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Using satellite observations from 2003 to 2020 and cruise observations in 2019 and 2021, this study reveals an unexpected minor role of upwelling in seasonal and interannual variations in chlorophyll-a (Chl-a) concentrations in the coastal upwelling region east of Hainan Island (UEH) in the northwestern South China Sea (NWSCS). The results show strong seasonal and interannual variability in the Chl-a concentration in the core upwelling area of the UEH. Different from the strongest upwelling in summer, the Chl-a concentration in the UEH area reaches a maximum of 1.18 mg m−3 in autumn and winter, with a minimum value of 0.74 mg m−3 in summer. The summer Chl-a concentration increases to as high as 1.0 mg m−3 with weak upwelling during El Niño years, whereas the maximum Chl-a concentration in October increases to 2.5 mg m−3 during La Niña years. The analysis of environmental factors shows that compared to the limited effects of upwelling, the along-shelf coastal current from the northern shelf and the increased precipitation are crucially important to the Chl-a concentration variation in the study area. These results provide new insights for predicting marine productivity in upwelling areas, i.e., multiple mechanisms, especially horizontal advection, should be considered in addition to the upwelling process. [ABSTRACT FROM AUTHOR]

Using high‐resolution full‐water column velocity data at a site of 60‐m depth on the northeastern shelf of the South China Sea, we observed high‐frequency internal waves (HIWs) occurring continuously on the continental shelf during the 15‐hr observation on 13‐14 July 2011. This phenomenon is in contrast to the regular occurrence of internal solitary waves (ISWs) that are generally phase‐locked to the tides. We hypothesize that the continuously occurring HIWs are generated from shoaling ISWs via fission. Analysis of the Moderate Resolution Imaging Spectroradiometer (MODIS) true‐color images supports this hypothesis. The mechanism of ISW fission is then investigated using an analytical model as well as a two‐layer weakly nonlinear numerical model based on the regularized long‐wave (RLW) equation. The results are also compared with those from a fully nonlinear continuously stratified numerical model. Simulation results are generally consistent with the field measurements, explaining the observations of the continuous occurrence of HIWs. Sensitivity experiments are conducted to investigate the role and effects of the initial wave amplitude, pycnocline depth and thickness, bathymetry, and viscosity on ISW fission. Although all these factors impact the properties of the shoaling ISWs, both the two‐layer and continuously stratified models suggest that fission mainly occurs near the critical point region, highlighting the importance of pycnocline depth. However, simulations using the two‐layer model suggest that fission occurs after ISWs pass through critical points, whereas the fission location predicted by the continuously stratified model is generally offshore of critical points. This points to the deficiency of the two‐layer model in simulating small‐amplitude short trailing waves, large‐amplitude ISWs, wave breaking, and higher‐mode waves. Key Points: High‐frequency internal waves were continuously recorded from moored observations in July 2011 in the northeastern South China SeaSatellite images indicate that the observed high‐frequency internal wave trains were generated from fission of internal solitary wavesThe ISW fission characteristics are investigated using an analytical model, a two‐layer weakly nonlinear numerical model, and a continuously stratified fully nonlinear numerical model [ABSTRACT FROM AUTHOR]

For investigation of internal solitary waves (ISWs) in the South China Sea (SCS), most cruise observations are concentrated from Luzon Strait to Dongsha Atoll in the northeastern SCS but few on the continental slope far away from the west of Dongsha Atoll. In this study, we use 1‐year long mooring data to determine dynamic and statistical features of the ISWs on the shelf slope of the northwestern SCS. The analysis results of the mooring data reveal that the ocean internal waves on the shelf slope of the northwestern SCS have physical properties of highly nonlinear waves, which are well described by the solutions of the Korteweg‐de Vries equation. The mean nonlinear phase speeds of mode‐1 and mode‐2 ISWs are 1.38 ± 0.14 and 0.66 ± 0.12 m/s, respectively. The major direction of mode‐1 ISWs is northwestward 305° ± 21°. Strong ISW currents force the major direction of total current velocities to turn 67.5° in the upper layer and 135° in the lower layer. The monthly occurrence frequency distribution of ISWs shows a peak in July with a maximum frequency of 16.2% and a trough in March with a minimum frequency of 3.3%. Mode‐2 ISWs appear most in December, accounting for 50% of total mode‐2 ISWs. The largest mode‐2 ISW on record up today with the depressed amplitude as large as 91 m, and the elevated amplitude of 73 m was observed at mooring station. These new findings and new data are of significance to local internal wave prediction model development. Plain Language Summary: Unlike the sea surface waves, the ocean internal waves are a sort of wave motions occurring in the ocean interior. Strong internal waves carry strong currents and huge power, which may greatly impact the normal ocean and cause harm to underwater navigation and ocean engineering facilities. Thus, it is a necessary work to clarify the internal wave activities in an area of interest. We deployed a chain of instrument system moored at the sea floor on the continental slope of the northwestern South China Sea, where is rich in oil and gas resources, to observe the internal waves for 1 year from June 2016 to July 2017. Using the data of observed 684 internal waves, we analyze their physical properties and statistical features. The results indicate that (1) the observed waves have typical features of internal solitary waves, (2) the maximum amplitude reaches near 100 m, and (3) strong internal wave currents force the major direction of normal current to turn about 70° and 135° in the upper and lower layers, respectively, and March and July are trough and peak months for internal wave occurrence. These data and results are of significance for underwater navigation safety and ocean engineering designs. Key Points: Nonlinear properties of mooring‐measured mode‐1 and mode‐2 IWs and wave packets in SCS are verified by KdV equation solutionsStatistical features of occurrence frequency, amplitudes, and phase speeds of ISWs are derived from the 1‐year long mooring dataModulation of the internal waves to background current is measured for ocean engineering applications [ABSTRACT FROM AUTHOR]

This study uses high resolution MEdium Resolution Imaging Spectrometer (MERIS) satellite data to analyze sub-mesoscale ocean vortex trains (OVTs) in the western boundary of the South China Sea (SCS). Thirteen cases of vortex trains on the lee side of the Phú Quý Island were observed in the boreal summer from May to August of 2005−2010. The average diameter of the vortices is 28.1 ± 13.8 km. The average spacing distance estimated between two consecutive vortices is 66.3 ± 27.8 km. The dynamic parameters of a case observed on July 26, 2009 were extracted using a theoretical model. The aspect ratio is 2.8. The ratio of the propagation velocity of the vortex to the undisturbed flow velocity is 0.73. We use a nested high-resolution numerical model with the finest grid of 500 m, to analyze the dynamics of OVT induced by the local topography in the western SCS (WSCS). The results give a propagation velocity of vortex 0.49 of m s −1 and a mean current velocity of undisturbed flow 0.58 m s −1 . Therefore, the ratio is 0.84. The shedding frequency is determined to be 1.14 × 10 −5  s −1 , corresponding to 1.02 day. The results also suggest that the sub-mesoscale cyclonic vortices along the OVT in the WSCS are characterized by low temperature and high Rossby number. The strong vertical vorticity and associated horizontal velocity shear of an individual vortex brings uplifted cold water on its leading side and warm water on its trailing side, which could drive an enhanced vertical exchange between the surface and oceanic interior. [ABSTRACT FROM AUTHOR]

This study investigates the mechanism of mesoscale eddies observed in the South China Sea (SCS) deep basin, which are characterized by high-frequency occurrence, long life, and regular distribution patterns. Geophysical fluid dynamics predicts that for an enclosed ocean basin, there exist the Rossby normal modes if considering the Earth rotation effect. We use a rectangular ocean basin with a uniform depth to simulate the SCS deep basin and obtain 2D (x - y) sea level distribution patterns of the Rossby normal modes, consisting of alternating cyclonic and anticyclonic eddies, as well as their dynamic parameters. The periods of the modes are evidenced by the Fourier period power density spectra of satellite altimeter sea level anomaly (SLA) data from 1993 to 2015. The 2D normal mode patterns are evidenced by SLA images. The phase speeds are evidenced by that derived from the westward movement of the wave/eddy patterns on SLA time-series images. [ABSTRACT FROM AUTHOR]

This study deals with winter storm-induced continental shelf waves (CSWs) in the northern South China Sea in winter 2009 using tidal gauge data and along-track satellite altimeter data. The results show that the periods of CSWs propagating along the coast from Kanmen to Shuidong are in bands of 62 and 133 h. The phase speeds are in a range from 8 to 13 m s −1 between Kanmen and Shantou, from 9 to 11 m s −1 between Shantou and Huizhou and from 10 to 15 m s −1 between Huizhou and Shuidong, respectively. Satellite altimeter captured along-track sea level anomaly during the CSW events. Further analysis using the theoretical cross-shore CSW modes to fit the along-track sea level anomaly data indicates that the first three wave modes play important roles during the CSW events and the first mode is a dominant component. [ABSTRACT FROM AUTHOR]

A near-inertial oscillation (NIO) burst event in the west South China Sea (SCS) was observed by an upward-looking mooring Acoustic Doppler Current Profiler (ADCP) in summer 2004. The mooring station was located at 13.99°N, 110.52°E. The spectral analysis reveals that typhoon Chanchu is a major mechanism in triggering the NIO burst event. Before typhoon Chanchu passed over, the NIO signals were quite weak. The NIO band becomes the most energetic constituent of the circulation during the typhoon-wake period. The average peak power density (PD) reaches (5.3 ± 2.6) × 10 cms (cycles per hour, cph) with a maximum value of 9.0 × 10 cms cph, i.e., 3.1 times higher than that of diurnal tide (DT), (1.7 ± 0.5) × 10 cms cph. At the upper (80 m) and sub-upper (208 m) layers, the central frequency of the NIO band is 0.022 cph with a blueshift of about 9% above the inertial frequency f (0.02015 cph). At the lower layer (400 m), the central frequency of the NIO band is 0.021 cph with a blueshift of about 4% above the inertial frequency. The blueshifts are explained partially by the Doppler shift induced by the vorticity of mesoscale eddies. During the after-typhoon period, a resonance-like process between NIO and DT is observed in the upper layer. As the NIO frequency approaches the DT subharmonic frequency (0.5K), the PD of the NIO band rises sharply accompanied by a sharp drop of the PD of the DT band. The PD ratio of the two bands increases from 4.5 during the typhoon-wake period to 8 during the after-typhoon period, indicating the effect of the parametric subharmonic instability (PSI) mechanism. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) data derived from satellite and in situ measurements are used to study the thermal variability in the South China Sea (SCS). Time-frequency-energy distributions, periods of variability, and trends are computed by the Hilbert-Huang transform method. The SST trend from 1982 to 2005 is 0.276°C per decade in the SCS which is higher than 0.144°C per decade in the western Pacific warm pool (WPWP). The warm pool (SST ≥ 28°C) area in the SCS has increased by 0.20 × 10 km per decade. The SST and area of the warm pool in the SCS are strongly correlated, respectively, with the SST and area of the WPWP with a time lag of 1 month, suggestive of a strong connection between these two warm pools. Once the annual cycle is eliminated, decadal oscillations dominate the variability of SST and warm pool area in the SCS. [ABSTRACT FROM AUTHOR]

With observational data from three Acoustic Doppler Current Profiler (ADCP) moorings, we detected strong near-inertial oscillations (NIO) in the continental shelf region of the northern South China Sea in July 2008. The amplitude of the near-inertial current velocity is much greater than that of diurnal and semi-diurnal tides. The power of the NIOs is strongest in the intermediate layer, relatively weak in the surface layer, and insignificant in the near-bottom layer. The spectral analysis indicates that the NIOs have a peak frequency of 0.0307 cph, which is 2% lower than the local inertial frequency, i.e., a red-shift. The near-inertial wave has an upward vertical phase velocity, which involves a downward group velocity and energy flux. The estimated vertical phase velocity is about 43 m day, corresponding to a vertical wave length of about 58 m. The horizontal scale of the NIOs is at least hundreds of kilometers. This NIO event lasted for about 15 days after a typhoon's passage. Given the northeastward background flow with significant horizontal shear, both Doppler shift and shear flow modulation mechanisms may be responsible for the red-shift of the observed NIOs. For the shear flow mechanism, the observed negative background vorticity and the corresponding effective Coriolis frequency reduce the lower limit of admissible frequency band for the NIOs, causing the red-shift. Meanwhile, the mooring area with the broadened frequency band acts as a wave-guide. The trapping and amplification effects lead to the relatively long sustaining period of the observed NIOs. [ABSTRACT FROM AUTHOR]

Satellite altimeter sea level data from 1993 to 2008 are used to analyze the interaction of nonlinear Rossby eddies with the Kuroshio at the Luzon Strait (LS). The sea level anomaly data show that the west Pacific (WP) is a source of nonlinear Rossby eddies, and the South China Sea (SCS) is a sink. The LS serves as a gateway between the two. The scale analysis indicates that eddies with a radius larger than 150 km are strong enough to significantly alter the Kuroshio and are able to modify the local circulation pattern. Statistical analysis indicates that the probability for eddies to penetrate through the Kuroshio may reach at least 60%. A case study of an anticyclonic mesoscale eddy passing through the LS in June-July 2004 indicates that the Kuroshio behaves as an unsteady flow with its stream path frequently modified, in a way of cutting off, meandering and branching during its interaction with the eddy. We therefore suggest that nonlinear Rossby eddies may play a significant role in modification of the local circulation system near the LS and in exchanges of the mass, momentum and energy between the WP and the SCS. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses the regional environmental oceanography in the South China Sea (SCS).

An introduction is presented in which the editor discusses various reports within the issue on topics including near-inertial oscillation (NIO) in the South China Sea (SCS), the water mass in the Taiwan Strait, and the thermal variability in the SCS.