M 2 ${M}_{2}$ Seasonal Variability in Northwestern Europe: Characteristics and Drivers

Abstract

The $M_2$ tide displays large seasonal variability in Europe, particularly in the North Sea. The tide is there larger in summer than in winter. However, there is no consensus on the physical drivers leading to such large values, atmosphere circulation and stratification being two good candidates. We analyzed hourly sea level data from observations at 35 tide gauges in Europe. The amplitude of $M_2$ seasonal cycle is the largest in the southern North Sea, reaching typically 4–6 cm. This cycle is well reproduced by a barotropic model, forced with the tidal potential and the atmosphere only. This suggests a minor role of the stratification. We show that large seasonal cycles in the southern North Sea are first due to gravitational nonlinear effects. The atmosphere also plays a role, but locally and in a smaller extent.