Capturing the incipient breaking of a convectively unstable internal solitary wave of depression shoaling in the South China Sea

The incipient breaking of a convectively unstable Internal Solitary Wave (ISW) of depression is explored through fully nonlinear and non-hydrostatic simulations, based on a high resolution/accuracy deformed spectral multidomain penalty method. The simulation incorporates field measurements of bathymetry and time-averaged background stratification and current sampled in the South China Sea. The ISW shoals over a gentle slope (< 3%), and as the wave adjusts to the changing bathymetry, an unstable region forms, characterized by the entrapment of heavier-over-light fluid. The convectively unstable fluid configuration is caused by the stretching of the near-surface vorticity layer that is generated by the baroclinic background current. Field observations in the South China Sea indicate that the unstable region within the wave may persist for more than 10 km of wave propagation distance and drive turbulent-induced mixing, estimated to be up to four times larger than that in the open ocean. Motivated by the particular observations and building on previous two-dimensional simulations by the authors, in this work, shoaling is simulated in three dimensions. Visualization of the initial three-dimensional instability reveals a transitional structure that develops within the overturning isopycnals in the lateral direction. The evolution of this lateral instability is compared with that of the wave-scale convective overturn in the ISW interior. Through this study, a preliminary understanding of the formation of recirculating cores in ISWs, the driver for subsequent turbulence, mixing, and particle transport in the interior is obtained.