Coupling between sub-mesoscale eddies, internal waves, and turbulence in the deep Mediterranean: A spectral investigation

Abstract. Interaction between energy-abundant (sub-)mesoscale eddies and internal waves can lead to turbulence generation and may prove important for replenishment of nutrients for deep-sea life and circulation. However, observational evidence of such interaction is scarce and precise energy transfer is unknown. In this paper, an extensive spectral study is reported using mooring data from nearly 3000 high-resolution temperature sensors in about half-a-cubic hectometer of seawater above a deep flat Northwestern-Mediterranean seafloor. The number of independent data records partially improves statistics for better determination of spectral slopes, which however do not show a roll-off to the viscous dissipation range of turbulence. The spectra hardly show power-laws ω^p having exponent p = -5/3 representing an inertial subrange that evidences shear-induced isotropic turbulence. Instead, they are dominated by p = -7/5 representing a buoyancy subrange, which evidences convection-induced anisotropic turbulence. In contrast with p = -5/3 that indicates a downgradient cascade of energy, p = -7/5 characterizes by an ambiguous cascade direction. At height h < 50 m above seafloor, p = -7/5 is found adjacent to instrumental noise. The p = -7/5 is also found in the sub-mesoscale/internal wave band that is elevated in variance by one order of magnitude. It is reasoned that this sub-inertial range cannot represent isotropic motions, hence p ≠ -5/3 at all heights, and a new deep-sea energy cascade is proposed between mesoscales and turbulence dissipation. Only higher up in more stratified waters an inertial subrange is formed. The transition from internal waves into large-scale turbulence follows p = -2, while a higher-frequency transition from 0 to π phase change reflects overturns of slanted convection or standing-wave breaking leading to isotropic turbulence.