Formation of the Baltic Sea level spectrum

Abstract

The Baltic Sea is an internal shelf basin of theAtlantic Ocean. Almost complete closure of the basin,which is connected with the open ocean only throughshallow narrow Danish straits, leads to the formationof sea level variation absolutely not typical for the openocean [1]. The fluctuation of the sea level is the oceanbreathing [2] formed under the influence of differentfactors: tidal and meteorological forcing, componentsof the water balance, and variations in the water density. These factors cover a wide range of periods; therelative role of different processes varies in differentbasins.In order to understand the water dynamics of theBaltic Sea, one has to have an idea about the formationof the entire spectrum of the sea level fluctuations withperiods from hours to tens of years. According to [3],one can distinguish the following five variability rangesin the classification of the sea level fluctuations withperiods smaller than 10 years: mesoscale (from 2 hoursto 2 days), synoptic (from 2 days to one month), intraannual (from 1 month to 1 year), seasonal (annualperiod and divisible harmonics), and interannual(from 1 year to 10 years). The tides make the greatestenergetic contribution to the dispersion of the sea levelfluctuations in the open ocean (approximately 85%).Due to the narrowness of the Danish straits, the tidesalmost do not propagate to the Baltic Sea and themesoscale variability of the sea level is determinedmainly by internal oscillations, which are generatedwithin the sea [4]. However, the longrange fluctuations of the sea level penetrate from the North Seaalmost without distortions.The research object of this work is the North Sea–Baltic Sea system. Such an approach allows us to studyin detail the mechanisms of the formation of the BalticSea level spectrum in various frequency ranges and todistinguish its main peculiarities.It is convenient to consider the sea level fluctuations as a probability process. One can use the spectralanalysis methods of random processes to estimate thefrequency distribution of the energy of sea level fluctuations. The fluctuations of the sea level are not strictlya stochastic process because tides have a strong influence on them. The tide is a regular deterministic process, which is manifested in the sea level spectra assharp deltashaped peaks (discrete spectrum). Theseasonal fluctuations also have a discrete spectrum, inwhich the annual and semiannual harmonics dominate, although these fluctuations are not strictly deterministic [5]. The pole tide is a special type of sea leveloscillations, which is anomalous in the Baltic andNorth seas [6]: this process also has a harmonic character (with a period of approximately 14 monthscaused by the Chandler wobble of the Earth’s axis ofrotation); however, its dominant period has specifictime variations. The other components of the sea levelfluctuations are characterized by random noise andform a continuous energy distribution by frequency(continuum).In this work, we apply spectral analysis to describethe spectrum of the sea level variations and estimatethe energy contribution of different frequency bandsto the total dispersion of fluctuations. With this inmind, we considered longterm time series of hourlyand monthly observations of the sea level at fourcoastal tide gauge stations: Cuxhaven (Germany) inthe North Sea, Stockholm, Furuogrund, and Kronstadt in the Baltic Sea (Table 1, Fig. 1) Joint application of longterm monthly mean time series togetherwith the hourly data of high quality (the percentage ofgaps at all stations did not exceed 1%) gave us the possibility to estimate the energy of sea level fluctuationsin a wide range of periods with the maximum highspectral resolution.The Danish straits separating the Baltic Sea fromthe North Sea serve as a lowfrequency filter that letthe longperiod sea level fluctuations propagate to theBaltic Sea (longterm and seasonal) and suppress thesynoptic and mesoscale sea level fluctuations [7]. Thisis the reason why synoptic and mesoscale sea levelvariations are formed within the Baltic Sea and arecharacterized as eigen (internal) oscillations [8]. Thestructure of internal oscillations of the Baltic Sea level