For the Baltic Sea, the monitoring data from recent years, which dissolve the seasonal season, show that all basic hydrographic parameters of the water column monitored in practice are strongly influenced by natural variability. The most striking feature here is the strong seasonal variability in the surface layer, which manifests up to the depth of the halokline. The sea surface temperature, averaged over the area of Baltic Sea waters and based on corresponding datasets issued by the BSH (‘ST data’), shows large interannual variations during the assessment period. This behaviour is overshadowed by a sustained increase in temperature, which is indicative of global ocean warming. However, SST fluctuations within a year are very large. The range is from 2 °C in climatological February and March to 18 °C in climatological August. It thus represents the largest fluctuations in temperature. During the 2016-2021 assessment period, the monthly surface temperatures for the German EEZ were within the range of ± 2 standard deviations. However, the months of June and July of 2021 were warmer. The annual mean of the near-surface water temperature at the Kiel lighthouse, as presented in the 2018 status assessment, has remained at an elevated level compared to the long-term average since the late 1980s, after a short-term collapse in the years 2010-2013. They reached their (previous) maximum in 2020 at 11.50 °C. The time trend in this series is 0.040 °C ± 0.024 °C per year. Unlike the surface layer, the deep waters of the Baltic Sea below the Halokline are not determined by seasonality, but by the Baltic single-flow events (MBI’s – Major Baltic Inflows). These are variable in time and can take place at different times of the year. The existing MARNET stations in the German Baltic Sea can capture these events, but do not allow a comprehensive assessment of the impact of the MBIs. For the assessment of the intensity of these single flow events, the MARNET station “Darßer Schwelle” is of paramount importance, as approximately 70 % of the water exchange takes place over this strait. The temporary observation of temperature changes in the ground waters at the MARNET stations must therefore always be seen in the context of the time of single flow events. No major inflow has taken place since 2017 (e.g. Mohrholz 2018). Several smaller, short-term events have an impact on the dynamics in the western Baltic Sea, generating increased variability there (IOW 2022). To date, climate change has had no obvious impact on single flow events (Mohrholz 2018). Systemic effects on biology and ecology as a result of hydrographic changes can only be expected once the natural variability of the limiting environmental factors, such as temperature and salinity, is persistently exceeded. In general, surface temperature and salinity show a high correlation with the NAO index. Comparing winter values (December, January, Feb-8 ruar) of years with a high positive NAO index with years with a low negative NAO index gives differences in surface temperature up to 3 °C and surface salt content up to 3 psu. This natural variability is significantly higher than the projected climate-related changes up to the end of the century; however, both processes overlap and need to be considered in their entirety. IOW 2022: Barotropic saltwater intrusions into the Baltic Sea (SBI) 1887 – 2021. https://www.io-warnemuende.de/major-baltic-inflow-statistics.html Mohrholz, V. (2018): Major Baltic inflow statistics-revised. Frontiers in Marine Science, 5, 384, https://doi.org/10.3389/fmars.2018.00384.

Systemic effects on biology and ecology as a result of hydrographic changes can only be expected once the natural variability of the limiting environmental factors, such as temperature and salinity, is persistently exceeded. In general, surface temperature and salinity show a high correlation with the NAO index. Comparing winter values (December, January, Feb-ruar) of years with a high positive NAO index with years with a low negative NAO index gives differences in surface temperature up to 3 °C and surface salt content up to 3 psu. This natural variability is significantly higher than the projected climate-related changes up to the end of the century; however, both processes overlap and need to be considered in their entirety.

For the Baltic Sea, the monitoring data from recent years, which dissolve the seasonal season, show that all basic hydrographic parameters of the water column monitored in practice are strongly influenced by natural variability. The most striking feature here is the strong seasonal variability in the surface layer, which manifests up to the depth of the halokline. The sea surface temperature, averaged over the area of Baltic Sea waters and based on corresponding datasets issued by the BSH (‘ST data’), shows large interannual variations during the assessment period. This behaviour is overshadowed by a sustained increase in temperature, which is indicative of global ocean warming. However, SST fluctuations within a year are very large. The range is from 2 °C in climatological February and March to 18 °C in climatological August. It thus represents the largest fluctuations in temperature. During the 2016-2021 assessment period, the monthly surface temperatures for the German EEZ were within the range of ± 2 standard deviations. However, the months of June and July of 2021 were warmer. The annual mean of the near-surface water temperature at the Kiel lighthouse, as presented in the 2018 status assessment, has remained at an elevated level compared to the long-term average since the late 1980s, after a short-term collapse in the years 2010-2013. They reached their (previous) maximum in 2020 at 11.50 °C. The time trend in this series is 0.040 °C ± 0.024 °C per year. Unlike the surface layer, the deep waters of the Baltic Sea below the Halokline are not determined by seasonality, but by the Baltic single-flow events (MBI’s – Major Baltic Inflows). These are variable in time and can take place at different times of the year. The existing MARNET stations in the German Baltic Sea can capture these events, but do not allow a comprehensive assessment of the impact of the MBIs. For the assessment of the intensity of these single flow events, the MARNET station “Darßer Schwelle” is of paramount importance, as approximately 70 % of the water exchange takes place over this strait. The temporary observation of temperature changes in the ground waters at the MARNET stations must therefore always be seen in the context of the time of single flow events. No major inflow has taken place since 2017 (e.g. Mohrholz 2018). Several smaller, short-term events have an impact on the dynamics in the western Baltic Sea, generating increased variability there (IOW 2022). To date, climate change has had no obvious impact on single flow events (Mohrholz 2018). Systemic effects on biology and ecology as a result of hydrographic changes can only be expected once the natural variability of the limiting environmental factors, such as temperature and salinity, is persistently exceeded. In general, surface temperature and salinity show a high correlation with the NAO index. Comparing winter values (December, January, February) of years with a high positive NAO index with years with a low negative NAO index gives differences in surface temperature up to 3 °C and surface salt content up to 3 psu. This natural variability is significantly higher than the projected climate-related changes up to the end of the century; however, both processes overlap and need to be considered in their entirety. IOW 2022: Barotropic saltwater intrusions into the Baltic Sea (SBI) 1887 – 2021. https://www.io-warnemuende.de/major-baltic-inflow-statistics.html Mohrholz, V. (2018): Major Baltic inflow statistics-revised. Frontiers in Marine Science, 5, 384, https://doi.org/10.3389/fmars.2018.00384.