Preprints
https://doi.org/10.5194/egusphere-2025-4735
https://doi.org/10.5194/egusphere-2025-4735
06 Oct 2025
 | 06 Oct 2025
Status: this preprint is open for discussion and under review for Ocean Science (OS).

Response of a semi-enclosed sea to perturbed freshwater and open ocean salinity forcing

Lars Arneborg, Magnus Hieronymus, Per Pemberton, Ye Liu, and Sam T. Fredriksson

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.

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Lars Arneborg, Magnus Hieronymus, Per Pemberton, Ye Liu, and Sam T. Fredriksson

Status: open (until 01 Dec 2025)

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Lars Arneborg, Magnus Hieronymus, Per Pemberton, Ye Liu, and Sam T. Fredriksson
Lars Arneborg, Magnus Hieronymus, Per Pemberton, Ye Liu, and Sam T. Fredriksson

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Short summary
Climate change influences estuaries and their ecosystems not only by rising temperatures but also by salinity changes. This modeling study highlights the magnitude and causes of the Baltic Sea salinity sensitivity to fresh water forcing and changed ocean salinity. The large sensitivity to increased fresh water forcing is shown to be caused by dilution of fresh water within the estuary, recirculation of much of the outflow water back into the estuary, as well as decreasing inflows of ocean water.
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