Review of « Baltic Sea freshwater content » by Urmas Raudsepp et al.

This article is a study of the Baltic Sea freshwater content, based on outputs of re-analyzed models. I think the study is clear and simple, and deserves publication, but there are some points which require further explanation

General comments :

You mention several times that the FWC is affected by the sea ice cover. That might be true from a seasonal point of view, but I fail to understand why this would have any effect from an inter-annual point of view since the freshwater stored in sea ice is totally released every year in the water column. That is really the only thing that appears strange for me in this article, and that I think requires further explanation.

Another point, less critical though, is the explanation of the decrease of FWC in the Baltic Proper, which is especially obvious for the deeper parts. You relate this point to an intensification of salt inflows to the Baltic, could you please explicitate what is the scientific consensus, is it related to climate change and/or sea level rise ?

Specific comments :

Line 60, can you explicitate the FWC formula so that the reader does not need to read Boyer 2007. Basically my understanding is that your formula is equivalent to saying that :

FWC= rho(Sref,Tref,p)/rho(0,Tref,p) * (S-Sref)/S

which is very different from the usual formula (Sref-S)/Sref since now the variability in time becomes part of the denominator. It would be nice to know what this formula mean, especially why is there a ratio of densities ?

Figure 2. It would be nice to have in the figure a reminder of what are BOB, BOS etc. Same for Figure 4.

In this study, freshwater contents of the various sub-basins of the Baltic Sea from the BALMFC CMEMS reanalysis data 1993-2020 were calculated following the method by Boyer et al. (2007). The authors investigated trends in freshwater content per sub-basin and vertically in the water column, as well as seasonal climatologies of freshwater content. In the discussion, trends were attributed to river discharge, net precipitation and sea ice volume changes.

Reanalysis data are well suited for the analysis of the ocean conditions and the detection of trends in three dimensions and for the calibration and evaluation of ocean circulation models. However, reanalysis products are generally not good for attribution studies because quantities are not conserved due to the assimilation methods used. As Baltic Sea models often have large biases in salinity due to artificial numerical diffusion (Burchard and Rennau, 2008), data assimilation results in artificial sources and sinks in salinity. Hence, any attribution analysis and discussion of causes of detected changes are difficult. Furthermore, the authors considered only river discharge and net precipitation data while wind fields were not analyzed although several previous studies claim that the seasonality in juvenile freshwater propagation or multi-decadal variability in freshwater content are controlled by the wind (the latter at least partly).

The provided explanation that melting sea ice could have contributed to the observed positive trends in freshwater content is wrong. In contrast to the Arctic Ocean, in the Baltic Sea multi-year sea ice does not exist. Averaged over one year, the freshwater extraction and freshwater supply is balanced.

The first part of the introduction suggests a relevance of the study for the impact of climate change. However, previous studies found a pronounced multi-decadal variability in salinity and freshwater content of the Baltic Sea (e.g. Winsor et al., 2001). Hence, trends during the rather short period of existing reanalysis data (1993-2020) only describe the natural variability and cannot be used for the analysis of systematic changes.

Methodologically, the study has gaps. Significance levels of trends are not provided. For me the rationality of the correlation analysis for the understanding of the observed variability is not clear. What have you learned?

Furthermore, the manuscript suffers from missing references (e.g. Winsor et al. 2001) and phrases that need to be revised (e.g. line 47, line 170).

In summary, the study in the current version is rather descriptive and does not provide any new insights into the causes of observed trends and variability in freshwater content. Hence, I recommend rejection.

References

Boyer, T., Levitus, S., Antonov, J., Locarnini, R., Mishonov, A., Garcia, H. and Josey, S.A., 2007. Changes in freshwater content in the North Atlantic Ocean 1955–2006. Geophysical Research Letters, 34(16).

Burchard, H., & Rennau, H. (2008). Comparative quantification of physically and numerically induced mixing in ocean models. Ocean Modelling, 20(3), 293-311.

Winsor, P., Rodhe, J., & Omstedt, A. (2001). Baltic Sea ocean climate: an analysis of 100 yr of hydrographic data with focus on the freshwater budget. Climate Research, 18(1-2), 5-15.

The research on the fresh water content is a useful approach when addressing estuarine basins.  The proposed study aims to make a step towards understanding the evolution of fresh water content in the Baltic Sea based on analysis of BALMFC data. Unfortunately, the manuscript does not provide (1) any validation and error estimates, and (2) sufficient depth of analysis. With the available from BALMFC data, authors could do much more than just a simple diagnostics.

1.  Abstract: „Copernicus regional reanalysis“, right term? May be better say Copernicus Baltic Sea regional reanalysis.
2. Line 60 big and small s.
3. Estimate the errors in the model by analyzing available in situ and satellite data and data from the reanalysis. This would show how credible your results are.
4. The analysis or fresh water content is show-and-tell like. No deep physical explanation of reasons, dynamics etc. are proposed.
5. It is not always clear whether authors present their own results or results of others. In Line 116-116: “The variability as well as negative trends are strongest in the southern and the northern Baltic Proper (Fig. 4e,c). The decrease of the FWC is explained by the saline water transport from the North Sea to the Baltic Sea by the Major Baltic Inflows (Mohrholz, 2018), large barotropic inflows (Lehmann et al., 2017) and smaller inflows of barotropic origin (Lehmann et al., 2022).” How exactly, what exactly. Use quantitative analyses (BALMFC) to support these statements. Are authors sure that, in BALMFC, the above conclusion hold. This is particularly important for the straits transport. Discuss the realism of straits transports. You may compare with the estimates of Mohrholz et al. (2015), Gräwe et al. (2915), and Stanev et al. (2018). An important question is whether BALMFC correctly represents straits transports.
6. Authors mix introduction and results, one example (there are also others): “. Deep layer water in the Gulf of Finland originates from the sub-halocline layer (110–120 m) of the central Baltic Proper (Liblik et al., 2018).” Please, move this in the introduction and restructure your paper. Alternatively, show this from the results of BALMFC, which is preferable. There are in the paper other similar cases. I am not sure whether what a model different from NEMO simulates (or one data set shows) is reproduced by the BALMFC. Support your conclusions with the results from NEMO.
7. Much of what is said in the part “Results” cannot be derived from the results: “In winter, the salt wedge withdraws from the interior of the gulf, the mean salinity decreases and FWC increases.” Does this follow from BALMFC or other model. Maljutenko and Raudsepp (2019) is not in the reference list.
8. The paragraph starting in line 157 is unclear. Kattegat-Golf of Riga-The whole basin. Author have to try to carefully explain their idea.

References:

Gräwe, U., Naumann, M., Mohrholz, V., Burchard, H., 2015. Anatomizing one of the largest saltwater inflows in the Baltic Sea in December 2014. J. Geophys. Res. 120, 7676–7697

Mohrholz, V., Naumann, M., Nausch, G., Krüger, S., Gräwe, U., 2015. Fresh oxygen for the Baltic Sea – an exceptional saline inflow after a decade of stagnation. J. Mar. Sys 2015.

Stanev E, Pein J, Grashorn S, Zhang Y, Schrum C, 2018: Dynamics of the Baltic Sea Straits via Numerical Simulation of Exchange Flows, Ocean Modelling 131:40-58