Three‐Dimensional Characteristics and Dominant Forcing Mechanisms of Third Waves Generated by Nonlinear Near‐Inertial Wave and Internal Tide Interactions

Abstract Nonlinear interactions between internal tides (ITs) and near‐inertial waves (NIWs) generate the “third waves” at frequencies corresponding to the sum of IT and NIW frequencies. The third waves drive oceanic energy cascade, yet under what conditions these waves are generated remain poorly quantified. This study employs an integrated approach combining multi‐mooring observations, causality analysis, and high‐resolution regional simulations (i.e., 2 km, 60 levels) in the Northern South China Sea (NSCS) to address this knowledge gap. Both field observations and numerical simulations reveal that at stations near the tropical cyclone (TC) track, third‐wave kinetic energy doubled after TC passage, while vertical penetration depths dramatically shoaled. Causality analysis demonstrates that NIWs dominate third‐wave generation at these sites, exceeding IT contributions by a factor of 9.4. In contrast, tide‐dominated stations farther from TC path showed slight energy increases with balanced NIW‐IT contributions. This study provides the first three‐dimensional, causality‐constrained perspective on how TC and ITs selectively control third‐wave generation through nonlinear interactions.