By analyzing 22 years (1993–2015) of daily eddy data, statistics of surface eddy properties were refreshed in the South China Sea. More than 7,000 of historical Argo profiles were collocated into eddy‐centered coordinates to reveal the composite mean three‐dimensional structure of eddies. The results indicate that eddies of both polarities have long conical shape, with a maximum (minimum) density anomaly of 0.55 kg/m³ (−0.51 kg/m³) at 60 m (90 m) in the composite cyclonic (anticyclonic) eddy. Temperature and salinity anomalies also peak at eddy cores, with values of −1.5 °C and 0.15 psu in the cyclonic eddy and 1.4 °C and −0.16 psu in the anticyclonic eddy. The temperature and density anomalies extend vertically to 400–500 m, while the salinity anomalies are apparent only in the upper 150 m. The temperature anomalies contribute about 90% of the density anomalies. Mixed layer depths in cyclonic eddies are on average 15 m shallower than those in anticyclonic eddies. The rotation of the composite cyclonic (anticyclonic) eddy generates meridional heat transport of 1.4 × 10¹² W (−3.1 × 10¹² W) and salt transport of −4.0 × 10⁴ kg/s (5.6 × 10⁴ kg/s). More than 90% of the heat and salt transports are concentrated in the upper 300 and 100 m, respectively. Compared to the meridional transports, the westward propagation of eddies results in zonal heat and salt transports on the same orders of magnitudes. The westward propagation of eddies also generates a basin‐scale westward water transport of 1.4 Sv, equivalent to about 30% of the annual‐mean Luzon Strait transport.

Large‐scale circulations are quite typical processes in the northern South China Sea (SCS) deep basin, yet their impacts on the variation of internal solitary waves (ISWs) remains poorly understood. We, here, focus on impacts of the upstream SCS western boundary current (SCSwbc) and three typical circulation patterns associated with different Kuroshio intruding paths on ISW propagation in this region. We show that the ISW modulated by the upstream SCSwbc gets a speedup, which is comparable to that induced by Coriolis effect, while the ISW amplitude shows an obvious reduction. Statistically, there is approximately one‐third time of 23 years (1993–2015) when circulation‐induced changes of wave speed exceed Coriolis‐induced changes. Specifically, the looping circulation pattern has the highest impact on significant speedup and amplitude reduction among the three patterns, while the leaping pattern has the lowest impact. These differences in wave characteristics caused by different circulation patterns result from the wave scattering, focusing and Doppler effects.

The vertical structure of eddies in the Leeuwin Current system affects the eddy volume, heat, and salt transport, even the ecosystem. However, the understanding of eddy vertical structure and eddy‐induced transport here are still very limited. In this study, satellite observed sea surface heights were combined with decade‐long in situ measurements of Argo floats to study the vertical structure of mesoscale eddies in the LC system and their volume, heat, and salt transport. A novel eddy reconstruction method, which considers the influences of both the eddy and background flow, is devised to study the three‐dimensional structure of eddies. Result shows that, in LC system, anticyclonic eddies (AEs) are usually surface‐intensified, with the geostrophic velocity decreasing sharply below the mixed layer, while cyclonic eddies (CEs) are subsurface‐intensified, with a maximum speed at 240 m. The density anomaly core of the average AE (CE) is at a depth of 130 m (650 m) with a density anomaly of −0.51 (0.24) kg/m³. The volume‐integrated eddy kinetic energy and available potential energy of the average CE are much larger than those of the average AE. The average lifespan of CEs is significantly longer than that of AEs, which can be explained by the deeper vertical scale of CEs. The offshore volume transport by eddy drift across the coastal (107°E) section is 9.05 Sv (12.5 Sv). The heat and salt onshore transport by eddy drift across the coastal (107°E) section are, respectively, 10.6 Tw and 143.1 ton/s (17.1 Tw and 241.0 ton/s).

Nonlinear interaction between near‐inertial waves (NIWs) and diurnal tides (DTs) after nine typhoons near the Xisha Islands of the northwestern South China Sea (SCS) were investigated using three‐year in situ mooring observation data. It was found that a harmonic wave (f + D₁, hereafter referred to as fD₁ wave), with a frequency equal to the sum of frequencies of NIWs and DTs (hereafter referred to as f and D₁, respectively), was generated via nonlinear interaction between typhoon‐induced NIWs and DTs after each typhoon. The fD₁ wave mainly concentrates in the subsurface layer, and is mainly induced by the first component of the vertical nonlinear momentum term, the product of the vertical velocity of DT and vertical shear of near‐inertial current (hereafter referred to as Component 1), in which the vertical shear of the near‐inertial current greatly affects the strength of the fD₁ current. The larger the Component 1, the stronger the fD₁ currents. The background preexisting mesoscale anticyclonic eddy near the mooring site may also enhance the vertical velocity of DT and thus Component 1, which subsequently facilitates the nonlinear interaction‐induced energy transfer to the fD₁ wave and enhances the fD₁ currents after the passage of a typhoon.

Internal solitary waves (ISWs) are a ubiquitous phenomenon in the dynamic ocean system, which play a crucial role in driving transport through turbulent mixing. Over the past few decades, numerical modelling has become a vital approach to investigate the generation mechanism and spatial distribution of ISWs. The northern South China Sea (NSCS) has been treated as a physical oceanographic focus of ISWs in massive numerical studies since the last century. However, there has been no systematic evaluation of a reliable three-dimensional (3D) model about accurately reproducing ISW characteristics in the NSCS. In this study, we implement a 3D ISW forecasting model in the NSCS and quantitatively evaluate the requirements of factors (i.e. model resolution, tidal forcing, and stratification selection) in accurately depicting ISW properties by comparison with observational data at a mooring station in the vicinity of the Dongsha Atoll. Firstly, the 500 m resolution model can basically reproduce the principal ISW characteristics, while the 250 m resolution model would be a better solution to identify wave properties, specifically increasing 40 % accuracy of predicting characteristic half-widths. Nonetheless, a 250 m resolution model spends nearly 5-fold the computational resources of a 500 m resolution model in the same model domain. Compared with the former two, the model with a lower resolution of 1000 m severely underestimates the nonlinearity of ISWs, resulting in an incorrect ISW field in the NSCS. Secondly, the model with 8 (or 13) primary tidal constituents can accurately reproduce the real ISW field in the NSCS, while the one with four main harmonics (M2, S2, K1 and O1) would underestimate averaged wave-induced velocity for about 38 % and averaged mode-1 wave amplitude for about 15 %. Thirdly, the model with the initial condition of field-extracted stratification gives a better performance in predicting some wave properties than the model with climatological stratification, namely 13 % improvement of arrival time and 46 % improvement of characteristic half-width. Finally, background currents, spatially varying stratification and external (wind) forcing are discussed to reproduce a more realistic ISW field in the future numerical simulations.

The bumpy continental slope/shelf topography is a quite common feature in the northern South China Sea (SCS), yet its effect on the shoaling internal solitary waves (ISWs) remains poorly understood. Therefore, numerical simulations by a fully nonlinear, nonhydrostatic model are carried out to explore the bumpy continental slope/shelf topographic effects on the transbasin evolution of large‐amplitude ISW in the northern SCS. It is found that the prominent bumps over both continental slope and shelf regions play significant roles in modulating the evolution of transbasin ISW in the northern SCS. The bump over the continental slope is capable of triggering a solitary‐like mode‐2 internal wave packet, while the bump over the continental shelf can result in three wave groups, including a leading group of rank‐ordered mode‐1 ISW packet and two following groups of non‐rank‐ordered mode‐1 ISW packet and mode‐2 internal waves. The bumps can cause a peak‐to‐peak difference of the energy decay rate of ISW up to 10–20 kW/m over continental slope region and 3–5 kW/m over continental shelf region. The wave kinetic energy (KE) is found to exceed the available potential energy (APE) by as much as 50% over the continental shelf break region. Over the shelf region, however, the bumps can first make the KE drop to as low as only 80% of the APE, but later the KE might bounce back to approximately 1.1–1.2 times of the APE. Both onshore‐ and offshore‐propagating beam‐like disturbances are found to be excited by the bumps. Except for the onshore‐propagating mode‐2 ISW packet, the reflected offshore‐propagating waves in different internal modes are also formed. These onshore‐ and offshore‐propagating multimodal internal waves can be clarified by the beam scattering and local generation mechanism.