Evolution of High-Frequency Nonlinear Internal Waves over a Realistic Continental Shelf: Observational Evidence and Dynamical Diagnostics

Zheng Ranshuai¹ and Bai Xiaolin¹
1.State Key Laboratory of Marine Environmental Science, and College of Ocean and Earth Sciences, Xiamen University, Xiamen, China

ABSTRACT
Internal wave breaking is a key energy source for ocean interior turbulent mixing, and its propagation and evolution over continental shelves directly determine the location, intensity, and efficiency of energy dissipation. As internal waves propagate shoreward, shoaling topography, stratification variations, and background currents jointly modify their waveform, modal structure, and energy budget, making the shelf zone a critical regime for internal wave energy cascade toward turbulent mixing. In addition to topographic effects, background currents can further affect internal wave propagation and energy evolution by modifying the propagation environment and flow shear. However, current studies on the modulation of internal wave propagation over shelf regions by background currents are mainly based on theoretical analyses and idealized models, while observational studies under realistic oceanic background current conditions remain limited.

Based on shipboard observational data collected in August 2024 over the shelf region south of the Taiwan Strait, together with satellite remote sensing imagery, this study investigates the propagation characteristics and dynamical mechanisms of high-frequency nonlinear internal waves in this region. Through the Taylor-Goldstein equation, weakly nonlinear KdV theory, combined with Richardson number diagnostics and energy flux estimates, this study systematically examines the joint control of stratification and background shear on internal wave propagation and stability.

The results show that the observed high-frequency nonlinear internal wave events exhibit significant dynamical differences, with their propagation speed, waveform, and related characteristics jointly controlled by stratification, nonlinear enhancement, dispersion effects, and background shear. In particular, background shear plays an important role in modulating the development of internal wave stability and the redistribution of energy, thereby further influencing differences in propagation patterns and evolutionary pathways. This study provides observational and dynamical evidence for the propagation, stability development, and energy transport mechanisms of high-frequency nonlinear internal wave under the combined influence of stratification and background shear in realistic shelf environments, and helps deepen understanding of the internal wave energy cascade and its conversion into turbulent mixing.