Characterization of the sub-mesoscale energy cascade in the Alboran Sea thermocline from spectral analysis of high-resolution MCS data

Abstract

Part of the kinetic energy that maintains ocean circulation cascades down to small scales until it is dissipated through mixing. While most steps of this downward energy cascade are well understood, an observational gap exists at horizontal scales of 1000-10 m that prevents characterizing a key step in the chain: the transition from anisotropic internal wave motions to isotropic turbulence. Here we show that this observational gap can be covered using high-resolution multichannel seismic (HR-MCS) data. Spectral analysis of acoustic reflectors imaged in the Alboran Sea thermocline evidences that this transition is caused by shear instabilities. In particular, we show that the averaged horizontal wavenumber spectra of the reflectors vertical displacements display three subranges that reproduce theoretical spectral slopes of internal waves [$\lambda x$>100 m], Kelvin-Helmholtz-type shear instabilities [100 m>$\lambda x$>33 m], and turbulence [$\lambda x$<33 m], indicating that the whole chain of events is occurring continuously and simultaneously in the surveyed area. The obtained spectra also allow to obtain high-resolution maps of diapycnal diffusivity, so that the levls of mixing, in the different parts of the thermocline