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Internal solitary waves are ubiquitous in coastal regions and marginal seas of the world’s oceans. As the waves shoal shoreward, they lose the energy obtained from ocean tides through globally significant turbulent mixing and dissipation and consequently pump nutrient-rich water to nourish coastal ecosystem. Here we present fine-scale, direct measurements of shoaling internal solitary waves in the South China Sea, which allow for an examination of the physical processes triggering the intensive turbulent mixing in their interior. These are convective breaking in the wave core and the collapse of Kelvin–Helmholtz billows in the wave rear and lower periphery of the core, often occurring simultaneously. The former takes place when the particle velocity exceeds the wave’s propagating velocity. The latter is caused by the instability induced by the strong velocity shear overcoming the stratification. The instabilities generate turbulence levels four orders of magnitude larger than that in the open ocean.

Analytical and numerical scattering models with accompanying digital representations are used increasingly to predict acoustic backscatter by fish and zooplankton in research and ecosystem monitoring applications. Ten such models were applied to targets with simple geometric shapes and parameterized (e.g., size and material properties) to represent biological organisms such as zooplankton and fish, and their predictions of acoustic backscatter were compared to those from exact or approximate analytical models, i.e., benchmarks. These comparisons were made for a sphere, spherical shell, prolate spheroid, and finite cylinder, each with homogeneous composition. For each shape, four target boundary conditions were considered: rigid-fixed, pressure-release, gas-filled, and weakly scattering. Target strength (dB re 1 m(2)) was calculated as a function of insonifying frequency (f = 12 to 400 kHz) and angle of incidence (θ = 0° to 90°). In general, the numerical models (i.e., boundary- and finite-element) matched the benchmarks over the full range of simulation parameters. While inherent errors associated with the approximate analytical models were illustrated, so were the advantages as they are computationally efficient and in certain cases, outperformed the numerical models under conditions where the numerical models did not converge.

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[1] A research cruise was carried out over the Heng-Chun Ridge during June 27-July 1, 2010, near 21 34′N, 120 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 À1 and the primarily zonal barotropic tidal currents exceeding 140 cm s À1 . 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.