Finescale Measurements of Kelvin–Helmholtz Instabilities at a Kuroshio Seamount

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Abstract Finescale properties of Kelvin–Helmholtz (KH)-like shear instabilities on the trailing edge of a nonlinear lee wave generated by the Kuroshio impinging on a seamount were measured using a towed CTD chain, shipboard ADCP, and echosounder. Lee-wave vertical velocity amplitudes vary in phase with the upstream semidiurnal along-stream current. The instabilities are analogous to atmospheric billows induced by a recirculation on the trailing edge of mountain lee waves. A total of 135 KH billows were identified in a 4-day-long time series roughly 300 m downstream of the center of the lee wave. The KH billows have heights H = 52 ± 11 m, widths L = 162 ± 72 m, and aspect ratios H / L = 0.39 ± 0.18. Positive reduced shear squared S 2 − 4 N 2 (where S is the vertical shear magnitude and N is the buoyancy frequency) in the shear-stratified billows suggests actively growing instabilities, with comparable contributions from across- and along-flow vertical shear. Billow cores are convectively unstable ( N 2 < 0). Large turbulent kinetic energy dissipation rates are inferred from density overturns. Density, shear, and inferred turbulence properties vary with billow aspect ratios. As H / L increases, density gradients smear out. For 122 billows with H / L < 0.6, dissipation rates increase by one order of magnitude with increasing H / L . These observations of ∼1-m vertical and ∼5-m horizontal resolution billow structures and density overturn dissipation rates can provide a reference for future high-Reynolds-number direct numerical simulations.

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