Tidal currents as estimated from ADCP measurements in “practically non-tidal” Baltic Sea

The Baltic Sea has been usually described as “practically non-tidal sea”. The statement is motivated by the landlocked intracontinental location of the sea connected to the ocean (North Sea) via narrow and sallow Danish Straits (Belt Sea). For most of the non-south-western Baltic Sea the tidal amplitude of the sea level is less that 10 cm except in the eastern Gulf of Finland (GoF) where the amplitudes greater than 10 cm have been reported. The knowledge about the horizontal motion of water associated with the water level oscillations (tidal currents) is insufficient till now. We estimated tidal currents from direct current measurements performed at three locations along the southern coast of the GoF using a bottom-mounted ADCP. The rotary spectra of the current velocity revealed significant energetic peaks with the periods of the dominating seiches, tides and inertial oscillation (13.9 h). In the bottom layer the diurnal tides (O₁ and K₁) dominated over the semi-diurnal (M₂ and S₂) constituents with roughly twice larger current velocity amplitudes (about 7.2 and 3.5 cm/s respectively). In the upper layer the both, diurnal and semi-diurnal, tidal amplitudes were similar - about 5 up to 6 cm/s. The autocorrelation function of the diurnal current constituent (band-pass filtered) revealed 14 day period pointing to their intensification by tropic tides. Some 3D simulations of the Baltic Sea salinity in the deep layer below the halocline by a model with the effect of tides included have shown the salinity values smaller than the outcome of the model with the tides excluded. The latter probably implies the importance of tidal currents to the vertical mixing of bottom layer salt and nutrients. Thus, in contrast to the intermittent wind and density-driven currents in the bottom layer the tidal currents are always present and can substantially contribute to the vertical mixing and in this way have an effect on the - utrophication - the most discussed ecological problem of the Baltic Sea.