The topic of long term oscillations in annual mean sea level variability has been tackled in many publications. Since the beginning of these studies, two kinds of oscillations have been apparent. First, there are changes in sea-level which have a physical background, i.e. an eleven-year period connected with solar activity, the pole and nodal tides. The second group consists of periods of oscillation which have in general no clear physical background, but are visible in the spectral characteristics; they have a periodicity interval mainly of 2 - 10 years. They have no stable periodicity, because they are detected in relatively short measurement series. Another source of periodicity dissipation is the weak and unstable forcing. The essential problem in the research of these oscillations has been connected with the possibility of dividing the forcing between local and large-scale phenomena.

For the Baltic, this difficulty can be overcome by introducing the annual mean Baltic basin sea-level (AMBBSL) to the computations. Spatial averaging removes local forcings, leaving only large-scale phenomena, and confirms the real existence of the computed peaks. AMBBSL were computed for the period 1897-1986 on the basis of the monthly mean Baltic basin sea- levels (MMBBSL) at Kungholmsfort, Stockholm, Nedre Gavle, Ratan, Oulu and Helsinki (Spencer and Woodworth, 1993). MMBBSL were determined by the EOF method (Wroblewski, 1992). Fourier transforms of AMBBSL determined the highest peaks in the amplitude spectrum with periods of 5.6, 4.5 and 3.6 years; the relevant amplitudes were 2.0, 2.2 and 2.2 cm, and the phases -108, -63 and 30 degrees.

For comparison, a similar Fourier transform was computed for Stockholm data collected during the same period. Stockholm is considered to be the most representative tide gauge station as regards MMBBSL changes. The highest peaks were obtained for the above periods and had amplitudes of 2.4, 2.3 and 2.3 cm with the same phases. The highest amplitudes in both computations had a 5% significance level estimated by the bootstrap method ( Efron, 1981; Efron and Gong, 1983 ). In the both computations, the 'eleven year' period was computed with a periodicity at 10 years, its amplitude was in the 1.7 - 1.9 cm interval and significance 10%. The computations suggest that there is a need for forcing analysis of mean sea-level oscillations in the Baltic basin over the period of many years. Up to now, they have generally been thought to be the result of the action of the Baltic sea-level forcing elements or the addition of particular oscillations.

References

Efron B., 1981, Nonparametric standard errors and confidence intervals, Can. J. Statist., 9, 139-172.

Efron B., and Gong G., 1983, A leisurely look at the bootstrap, the jackknife and cross-validation, Am. Statist., 37, 36-48.

Spencer N.E., and Woodworth P.L., 1993, Data Holdings of the Permanent Service for Mean Sea Level. Birkenhead: Permanent Service for Mean Sea Level. 81 pp.

Wroblewski A., 1992, Application of EOF in Determining Basin Mean Sea Level Using Computations for the Baltic as an Example. Sea Level Changes: Determination and Effects, American Geophysical Union and IUGG, 23-28.