An Analysis of Annual Sea Level Variations in European Waters

Summary Regression analysis has been applied to all long series of annual mean sea level heights for European stations in order to estimate the components due to secular variation, air pressure variations and the nodal tide. The multiple correlation coefficients are in general found to lie between 0.8 and 1.0. Secular variations (rise or fall in sea level relative to the land) are well defined; they range from almost 90 cm/century fall at the head of the Gulf of Bothnia to a rise of 20 cm/century in the southern North Sea, English Channel and Mediterranean. The variations are consistent enough to enable isopleths to be constructed. The results are used to predict the probable value of mean sea level at certain stations in 1970. Expressions are obtained for annual mean sea level anomalies, arising from variations in the air pressure distribution, which enable the computation of ‘normal’ mean sea level (under normal atmospheric conditions) and ‘isobaric’ mean sea level (on the assumption of an isobaric atmosphere). The isobaric estimates represent an approximate to the geoid in European waters, though clear indications exist of a slope due to the spatial distribution of sea water density; the principal isobaric surface slopes are found to be more than 20 cm upwards from the North Sea into the Baltic, 8 cm downward to the north along the Norwegian coast, and 10 cm downward from the English Channel into the Mediterranean. The permanent deformation of the sea surface due to prevailing winds and air pressure gradients is well marked, rising more than 20 cm from the Mediterranean to the Gulf of Bothnia. Estimates of the nodal tide (period 18.6 years) suggest its existence in the equilibrium form, but the degree of scatter in the results is too great to permit a more definite conclusion.