Baltic Saline Barotropic Inflows (SBI) 1887 - 2024

Major Baltic inflow events (MBI) transport large amounts of saline water into the Baltic. They are the solely source for deep water ventilation in the central Baltic basins, and control to a large extent the environmental conditions below the halocline. Till 2015 the available time series of MBI frequency and intensity depicted strong decrease of MBI frequency after the 1980ies, followed by long lasting stagnation periods in the central Baltic basins.
The expected long term decrease in mean salinity of the Baltic was not observed. This was explained by a compensation effect of increased frequency of baroclinic saline inflows. Also the frequency of large volume changes of the Baltic has not changed (Lehmann and Post, 2015), and recent model studies predict a slight increase of MBI frequency with warming climate and rising sea level. Additionally, the lack of minor MBI events since the early 1980ies pointed to a bias in the MBI time series.

Triggered by the exceptional MBI observed in December 2014 (Mohrholz et al., 2015) the MBI time series was revised. Using long term data series of sea level, river discharge, and salinity from the Belt and Sound a continuous time series of barotropic inflows was constructed for the period from 1886 till present (DS5). A comparison with the MBI time series FM96 of Fischer and Matthäus (1996) revealed significant differences in the period since the 1980ies (Figure 1). The reasons for the deviations between both time series are mainly the lack of appropriate data between 1976 and 1991, and the change in observation methods afterwards, which caused a systematic bias in the inflow statistics.

The frequency distribution of MBI in the revised time series depicts no statistically significant difference for three subsequent forty year periods (Figure 2). The inflow class distribution follows the expected exponential distribution.

In contrast to earlier investigations the revised MBI time series depicts no significant long term trend in MBI frequency and intensity, contradicting the hypothesis that climate change caused a decreasing MBI frequency.
However, there exists a multi-decadal variability of MBI. Periods with reduced MBI frequency were identified. The revised MBI time series was verified with observations of dissolved oxygen and salinity in the bottom layer of the Bornholm basin.
Until today climate change has no obvious impact on the MBI related oxygen supply to the central Baltic Sea. The increased eutrophication during the last century is most probably the main driver for temporal and spatial spreading of suboxic and anoxic conditions in the deep layer of the Baltic Sea.
 

Figure 3: Time series of barotropic inflow events from 1886 to 2024 (blue bars). The height of the bars describes the intensity of the individual events in Gt of imported salt. The red curve represents the average annual salt import of all events classified as MBI in Gt of salt per year. The dashed black curve shows the mean annual salt import of all barotropic inflow events in Gt of salt per year.

Data set description

The data set consists of two products. The time series of daily barotropic transports through the Belt and the Sund between 1886 and January 2024, and a list of all barotropic inflow events through the Danish straits.
The data are supplied in ASCII, Matlab and NetCDF format. The transport time series (SBI_1887_2024_trans.*) consists of three parameters:

The event list (SBI_1887_2024.*) provides for each barotropic inflow event the following parameters: