Baltic Sea Subsurface Salinity trend from Reanalysis

'''DEFINITION'''The subsurface salinity trends have been derived from regional reanalysis and forecast modelling results of the Copernicus Marine Service BAL MFC group for the Baltic Sea (product reference BALTICSEA_MULTIYEAR_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.'''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 metres (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 (Lehmann et al., 2022). 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 1993-2022 period shows a positive trend in the entire depth range. The salinity trend is practically negligible in the upper mixed layer of the sea. Then increases monotonically from 0.01 to 0.04 per year within the extended halocline layer between 40-120 m. The salinity trend is almost constant of 0.04-0.05 in the deep layer. In the halocline and in the deep layer, i.e. at the depth range between 60 and 250 metres the confidence interval of the trend values is wide, which indicates that this depth range is relatively dynamic.'''Figure caption'''The vertical profile of horizontally averaged subsurface salinity trends for period of 1993-2022. The salinities from Copernicus Marine Service regional reanalysis product (BALTICSEA_MULTIYEAR_PHY_003_011) have been averaged over the Baltic Sea domain (13 °E - 31 °E; 53 °N - 66 °N; excluding the Skagerrak strait). The shaded area shows 95% confidence interval of calculated trend values.'''DOI (product):''' https://doi.org/10.48670/moi-00207