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

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 perform a sensitivity study where fresh water forcing and salinities at the outer boundaries of the North Sea are perturbed in a systematic way in order to obtain a second-order Taylor polynomial of the statistical steady state mean salinity. The polynomial is constructed based on perturbations of 57-year long hindcast runs 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 analyzed in terms of a total exchange flow analysis in the entrance region, and it is found that the results to a large degree can be explained by (1) recirculation in the entrance region where the inflow water consists of two parts outflowing Baltic water and one part North Sea water, and (2) partitioning of increased (decreased) net outflow on increased (decreased) outflow and decreased (increased) inflow according to the fraction of time these occur.