A B S T R A C T The low-frequency sea level spectrum in the Baltic Sea has been analysed based on long-term time series of sea level data (15Á124 yr) from three tide gauge stations in the Baltic Sea and two stations in the North Sea. The principal periodicities detected in the spectrum are seasonal and tidal oscillations including the pole tide with a period of about 14 months. Cross-spectral analysis has been applied to estimate the frequency response of sea level oscillations in the Baltic

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... relative to the North Sea. It is demonstrated that the basic factor in the formation of the low-frequency sea level spectrum in the Baltic Sea is the barotropic water exchange through the Danish straits. The limited throughput of these straits plays the role of a natural low-pass filter for the sea level variations: high-frequency sea level variations from the North Sea are effectively damped, while the lowfrequency signal can pass through into the Baltic Sea almost undisturbed. A simple model of the barotropic water exchange used in the study allows us to estimate the parameters of the filter. It is shown that the cutoff frequency is about 0.014 cpd (74 d period): the energy of sea level oscillations at this frequency is reduced by one half after their penetration into the Baltic Sea. This study contributes to quantifying extreme sea level events in the Baltic Sea in height and time to improve their predictability.