Effects of Equatorial Undercurrent Shear on Upper-Ocean Mixing and Internal Waves

Abstract

The effect of equatorial undercurrent (EUC) shear on equatorial upper-ocean mixing is studied using a large eddy simulation (LES) model. This study consists of five numerical experiments of convection with various initial shear profiles: 1) full background shear (EUC shear), 2) same as 1 but with a surface cooling rate reduced by a factor of 10, 3) no shear, 4) stable part the background shear only (velocity constant above 30 m where Ri < 1/4 in experiment 1), and 5) unstable part of the background shear only (velocity constant below 30 m). It is found that flow evolution crucially depends on the background shear. Removal of all or part of the shear profile dramatically degrades the realism of the results. Convection in the mixed layer triggers shear instability, which in turn radiates gravity waves downward into the upper thermocline. Local shear instability can be triggered by downward-propagating internal waves in a marginally stable environment. This local shear instability is the cause of mixing well below the mixed layer. When complete EUC shear is present, internal waves with wavelengths of 200?300 m are generated below the boundary layer, in agreement with observations and linear instability analysis. The total shear profile determines the characteristics of the waves. When the stable shear, or the portion of the shear with Ri > 1/4, is eliminated, the internal waves have smaller wavelengths (about 80 m). When the unstable shear, or the portion of the shear with Ri ≤ 1/4, is eliminated, the intensity of internal waves below the boundary layer is much reduced, but the wavelengths are much larger than the case of convection without shear. In the absence of large-scale forcing to maintain the surface shear, the bulk of the kinetic energy from the mean shear is released in just a few hours after the onset of convection and shear instability. Turbulent kinetic energy budgets with and without shear show some similarities during the early stage of convection but show dramatic differences when the turbulence is fully developed. Namely, the turbulent transport and pressure transport terms are important in the case of convection without shear but are negligible in the case of convection with EUC shear, even though the surface forcing is the same. Local shear instability in a marginally stable mean flow environment is shown to play an important role in transporting heat and momentum into the stratified region below the mixed layer. Turbulence and waves generated by the mean shear instability are shown to be more effective than convective plumes in triggering local instability in the marginally stable region below the mixed layer.