On the vertical structure of the low-frequency oscillations of currents in the Gulf of Finland

The aim of this study was to determine the depths which were most influenced by low-frequency motions and to reveal the processes behind the observed distribution of kinetic energy. The long term records of five bottom-mounted ADCP deployed along the Gulf of Finland (GoF) in different regions were studied. Two ADCP of five were deployed along the thalweg of the gulf at the same time period (winter 2011/2012) while the remaining three were located closer to the southern coastal slope of the GoF. Those ADCP were deployed at different time periods. It was found that in many cases the low-frequency oscillation of currents, not found in the surface layer, was a dominating phenomenon below the pycnocline. The maximum amplitudes of those motions were observed sometimes directly near the bottom and sometimes in the middle of the bottom layer (the layer below the pycnocline). Still there were also observed time periods when the low-frequency variability did not exist. The frequency of oscillations varied by region in the GoF and therefore the role of bottom topography in determination of the frequency of oscillations could be expected. Furthermore, a hypothesis that some of the observed low-frequency oscillations were bottom-trapped topographic waves was proposed. As a source of energy for these periodic processes the varying wind conditions were expected. Under the ice cover the current variability in the low-frequency range was not observed. For the comparison model results of bottom-trapped topographic waves for one region of the GoF were discussed. The vertical distribution of amplitudes of measured currents in some cases was similar to the modelled ones but in many cases a more complicated pattern was observed.