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.

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.

<p>Due to climate change, the occurrence of extreme events such as typhoons, marine heat waves, storminess, and cold waves is increasing in many regions, and these events could dramatically change with significant impacts on the marine environment (e.g., ocean circulation). The East Korea Warm Current (EKWC) has been recognized to flow along the western boundary current of the East Sea (Japan Sea). To examine the variations of EKWC, six bottom-mounted current profiler moorings were operated off the east coast of Korea (the Hupo Bank and Wangdolcho) since June 2021. The observed mean current speed and their principal axis were 0.40 m/s and 58<sup>o </sup>(counterclockwise from the east) implying northeastward EKWC over the region. In August 2021, an unprecedentedly strong surface current was observed with a maximum of 1.89 m/s and observed currents showed similar variability at mooring sites and several depths. This strong current lasted for about a month, and then rapidly disappeared within a few days. At that time, the geostrophic currents based on satellite-altimetry has a strong current pattern with the Inertial Boundary Current pattern, which is one of the EKWC patterns that flows strongly northward currents closer coast. In addition, the highest mean speed of the EKWC near the mooring sites from 1993 was found in August 2021. The high-speed period was similar to the period of the North Pacific marine heat waves that were already reported, and the low-speed period was related to typhoon passage. This study reported the results of observed EKWC for two years from 2021 and the unprecedentedly enhanced EKWC in August 2021. In particular, it can be a case in which the rapid changes of western boundary currents interact with extreme events such as marine heatwaves and typhoons.</p>

<p>Oceanic nonlinear internal waves (NLIWs) play an important role in regional circulation, biogeochemistry, energetics, vertical mixing, underwater acoustics, marine engineering, and submarine navigation, most commonly generated by the interaction between barotropic tides and bathymetry. Here, we present characteristics of first mode NLIWs observed using high-resolution in-situ data collected using moored and underway temperature sensors in a relatively flat bottom in the northeastern East China Sea during May 15-28, 2015. During the experiment, totally 34 events of first mode NLIWs were identified and characterized with amplitude of 4–16 m, characteristic width of 310–610 m, propagation speed of 0.53–0.56 m s<sup>-1</sup>, and propagation direction (mainly southwestward propagation), respectively. Most NLIWs were observed during period of spring tide with phases locked to semidiurnal barotropic tides. Generation and propagation of the first mode NLIWs observed in the region are discussed in relation to satellite images and historical hydrographic data collected in the region. Our results support significance of first mode NLIWs and their interactions on turbulent mixing and regional circulation particularly in a broad and shallow continental shelves where the NLIWs generated from multiple sources propagate into multi-directions experiencing wave-wave interactions.</p>

<p>Oceanic nonlinear internal waves (NLIWs) play an important role in regional circulation, marine biogeochemistry, energetics, vertical mixing, underwater acoustics, marine engineering, and submarine navigation, most commonly generated by the interaction between barotropic tides and bathymetry. However, our understanding of their characteristics, generation, and propagation is still far from complete in many water bodies. Here, we present the characteristics of NLIWs observed from moored and underway observation in the northern East China Sea during May 15-28, 2015 and discuss their generation and propagation. The NLIWs observed during the experiment were characterized by an amplitude ranging from 4 to 16 m, width ranging from 380 to 600 m, and propagated southwestward at a speed of 0.64–0.72 m s<sup>−1</sup>. Groups of NLIWs were predominantly observed during, or a couple of days after, the period of spring tides, with a time interval 24–96 min shorter than the canonical semidiurnal period (12.42 h; M<sub>2</sub>); this is in contrast to those found in many other regions that have a phase-locking to the barotropic semidiurnal tides. The remote generation and propagation of the NLIWs from potential generation sites into the study area under time-varying stratification support the fact that the time interval departed from the semidiurnal period. Our results have substantial implications for turbulent mixing and ocean circulation in regions where the shelf is broad and shallow. The NLIWs generated from multiple sources propagate in multiple directions with propagating speeds varying over days depending on stratification. </p>