Acoustic Doppler Current Profiler measurements of tidal phase and amplitude in Cook Strait, New Zealand

Abstract

Strong tidal flows are observed in Cook Strait which separates the North and South Islands of New Zealand. The high velocities within the 30 km wide Strait result from a 140° phase difference in the M 2 tide between the ends of the Strait. Extraordinarily 135° of this phase difference occurs over just 40 km in the narrowest section of the Strait. Measurements from a ship mounted Acoustic Doppler Current Profiler (ADCP) over a single tidal cycle are used to determine the horizontal and vertical variation of tidal phase and amplitude in the Strait. Results show that tidal velocity amplitude ranges from 70 cm s −1 on the west of the Strait to 140 cm s −1 on the east. There was little amplitude variation over most of the water column. The eastern side of the Strait led the west by 20°. Near bottom velocity led surface velocity by approximately 10° due to the effect of bottom friction on the oscillating flow. Results from a subsequent 1 month deployment of ADCPs on the same line as the ship track are used to hindcast the semi-diurnal tide on the day of the shipboard measurements. The shipboard measured semi-diurnal tidal amplitude and phase agree extremely well with the hindcast composite of the three largest tidal constituents. Thus shipboard measurements over a single tidal cycle were able to accurately determine the horizontal and vertical variation of phase and amplitude of the semi-diurnal tide in Cook Strait.