Observations of a mode-2 nonlinear internal wave on the northern Heng-Chun Ridge south of Taiwan

A research cruise was carried out over the Heng-Chun Ridge during June 27-July 1, 2010, near 21 degrees 34'N, 120 degrees 54'E, about 35 km south of Taiwan. The goal of the cruise was to determine if the location is an active generation site for internal tides and high-frequency nonlinear internal waves (NLIWs) in the northeastern South China Sea (SCS). The method was to sample a series of across-ridge sections using an underway conductivity-temperature-depth (UCTD) profiler and to conduct a time series at a fixed point atop the ridge using a CTD with lowered acoustic Doppler current profiler (LADCP) instrumentation. A hull-mounted ADCP and acoustic backscatter device were also operated throughout the cruise. The site was a very high energy region, with the northward Kuroshio Current exceeding 100 cm s(-1) and the primarily zonal barotropic tidal currents exceeding 140 cm s(-1). The most remarkable feature observed was a convex-type mode-2 NLIW with a westward-propagating core centered near 100 m depth. The wave was clearly visible in the velocity and backscatter data and had surface expressions visible both on radar and with the naked eye. The horizontal and vertical velocity structure was a good match for theoretical mode-2 waves in the SCS. The wave generation was consistent with local lee wave dynamics, which favored mode-2 generation over mode-1 at peak ebb tide given the currents, stratification, and bottom slope at the site. The wave could not be tracked farther west, and apparently did not escape the opposing Kuroshio.