Wave Nature and Modulation of Annual Fluctuations in the Level of the Baltic Sea

Purpose. The study is purposed at estimating the features of spatial-temporal variability of the characteristics of annual fluctuations of the Baltic Sea level based on satellite and contact measurements, their comparing with theoretical dispersion relations of the low-frequency waves of different types, as well as investigating possible mechanisms of the amplitude modulation of annual fluctuations of the Baltic Sea level. Methods and results. The hypothesis on a wave nature of annual fluctuations of the Baltic Sea level is tested and the reasons for their amplitude modulation are investigated based on the harmonic analysis of satellite altimetry data and the 132-year series of tide gauge sea level measurements in Stockholm. It is shown that the wave-like annual disturbances in the sea level field propagate from the southwest to the northeast at the speed from 0.06 to 0.36 m/s. A comparison of the estimated characteristics of annual waves and theoretical dispersion relations of different types of low-frequency waves has shown that they are identified as internal Kelvin waves over the most of the sea area and their characteristics agree with the theoretical dispersion relations of baroclinic topographic Rossby waves in rare cases only in the southwest of the sea. The perceptible interdecadal changes in the annual wave parameters in the sea level field were noted. As compared to the 1993-2021 period, the 1993-2002 decade is characterized by a decrease of the Sa harmonic amplitude by 1.5-3 times, later onset of the maximum of the sea level annual variation (about 1 month later), as well as a noticeable slowdown of the annual wave phase velocity in the southwest of the sea. Conclusions. The reasons for the amplitude modulation of annual waves in the sea level field are related to the impact of the oscillations with periods 352, 374 and 379 days which are identified in the form of small but significant amplitude maxima in the Fourier series spectra of sea level, wind speed and atmospheric pressure. One more mechanism of the amplitude modulation of annual waves is assumed to be related to the changes in frequency of the Baltic Sea natural baroclinic oscillations due to the interannual variations of its stratification.