Internal solitary waves within background shear currents in a three-layer fluid system

Abstract

Ocean shear currents are ubiquitous and have a profound effect on the characteristics of internal solitary waves (ISWs). A systematic study is conducted on how linear and nonlinear background shear currents affect the structure and induced dynamics of mode-1 and mode-2 ISWs in three-layer system. A simulation method for ISWs in background shear currents is developed by coupling the high-level Green-Naghdi (HLGN) theory with computational fluid dynamics (CFD). This hybrid approach enables the modeling of mode-1 and mode-2 ISWs under background shear currents with arbitrary vertical structures, while achieving a favorable balance between computational efficiency and domain size. For mode-1 ISWs, positive vorticity currents broaden the upper and lower wave profiles while attenuating the lower amplitude, effectively thinning the pycnocline, and reducing wave-induced horizontal velocity differences across the pycnocline, vertical velocity, and propagation speed. Negative vorticity currents induce the opposite effects. For mode-2 ISWs, positive vorticity background shear currents (relative to wave-induced vorticity at the trough) broaden the lower profile while narrowing the upper profile and reducing its amplitude, weakening velocity signatures and slowing propagation. Negative vorticity currents enhance these characteristics. Overall, background shear currents play a dominant regulatory role by significantly modifying the density stratification, shear distribution, and propagation characteristics of internal solitary waves. When nonlinear shear currents are considered, the resulting effects are weaker but exhibit increased complexity. These results deepen our understanding of wave-current interactions in stratified oceans and offer a useful basis for future investigations of complex stratified flows.