Semidiurnal tides observed in the western equatorial Pacific during the Tropical Ocean-Global Atmosphere Coupled Ocean-Atmosphere Response Experiment

The semidiurnal tide within a 100 km square region of the western equatorial Pacific centered at 1.8 degrees S, 156.1 degrees E is examined using shipboard survey and mooring data collected during the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). Baroclinic and barotropic tidal amplitudes and phases are estimated from the survey and mooring observations in the upper 300 m of the 1800 m deep ocean by specifying their horizontal and vertical structures. The barotropic tide is assumed to have zero horizontal wavenumber over the domain, while a component of the baroclinic tide that is phase-locked to the barotropic tide is determined by a searching method using plane wave fits to the data. The estimated barotropic tidal current is in good agreement with tide models derived from TOPEX/POSEIDON observations. The plane wave analysis indicates a dominant mode one baroclinic wave propagating toward the northeast. The second vertical mode can also be detected. Given the phase differences between the M2 and S2 constituents in the barotropic and baroclinic tides, the source of the baroclinic tidal signal is determined to be about 320 km southwest of the observing region, at a series of islands and shallow ridges. The combined estimates of the barotropic and baroclinic tides typically account for only 40-60% of the observed semidiurnal band,current variance in the mooring data, indicating the high degree of temporal and spatial variability of the baroclinic tide in this region. The results of this study suggest, however, that coherent barotropic and baroclinic tidal signals can be successfully distinguished in the deep ocean using shipboard survey data, even when the data are limited to the upper 300 m.