Tidal amphidrome movement and energy dissipation in the Irish Sea

Abstract

The positions of points of zero tidal range are normally plotted as the amphidromes of individual harmonic tidal constituents on a series of cotidal and coamplitude charts. However, the amphidrome may also be considered as a time-dependent position of zero tidal range for a complete tidal band. Daily movement of the semidiurnal tidal amphidrome in the southern Irish Sea, though a spring-neap cycle, extends over 70 km in a direction perpendicular to the entrance channel axis. At spring tides the amphidrome is degenerate, being located inland, but at neap tides the position moves into the Irish Sea. This movement is due to proportionally more energy being absorbed from spring than from neap tides. The area-averaged power law of energy dissipation is approximately cubic for small tidal ranges; however, it reduces towards a square law for large ranges, because dissipation is limited by the available inward energy flux. The model applied can be extended to explain why anomalously weak spring tidal currents occur in parts of the Celtic Sea.