The correct representation of the 18.61 year nodal tide is essential for an interpretation of the evolution of mean sea level, as errors cause misleading bias. The nodal tide is currently estimated by applying correction factors in harmonic analysis, which are derived from the equilibrium tide. From the equilibrium tide, correction values f for amplitude and u for phase are determined, which alter lunar tidal constituents, depending on the nodal cycle. This approach has proven to be valid for many tide gauges, even though the impact of the nodal tide in shelf seas has been shown to differ from their theoretical correction value. Hence, tidal constituents from tide records in the North Atlantic shelf region were analyzed for their nodal amplitude and phase lag with a new multiple, non-linear regression approach, which is able to approximate the nodal modulation quantitatively and its agreement to the theoretical equilibrium tide. Results show an overestimation of the lunar M and N constituents by the equilibrium of more than 2.7% in the Wadden Sea, while O and K are underestimated by 1 to 4.6%, which would produce an error of 2 to 5 cm e.g. in the German Wadden Sea. Additionally, a process-based model of the North Sea was applied at the diurnal minimum and maximum of the nodal cycle to calculate the spatial distribution of f and u. Model results reproduce a regionally varying pattern of f and u, indicating how the amplitude modulation of nodal constituents in shallow areas is distributed.

In many places around the world, tide gauges have been measuring substantial non-astronomical changes. Here we document an exceptional large spatial scale case of changes in tidal range in the North Sea, featuring pronounced trends between -2.3 mm/yr in the UK and up to 7 mm/yr in the German Bight between 1958 and 2014. These changes are spatially heterogeneous, suggesting a superposition of local and large-scale processes at work within the basin. We use principal component analysis to separate large-scale signals appearing coherently over multiple stations from rather localized changes. We identify two leading principal components (PCs) that explain about 69% of tidal range changes in the entire North Sea including the divergent trend pattern along UK and German coastlines, which suggest movement of the region’s semidiurnal amphidromic areas. By applying numerical and statistical analyses, we can assign a baroclinic (PC1) and a barotropic large-scale signal (PC2), explaining a large part of the overall variance. A comparison between PC2 and tide gauge records along the European Atlantic coast, Iceland and Canada shows significant correlations on time scales of less than 2 years, which suggests an external and basin-wide forcing mechanism. By contrast, PC1 dominates in the southern North Sea and originates, at least in part, from stratification changes in nearby shallow waters. In particular, from an analysis of observed density profiles, we suggest that an increased strength and duration of the summer pycnocline has stabilized the water column against turbulent dissipation and allowed for higher tidal elevations at the coast.