Spatial variations of mean sea level of the North Sea off the east coast of Britain

Abstract

Low frequency currents and bottom pressures off the east coast of Britain and sea levels from coastal stations, collected during May and June 1981, are used to investigate the spatial gradient of the mean sea level. The summer time was chosen for the experiment so that the relatively calm conditions would allow an accurate measurement of the mean sea level slope for comparison with the slope as determined by the traditional methods of geodetic levelling on land. Effects of the wind stress and pressure gradients on the mean sea level and on the balance of the equations of momentum are also considered. However, as hoped, weather was relatively calm during these months and its influence on the sea level was negligible. Low frequency changes in sea level at different sites were strongly correlated, as were both wind and atmospheric pressure. Principal component analysis showed that more than 88% of the variance of all the time series of each of these variables can be accounted for by the first empirical mode. However, low frequency currents at different sites are only weakly correlated and the first empirical mode cannot account for more than 30% of the total variance. The Coriolis, pressure gradient and bottom friction terms of the momentum equations are significantly correlated. Though the variance of the bottom-friction term is very small, there is a strong correlation between it and the Coriolis term, in both the longshore and offshore momentum equation. For low frequency variations, at least a five-fold increase in the coefficient of bottom friction is required to obtain the best balance for both equations of momentum, but it is difficult to justify such a large increase. There is no evidence to support the view that there is north-south slope of mean sea level as suggested by the geodetic levelling.