綠島附近海域流切變不穩定波之觀測及動力研究

Abstract

本論文使用在綠島周遭黑潮流域兩次的現場觀測資料研究切變不穩定之現象，此為一扮演將能量從大尺度的運動導入紊流消散的角色之運動。而此研究區域流速強約1至1.5 m s-1，使得當此強流遇到一在空間上快速變化的地形時〈海底山或島〉，則對此流造成較強的影響。實驗一中，分析一組於 2016年3月施放在黑潮流經之海底火山上的高垂直解析之溫度計及聲波都普勒流剖儀(ADCP)錨碇串，結果顯示：此切變不穩定波之邊緣有著很強的溫度梯度與回聲強度，可清楚觀測到垂直尺度約60公尺。由等溫度面之翻轉(overturn)對應至流矢有著渦旋(vortex)結構，使得高溫與低溫海水相互穿插。另一方面，在波邊緣有著次級不穩定(secondary instability)的渦旋翻轉尺度約10公尺，此可能為次級不穩定的第一個較清楚的現場觀測。依背景流及密度分層進行線性穩定性分析(LSA)，不穩定之成長率最大的模(mode)發生在波長90-100公尺及周期約10-13分鐘，與觀測符合。實驗二中，在2017年三月使用深水型剖面紊流儀(VMP-500)來量化切變不穩定波所伴隨的混合現象，而此切變不穩定波與綠島後方尾渦流有關。由漁探儀所捕捉到的波濤與紊流儀之資料對照，發現穿過切變不穩定波之直接觀測的紊流消散率高達10-4 W kg-1，其對應之紊流擴散係數則為10-1 m2 s-1。綜合以上兩個實驗，切變不穩定事件後的混合和紊流對於黑潮水的變異性增加扮演相當重要的角色。

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.