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Oxygen isotope (δ18O) of seawater is an excellent proxy for tracing the origins of water masses and their mixing processes. Combining with hydrographic observation, hybrid coordinate ocean model (HYCOM) analysis data, and seawater oxygen isotope, we investigated the source of the South China Sea Warm Current (SCSWC) in the southwestern Taiwan Strait and its underlying mechanism. Results show that the Kuroshio subsurface water (KSSW) can intrude the continental slope in the southwestern Taiwan Strait, and thereby climb up the continental slope coupled with upwelling. The δ18O-salinity relationship further indicates that in spring, the SCSWC in the southwestern Taiwan Strait originates from the upslope deflection of the slope current formed by the KSSW intrusion into the South China Sea, rather than from the west segment of the SCSWC formed to the east of Hainan Island. In addition, the southward flowing Zhe-Min Coastal Current (ZMCC) can reach as far as the Taiwan Bank (TB) and deflects offshore over the western TB at approximately 23.5°N, to some extent affecting the SCSWC. Moreover, this study reveals that seawater δ18O is exquisitely sensitive to the determination of the origin and transport of water masses as compared with traditional potential temperature-salinity plot (θ-S) and HYCOM analysis data. In addition, their coupling can more reliably interpret the mixing processes of shelf water masses. [ABSTRACT FROM AUTHOR]

The El Niño wind and Luzon Strait transport are important factors modulating the interannual variability of the southern and northern South China Sea (SCS) winter circulation, respectively. The joint effect of El Niño wind and westward Luzon Strait transport drives a dipolar gyre with an anticyclonic (cyclonic) circulation anomaly in the southern (northern) SCS, which enhances the cross-basin current that connects the SCS western boundary current with the eastern boundary of the SCS around the Mindoro Strait, and then effectively modulates the interannual variability of Mindoro Strait transport. An extreme El Niño and large Luzon Strait intrusion occurred in winter 2015/16, and mooring observations and remote sensing data confirmed the presence of an intense dipolar gyre. This resulted in an extreme eastward cross-basin current and outward SCS Mindoro Strait transport anomaly in 2015/16. [ABSTRACT FROM AUTHOR]

The South China Sea (SCS) is a large marginal sea connecting the Indian and Pacific oceans. Under the factors of monsoons, strait transport, and varied bathymetry, the SCS presents a three-layer structure and strong diapycnal mixing which is far greater than that in the open ocean. Theoretical analysis and observations reveal that internal tides, internal solitary waves, and strong winds are the sources of the strong mixing in the northern SCS. A major consequence of the strong mixing is an active mid-deep circulation system. This system promotes exchange of water between the SCS and adjacent oceans, and also regulates the upper layer of wind-driven circulation, making the 3 dimensional SCS circulation clearly different from that in other tropical and subtropical marginal seas. The mass transport capacity of the mid-deep circulation has a substantial impact on marine sedimentation, the biogeochemical cycle, and other processes in the SCS. This paper summarizes the recent advances in mid-deep sea circulation dynamics of the SCS, and discusses the opportunities and challenges in this area. [ABSTRACT FROM AUTHOR]

Topographic Rossby waves (TRWs) are reported to make a significant contribution to the deep-ocean current variability. On the northern South China Sea (NSCS) continental slope, TRWs with peak spectral energy at ~14.5 days are observed over about a year at deep moorings aligned east–west around the Dongsha Islands. The TRWs with a group velocity of O(10) cm s−1 contribute more than 40% of total bottom velocity fluctuations at the two mooring stations. The energy propagation and source are further identified using a ray-tracing model. The TRW energy mainly propagates westward along the NSCS continental slope with a slight downslope component. The possible energy source is upper-ocean 10–20-day fluctuations on the east side of the Dongsha Islands, which are transferred through the first baroclinic mode (i.e., the second EOF mode). These 10–20-day fluctuations in the upper ocean are associated with mesoscale eddies. However, to the west of the Dongsha Islands, the 10–20-day fluctuations in the upper ocean are too weak to effectively generate TRWs locally. This work provides an interesting insight toward understanding the NSCS deep current variability and the linkage between the upper- and deep-ocean currents. [ABSTRACT FROM AUTHOR]