Dynamic instability of stratified shear flow in the upper equatorial Pacific

Abstract

The role of stratified shear instability in maintaining the deep cycle of turbulence immediately below the equatorial mixed layer is examined by means of linear stability analysis. The Taylor‐Goldstein equation is solved numerically, using observed currents and stratification from the Tropical Instability Wave Experiment (TIWE). Multiple unstable modes are found, each associated with a local minimum of the gradient Richardson number. Wave radiation due to the unstable modes fluxes momentum downward through the Equatorial Undercurrent. The frequency, wavelength and propagation direction of the unstable modes are consistent with those estimated from observations of internal waves made during TIWE. These results, in combination with the previous observation that the internal waves are closely associated with deep cycle turbulence, lead us to conclude that shear instability is a dominant factor in the generation of the deep cycle.