A numerical experiment with a hydrodynamic model is performed to study free low-frequency barotropic oscillations of the Baltic Sea level. The model series are used to construct the fields of various probability characteristics of the free low-frequency oscillations of the sea level and flow velocity. A comparison of the empirical and theoretical dispersion relations of the topographic Rossby waves is carried out. Comparison has shown that the empirical and theoretical estimates of the characteristics of topographic waves do not coincide, while the empirical and theoretical values of dimensionless wavelengths of short Rossby waves agree quite well. It is concluded that the initial disturbance in the sea generates a standing gravity wave (seiche), whose energy dissipates under the influence of the nonlinear effects, Coriolis force, Earth's sphericity, and topographic effects, and motion becomes quasi-nondivergent; inertial currents decay and the motion is further determined mainly by horizontal-transverse gradient-vorticity waves.