Lars Axell

In this report, we examine how the oceanographic climate of the Baltic Sea and the Skagerrak–Kattegat region is expected to change and impact the sound speed in the future for two time periods, 2040–2050 and 2090–2099, compared with 2015–2024. The speed of sound in se-awater is primarily determined by three physical variables: temperature, salinity, and pressure. Because these parameters vary between marine environments, sound speed is not homogene-ous in the ocean. Changes in these variables with depth create complex vertical profles that infuence how sound propagates.    To model temperature and salinity, we use NEMO-Nordic, a high-resolution ocean model for the Baltic Sea, the North Sea, and the English Channel, based on the internationally developed NEMO system. The model is forced with atmospheric data from three climate models (EC-Earth, HadGEM, and MPI) under three CMIP5 scenarios (RCP2.6, RCP4.5, and RCP8.5), resulting in three model members for each scenario.    The sound-speed profles show clear seasonal inversions, with temperature — and thus sound speed — being lowest at the surface in winter and highest in summer. In all scenarios, sound speed increases over time, while salinity decreases, especially under RCP8.5, mainly due to an intensifed hydrological cycle. However, future salinity levels in the Baltic Sea are highly uncertain.     The trends are similar in the Skagerrak–Kattegat region and the Baltic Sea, but the changes appear earlier in the Baltic Sea (from around 2040) and intensify toward the end of the century, particularly in RCP8.5.