'''DEFINITION'''

The subsurface salinity trends have been derived from regional reanalysis and forecast modelling results of the CMEMS BAL MFC group for the Baltic Sea (product references BALTICSEA_REANALYSIS_PHY_003_011). The salinity trend has been obtained through a linear fit for each time series of horizontally averaged (13 °E - 31 °E and 53 °N - 66 °N; excluding the Skagerrak strait) annual salinity and at each depth level (Mulet et al., 2018).

'''CONTEXT'''

The Baltic Sea is a brackish semi-enclosed sea in North-Eastern Europe. The surface salinity varies horizontally from ~10 near the Danish Straits down to ~2 at the northernmost and easternmost sub-basins of the Baltic Sea. The halocline, a vertical layer with rapid changes of salinity with depth that separates the well-mixed surface layer from the weakly stratified layer below, is located at the depth range of 60-80 meters (Matthäus, 1984). The bottom layer salinity below the halocline depth varies from 15 in the south down to 3 in the northern Baltic Sea (Väli et al., 2013). The long-term salinity is determined by net precipitation and river discharge as well as saline water inflows from the North Sea (BACCII Author Team, 2015). Long-term salinity decrease may reduce the occurrence and biomass of the Fucus vesiculosus - Idotea balthica association/symbiotic aggregations (Kotta et al., 2019). Changes in salinity and oxygen content affect the survival of the Baltic cod eggs (Raudsepp et al, 2019; von Dewitz et al., 2018).

'''CMEMS KEY FINDINGS'''

The subsurface salinity over the 1993-2020 period shows no trend in the surface layer of 40-m. Then, the trend increases monotonically until the depth of 80-m, where it reaches the value of about 0.04 per year. Below that depth the salinity trend is uniform within the uncertainty limits.

'''DOI (product):'''

https://doi.org/10.48670/moi-00207