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The eddy‐abundant circulation in the northern South China Sea (NSCS) tends to be dynamically complex due to monsoon forcing, Kuroshio intrusion, and emitted internal waves from the Luzon Strait. This study uses 13‐year shipboard acoustic Doppler current profiler measurements (2004–2016), orbital altimeter data, and high‐resolution model output to perform wave‐vortex decompositions and investigate the scale of transition from dominantly geostrophic flows to internal wave motions in the northern South China Sea. The upper ocean kinetic energy spectra transition on scales exceeds 200 km. This large scale of transition is attributed to the energetic low‐mode internal waves (e.g., internal tides and inertia‐gravity waves). However, inconsistencies in the decomposition reveal that the assumptions of homogeneity and isotropy required for the 1‐D decomposition (Bühler et al., 2014) are sufficiently violated at smaller scales to affect the subdominant member of the decomposition on scales below 100 km. A method for direct quantification of the consequences by degree of violation using bootstrapping of the 2‐D model data is developed and illustrated. Observed and modeled sea surface height spectra flatten at scales smaller than 125 km, which is found in the model to be due to the coherent, semidiurnal internal waves. The large scale of transition between geostrophic and wave motions in the South China Sea is an irreducible uncertainty for altimeter velocities (e.g., the Surface Water and Ocean Topography mission). Plain Language Summary: The scale transition from balanced geostrophic flows to unbalanced motions is of great importance for understanding the circulation system of the South China Sea where both eddies and internal waves are particularly active. Based on observations and high‐resolution simulation, this paper investigates the wave number spectra of kinetic energy and sea surface height variance in the northern South China Sea. The large transition length scale (>200 km) from balanced geostrophic flows to unbalanced motions is due to strong low‐mode internal waves in the upper ocean. The violations of the assumptions for the 1‐D decomposition (stationarity, homogeneity, and horizontal isotropy) are also assessed using the 2‐D model data. With the model output, the low‐mode semidiurnal internal tides are found to have significant impacts on the sea surface height variance at scales of ~125 km. These results are directly relevant to the efficiency of diagnosing geostrophic flows from altimeters (e.g., the upcoming Surface Water and Ocean Topography mission). Key Points: The transition of KE from geostrophic flows to unbalanced motions is found to be larger than 200 km in the upper ocean of the NSCSThe frequency‐wavenumber analysis reveals the dominated processes for the transitionThe semidiurnal internal tides are found to have a significant contribution to the variance of sea surface height in the NSCS [ABSTRACT FROM AUTHOR]

The external inflow/outflow through straits on the periphery of the South China Sea (SCS) and the associated internal response of vertical transport over the broad continental slope form and sustain a cyclonic‐anticyclonic‐cyclonic (CAC) circulation in the upper‐middle‐lower layer in the SCS. We conduct a process‐oriented numerical study to investigate the underlying coupled external‐internal dynamics that remains unknown, despite that the dynamics plays a critical role in forming and sustaining the CAC circulation. External sandwich‐like inflow‐outflow‐inflow in the water column through the Luzon Strait forms a three‐dimensional CAC slope current over the curving slope topography in the SCS basin, in which the downward/upward transport associated with the cross‐slope motion is established. The along‐slope current over the basin provides vorticity through the beta effect and accounts for the most external planetary vorticity input through the strait in the upper layer of the SCS, while the vorticity induced by the vertical transport is the major response to the external vorticity flux in the middle and lower layers. We illustrate the critical role of the vertical transport in linking the vorticity among the different layers for the development and sustenance of the CAC circulation. The vertical transport is associated with the cross‐slope motion due to slope current‐topography interaction, which involves mainly in bottom frictional transport and geostrophic cross‐isobath transport by the pressure gradient force in the along‐slope direction. Pressure gradient force is generated by nonlinear vorticity advection and the beta effect of the background slope current. Plain Language Summary: The unique cyclonic‐anticyclonic‐cyclonic (CAC) circulation in the upper‐middle‐lower layer has been recognized in South China Sea (SCS) and the previous studies mainly focused on the links between the layered circulation and external forcing through the strait on the periphery of the SCS. However, the internal processes as in response to the external forcing and the role of coupled external‐internal dynamics in regulating the CAC circulation have not been adequately addressed. Using process‐oriented numerical simulation, this study explores the formation process of the CAC circulation and investigate the underlying coupled external‐internal dynamics. Driven by the external inflow‐outflow‐inflow in the water column through the Luzon Strait, the three‐layer CAC slope current is formed with the establishment of downward/upward transport. The vertical transport between the different layers mainly occurs over the slope because of the cross‐slope motion, which is induced by the bottom friction and pressure gradient forcing in the along‐slope direction. The vertical transport plays a critical role in linking the vorticity among different layers for the development and sustenance of the CAC circulation particularly in the middle and lower layers. This study improves the understanding of dynamics in the layered ocean circulation of the SCS. Key Points: The layered circulation in SCS is formed and sustained by the coupled dynamics of external forcing through straits and internal response of vertical transportThe vertical transport is associated with the cross‐slope motion due to slope current‐topography interaction and links the circulation in the different layersThe vorticity induced by the internal vertical transport is the major response to the external vorticity flux in the middle and lower layers [ABSTRACT FROM AUTHOR]

An intensive field observation experiment using 12 Chinese gliders equipped with conductivity-temperature-depth (CTD) sensors and 62 expendable CTD probes (XCTDs) was performed to investigate the 3-D structure and time evolution of an anticyclonic eddy in the northern South China Sea (NSCS). The observed results showed that the anticyclonic eddy had a horizontal radius of about 80 km at surface and a vertical depth of impact of more than 1000 m. The largest temperature and salinity anomalies compared with the averaged values of the temperature and salinity profiles were 3.5°C and 0.4 psu at 120 m depth, respectively. Combined analysis of altimeter sea level and water mass properties indicated that the anticyclonic eddy was shed from the Kuroshio loop current. The vertical axis of the anticyclonic eddy tilted from surface to the observed maximum depth (1000 m) along its translation direction against the 2000 m isobath. The center of the anticyclonic eddy remained in the region east of Dongsha Island for more than half a month. During this time, the long axis direction of the eddy changed from across the slope to along the slope. Then, the eddy moved southward along the 2000 m isobaths. Both the geostrophic current and temperature distribution revealed that the eddy intensity weakened during the observation period gradually. These observations indicated strong interaction between the anticyclonic eddy and the slope topography of Dongsha Island. [ABSTRACT FROM AUTHOR]

The Code for Unplanned Encounters at Sea [CUES 2014 Code for Unplanned Encounters at Sea (Western Pacific Naval Symposium, 22 April 2014] was finally adopted by the Western Pacific Naval Symposium in 2014. It is an important step toward improving confidence between navies. However, as a non-legally binding document with unclear geographical scope which only applies to naval vessels and aircraft, CUES 2014 is not comprehensive or potent enough, especially in the East and South China Seas where maritime disputes are unresolved and most maritime incidents involve civilian law enforcement vessels and aircraft. This article provides a geostrategic picture of the East and South China Seas, the background of the code, and analyses the practical limitations of CUES 2014 in the context of the East and South China Seas. [ABSTRACT FROM AUTHOR]

A process-oriented numerical modeling study was conducted to investigate the formation and underlying forcing of an anticyclonic eddy train observed in the northern South China Sea. Observations showed that long-lived anticyclonic eddies formed an eddy train along an eastward separated jet across the northern South China Sea in summer. The eddy train plays a critical role in regulating ocean circulation in the region. Forced by the southwesterly monsoon and prevailing dipole wind stress curl in the summer, the northward coastal jet separates from the west boundary of the South China Sea basin and overshoots northeastward into the basin. The anticyclonic recirculation of the separated jet forms the first anticyclonic eddy in the eddy train. The jet meanders downstream with a strong negative shear vorticity that forms a second and a third anticyclonic eddy along the jet's path. These three eddies form the eddy train. These eddies weaken gradually with depth from surface, but they can extend to approximately 500 m deep. The inherent stratification in the region regulates the three-dimensional scale of the anticyclonic eddies and constrains their intensity vertical extension by weakening the geostrophic balance within these eddies. Analyses of the vorticity balance indicate that the eddy train's negative vorticity originates from the beta effect of northward western boundary current and from the subsequent downstream vorticity advection in the jet. The jet separation is a necessary condition for the formation of the eddy train, and the enhanced stratification, increased summer wind stress, and associated negative wind stress curl are favorable conditions for the formation of the anticyclonic eddies. [ABSTRACT FROM AUTHOR]

Using high-resolution in situ data from gliders, satellite data of sea level anomaly and geostrophic currents, we presented the detailed structure of an anticyclonic eddy during spring 2015 in the northern South China Sea. The impact depth of the anticyclonic eddy reached about 1000 m and had a maximum temperature anomaly of about 3°C at ∼120 m and maximum salinity anomaly of more than 0.3 psu in the mixed layer. The maximum geostrophic velocities perpendicular to the glider path were about 0.3 m s−1at 100 m. The estimated radius was about 72 km and the translation velocity was about 5.2 cm s−1. The rotational speed of the eddy was estimated to be 0.35 m s−1. The high temperature and large salinity of the anticyclonic eddy indicated it did not originate from the South China Sea locally. The analysis of water mass indicated the character of the eddy water was similar to Kuroshio water, and the time evolution of the sea level anomaly and surface geostrophic velocity anomaly further validated that it originated from the Kuroshio intrusion as a loop current to the southwest of Taiwan. [ABSTRACT FROM PUBLISHER]

Based on the satellite altimeter data, it is found that the topography trapped wave (TTW) propagates along the continental shelf in the northern South China Sea (NSCS). The TTW obviously slows down while passing by the Dongsha Islands. Using a quasi-geostrophic equation including the topography effect, we found that the topographic gradient induced by the Dongsha Islands determines the propagation speed, leading to the slowdown of the TTW. [ABSTRACT FROM AUTHOR]

The interannual variability of the upper-ocean circulation forced by seasonally varying monsoonal wind is investigated in a two-layer quasigeostrophic (QG) model, with the aim to understand the low-frequency variability of the South China Sea (SCS) circulation. It is demonstrated that the seasonally varying monsoonal wind can force the upper-ocean circulation with significant internal variability, which is mainly associated with the intrinsic nonlinear dynamics of the summer double-gyre system. This arises from the fact that the intrinsic variability, characterized by the Rossby wave adjustment in the winter single-gyre system, is much weaker than that in the summer double-gyre system driven by the intergyre eddy potential vorticity flux through barotropic instability. [ABSTRACT FROM AUTHOR]

Two deep channels that cut through the Luzon Strait facilitate deep (>2000 m) water exchange between the western Pacific Ocean and the South China Sea. Our observations rule out the northern channel as a major exchange conduit. Rather, the southern channel funnels deep water from the western Pacific to the South China Sea at the rate of 1.06 ± 0.44 Sv (1 Sv = 106 m3s−1). The residence time estimated from the observed inflow from the southern channel, about 30 to 71 years, is comparable to previous estimates. The observation-based estimate of upwelling velocity at 2000 m depth is (1.10 ± 0.33) × 10−6 ms−1, which is of the same order as Ekman pumping plus upwelling induced by the geostrophic current. Historical hydrographic observations suggest that the deep inflow is primarily a mixture of the Circumpolar Deep Water and Pacific Subarctic Intermediate Water. The cold inflow through the southern channel offsets about 40% of the net surface heat gain over the South China Sea. Balancing vertical advection with vertical diffusion, the estimated mean vertical eddy diffusivity of heat is about 1.21 × 10−3 m2s−1. The cold water inflow from the southern channel maintains the shallow thermocline, which in turn could breed internal wave activities in the South China Sea. [ABSTRACT FROM AUTHOR]