New Generation Site and Estimated Propagation of Nonlinear Internal Waves Under Varying Background Stratification in the Northeastern East China Sea

This study investigates the generation and propagation of nonlinear internal waves (NLIWs) in the northeastern East China Sea (ECS), focusing on the impact of time-varying background stratification. Using satellite imagery and tide-generating body force, 327 groups of NLIWs were identified from 93 MODIS images collected between 2015 and 2019 and classified into three distinct types based on their propagation directions—Type-A (∼42%), Type-B (∼38%), and Type-C (∼20%)—each originating from different generation sites. Type-B NLIWs were found to originate from a newly identified site near southwestern Fukue Island reported here for the first time, whereas Type-A and Type-C originated from the northern Okinawa Trough and southwestern Jeju Island, respectively. Seasonal variations in NLIW propagation speed using historical CTD data revealed an increase in the speed from winter to spring and fall, peaking in summer due to strong stratification. Although spatial distribution varied by season, propagation was generally faster in the eastern region than in the western region attributed to deeper thermoclines and higher density gradients. During field observation in May 2015, the observed NLIWs had an amplitude of 5–6 m, a characteristic width of 380–450 m, southwestward propagating directions, and a speed of ∼0.54 m s⁻¹. An empirical model, used to track and predict these waves, confirmed their generation locations from southwestern Fukue Island (Type-B) and indicated fast arrival speeds, influenced by time-varying stratification conditions during the stratification period. These findings highlight the impacts of varying background conditions on NLIW propagation and would facilitate their prediction.