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

Abstract

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 mi...