Oceanic internal waves are known to be important to the understanding of underwater acoustics, marine biogeochemistry, submarine navigation and engineering, and the Earth’s climate. In spite of the importance and increased knowledge of their ubiquity, the wave generation is still poorly understood in most parts of the world’s oceans. Here, we use satellite synthetic aperture radar images, in-situ observations, and numerical models to (1) show that wave energy (having relatively high amplitude) radiates from a shallow sill in the East China Sea in all directions, but with a significant time lag dependent on background conditions, (2) reveal that wave fronts are locally formed with often favorable conditions for re-initiation, and (3) demonstrate the resulting variety of wave patterns. These findings would be the case for any broad shelf having shallow sills with time-varying conditions, and therefore have significant implications on the redistribution of energy and materials in the global as well as regional ocean.

The Changjiang Diluted Water (CDW) plays a crucial role in shaping the hydrography and ecosystem dynamics of the East China Sea (ECS), particularly during summer when freshwater discharge enhances stratification and modulates biogeochemical processes. Despite its importance, the detailed progression of CDW and its short‐term impacts remain poorly understood due to the limited availability of high‐resolution observations. Using high‐resolution in situ observations from the Ieodo Ocean Research Station in summer 2020, we examined the sequential evolution of CDW and its effects on the upper‐ocean structure and phytoplankton blooms. CDW evolution was categorized into six distinct phases, characterized by abrupt shifts in salinity and stratification, driven by monsoonal winds, tides, and typhoon‐induced mixing. CDW intrusion formed a persistent barrier layer that suppressed vertical mixing and trapped surface heat, leading to increased sea surface temperatures. Nutrient influx from CDW, enriched in nitrate and phosphate, promoted the proliferation of diatoms relative to picoplankton and dinoflagellates, although all groups increased during the CDW‐influenced period. Internal wave‐induced mixing facilitated nutrient replenishment from deeper layers, partially alleviating phosphate deficiency inherent in CDW and enhancing phytoplankton productivity. Typhoon Bavi disrupted stratification through strong vertical mixing, resulting in a high‐salinity state that differed from preintrusion hydrography. This study provides a time‐resolved view of CDW‐driven variability and demonstrates how episodic freshwater inputs, coupled with physical forcing, regulate stratification, nutrient supply, and ecosystem response in the ECS.

Propagation speed and direction of nonlinear internal waves (NLIWs) are important parameters for understanding the generation and propagation of waves, and ultimately clarifying regional ocean circulation. However, these parameters cannot be directly measured from in-situ instruments, but can only be estimated from post-processing in situ data. Herein, we present two methods and an optimal approach to estimate the propagation speed and direction of waves using underway and moored observations. The Doppler shift method estimates these parameters from apparent observations concerning a moving ship using the Doppler shift induced by the changing relative distance of the NLIWs from the moving ship. The time lag method estimates the parameters using the distance between two locations of the NLIW observed at different times and the time lag. To optimize the speed and direction of NLIWs, the difference in the propagation direction independently estimated by the two methods needs to be minimized concerning the optimal propagation speed to yield the optimal propagation direction. The methods were applied to two cases observed in the northern East China Sea in May 2015 and August 2018. This study has practical significance for better estimating the propagation speed and direction of NILWs particularly over a broad continental shelf.

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.

Many countries worldwide promote artificial reef projects to increase and preserve fishery resources; however, how artificial reefs form fisheries is unclear. Nevertheless, specific hydraulic features of artificial reefs may attract fish. We selected an underwater reef as a research site to clarify this hypothesis. In this study, environmental conditions around the underwater reef were modeled and quantitatively assessed using numerical analysis. We identified two hydraulic features related to fish attraction: the wake region and the local upwelling region. Their spatial distributions were superimposed on the path of a fishing vessel that was monitored using an automatic identification system (AIS). We showed that various hydraulic characteristics (such as wake region, local upwelling region, and flow velocity) identified in the path of the fishing vessel can be quantitatively evaluated. Increasing amounts of information from the AIS can be used to identify the hydraulic features that attract the most fish and therefore improve the productivities of artificial reefs.

This study investigates the variability of coastal currents off the east coast of Korea, specifically in the Wangdolcho area of the East/Japan Sea, from June 2021 to October 2022. The observations revealed significant fluctuations in depth-averaged current velocity, with a peak of 0.81 m/s in August 2021 and a minimum of −0.05 m/s in August 2022. These year-to-year variations highlight the complex dynamics of coastal currents influenced by regional wind patterns and mesoscale eddies. In the summer of 2021, the development of offshore eddies, coupled with variable alongshore wind stress, led to increased current velocities and alternating upwelling and downwelling conditions, resulting in abrupt changes in current intensity. Conversely, in the summer of 2022, the negative vorticity in the Korea Strait and negative wind stress curl along the coast likely caused the East Korean Warm Current to shift further offshore, resulting in a weakened southward flow along the coast. This study emphasizes the need for long-term monitoring to better understand the coastal current dynamics and their environmental impacts. The anticipated completion of the Wangdolcho Ocean Research Station is expected to greatly improve monitoring capabilities, providing continuous and comprehensive data that will enhance our understanding of coastal currents and their broader impacts on the marine environment.