A Combined Modeling and Measurement Approach to Assess the Nodal Tide Modulation in the North Sea

Abstract

AbstractThe 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, nonlinear 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 M2 and N2 constituents by the equilibrium of more than 2.7% in the Wadden Sea, while O1 and K2 are underestimated by 1–4.6%, which would produce an error of 2–5 cm for example, 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 a spatial distribution of f and u. Results confirm the spatially varying nodal satellite modulation in friction dominated, shallow water regions.