Barotropic response of the Baltic Sea dynamic system to sea level fluctuations in the Kattegat strait

With the help of numerical experiments with a three-dimensional barotropic hydrodynamic model, the response of the level surface of the Baltic Sea to different-scale level disturbances propagating into the Baltic through the Danish straits from the North Sea is studied. For this, 53 harmonic oscillations with amplitudes of 10 cm and periods from 3 hours to 1 year, which in the article are called "external", in contrast to the perturbations of the level inside the sea induced by them. The same amplitudes for all initial oscillations were set so that when analyzing the results, quantitative differences in the amplitude spectra of "external" and "internal" oscillations were clearly visible. It is shown that, with the exception of the frequency range of natural barotropic oscillations of the Baltic Sea, in the Danish Straits there is an almost complete filtration of oscillations specified at the boundary of the region with periods from 3 hours to 10 days. In the range of periods from 15 to 35 hours, there is an increase in "internal" oscillations due to the resonance of the initial oscillations with the natural oscillations of the Baltic. In the mesoscale frequency range (periods from hours to several days), the response of the level surface of the Baltic Sea to the impact of "external" oscillations is manifested in the generation of progressive-standing Kelvin waves with pronounced amphidromic systems and antinodes. For oscillations with periods of more than 10 days, with a decrease in frequency, the filtering effect of the Danish Straits is weakened and for periods of about 60 days, their amplitudes decrease by only 50%. In the range of seasonal variability, hydraulic resistance in the Danish Straits has the least effect on the propagation of "external" sea level fluctuations, reducing their amplitudes by only 6 - 22%. The highest amplitude is observed in semi-annual oscillations, which are amplified due to the influence of the semi-annual overtone in the annual oscillation generated by nonlinear effects.