Shear-generated turbulence in the equatorial Pacific produced by small vertical scale flow features

Abstract

We investigate the characteristics of shear-generated turbulence in the natural environment by considering data from a number of cruises in the western equatorial Pacific. In this region, the vertical shear of the flow is dominated by flow structures that have a relatively small vertical scale of O(10 m). Combining data from all cruises, we find a strong relationship between the turbulent dissipation rate, E, vertical shear, S, and buoyancy frequency, N. Examination of E at a fixed value of Richardson number, Ri=N-2/S-2, shows that Eut2N for a wide range of values of N, where u(t) is an appropriate velocity scale which we assume to be the horizontal velocity scale of the turbulence. The implied vertical length scale, (v)=u(t)/N, is consistent with theoretical and numerical studies of stratified turbulence. Such behavior is found for Ri<0.4. The vertical diffusion coefficient then scales as kappa nu proportional to u(t)(2)/N at a fixed value of Richardson number. The amplitude of E is found to increase with decreasing Ri, but only modestly, and certainly less dramatically than suggested by some parameterization schemes. Provided the shear generating the turbulence is resolved, our results point to a way to parameterize the unresolved turbulence.