[A-CG40] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG40-P03] Temporal variation of energetic turbulence on a seamount flank in Tokara Strait

★Invited Papers

*Anne Takahashi¹, Ren-Chieh Lien², Eric Kunze³, Anda Vladoiu², Barry Ma², Hirohiko Nakamura⁴, Ayako NIshina⁴, Eisuke Tsutsumi⁴, Ryuichiro Inoue⁵, Takeyoshi Nagai⁷, Takahiro Endoh⁶, Sachihiko Itoh¹ (1.Atmosphere and Ocean Research Institute, The University of Tokyo, 2.Applied Physics Laboratory, University of Washington, 3.NorthWest Research Associates, 4.Faculty of Fisheries, Kagoshima University, 5.Japan Agency for Marine-Earth Science and Technology, 6.Research Institute for Applied Mechanics, Kyushu University, 7.Tokyo University of Marine Science and Technology)

Keywords:flow-topography interactions, turbulence, parameterizations, mooring observations

Turbulence is highly intermittent, but temporal variability of turbulence has been less-frequently sampled than spatial variability. In this study, 6-month ADCP and CTD mooring observations were conducted in the wake of Hirase Seamount in Tokara Strait, where Kuroshio and tidal currents impinge on steep topography to excite vigorous turbulence. Horizontal velocities, temperature and salinity sampled every 2.5 mins in 6-m vertical bins over depths of 50–320 m for the ADCP and 240-306 m for the CTDs are used to investigate statistics and temporal variability of strong turbulence. Turbulent kinetic energy dissipation rates ε are estimated from Thorpe-scale (Th; Thorpe 1977) and reduced-shear (RS; Kunze et al. 1990) parameterizations in 1-hour windows. While 1-h estimates ε_Th and ε_RS are scattered with respect to each other, 48-h moving-averages <ε_Th > and <ε_RS > are correlated with R=0.64 (p<0.01). The frequency spectrum of log₁₀(ε_RS ) exhibits a sharp peak at semidiurnal and broad enhancement in subinertial frequencies. Estimated ε_Th and ε_RS range from 10^-8 to 10^-4 W/kg, while <ε_Th > and <ε_RS > from O(10^-7 W/kg) to O(10^-6 W/kg). Temporal variation in <ε> correlates with the current speed u at 200–300 m depths with R=0.62, but less so at 100–200 m with R=0.51 and 50–100 m with R=0.34. The observed relationship ε∝ u³ is consistent with observations in wake eddies generated at the Palau island chain (MacKinnon et al. 2019).