Role of the barotropic water exchange in the formation of the Baltic Sea level spectrum

Abstract

The spectrum of low-frequency sea level variations was analyzed on the basis of long-term (15–124 years) time series of sea-level data from three tide gauge stations in the Baltic Sea and two stations in the North Sea. The principal periodicities revealed in the spectrum are seasonal as well as the tidal oscillations including the pole tide with a period of about 14 months. The response function of the Baltic Sea level to variations in the North Sea level is calculated using a cross spectral analysis. It is shown that the barotropic water exchange through the Danish Straits is a basic factor in the formation of the low-frequency sea level spectrum in the Baltic Sea. 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 low frequency signal can pass through into the Baltic Sea almost undisturbed. The simple model of the barotropic water exchange used in the study enables us to estimate the parameters of this filter. It is shown that the cutoff frequency is about 0.014 cpd (a period of 74 days): the energy of the sea level oscillations at this frequency is reduced by one half after their penetration into the Baltic Sea.