The M2 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 M2 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.

Key Points

• M2 displays a strong seasonal cycle in the North Sea with amplitude larger than 5 cm
• The seasonality is well reproduced by a barotropic model forced with the tidal potential and the atmosphere only
• The seasonal cycle is mainly due to gravitational nonlinear effects in a large part and atmospheric effects in a smaller extent

Plain Language Summary

The tide is changing over a year, the amplitude of the tide being larger in summer than in winter in the North Sea. We try here to understand what are the reasons that explain such changes over a year. For this purpose, we analyze sea level data from 35 tide gauges in Europe. We found that the largest changes occur in the southern North Sea. Such changes are well reproduced by a model, whose inputs are the tidal potential and the atmosphere only, and which does not consider the stratification of the waters. This means that the stratification only plays a minor role. We show that the tide changes over a year, first because of the gravitational tide (some nonlinear components introducing some seasonal modulations). The atmosphere also plays a role, but locally and in a smaller extent.