A fully nonlinear three-dimensional hydrodynamic model of the Irish Sea is used to examine the processes determining the spatial variability of the M4 tide in the Irish Sea. The model is based upon a mixed spectral/finite difference approach, with an expansion of functions in the vertical (yielding a continuous current profile) and a finite difference grid in the horizontal. The model has a sufficiently fine grid to enable the nonlinear advective terms to be resolved in their finite difference form, and the spatial variability of the M4 tide to be represented. Computed M4 elevations and currents determined using a radiation open boundary condition with both M2 and M4 elevation and current input and a flow dependent viscosity are found to be in satisfactory agreement with observations considering the limited grid resolution of the model, and the level of noise in the M4 tide as a percentage of the signal is much higher than for M2. This agreement is good enough to establish the credibility of using the model to examine the processes influencing the M4 tide, which are discussed in detail. The importance of an M4 current input to the radiation condition, and the difficulty of using an elevation specified open boundary condition to reproduce the M4 tide in this region are considered in some detail.

The nonlinear advective terms are shown to have a limited local influence (namely, around headlands) in generating the M4 tide. The influence of wind effects upon M4 tidal currents, particularly through the nonlinear term associated with a time dependent eddy viscosity, is examined, and shown to produce a significant increase in the M4 near surface tidal current, and a modification of the current profile. In essence, the change of the M4 tidal current by meteorological effects will give rise to noise in any observed M4 current due to the random nature of the wind fields.