The existence of seasonal variations in the main tidal constituents has bee
n known for a long time from coastal tide gauges. In this paper we extend t
he amount of observations from the relatively limited number of coastal tid
e gauge observations to also include the large amount of offshore informati
on that have become available from altimetry. These observations are compar
ed with results from a hydrodynamical model applied for the north-west Euro
pean shelf. The model clearly identifies the seasonal variation in the M-2
constituent as a shallow water effect with a large part of the variation ex
plained as a barotropic signal having high dependency on the meteorological
field over the area.