||To test the feasibility of measuring small-scale turbulence from a conventional shipboard CTD, a self-contained package carrying velocity shear probe, FP07 thermistor and micro-conductivity probe was attached to the CTD in the field measurements. Four cruises were conducted, and the studied areas consist of waters off Kaohsiung, Kao-ping Submarine Canyon, Peng-hu Channel and the continental shelf of northern South China Sea (SCS). Data from various sea states and flow conditions were analyzed to investigate the effect of noise induced by the vibrations of the platform on the microstructure turbulence. Dissipation rates of turbulent kinetic energy (ε) and temperature variance (χ_T) as well as the vertical eddy diffusivity were estimated and compared with those derived from the Thorpe scale method.|
Our results indicate that vibrations from the CTD rosette and cables will contaminate shear spectra significantly. After some quality control procedures, a small number of good shear data can be selected which are free from the platform vibrations with small tilt angles. The measured shear spectra fit well with the Nasmyth Empirical Spectrum. Values of χ_T derived from ε are consistent with those measured directly from the thermistor, thus confirming the feasibility of thermistor in measuring turbulence. On the other hand, ocean turbulence measured directly by the micro-conductivity probe shows less satisfactory results.
The second part of this study is using the CTD-microrider to measure the turbulence properties of internal waves (IW) in the continental shelf of the northern SCS. With the R/V OR3 following the IW, CTD casts were conducted in two styles of fixed depth and tow-yo. When the microrider was fixed at a depth of 80 m, it was found that strong horizontal temperature gradient occurs during the passage of IW, and maximum χ_T reached a value of approximately 10-4 K2s-1. When the microrider was doing tow-yo, it was found that intensive density variation associated with vertical overturns occur behind the IW passage accompanied with large eddy diffusivity. Maximum value of ε_χ as large as 10-3 Wkg-1 was obtained at the lower layer of the backward face of IW, implying strong turbulent mixing induced by the IW.