wind or surges? How often does it happen? The model system used here should be able to reproduce those dense water overflows. Just initialising the salinity and temperature fields at some instant of time and simulating for a short period might completely miss the dynamics. Here, just one day is used to let the model adjust to the initial fields, a time span that should be by far shorter than the deep water renewal time.

As for the validation, the results are very poor. Tidally resolved velocity measurements are not compared to model results, and the comparison between simulated and observed residual velocity profiles is very bad. Salinity observations are not available during the simulation period. A comparison to observed salinity profiles obtained during several other years is made, shows big differences to the model results and a high variability (and makes no sense anyway). With this, the model results are nit validated at all, and do probably not reflect the dynamics of the sound under consideration.  

The paper is lacking motivation. In the introduction, a clear scientific problem needs to be presented on the background of the state of the art. Here, however, very little state of the art is given, a problem is not clearly identified and hypotheses are not offered.  

Altogether, a non-validated model is used that obviously does not reproduce the hydrodynamic regime. The key question of the regime (deep water renewal) is not addressed. The authors admit that the data situation is poor for the sound and they “recommend that a more targeted field experiment is implemented that can reveal more of the strengths and weaknesses of the model”. I agree with this statement and do additionally recommend to extend the simulation period to reproduce deep water renewal events. It would be good to deploy long-term moorings including CTDs and ADCPs that can monitor such inflows events.

Therefore, I recommend to reject this manuscript at the present time and repeat the model study once a better data situation is given.

Specific comments

3: Specify for which partial tide you have the amphidromic region. You probably mean the M2 tide, but please specify.

5: I would prefer “volume transport”, because I think “flux” is reserved for “transport per unit area”.

9: How can you verify transports with sea level observations?

10/11: reformulate this as a sentence.

26: “reducing the runoff into the southern part of the sound while the northern part has received more freshwater”. What is the mechanism here and how do you know?

29/30: typo “estaury”, here and at many other locations.

37/38: “when the circulation is more similar to that of a non-sill estuary”: At this point the reader has no idea of the salinity distribution in this sound. A better motivation is needed for the choice of the winter for this case study.

39-45: The review article by Farmer and Freeland does actually discuss tides as an important process of fjord dynamics (see their section 4).

48-49: You can also have “a strong periodically varying barotropic pressure gradient through the estuary” when the amphidromic points are far away. So, at this point I do not see any special influence of the proximity of the amphidromic point apart from the fact that the M2 tide is weak.

56-61: I would move this paragraph to the “Materials” section, since the introduction should serve more general purposes and introduce the problem, give hypotheses, etc.

64/65: “One aim of this study was therefore to validate the model against these observations.”: This is not a sufficient aim for a study to be published in a peer-reviewed international journal. Also the next sentence is not sufficient as motivation.

82/83: Could you also give the runoff in m3/s which is more common.

83: What do you mean with constant daily run-off? I suppose that the run-off has to be given a every barotropic model time step which is much shorter than one day.

84/85: Not clear how the spin-up of the model can be as short as one day. How are the initial conditions for the high-resolution simulation been initiated? I guess from the level-2 nest. This needs to be explained. Since the residence time of the deep water in the sound must be much longer than one day, I wonder how good the quality of the initial condition is. Have they been validated by observations?

96-113: I do not see any agreement between observed and simulated velocity profiles. The model results show a residual flow that is directed northwards, but the observations do not show that at all. I find it also strange to report on a study of tidal flow, have tidal flow observations at hand, but state that “a model-observation comparison of instantaneous velocities is not very meaningful”. The key issue in tidal simulations is to reproduce tidal phases and amplitudes. This requirement is not met here.

115-130: Simulated salinity is here compared to observations that have been made outside the simulation period. Since salinity at the bottom should vary substantially with deep water renewal events, any similarity between observed and simulated salinity would be pure random. With this, no validation of the salinity field has been made. I wonder, if the bottom-mounted ADCP’s should have included a CTD such that at least bottom salinity and temperature could be validated.

I am stopping here with my detailed review, since I do not think that it makes sense to deeply analyse results of a non-validated model.