Mooring Observation and Numerical Simulation of the Oblique Wave-Wave Interactions in the Andaman Sea

Internal solitary waves (ISWs) typically exhibit a strong tendency to preserve their waveforms and amplitudes upon collision, but this preservation breaks down when two ISWs obliquely interact. Although oblique wave-wave interactions have been frequently observed in the oceans worldwide and studied extensively through theoretical research and satellite imagery, the fundamental underwater dynamics and spatiotemporal evolution of these interactions remain poorly understood. This study presents a detailed investigation of oblique ISW interactions in the southern Andaman Sea, integrating long-term in situ mooring data with high-resolution 3D numerical simulations. The observations show that when two ISW packets interact obliquely, they merge into a new, chaotic waveform with significant disruption to the horizontal velocity field. The resulting merged ISW is asymmetric, narrow, and steep, exhibiting a particular velocity structure with enhanced current shears. It is worth noting that the amplitude (energy) of the merged ISW is approximately 25% (79%) larger than the combined amplitude of the two incident ISWs, highlighting the strong nonlinearity that has been long proposed by previous theoretical studies. Theoretical analyses suggest that the observed interaction is strong and non-phase-conserving, and the resulting merged ISW is a part of an evolving Mach stem. Additionally, model simulations indicate that ISWs originating from three distinct source regions interact in multiple ways, each producing unique variations in crest patterns and wave intensity, which is far more complex than earlier theoretical predictions. This study underscores the critical role of oblique wave-wave interactions in altering the 3D characteristics of ISWs, including both their underwater dynamics and along-crest variability.