Variance of Bottom Water Temperature at the Continental Margin of the Northern South China Sea

Abstract

AbstractHigh‐resolution temperature variation was examined about 40 days near the ocean bottom at 20 sites within an area of 110 × 120 km2 close to the continental margin of the northern South China Sea (depth ≈ 4,000 m). The monitoring depth (0.5 m above the bottom) was presumably within the lower part of the bottom mixed layer, compared to its median thickness of 63 m. At all sites, the bottom water had temperature variance less than 0.01°C, and it alternatively varied in the near‐quiescent and internal wave states with time interval of diurnal period or even longer. Spectra of temperature variation showed that diurnal frequency dominated over the internal wave band. At different sites, the transition between internal wave band of scaling −2 and the inertial subrange turbulence of scaling −5/3 cannot easily be discriminated, attributed to the different intermittent presences of internal waves (tides). However, the resolved viscous‐convective and viscous‐diffusive subranges in the temperature spectrum provide the opportunity to evaluate the thermal (χ) and kinetic energy (ɛ) dissipation rates near the ocean bottom at each site. Close to a seamount, χ and ɛ were inferred to be about 1.2 × 10−12 °C2s−1 and 7.2 × 10−10 m2s−3, respectively, and they were strongly modulated by the diurnal internal tides. In the flat abyssal plain, χ and ɛ were much weaker, about at 3.2 × 10−14 °C2s−1 and 1.7 × 10−11 m2s−3, respectively. The spatial distribution of the nominal thermal χ, kinetic energy ɛ and the turbulent diffusivity Kρ indicated that they varied in the ranges [3.9 × 10−15 1.9 × 10−12] °C2s−1, [2.0 × 10−12 1.1 × 10−9] m2s−3, and [8.3 × 10−6 5.2 × 10−3] m2s−1, respectively, representing the different influences of internal waves and local topography.