Characterization of Nonlinear Internal Waves in the Southwest Sulu Sea: Insights from Satellite and In-Situ Observation

Abstract

Nonlinear internal waves, also known as solitons, are significant oceanographic phenomena that can influence offshore operations, marine navigation, and environmental stability. The Sulu Sea, shaped by complex bathymetry and tidal forcing, is recognized for the frequent occurrence of solitons; however, detailed characterization of these waves remains limited. This study addresses that gap by investigating the occurrence, structure, and propagation behaviour of nonlinear internal waves in the region. A combination of satellite-based and in-situ observational techniques was employed to provide a comprehensive analysis. Sentinel-1 synthetic aperture radar (SAR) imagery was used to detect surface expressions of internal waves, while a moored Acoustic Doppler Current Profiler (ADCP) recorded subsurface current variations over a three-month period to capture soliton events. The satellite analysis revealed internal wave signatures in 72-77% of the images examined. In-situ measurements identified over 100 soliton events, with individual wave durations ranging from 4 to 12 minutes and wave packets lasting up to 3 hours. Soliton propagation was predominantly shoreward toward the southwest, with some events directed west and south. SAR imagery indicated soliton packet spacing between 15-20km, and inter-soliton spacing of 250-400m, with estimated propagation speeds reaching up to 0.45m/s. The predominance of upward-propagating solitons, which are typically less visible in satellite imagery, highlights the critical role of in-situ measurements. These findings confirm the presence of distinct soliton types and emphasize the value of integrating satellite and in-situ methods to capture the full spectrum of internal wave dynamics, particularly in the Southwest Sulu Sea.