A method based on the Okubo–Weiss parameter was used to detect subsurface eddies (SSEs) with an eddy-resolving ocean general circulation model. Statistical analyses showed that SSEs are ubiquitous in the northwestern Pacific Ocean. Three regions were found to have high probability of SSE, which are as follows: the latitudinal band between 9°N and 17°N, the Kuroshio extension region, and the area east of the Ryukyu Islands. Although surface eddies (SEs) were found distributed widely within the zonal band of the Subtropical Counter Current, few SSEs were found there. In contrast, few SEs were found to the east of The Philippines, whereas SSEs were abundant. The kinetic energy contained within SSE was found comparable in magnitude with that of SE. During 1993–2013, about 2569 and 2099 SSEs (at a depth of about 400 m) were observed to be anticyclonic and cyclonic, respectively; thus, SSEs tended to be anticyclonic. The mean radius, lifespan, and propagation speed of SSE in this study were about 60 km, 50 days, and 6.6 cm/s, respectively. The propagation speed showed a wave-like decrease with increasing latitude. Some long-lived SSEs were found to persist for longer than 4 months and to move thousands of kilometers. About 89% of SSEs were nonlinear for at least half their lifespan, which implies that SSE can trap interior fluid during translation. Trajectories revealed that SSEs propagate nearly due west with only small meridional deflection. The findings of this study will contribute to the enrichment of our knowledge regarding SSE in the northwestern Pacific Ocean.

The southern Yellow Sea Cold Water Mass (YSCWM) and the Yellow Sea Warm Current (YSWC) are prominent features in the southern Yellow Sea. The traditional viewpoint is that the southern YSCWM exists in the warm half of the year and the YSWC exists in the cold half of the year; the most existing studies have regarded these features as independent ocean phenomenon. Through marine investigations conducted in the recent years in the southern Yellow Sea, the southern YSCWM was found to exist in the early winter in 2010, and the YSWC was observed in the early summer in 2013, 2014. For approximately half the year, the southern YSCWM and YSWC existed simultaneously, and the YSWC was located in a slightly high-temperature region in the middle of the southern YSCWM. A sandwich-shaped temperature structure formed in the middle of the southern Yellow Sea from spring to autumn. This article studies the seasonal variation characteristics of the three-dimensional southern YSCWM and YSWC structures using ocean survey data collected in the recent years. The new perspective of the seasonal variations in the southern YSCWM and YSWC obtained herein will promote the recognition of southern Yellow Sea water exchanges and support biogeochemical cycle research in the Yellow Sea.

Mesoscale eddies can alter the propagation of wind-generated near-inertial waves (NIWs). Different from previous studies, the subsurface mooring observed NIWs are generated outside an anticyclonic eddy (ACE) and then interact with the arriving ACE. It is found that with the arrival of the ACE, the NIWs accelerate to propagate downward and the maximum vertical wavelength and group velocity of NIWs reach ∼500 m and ∼35 m day⁻¹, respectively. When entering the core of the ACE, the near-inertial energy is trapped and finally stalls at a critical depth, which basically corresponds to the base of the ACE located at around 750-m depth. Through a ray-tracing model and dynamic analyses, this critical depth is much deeper than that of NIWs generated directly inside an ACE. By using depth–time varying stratification and relative vorticity, ray-tracing experiments further demonstrate that NIWs generated outside and passed over by an ACE can propagate to deep depths. Furthermore, energy budget analyses indicate that the net energy transfer from the ACE to NIWs plays an important role in the enhancement of downward-propagating near-inertial energy and its long-term persistence (∼45 days) in the critical layer. Within the critical layer, the enhancement of shear instability and nonlinear interactions among internal waves account for the loss of the trapped near-inertial energy and provide energy for furnishing deep ocean mixing. Significance Statement: The interactions between near-inertial waves and a westward-moving anticyclonic eddy are investigated in this study. Knowledge about the propagation of near-inertial waves continues to be a topic of interest because near-inertial waves transfer energy from the mixed layer to the interior ocean, which is an important source of turbulent mixing. While much is known about how near-inertial energy propagates inside an anticyclonic eddy, few studies have examined how near-inertial energy propagates when it is generated outside an anticyclonic eddy and then enters the arriving anticyclonic eddy. In this study, the deep propagation and trapping of near-inertial energy by a westward-moving anticyclonic eddy is observed, which contributes greatly to the energy budget and the deep-ocean mixing.