| 06 Oct 2025
Response of a semi-enclosed sea to perturbed freshwater and open ocean salinity forcing
Abstract. The sensitivity of Baltic Sea salinities to changed fresh water forcing and other forcing factors have been debated during the last decades, since changed salinities would have large impacts on the marine ecosystems, and since this parameter still shows a high degree of uncertainty in regional climate projections. In this study we performed a sensitivity study where fresh water forcing and salinities at the outer boundaries of the North Sea were perturbed in a systematic way in order to obtain a second-order Taylor polynomial of the statistical steady state mean salinity. The polynomial was constructed based on perturbations of a 57-year long hindcast run for the period 1961–2017 with a regional ocean model covering the North Sea and the Baltic Sea. The results show that the Baltic sea is highly sensitive to fresh water forcing and that only about one third of the boundary salinity change propagates into the Baltic Sea. The results are also analysed in terms of a total exchange flow analysis in the entrance region, and it is found that the Baltic Sea salinity sensitivity to a large degree can be explained by increased freshwater input causing (1) dilution inside the Baltic Sea, (2) decreased inflows caused by changes to the mean sea level gradient in the entrance region, and (3) reduced inflow salinities due to recirculation of outflowing Baltic water in the entrance region where the inflow water consists of about two parts outflowing Baltic water and one part North Sea water.
The manuscript investigates the sensitivity of the salinity of the Baltic Sea to changes in the freshwater forcing and the salinity at the open boundaries in a model setup comprising the Baltic Sea and the North Sea. As mentioned by the authors, several studies have already been performed on the sensitivity to freshwater forcing. However, considering the salinity at the open boundary is a novel approach and also the idea of approximating the salinity of the Baltic Sea by a Taylor polynomial depending on the effects of the two forcings under consideration is new. Finally, the study adds novel insights to the water exchange between the North Sea and the Baltic Sea. All in all, the study meets the quality and scope of Ocean Science.
General comments
1. The study could provide a bit more background / context information, e.g., about typical variations of freshwater forcing and boundary salinities, about how they are expected to evolve in the future, whether there is any kind of interdependence etc… . Why did you choose specifically those two parameters? Aren’t other parameters like wind patterns / sea level rise more important for the salinity in the Baltic Sea than the boundary salinity in the North Sea (the importance of sea level rise is at least mentioned at the end of the text)? What exactly is the use of the Taylor polynomial? To explore the parameter space without having to run simulations for every combination of parameters? Are there alternative approaches and if so, why did you choose this particular approach? A final assessment of how this study advances the existing knowledge at the end of the manuscript would also be great.
2. There could be a few more references. Others have already looked at, for instance, the impact of runoff on inflows via changed sea level gradients; maybe, there are also more studies on the water exchange between the North Sea and the Baltic Sea or about the salinity at the North Sea boundary. See suggestions in the attached pdf.
3. The model validation could be a bit more comprehensive. For instance, a validation of transports in the entrance area of the Baltic Sea would be great as they are important for the study. The salinities in the central Baltic Sea look quite good (with some exceptions mentioned in the detailed comments) but they might be “right for the wrong reason”. In addition, possible inaccuracies introduced by the Taylor polynomial approach, namely by the short averaging period / nonstationarity of the time series in this period, the discrete differences, and the truncation of the polynomial, are only discussed very briefly (in Figure 7) and might deserve a few more sentences.
In case the suggested modifications lead to a too long manuscript, less important parts might be moved to a supplementary file.
Detailed comments can be found in the attached pdf.