Abstract
Abstract. Two research cruises were conducted from the R/V OCEAN RESEARCHER 3 during 05–16 August 2011 to study the generation and propagation of high-frequency nonlinear internal waves (NLIWs) over the northern Heng-Chun Ridge south of Taiwan. The primary study site was on top of a smaller ridge about 15 km wide by 400 m high atop the primary ridge, with a sill depth of approximately 600 m. A single mooring was used in conjunction with shipboard observations to sample the temperature, salinity and velocity structure over the ridge. All the sensors observed a profusion of mode-2 NLIWs. Some of the waves were solitary, while others had as many as seven evenly spaced waves per packet. The waves all exhibited classic mode-2 velocity structure with a core near 150–200 m and opposing velocities in the layers above and below. At least two and possibly three most common propagation directions emerged from the analysis, suggesting multiple generation sites near the eastern side of the ridge. The turbulent dissipation due to overturns in the wave cores was very high at order 10−4–10−3 W kg−1. The energy budget suggests that the waves cannot persist very far from the ridge and likely do not contribute to the South China Sea transbasin wave phenomenon.
Highlights
Since 1999, a series of joint programs between Taiwan and the United States have been studying the world’s largest high-frequency nonlinear internal waves (NLIWs) in the northeastern South China Sea
The results clearly suggested that the ocean’s behavior at the northern HC ridge site was different from the Batan/Itbayat Island passage on the eastern ridge, and merited a second visit
The results from RYRB12 suggested that westwardpropagating higher-mode internal tides and NLIW are generated at the site via lee waves forming east of the ridge on the stronger ebb tide
Summary
Since 1999, a series of joint programs between Taiwan and the United States have been studying the world’s largest high-frequency nonlinear internal waves (NLIWs) in the northeastern South China Sea. It is generally accepted that the internal tide is formed by the flux of the barotropic tide in a stratified fluid across abrupt topography. The initial pycnocline depressions that propagate away from the topography can form either as lee waves on the ebb tide (Maxworthy, 1979; Farmer and Smith, 1980; Apel et al, 1985) or as a downward surge off topography on the flood tide (Lee and Beardsley, 1974; Scotti et al, 2007; Lai et al, 2010). What happens depends on the strength of the forcing, the slope of the wave ray path relative to the topography, and the tidal excursion. It has been demonstrated that relatively weak forcing over wide topogra-
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