Transport by turbulent subsurface cores in internal solitary waves
Abstract
Internal solitary waves (ISWs) are known to induce material transport through various mechanisms, with important implications for physical and ecological processes. We report direct numerical simulations of an ISW under conditions favorable for the development of a subsurface core, a situation particularly suitable for significant horizontal transport. The results show that the core is dynamically active and strongly three-dimensional, characterized by persistent turbulence. Analysis based on passive tracers reveals that the core exhibits continuous entrainment of ambient fluid, followed by turbulent mixing, temporary retention, and eventual leakage. Entrainment occurs primarily across the upstream–upper core boundary and is closely associated with shear and convective instabilities that persist throughout the wave evolution. Once entrained, fluid is redistributed by turbulence within the core and remains trapped for a finite duration before being released downstream. Increasing the background shear strength by a factor of four increases the entrainment by approximately 67.4%, and reducing the wave amplitude by 9.1% decreased the entrainment by approximately 10.4%. Our simulations reveal that subsurface cores in ISWs under shear currents should not be considered as closed-streamline trapped cores but rather as turbulent, leaky transport systems.