NOTES AND CORRESPONDENCE Evaluation of Tidal Removal Method Using Phase Average Technique from ADCP Surveys along the Peng-Hu Channel in the Taiwan Strait

Three cruises with shipboard Acoustic Doppler Current Profiler (ADCP) were performed along a transect across the Peng-hu Channel (PHC) in the Taiwan Strait during 2003 2004 in order to investigate the feasibility and accuracy of the phase-averaging method to eliminate tidal components from shipboard ADCP measurement of currents. In each cruise measurement was repeated a number of times along the transect with a specified time lag of either 5, 6.21, or 8 hr, and the repeated data at the same location were averaged to eliminate the tidal currents; this is the so-called “phase-averaging method”. We employed 5-phase-averaging, 4-phase-averaging, 3-phase-averaging, and 2-phase-averaging methods in this study. The residual currents and volume transport of the PHC derived from various phase-averaging methods were intercompared and were also compared with results of the least-square harmonic reduction method proposed by Simpson et al. (1990) and the least-square interpolation method using Gaussian function (Wang et al. 2004). The estimated uncertainty of the residual flow through the PHC derived from the 5-phase-averaging, 4-phase-averaging, 3-phase-averaging, and 2-phase-averaging methods is 0.3, 0.3, 1.3, and 4.6 cm s, respectively. Procedures for choosing a best phase average method to remove tidal currents in any particular region are also suggested.