Abstract

In the Baltic Sea, salinity and its large variability, both horizontal and vertical, are key physical factors in determining the overall stratification conditions. In addition to that, salinity and its changes also have large effects on various ecosystem processes. Several factors determine the observed two-layer vertical structure of salinity. Due to the excess of river runoff to the sea, there is a continuous outflow of water masses in the surface layer with a compensating inflow to the Baltic in the lower layer. Also, the net precipitation plays a role in the water balance and consequently in the salinity dynamics. The salinity conditions in the sea are also coupled with the changes in the meteorological conditions. The ecosystem is adapted to the current salinity level: a change in the salinity balance would lead to ecological stress of flora and fauna, and related negative effects on possibilities to carry on sustainable development of the ecosystem. The Baltic Sea salinity regime has been studied for more than 100 years. In spite of that, there are still gaps in our knowledge of the changes of salinity in space and time. An important part of our understanding of salinity are its long-term changes. However, the available scenarios for the future development of salinity are still inaccurate. We still need more studies on various factors related to salinity dynamics. Among others more knowledge is needed, e.g. from meteorological patterns in various space and time scales and mesoscale variability in precipitation. Also, updated information on river runoff and inflows of saline water is needed to close the water budget. We still do not understand accurately enough the water mass exchange between North Sea and Baltic Sea and within its sub-basins. Scientific investigations of the complicated vertical mixing processes are additionally required. This paper is a continuation and update of the BACC II book which was published in 2015, including information from articles issued until 2012. After that, there have been many new publications on the salinity dynamics, not least because of the Major Baltic Inflow which took place in December 2014. Several key topics have been investigated, including the coupling of long-term variations of climate with the observed salinity changes. Here the focus is on observing and indicating the role of climate change for salinity dynamics. New results of MBI-dynamics and related water mass interchange between the Baltic Sea and the North Sea have been published. Those studies also included results from the MBI-related meteorological conditions, variability in salinity and exchange of water masses between various scales. All these processes are in turn coupled with changes in the Baltic Sea circulation dynamics.

Highlights

  • The Baltic Sea salinity is a physical variable, but it describes in an integrated way the simultaneous effects of the energy and water cycles in the sea; some of these features are just typical for the Baltic Sea, as the low mean level of salinity and its pronounced variability

  • Due to the excess of river runoff to the sea, there is a continuous outflow of water masses in the surface layer

  • The net precipitation over the sea plays a role in the water balance and in the salinity dynamics

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Summary

Introduction

The Baltic Sea salinity is a physical variable, but it describes in an integrated way the simultaneous effects of the energy and water cycles in the sea; some of these features are just typical for the Baltic Sea, as the low mean level of salinity and its pronounced variability. An essential role in salinity dynamics is played by the barotropic water exchange which comprise irregularly Major Baltic Inflows (MBIs, Matthäus and Franck, 1992) and Large Volume Changes (LVCs, Lehmann et al, 2017) These inflows have a significant impact on the modification of the observed patterns of stratification and oxygen conditions. We will summarize here the earlier findings of salinity dynamics to set up the basis of our current understanding: There was a decreasing trend of the mean salinity of the Baltic Sea both in the early 1900s and later during the century (1980s and 1990’s); the latter is coupled with a complete lack of MBIs during 1983-1993 During those periods, freshwater inflows were more extensive than on average and zonal winds were stronger than normal, showing a very long-term natural variability in the highly dynamic system. Most probably, such events have occurred before but could not be observed due to shortcomings in the observational strategy

Atmospheric forcing driving the salinity dynamics of the Baltic Sea
Large Volume Changes and Major Baltic Inflows
The cold intermediate layer
Salinity dynamics of the eastern Gotland Basin and the Gulf of Riga
Salinity dynamics of lagoons
Salinity dynamics of the Gulf of Finland
Salinity dynamics of the Gulf of Bothnia
Development of the mean salinity
Internal circulation and stratification
The specific role of precipitation and river runoff
The role of sea level change due to global warming
Oxygen conditions
Environmental interaction between fish/larvae and salinity dynamics
Present knowledge gaps
Findings
Key messages
Full Text
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