Observations of the Phase of Tidal Currents along a Strait

Abstract

The phase of the M₂ surface elevation tide changes by 100° along a 40-km length of Cook Strait, New Zealand. Acoustic Doppler Current Profiler measurements are presented that show a change of only 12° in the phase of depth-averaged semidiurnal tidal currents along this same length. This phase change supports the general result developed by Vennell, which states that the phase of the cross-sectional average velocity is approximately constant along a “short” strait of variable cross-sectional area. Vennell also developed a relationship between the constant phase of tidal currents along the strait and the amplitude and phase of the surface elevation tide at the ends of a short strait. The velocity phase given by this relationship is shown to agree well with the observed tidal velocity phase in Cook Strait and one other strait.