Trapped Core Formed within Second-mode Nonlinear Internal Waves over the Shelf Break of the East China Sea

Second-mode nonlinear internal waves (NLIWs) were observed over the shelf break of the East China Sea in July 2018. Two acoustic Doppler current profilers (ADCPs) mounted in trawl-resistance bottom mounts were deployed in an upward looking configuration on the seabed, where the water depth is about 120 m. During the observation period of two days, wave trains arrived at an interval of approximately one day, suggesting that these NLIWs are generated in association with the diurnal tide. The amplitude, propagation direction, and propagation speed of the second-mode NLIW were estimated from along-beam velocities using the iterative method that takes into account the effects of the background flow as well as the inevitable beam-spreading of the ADCP. The results of the iteration show that second-mode NLIWs propagate west-northwestward at a speed of about 0.3 ms^-1, consistent with a 6-hour lag of the arrival time between two ADCPs spaced 7.4 km apart, perpendicular to isobaths of the continental slope. The amplitude and maximum velocity of the leading wave are estimated to be 20 m and 0.4 ms^-1, respectively. The estimated maximum velocity along the propagation direction exceeds the propagation speed, resulting in the formation of the subsurface trapped core with closed streamlines in the depth range of 60-80 m. These results indicate that the observed second-mode NLIWs can transport tracers in addition to energy over the continental shelf before dissipating.