Generation and Propagation of Internal Solitary Waves in Background Shear Currents: Incorporating the Free-Surface Effect

Abstract

Internal solitary waves (ISWs) are ubiquitous in the ocean and play an important role in ocean mixing and energy transport. While the generation and evolution of ISWs have been extensively studied using two-layer models, the combined effects of a free surface, multilayer stratification, and background shear currents remain insufficiently understood. This study develops a three-layer stratified ocean model that explicitly incorporates free-surface effects to investigate the generation and propagation of ISWs induced by an initial density disturbance, both with and without background shear currents. Both depression-type and elevation-type ISWs are examined to represent typical oceanic conditions and to enable a systematic comparison of their dynamics. Background shear currents introduce a pronounced directional asymmetry in ISW dynamics: ISWs propagating in positive vorticity currents are broader and slower, whereas those propagating in negative vorticity currents are narrower and faster. Compared with the two-layer system, the presence of a pycnocline in the three-layer configuration leads to more complex current-wave interactions, particularly for ISWs propagating in negative vorticity currents, resulting in enhanced variability in wave profiles. Notably, although depression-type and elevation-type ISWs exhibit similar internal dynamics, their free surface responses differ substantially; under strong background shear, elevation-type ISWs propagating in negative vorticity currents may even undergo a reversal of surface polarity. These findings highlight the critical role of background vorticity and multilayer stratification in shaping ISW dynamics and surface signatures, providing new insights for the interpretation and modeling of ISWs in realistic oceanic environments.