Baltic Sea Subsurface Salinity trend from Reanalysis

'''DEFINITION'''

Subsurface salinity trend ocean monitoring indicator was introduced in Copernicus Marine Service Ocean State Report, Issue 2 (Mulet et al, 2018) and is derived from regional reanalysis product 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 from 1993 to 2024 exhibits distinct variations at different depths. In the surface layer up to 25 meters, which aligns with the average upper mixed layer depth in the Baltic Sea, there is no discernible trend. The salinity trend increases steadily from zero at a 25-meter depth to 0.04 per year at 70 meters. The most pronounced trend, 0.045 per year, is found within the extended halocline layer ranging from 70 to 150 meters. It is noteworthy that there is a slight reduction in the salinity trend to 0.04 per year between depths of 150 and 220 meters. Although this decrease is minor, it suggests that salt transport into the extended halocline layer is more pronounced than into the deeper layers. The Major Baltic Inflows are responsible for the significant salinity trend of 0.05 per year observed in the deepest layer of the Baltic Sea.

'''DOI (product):''' https://doi.org/10.48670/moi-00207