Effects of Turbulent Mixing and Horizontal Shear on Double-Diffusive Interleaving in the Central and Western Equatorial Pacific

Abstract

The effects of small-scale turbulence and the horizontal shear on double-diffusive interleaving are treated in the framework of a linear stability problem in the equatorial ?-plane approximation. The model predicts the decrease of the vertical wavenumber (mmax) and growth rate (?max) for the fastest-growing intrusion with the increase in the turbulent diffusivity and predicts increased ?max and decreased mmax with the horizontal shear of the mean zonal flow. The effects of turbulent mixing are analyzed at different values of the Prandtl number, and the criteria of instability are obtained analytically for some simple cases. Special attention is focused on the influence of the horizontal shear on instability, as with the f-plane models of interleaving. The CTD profiling data obtained from Korean and Russian research vessels are used to prove the model. A closely spaced CTD section across the equator at 176°16?W shows several intrusive layers with high lateral coherence throughout the section, which are suddenly broken inside a narrow latitudinal band. The model suggests that the breakdown results from enhanced turbulent mixing in the wake behind Baker Island, which is situated just 10 n mi west of the section. The CTD data collected in the section across the equator at 152°30?E reveal an intensification of intrusions in a latitudinal band where the Equatorial Undercurrent is found. In accordance with the model, the intensification supposedly results from the effect of the horizontal shear attributed to the undercurrent. In terms of the applications of the model results, the value of the turbulent mixing ratio is estimated in the wake behind Baker Island, and mechanisms of intrusive layering are discussed. It is found that the intrusions observed in a meridional section, from 2° to 1°S at 176°16?W, correspond satisfactorily to the 2D baroclinic instability triggered by double diffusion.