Abstract
The observation of interleaving of water masses within the thermocline of the equatorial Pacific raises questions as to why the interleaving is present and what impact it has on mixing and the broader scale structure of the flow and tracer fields. Individual layers are of order 10m thick and stretch for order 200 km and 1000 km in the meridional and zonal directions, respectively. Here we investigate the stability of time varying equatorial flows in the form of a simple oscillating sheared flow and a more complicated superposition a Gaussian jet (to model the Equatorial Undercurrent) and an equatorial planetary wave (a low vertical mode Yanai wave). The conditions for which the flow is unstable and produces layering are investigated. A linear instability analysis reveals that inertial instability, parametric sub-harmonic instability and a mixed instability dependent on both the shear and time variability of the background flow, can all produce structures with small vertical scales similar to those observed. Numerical experimentation shows the subsequent development produces extensive layering, with substantial mixing of momentum, potential vorticity and tracer fields. The equilibrated state depends strongly on the flow configuration. Our results suggest that instabilities that produce small vertical scale structures in the stratified ocean are likely to play a significant role in the larger scale dynamics of the equatorial Pacific leading to scale interactions on inter-annual timescales.