黑潮流經海底山所引發之紊流混合及其時空變異

Abstract

本研究分別使用了2020年10月及2021年9月分別在新海研1號及新海研2號上進行的現場觀測來探討臺灣東方海域黑潮與海底山的交互作用，並特別著重於海流流經陡峭地形所產生的小尺度物理過程。觀測資料顯示，當黑潮流過綠島西北方高度約200公尺的海底山時，海底山的下游處除了會形成背向波 (lee wave) 外，也可以透過聲納回波影像發現一長串由山峰向下游延伸數公里的凱文赫姆茲不穩定波 (Kelvin-Helmholtz billow)，而背向波所造成的流場、水文結構變形將會加速該不穩定波的成長。上述小尺度海水運動最終將會演變成紊流，使得海底山下游地區的紊流動消散率 (turbulent kinetic energy dissipation rate) 明顯較上游地區大2個數量級 (O ~ 10-5 W kg-1)，且其伴隨的紊流混合可能會改變當地的水文性質。此外，紊流動消散率在我們的實驗期間的變化和近岸潮位資料高度相關，因此推論黑潮流經陡峭海底山所衍生的紊流反應會受到潮汐調節，且理查遜數(Ri)的機率分布大致落於臨界值0.25附近，說明當地之流剪切力與紊流混合之間的平衡對於切變不穩定 (shear instability) 而言處於界穩定狀態 (marginally unstable state)，使得不穩定波及高紊流動能消散率 (O ~ 10^-5 W kg-1) 得以在實驗期間不斷地被觀測到。最後，我們將觀測結果所估計出的渦流擴散係數 (eddy diffusivity) 和前人研究中提出的經驗函數做比較，期望能對改善紊流混合的參數化有所幫助。

Two scientific cruises were conducted to investigate the flow-topography interactions along the Kuroshio over a seamount off the eastern coast of Taiwan and near Green Island. When the Kuroshio flowed over the 200-m-depth seamount, echo sounder captured the signals of lee waves and shear instability above the seamount and at the immediate lee of the seamount. These small-scale processes further induced vigorous turbulent mixing. Results of microstructure profiling revealed that turbulent kinetic energy dissipation rate (ϵ) varied in both space and time around the seamount. That is, ϵ was O(10^-5 W kg-1) downstream, which was 100 times larger than upstream of the seamount. Besides, ϵ was strongly modulated by the tide, i.e., strong and weak ϵ occurred at low and high tides, respectively. Using a simplified one-dimensional diffusion model, we demonstrated that turbulent mixing plays a role in shaping the downstream hydrographic structure. The lee wave is likely to create a circumstance favoring a higher growth rate of shear instability, which was examined using linear stability analysis. Mostly, the criterion for the occurrences of shear instability, Richardson number Ri < 0.25, was satisfied downstream. The distribution of Ri fluctuated around a central value near 0.25, which is a typical feature of the status of marginal instability, and this can be interpreted as a cyclic circumstance in a balance between shear forcing and turbulent mixing. Finally, we compared parameterizations of turbulent mixing based on the Ri between previous empirical relationships and our observations. This may help to improve the parameterization of turbulent mixing induced by flow encountering abrupt topography.