Separation of tidal and subtidal currents in ship-mounted acoustic Doppler current profiler observations


  • Julio Candela,

  • Robert C. Beardsley,

  • Richard Limeburner


A simple method is developed to analyze current measurements obtained from a moving platform. The need for this is motivated by the now common use of the ship-mounted acoustic Doppler current profiler (ADCP) to acquire absolute velocity data during an oceanographic survey of a given region. The full potential of this new measurement technique is severely hindered when the presence of high-frequency phenomena (e.g., tidal or inertial motions) prevents a clear visualization of the lower-frequency current structure of interest. Our analysis method is based on a spatial interpolation scheme, using arbitrary functions, that allows for the tidal current time variability, which then permits the tide-induced motions to be subtracted from the ADCP data to yield the subtidal current field. The method also allows nearby moored and drifter current measurements (if available) to be combined with the shipboard ADCP observations in a single analysis to obtain the best description of the tidal and subtidal currents. To illustrate this method, we present results from the analysis of ADCP data taken during oceanographic surveys in two different continental shelf regions, the East China Sea and the Amazon shelf. A 5-day conductivity-temperature-depth (CTD) and ADCP survey was made in the East China Sea near the mouth of the Yellow Sea during January 1986. There the currents were essentially barotropic and dominated by the semidiurnal tide. The ADCP-derived cotidal chart for the M2 (12.42 hours) component agrees well with existing charts derived empirically from sea level observations or from regional numerical models. The ADCP-derived steady flow is also consistent with the observed density field and indicates little flow in or out of the Yellow Sea and a transport of about 1 Sv toward the Tsushima Strait. Two CTD-ADCP surveys lasting 21 and 23 days were conducted over the Amazon shelf during March and May, 1990. Simultaneously moored current observations were also obtained at three locations along a cross-shelf array 200 km north of the Amazon River mouth, for a common period of at least 2 months (February 12 to April 13). Over the shelf, tidal and subtidal currents were comparable in magnitude. Our analysis indicates that the tidal currents were essentially barotropic, semidiurnal, and polarized in the cross-shelf direction, increasing in magnitude toward the inner shelf where current values of more than 2 m/s are common. The ADCP-derived steady currents were aligned in the along-shelf direction and strongly influenced by the North Brazil Current (NBC). During both ADCP surveys, the northwestward flowing NBC was transporting more than 2 Sv over the shelf at depths shallower than 100 m.