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Climate of the Past An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/cp-2019-105
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/cp-2019-105
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 27 Sep 2019

Submitted as: research article | 27 Sep 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Climate of the Past (CP).

Explaining interdecadal salinity changes in the Baltic Sea in a 1850–2008 hindcast simulation

Hagen Radtke1, Sandra-Esther Brunnabend1,a, Ulf Gräwe1, and H. E. Markus Meier1,2 Hagen Radtke et al.
  • 1Department of Physical Oceanography and Instrumentation, Leibniz Institute for Baltic Sea Research Warnemünde,Rostock, Germany
  • 2Department of Research and Development, Swedish Meteorological and Hydrological Institute, Norrköping, Sweden
  • apresent address: Department of Research and Development, Swedish Meteorological and Hydrological Institute,Norrköping, Sweden

Abstract. The detection of historical long-term trends is often complicated by interdecadal variability in the time series of interest. A mechanistic understanding of the causes of this variability allows to separate the signals. Salinity of the Baltic Sea contains a dominant 30-year cycle with a peak-to-peak amplitude of around 0.4 g kg−1 at the surface. We use both analysis of empirical data and a numerical model reconstruction for the period of 1850–2008 to explain these changes. It is known that the 30-year periodicity coincides with a variability in river runoff. Periods of enhanced runoff are followed by lower salinities. We demonstrate, however, that the drop in mean salinity cannot be understood as a simple dilution of the Baltic Sea water by freshwater. Rather, the 30-year periodicity in river runoff occurs synchronously with a substantial variation in salt water import across Darss Sill. Fewer strong inflow events occur in periods of enhanced river runoff. This reduction in the import of high-salinity water is the main reason for the freshening of the water below the permanent halocline. In the bottom waters, the variation in salinity is larger than at the surface. As a consequence, the surface layer salinity variation is caused by a combination of both effects, a direct dilution by river water and a reduced upward diffusion of salt as a consequence of reduced inflow activity. It remains unclear whether the covariation in river runoff and inflow activity are only a spurious correlation during the historical period, or a mechanistic link exists between the two quantities, e.g. both are caused by the same atmospheric patterns.

Hagen Radtke et al.
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Hagen Radtke et al.
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Short summary
During the last century, salinity in the Baltic Sea showed a multidecadal oscillation with a period of 30 years. Using a numerical circulation model and wavelet coherence analysis, we demonstrate that this variation has at least two possible causes. One driver is river runoff which shows a 30-year variation. The second one is a variation in the frequency of strong inflows of saline water across Darss Sill which also contains a pronounced 30-year period.
During the last century, salinity in the Baltic Sea showed a multidecadal oscillation with a...
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