Abstract
The seasonal cycle accounts for about 40% of the total sea level variability in the Baltic Sea. In a climate change context, changes are expected to occur, not only in mean levels but also in the seasonal characteristics of sea level. The present study addresses the quantification of changes in the seasonal cycle of sea level from a set of century-long tide gauge records in the Baltic Sea. In order to obtain robust estimates of the changes in amplitude and phase of the seasonal cycle, we apply different methods, including continuous wavelet filtering, multi-resolution decomposition based on the maximal overlap discrete wavelet transform, auto-regressive-based decomposition, singular spectrum analysis and empirical mode decomposition. The results show that all methods generally trace a similar long-term variability of the annual cycle amplitudes, and we focus on discrete wavelet analysis as the natural counterpart of classical moving Fourier analysis. In contrast to previous studies suggesting the existence of long-term changes in the seasonal cycle, in particular an increase of the annual amplitude, we find alternating periods of high and low amplitudes without any clear indication of systematic long-term trends. The derived seasonal patterns are spatially coherent, discriminating the stations in the Baltic entrance from the remaining stations in the Baltic basin, for which zonal wind accounts for typically more than 40% of the variations in amplitude.
Highlights
The seasonal cycle is a ubiquitous feature of climate records in general, and sea level records in particular
The mean seasonal cycle is computed for each monthly tide gauge record by averaging the values for each calendar month
Before discussing the general shape and long-term changes of the seasonal cycle in Baltic sea level inferred using the different methods introduced in Section 3, we emphasise the necessity of discussing the separability of this variability component
Summary
The seasonal cycle is a ubiquitous feature of climate records in general, and sea level records in particular. In the Baltic Sea, the seasonal cycle of sea level is a well-known feature, with a pronounced minimum in spring Previous studies have suggested an increase in the amplitude of the seasonal sea level cycle in the Baltic Sea. Ekman and Stigebrandt (1990) found an increase in the seasonal amplitude from 7 to nearly 11 cm applying Fourier analysis to two distinct 80-yr periods of the Stockholm record. Plag and Tsimplis (1999) applied a moving harmonic analysis over time windows of fixed length (19 yr) and found over the North Sea and Baltic area regionally coherent Ekman and Stigebrandt (1990) found an increase in the seasonal amplitude from 7 to nearly 11 cm applying Fourier analysis to two distinct 80-yr periods of the Stockholm record. Plag and Tsimplis (1999) applied a moving harmonic analysis over time windows of fixed length (19 yr) and found over the North Sea and Baltic area regionally coherent
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