The role of intermittency in internal-wave shear dispersion

Abstract

This paper revisits a long-standing discrepancy between (i) 1-5-km isopycnal diffusivities of O(1) m2 s-1 based on dye spreading and (ii) inferences of O(0.1) m2 s-1 from internal-wave shear dispersion Kh ~ 〈Kz〉 〈Vz2〉/f2 in several studies in the stratified ocean interior, where 〈Kz〉 is the bulk average diapycnal diffusivity, 〈Vz2〉 the finescale shear variance, and f the Coriolis frequency. It is shown that, taking into account (i) the intermittency of shear-driven turbulence, (ii) its lognormality, and (iii) its correlation with unstable finescale near-inertial shear, internal-wave shear dispersion cannot necessarily be discounted based on available information. This result depends on an infrequent occurrence of turbulence bursts, as is observed, and a correlation between diapycnal diffusivity Kz and the off-diagonal vertical strain, or the vertical gradient of horizontal displacement, |χz| = |√Vz dt|, which is not well known and may vary from region to region. Taking these factors into account, there may be no need to invoke additional submesoscale mixing mechanisms such as vortical-mode stirring or internal-wave Stokes drift to explain the previously reported discrepancies.