DYNAMIC INSTABILITY OF STRATIFIED SHEAR-FLOW IN THE UPPER EQUATORIAL PACIFIC

The role of stratified shear instability in maintaining the deep cycle of turbulence immediately below the equatorial mixed layer is examine d by means of linear stability analysis. The Taylor-Goldstein equation is solved numerically, using observed currents and stratification fro m the Tropical Instability Wave Experiment (TIWE). Multiple unstable m odes are found, each associated with a local minimum of the gradient R ichardson number. Wave radiation due to the unstable modes fluxes mome ntum downward through the Equatorial Undercurrent. The frequency, wave length and propagation direction of the unstable modes are consistent with those estimated from observations of internal waves made during T IWE. 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.