Interactive comment on “Validation of the coupled physical-biogeochemical ocean model NEMO-SCOBI for the North Sea-Baltic Sea system” by Ruvalcaba -Baroni et al. 

The paper of Ruvalcaba-Baroni et al. represents an important step forward in coupled physical-biogeochemical modeling of the North Sea and Baltic Sea regions as a single domain. The combination of NEMO with SCOBI is logical and according to the authors’ knowledge is only the third such effort from the modeling community after the papers of Maar et al. (2011) and Daewel & Schrum (2013), based on DMI-BSHcmod/ERGOM and ECOSMO, respectively. Given the need for model ensembles to improve overall understanding of biogeochemical functioning of marine systems, this study is very relevant, even more so with current efforts to better integrate management of the two seas under study. 

The paper is generally of very high quality and builds upon the base of previous work in the same group with respect to both the development of NEMO-Nordic and SCOBI. Therefore I do not see any fundamental problems with the research and support its publication. That said, it is interesting to observe the discrepancies between SCOBI-modeled parameters and observational data in certain regions, which show the current limitations in knowledge and hopefully will guide the authors towards future improved iterations of the model. Some of these discrepancies deserve a bit more elucidation in the text, or at least better structuring of the sections, so as not to leave the most important discussion points to the end of the paper. There are also a small number of technical clarifications I would draw attention to, and some suggestions for alternative phrasing and setting the context of the study. These may all be considered minor revisions so I give them as a Line-by-Line list below. 

Kind regards, 

Tom Jilbert 

University of Helsinki, Finland 

Line 45-50: Not clear why primary producer assemblages are mentioned here for NS but no equivalent description for BS. I suggest to introduce the physical aspects first, then biogeochemical and finally the plankton assemblages. 

Line 66: Replace "recycling of benthic phosphorus minerals" with "recycling of phosphorus from sediments" 

Line 70-71: Please update this setting of the context with a citation of the following publication and potentially references therein: "The Baltic and North Sea Strategic Research and Innovation Agenda BANOS SRIA 2021 The final BANOS SRIA draft of the proposed, new, joint Baltic and North Sea Research and Innovation Programme -BANOS BANOS CSA Deliverable 1.5". 

Caption of Fig. 1: Capitalize SHARK 

Table 1: Modify “mg CHL m-3" to “mg Chl-a m-3" 

Figure 2 legend: Typo in “atmosphere” 

Line 183: Typo “100 kton N/yr” 

Line 195: Does this mean "reduced to 0.3 x and 0.75 x the original value, respectively"? Please clarify. Also clarify (as implied in Fig. 2) that the resulting nutrient flux to the sea is entirely in the dissolved fractions and comment briefly on simplifications with respect to e.g. fluxes of TP, that in reality are largely particulate. 

Line 286: Modify to e.g. "depending on their proximity to one another" 

Line 301: Typo "to analyze" 

Line 315: Replace “80s” with “1980s” 

Line 317: Typo “observations are too low” 

Line 326: Replace “don’t” with “do not” 

Line 346: Typo “statistically” 

Line 347: Remove "not studied here", it confusing to state this 

Line 349: Rephrase to e.g. "There is a lack of observational data for bottom water oxygen during the period 1975-1995" 

Line 407: Typo “less than” 

Line 420: Typo “extent” 

Line 421: Modify to “The main inference of...” 

Line 424: Modify to “"Despite such specificities of..." 

Line 457: Modify to “its” (no apostrophe) 

Line 471: Modify to "bias is" or "biases are" 

Line 497: Modify to “most models” 

Line 572: How could low oxygen theoretically inhibit primary production? Not easy to understand what is meant here. 

Line 574: The model vs. data discrepancy in the timing of the late winter/spring bloom in the Kattegat is one of the key question marks raised in the study, and indeed it is highlighted in Fig. 7. I think it deserves more elucidation at this point in the discussion, for example an assessment of the degree to which light or temperature might be inhibiting the early onset of the bloom in the model. The authors return to this in Section 3.6 “Future work and data gaps” but in the current version the reader is left hanging for an explanation after it is established that there is no nutrient limitation at the time in question. 

Line 590: Again please check the BANOS SRIA for up-to-date statements about the need for integrated modeling of BS-NS system 

Line 595: Citations in brackets 

Line 604: Modify to "in Ford et al. (2017)..." 

Line 616-623: This is a very important section of the discussion: the model assumes (if I understand correctly) that a certain bioavailable fraction of nutrients enters the sea, directly into the dissolved phase state variables PO4, DSi, NO3, NH4 (Fig. 2). This is of course a large simplification of reality, where there is transfer between bioavailable and non-bioavailable fractions within the coastal filter, as well as heterogeneous removal of nutrients. Different types of coastlines may behave quite differently in this regard, see eg. Asmala et al., L&O 62 (2017). Are there any of the observed model vs. data discrepancies e.g. in near shore nutrient or Chl-a concentrations, that could be affected by this simplification? If so, it would deserve some mention higher up in the discussion for those specific areas. 

Line 666: typo “therefore” 

Appendix Line 737: Important: should this read "BOPREM-BIP decreases with increasing salinity and decreasing bottom oxygen concentrations..." ? That is the implication of the equations and following text, and the logical relationship. 

Appendix Line 771: Typo “from” 

Appendix Table A2: Comment on the validity of using a single porosity value for entire NS-BS system.