Seasonal characteristics and nonlinear interaction of internal tides (ITs) near the Luzon Strait in the northeastern South China Sea are investigated using 285 day, in situ observation data. It is found that ITs, which are dominated by the first‐mode wave throughout the year, are the strongest in subsurface layer. Baroclinic incoherent diurnal (semidiurnal) variance accounts for about 85.7% (78.3%) of diurnal (semidiurnal) ITs. The amplitude and seasonal variation of the diurnal ITs are more prominent than those that are semidiurnal, e.g., the largest kinetic energy densities of diurnal and semidiurnal baroclinic tidal currents are 2.81 and 0.83 KJ/m2 in winter, respectively. It is considered that there are two reasons for the significantly enhanced ITs in winter: (1) it may be due to the Kuroshio intrusion, and (2) the enhanced diurnal ITs may be due to the enhanced diurnal barotropic tidal currents, while the enhanced semidiurnal ITs may be caused by the strong nonlinear interaction between diurnal IT constituents O1 and K1 due to their high vertical shears in subsurface layer. Thus, harmonic semidiurnal constituent D2 with a similar frequency of constituent M2 is induced; it subsequently enhances the semidiurnal ITs in this subsurface layer, and the associated energy is carried downward to enhance the semidiurnal ITs in the upper and lower layers.

An idealized eddy-resolving ocean basin, closely resembling the North Pacific Ocean, is simulated using MITgcm. We identify rotationally coherent Lagrangian vortices (RCLVs) and sea surface height (SSH) eddies based on the Lagrangian and Eulerian framework, respectively. General statistical results show that RCLVs have a much smaller coherent core than SSH eddies with the ratio of radius is about 0.5. RCLVs are often enclosed by SSH anomaly contours, but SSH eddy identification method fails to detect more than half of RCLVs. Based on their locations, two types of eddies are classified into three categories: overlapping RCLVs and SSH eddies, nonoverlapping SSH eddies, and nonoverlapping RCLVs. Using Lagrangian particles, we examine the processes of leakage and intrusion around SSH eddies. For overlapping SSH eddies, over the lifetime, the material coherent core only accounts for about 25% and about 50% of initial water leak from eddy interior. The remaining 25% of water can still remain inside the boundary, but only in the form of filaments outside the coherent core. For nonoverlapping SSH eddies, more water leakage (about 60%) occurs at a faster rate. Guided by the number and radius of SSH eddies, fixed circles and moving circles are randomly selected to diagnose the material flux around these circles. We find that the leakage and intrusion trends of moving circles are quite similar to that of nonoverlapping SSH eddies, suggesting that the material coherence properties of nonoverlapping SSH eddies are not significantly different from random pieces of ocean with the same size. Plain Language SummaryMaterial coherent transport is widely estimated based on the Eulerian methods, but the organization of Eulerian eddies determines Eulerian eddies cannot keep material coherence during the lifespan. In this study, we examine the relationship between eddy detection methods and material coherent transport and quantitatively estimate the Eulerian eddy leakiness in material transport. We compare the statistical features of rotationally coherent Lagrangian vortices and sea surface height (SSH) eddies identified from a high-resolution idealized model based on two different frameworks. We find that the Eulerian method fails to detect many Lagrangian structures and overestimate the real coherent core. Besides, using Lagrangian particles, we quantify the material flux across SSH eddies boundaries. Our results suggest that SSH eddies are far from coherent structures, with the leakage of initial water reach at least 50%. We also find that one type of SSH eddies is not significantly different from random pieces of ocean with the same size in material coherence properties. Our findings suggest that using Eulerian eddy boundaries to estimate coherent transport may be misleading.of coherent eddy transport based on different identification methods vary dramatically. In this paper, we aim to clarify the relationship between eddy identification methods and material coherent transport. Also, we seek to examine the extent to which the eddie...