Eulerian and Lagrangian characterization of a high-amplitude convectively unstable shoaling internal solitary wave in two dimensions.

High-amplitude Internal Solitary Waves (ISWs), shoaling over a realistic transect of the gentle bathymetric slope in the South China Sea, are subject to subsurface convective instability (Umax > C), which, in conjunction with the near-surface shear structure of the baroclinic background current, supports the development of a subsurface recirculating core. Through this core and its dynamic evolution, ISWs act as key drivers of material and mass transport. Via the one-way online coupling of a high-resolution, fully nonlinear non-hydrostatic flow solver integrated with a high-accuracy particle-tracking scheme, two-dimensional simulations of a single propagating ISW are conducted. The interaction between the formation and dynamic evolution of the ISW’s convectively-driven recirculating core, its associated vortical structures, and the trajectories of neutrally buoyant particles are examined. Particular emphasis is placed on identifying the primary entrainment pathway along a negative-vorticity layer at the rear of the ISW, as well as secondary entrainment routes subsequently emerging from the top and bottom of the core. In contrast, detrainment is found to occur primarily through a narrow channel in the ISW rear. These features are corroborated by Finite-Time Lyapunov Exponent (FTLE) analysis. The size and shape of the recirculating core are further examined using Lagrangian Coherent Structures (LCS), providing a complementary perspective to the classical Eulerian criterion based on Umax > C. Finally, long-range particle transport and residence times are quantified, reaching O(10 km) and durations on the order of hours, respectively, for a substantial fraction of entrained particles.