The existence of seasonal variations in the main tidal constituents has been known for a long time from coastal tide gauges. In this paper we extend the amount of observations from the relatively limited number of coastal tide gauge observations to also include the large amount of offshore information that have become available from altimetry. These observations are compared with results from a hydrodynamical model applied for the north-west European shelf. The model clearly identifies the seasonal variation in the M2 constituent as a shallow water effect with a large part of the variation explained as a barotropic signal having high dependency on the meteorological field over the area. ¿ 2001 American Geophysical Union