Seasonal variation in the main tidal constituent from altimetry

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.