Turbulence characteristics in the Penghu channel and adjacent seas

• Main Authors: Huan-Jie Shao, 邵煥傑
• Other Authors: Ruo-Shan Tseng
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/693yv2

博士 === 國立中山大學 === 海洋生物科技暨資源學系研究所 === 107 === Several water masses such as Kuroshio Branch Water and South China Sea water meet in the seas off southwestern Taiwan where the tidal current is strong and bottom topography is complicated. Energetic internal waves and internal tides are also ubiquitous in this area. These abundant oceanic features are the motivations for this study. In order to investigate temporal and spatial variations of turbulence properties in the bottom boundary layer (BBL) and its generation mechanism, measurements of turbulence dissipation, current, and stratification in an energetic, sloping tidal channel, the Penghu Channel (PHC), in the Taiwan Strait as well as the continental margins and Kaoping Submarine Canyons (KPSC) were conducted. It was found that the northern, constricted section of PHC exhibits a unique feature of semidiurnal cycle of turbulence and quarter-diurnal cycle of temperature in the BBL due to the fact that current speeds during the flood are much higher (about four times as big) than those during the ebb. Turbulent mixing in the BBL, produced mainly by the tidal current shear, has high values of dissipation (~10−5 W Kg−1) and eddy diffusivity and extends upward to approximately 40 m above the bottom during the flood. During the flood upslope flow, significant temperature drops and destratification of the near-bottom layer occur due to turbulence mixing associated with the shear instabilities, confirmed by the gradient Richardson number less than the critical value of 1/4. By contrast, stratification produced during the ebb is discernible only in the upper part of the BBL above the mixed layer. The stratification is weak (strong) during enhanced (suppressed) turbulence. The observed dissipation rate of turbulent kinetic energy is proportional to the cubic power of current speed, suggesting that the observed turbulence is generated via the boundary layer shear instability. Near-internal waves were observed in the mid layer of the PHC at the transition phase from flood to ebb tides by using acoustic backscattering data of echosounder. Analysis of turbulence and current profiles indicates the near-internal waves are corresponding to shear instabilities with TKE dissipation rate elevated to 10－７ W Kg－１. Obvious internal tides were observed in the continental margins south of PHC and in the KPSC. At the mouth of KPSC where water depth is about 500 m, maximum vertical displacement of isopycnal oscillations nearly 100 m was observed during the spring tide in December. The TKE dissipation rate and eddy diffusivity estimated from the Thorpe scale analysis during the spring tide in December are approximately three to four times as big as those during the neap tide in July. This is attributed to the fact that weak stratification and strong vertical shear result in elevated turbulence intensities. TKE dissipation rates in the KPSC can reach a maximum value of 10－６ W Kg－１, which is in balance with the baroclinic energy fluxes. Analysis of the ratio between the APE (available potential energy) and HKE (horizontal kinetic energy) reveals that the internal tides in the continental margins south of PHC and inside the KPSC are in the form of standing waves. High-resolution gridded output data from NRL/LZSNFS numerical model of US Naval Research Lab are used to compute spatial distribution of baroclinic energy fluxes in the PHC. Our results indicate that the baroclinic energy mostly propagates along the edge of continental margins. At the continental slope the baroclinic energy fluxes can reach a maximum value of -50 KWm－１, while those at the continental shelf and at northern PHC are only 0.44 and 0.02 KWm－１, respectively. This result implies that as the internal tides propagate northward toward the PHC interior, the baroclinic energy decays rapidly. In summary, the present study clearly indicates that the energetic turbulence in the BBL and mid layer of the PHC is generated primarily by the shear instability. On the other hand, the turbulence in the continental margins and in the KPSC is generated mostly by the internal tides and breaking of internal waves. Finally, turbulence characteristics observed by the shipboard CTD/MicroRider and by the free-fall VMP-250 are generally in consistent with each other and also agree qualitatively with that estimated from the Thorpe scale analysis.