Mode-2 solitary waves in continuous stratifications

Abstract

Computed herein are examples of mode-2 internal solitary waves to the Dubreil-Jacotin-Long (DJL) equation. These are localised, fully nonlinear waves in a continuously stratified, inviscid fluid under the Boussinesq and rigid-lid approximations. The equation is solved directly using finite differences on a periodic domain, enabling the computation of both generalised solitary waves, which possess oscillatory mode-1 tails, and embedded solitary waves, whose tails vanish. Two stratifications are considered. The first is a double pycnocline, generalising the three-layer system considered by Doak et al. (2022) by replacing each infinitesimal interface with a pycnocline of finite thickness. The second stratification, a surface mixed layer above an exponentially stratified layer, shows that embedded mode-2 solitary waves are not restricted to double-pycnocline stratifications. These are the first mode-2 solitary waves of the DJL equation computed for stratifications that are asymmetric about mid-depth.