Dynamic of Kelvin-Helmholtz (KH) Billows Observed around Green Island, east of Taiwan

• Main Authors: Jia-Xuan Chang, 張珈瑄
• Other Authors: Ming-Huei Chang
• Format: Others
• Language: en_US
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/ncu7wt

碩士 === 國立臺灣大學 === 海洋研究所 === 105 === Two field experiments were performed to observe the Kelvin-Helmholtz (KH) instability, mediating the energy of large-scale flow toward turbulent dissipation, in the Kuroshio southeast of Taiwan. This region has strong current speeds ranging from 1 to 1.5 m s–1 and is occupied by the rapidly changing topography containing the seamounts and islands, implying strong impacts of topography on flow. In experiment I, high resolution moored current and temperature measurements above a seamount revealed spiral cold water rolling up as a result of the vortex current structure in the vertical. It spawns strong water mixing resulting in significant density gradient from their upper and lower water, forming clear KH billows at a vertical of ~60 m. The secondary instability-like vortex structure was found in the upper billow core with a vertical scale ~10 m, which may be the first clear observation for the secondary instability. It is found that the linear stability analysis (LSA) fit our observations well, suggesting the wavelength of 90-100 m and the period of 10-13 mins. In experiment II, the shipboard echo sounder captured the KH billows in the leeward side of Green Island, related to island wake. The simultaneous observations of microstructure using vertical microstructure profiler (VMP) revealed strong turbulent dissipation and mixing. The dissipation rate of turbulent kinetic energy was O(10-4) W kg-1 corresponding to turbulent diffusivity O(10-1) m2 s-1. These field measurements summarize the water mixing via turbulence as a result of KH instabilities is expected to play a role to increase the diversity of water properties in the Kuroshio.