ACOUSTIC DOPPLER CURRENT PROFILER MEASUREMENTS OF TIDAL PHASE AND AMPLITUDE IN COOK STRAIT, NEW-ZEALAND

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-degrees phase difference in the M2 tide between the ends of the Strait. Extraordinarily 135-degrees 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-degrees. Near bottom velocity led surface velocity by approximately 10-degrees 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.