Some transient aspects of tidal propagation

The effects of transient forcing on the structure of some tidal modes are investigated using a time‐dependent numerical model. After presentation of the numerical scheme, some of the main features of tidal field variations are presented. The simulated tidal structures are interpreted as consisting of beats between tidal modes and transient waves near tidal periods. From these simulations, several types of information are obtained. Vertical group velocities are calculated and compared with those computed using the analytical expressions of Richmond (1975). Several possible definitions of the setup time are given, and the results from our model calculations are compared with those obtained by Bernard (1981). We found that the (2, 2) setup time is longer (several weeks) than the (1, 1) setup time, contrary to the Bernard conclusions. From these setup time estimates, some conclusions regarding the interpretation of observations and comparisons with steady state model calculations are drawn. Finally, similar results obtained for a two‐dimensional gravity wave are analyzed to clarify further basic mechanisms acting on tidal structure variations induced by transient forcing.