Measurements of turbulence and fossil turbulence near ampere seamount

Abstract

Abstract Measurements of temperature and velocity microstructure near and downstream of a shallow seamount are used to compare fossil turbulence versus non-fossil turbulence models for the evolution of turbulence microstructure patches in the stratified ocean. According to non-fossil oceanic turbulence models, all overturn length scales LT of the microstructure grow and collapse in constant proportion to each other and to the turbulence energy (Oboukov) scale LO and the inertial buoyancy (Ozmidov) scale L R ≡(ϵ/N 3 ) 1 2 of the patches; that is, with LTrms ≈1.2LR and viscous dissipation rate ϵ ≈ ϵ 0 ∗ . According to the Gibson fossil turbulence model, all microstructure originates from completely active turbulence with ϵ ⩾ ϵ 0 ≈ 3L T 2 N 3 (≈ 28ϵ 0 ∗ ) and L T /√6 ≈ L Trms , but this rapidly decays into a more persistent active-fossil state with ϵ0⩾ϵ⩾ϵF ≈ 30vN2, where N is the buoyancy frequency and v is the kinematic viscosity and, without further energy supply, finally reaches a completely fossil turbulence hydrodynamic state of internal wave motions, with ϵ ⩽ ϵF. The last turbulence eddies, with ϵ ≈ ϵF, vanish at a buoyant-inertial-viscous (fossil Kolmogorov) scale LKF that is much smaller than the remnant overturn scales LT for large ϵ0/ϵF ratios. These density, temperature, and salinity overturns with LT ≈ 0.6 LR0 ⪢ 0.6 LR persist as turbulence fossils (by retaining the memory of ϵo) and collapse very slowly. In the near wake below the summit depth of Ampere seamount, a much larger proportion of completely active turbulence patches was found than is usually found in the ocean interior away from sources. Dissipation rates ϵ and turbulence activity coefficients A T ≡ (ϵ/ϵ 0 ) 1 2 of microstructure patches were found to decrease downstream, suggesting that the active turbulence indicated by the patches with AT ⩾ 1 was caused by the presence of the seamount as a turbulence source. Therefore, the turbulence and mixing processes of ocean layers far away from turbulence sources probably have been undersampled by microstructure data sets lacking any AT ⩾ 1 patches. This is because large fractions of the mixing and viscous dissipation of the patches occur in short-lived active turbulence regimes that are too brief to be detected. Consequently, large underestimates of the true space-time average turbulence fluxes and turbulence and scalar dissipation rates may result if non-fossil turbulence models are assumed in ocean microstructure data interpretation.