Turbulence Generation via Nonlinear Lee Wave Trailing Edge Instabilities in Kuroshio‐Seamount Interactions

• Published in: Journal of geophysical research. Oceans Vol. 129; no. 9
• Main Authors: Yeh, Yu‐Yu, Chang, Ming‐Huei, Lien, Ren‐Chieh, Chang, Jia‐Xuan, Chen, Jia‐Lin, Jan, Sen, Yang, Yiing Jang, Vladoiu, Anda
• Format: Journal Article
• Language: English
• Published: 01.09.2024
• Subjects: Kuroshio; lee wave; rotor; seamount; turbulence
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2024JC020971

Physical processes behind flow‐topography interactions and turbulent transitions are essential for parameterization in numerical models. We examine how the Kuroshio cascades energy into turbulence upon passing over a seamount, employing a combination of shipboard measurements, tow‐yo microstructure profiling, and high‐resolution mooring. The seamount, spanning 5 km horizontally with two summits, interacts with the Kuroshio, whose flow speed ranges from 1 to 2 m s−1, modulated by tides. The forward energy cascade process is commenced by forming a train of 2–3 nonlinear lee waves behind the summit with a wavelength of 0.5–1 km and an amplitude of 50–100 m. A train of Kelvin‐Helmholtz (KH) billows develops immediately below the lee waves and extends downstream, leading to enhanced turbulence. The turbulent kinetic energy dissipation rate is O (10−7–10−4) W kg−1, varying in phase with the upstream flow speed modulated by tides. KH billows occur primarily at the lee wave's trailing edge, where the combined strong downstream shear and low‐stratification recirculation trigger the shear instability, Ri < 1/4. The recirculation also creates an overturn susceptible to gravitational instability. This scenario resembles the rotor, commonly found in atmospheric mountain waves but rarely observed in the ocean. A linear stability analysis further suggests that critical levels, where the KH instability extracts energy from the mean flow, are located predominantly at the strong shear layer of the lee wave's upwelling portion, coinciding with the upper boundary of the rotor. These novel observations may provide insights into flow‐topography interactions and improve physics‐based turbulence parameterization. Plain Language Summary A thorough grasp of the physics driving flow‐topography interactions and turbulent transition is crucial for precise parameterization in numerical models. This study explores energy transformation into turbulence as the Kuroshio encounters a seamount based on direct field observations and theoretical examination. We found that the Kuroshio affected by the seamount kick starts a series of processes wherein the Kuroshio transfers its energy into turbulence through the formation of lee waves and shear instability. As the flow reverses beneath the trailing edge of lee waves, a recirculation rotor is formed, enhancing the vertical shear and weakening the stratification. The strong vertical shear exceeds the limitations imposed by water column stratification, promoting the development of shear instability and ultimately leading to water roll‐ups and turbulence generation, which could dissipate the Kuroshio energy and mix the Kuroshio water. Our findings could help improve physics‐based turbulence parameterization skills for flow‐topography interaction processes in numerical models. Key Points Prominent nonlinear lee waves are formed as the Kuroshio interacts with the seamount east of Taiwan The enhanced shear related to the lee wave and the weakened stratification related to the rotor generate a shear unstable region The shear instability grows by extracting the mean flow energy at the upwelling portion of the lee wave, where ε O(10−7−10−4) W kg−1