09:30 〜 09:45
[AOS11-03] Energetic turbulence and internal waves in Tokara Strait
キーワード：Turbulence, Internal waves, Kuroshio, Internal lee waves
Internal lee waves generated by geostrophic flows interacting with small-scale topography are one leading energy sink for the 1-TW wind-forcing input to gyre-scale circulation and eddies. In this study, we examine the relationship between turbulence and finescale velocity variations above and downstream of where the Kuroshio interacts with a ∼O(10-km) wide seamount in the Tokara Strait. Data were collected during November 2019 using Chi-augmented EM-APEX profiling floats which measured temperature, salinity, horizontal velocity (u, v) and microstructure temperature variance dissipation rate χ from the surface to bottom at vertical resolutions of Δz ≈ 5 m. Finescale vertical velocity w is also estimated following Cusack et al. (2016).
Average turbulent kinetic energy dissipation rates ε and diapycnal diffusivities Kρ, inferred from χ, are at least an order of magnitude greater than open ocean thermocline values with ε ∼ 10-8 W/kg and Kρ ∼ 10-4 m2/s. They are further enhanced above the seamount (ε ∼ 10-7 W/kg and Kρ ∼ 10-2 m2/s) and in a ∼200-m layer below the surface mixed layer downstream of the seamount (ε ∼ 10-6 W/kg and Kρ ∼ 10-2 m2/s). The vertical wavenumber shear spectrum ΦSh (m) is an order of magnitude higher than the canonical GM level and resembles the saturated spectral slope –1 above a rolloff vertical wavenumber mc ≈ 0.01 cpm and shows no correlation with ε. The ratio of horizontal kinetic energy to vertical kinetic energy decreases from 100 at m = 0.01 cpm to 10 at m = 0.1 cpm, suggesting that the velocity field is more isotropic at small vertical scales. Dissipation rates ε are correlated within a factor of three with vertically-high-passed (m > 130 cpm) vertical velocity w and vertical divergence dw/dz, consistent with the large-eddy parameterization (e.g., Beaird et al. 2012). The internal-wave finescale parameterization scheme fails in this energetic regime.
Average turbulent kinetic energy dissipation rates ε and diapycnal diffusivities Kρ, inferred from χ, are at least an order of magnitude greater than open ocean thermocline values with ε ∼ 10-8 W/kg and Kρ ∼ 10-4 m2/s. They are further enhanced above the seamount (ε ∼ 10-7 W/kg and Kρ ∼ 10-2 m2/s) and in a ∼200-m layer below the surface mixed layer downstream of the seamount (ε ∼ 10-6 W/kg and Kρ ∼ 10-2 m2/s). The vertical wavenumber shear spectrum ΦSh (m) is an order of magnitude higher than the canonical GM level and resembles the saturated spectral slope –1 above a rolloff vertical wavenumber mc ≈ 0.01 cpm and shows no correlation with ε. The ratio of horizontal kinetic energy to vertical kinetic energy decreases from 100 at m = 0.01 cpm to 10 at m = 0.1 cpm, suggesting that the velocity field is more isotropic at small vertical scales. Dissipation rates ε are correlated within a factor of three with vertically-high-passed (m > 130 cpm) vertical velocity w and vertical divergence dw/dz, consistent with the large-eddy parameterization (e.g., Beaird et al. 2012). The internal-wave finescale parameterization scheme fails in this energetic regime.