| Summary: | Purpose. The main purpose of the paper is to estimate in a stationary approximation the magnitudes of the amplitude-phase characteristics of the sea level seasonal fluctuations in the Baltic Sea in 1971–2020, and to study the features and possible reasons for their non-stationarity.
Methods and Results. The description of seasonal fluctuations is substantiated by a sum of 4 harmonics: the annual (Sa), semiannual (Ssa), terannual (Sta) and quarterannual (Sqa) ones. The average values of the Baltic Sea level seasonal fluctuations, as well as their interannual variability are estimated using a harmonic analysis of the average daily values of the tide gauge data. The results indicate that the amplitudes of the harmonics Sa, Ssa, Sta and Sqa are reliably distinguished for all the coastal points under study. The features of the phase spatial distribution testify to a statistically reliable propagation of the sea level disturbances with a one-year period from the southwest to the north and to the northeast. For the other components of the sea level seasonal fluctuations (Ssa, Sta and Sqa), the spatial phase changes are very small and comparable to the standard errors of their calculation. The amplitude modulation with a period of about 20 years for the Ssa harmonic and approximately 2–10 years for the Ssa, Sta and Sqa ones is well pronounced in the interannual changes in the Baltic Sea level seasonal fluctuations. A significant negative trend for all the stations is observed in the time series of the Sa harmonic amplitudes in 1971–2020. In order to clarify the reasons of the revealed features of the Sa harmonic interannual changes, a non-stationary harmonic analysis of the atmospheric pressure series, wind and the sea level steric fluctuations is done in the study.
Conclusions. It is shown that the changes in the annual sea level fluctuations observed in 1971–2020 are mostly influenced by the anomalies of annual wind fluctuations, and to a lesser extent, by those of the atmospheric pressure.
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