Observations of internal tidal waves in the isolated seasonally stratified region of the western Irish Sea

Data from moored stations, sampled in a patch of isolated seasonal stratification in the western Irish Sea in 2002 and 2006, show the presence of large-amplitude internal waves with both semi-diurnal and supertidal frequencies. The barotropic and baroclinic signals were separated, and a semi-diurnal tidal fit was performed on the baroclinic signal. The orbital velocities and the group speed of the internal tide are shown to be of the same magnitude as the barotropic tide. This leads to a distortion of the wave properties because of the measurements being made at a fixed location in a moving media. The internal semi-diurnal tide takes the form of a mode I (two-layer) motion in the 2002 observations, whereas it exhibits a higher modal structure in 2006. A fit using an inertial component shows only very weak inertial motions compared to those of the tide. The internal tide consistently propagates southwards, parallel to the tidal mixing front, and it is suggested that it is generated over topography in the northern part of the stratified region. The flux of baroclinic energy is 170Wm-1 towards the south, of which about 90Wm-1 is found in the internal M2 tide. Measurements of the dissipation of turbulent kinetic energy and theoretical arguments indicate that the internal tide is unable to propagate over the entire stratified area before dissipating, but it does reach over a significant fraction of the area.