Assessment of M2 tidal amplitude changes in the North Sea : 1980-2023

Tidal amplitudes and phases are generally considered to be stationary. However, there is growing evidence from observations around the world for trends in tidal amplitudes that cannot be explained by the changes in tidal potential (i.e., the gravitational forces associated with the moon and sun). In earlier unreported work, a rapid decline in M2 tidal amplitude was reported along the Dutch coast. That work considered tide gauge measurements from 1980 up to 2017. The present study aims to investigate whether the decline in M2 tidal amplitude has continued in recent years and more generally how the amplitude has evolved since 2017. To this aim, we present an analysis of tidal records and their trends along the North Sea coasts for the period 1980-2023. We focus on M2 since this is the predominant constituent in most parts of the North Sea. We observe a gradual but strong decline in M2 amplitudes from 2007 to 2016. The strongest trends are in the order of a decline of 0.4-0.5 % per year, equal to a decrease of 3-4 mm/year. The decline in M2 amplitude is strongest along the Dutch coast, the German Bight, Skagerrak, and Kattegat and gradually decreases away from these areas. This decline adds up to 4-5% or 3-4 cm since onset along parts of the Dutch coast. From 2016 onwards we observe a reversal in the direction of the trend, leading to a stagnation or gradual increase in M2 amplitudes. The magnitude of the trend is smaller than in the preceding period and the spatial correlation is less outspoken. Since the cause for the decline in M2 tidal amplitude and subsequent reversal from 2016 onwards is unknown it is difficult to project the trend into the future. We emphasize that it is highly important to further investigate the mechanism behind these changes in tidal amplitude, for which several suggestions are given.