Abstract
Recent progress of physical oceanography in the South China Sea (SCS) associated with the western boundary current (WBC) and eddies is reviewed in this paper. It includes Argo observations of the WBC, eddy detection in the WBC based on satellite images, cross-continental shelf exchange in the WBC, eddy-current interaction, interannual variability of the WBC, air-sea interaction, the SCS throughflow (SCSTF), among others. The WBC in the SCS is strong, and its structure, variability and dynamic processes on seasonal and interannual time scales are yet to be fully understood. In this paper, we summarize progresses on the variability of the WBC, eddy-current interaction, air-sea interaction, and the SCSTF achieved in the past few years. Firstly, using the drifting buoy observations, we point out that the WBC becomes stronger and narrower after it reaches the central Vietnam coast. The possible mechanisms influencing the ocean circulation in the northern SCS are discussed, and the dynamic mechanisms that induce the countercurrent in the region of northern branch of WBC in winter are also studied quantitatively using momentum balance. The geostropic component of the WBC was diagnosed using the ship observation along 18°N, and we found that the WBC changed significantly on interannual time scale. Secondly, using the ship observations, two anti-cyclonic eddies in the winter of 2003/2004 in the northern SCS, and three anti-cyclonic eddies in the summer of 2007 along 18°N were studied. The results show that the two anti-cyclonic eddies can propagate southwestward along the continental shelf at the speed of first Rossby wave (∼0.1 m s−1) in winter, and the interaction between the three anti-cyclonic eddies in summer and the WBC in the SCS is preliminarily revealed. Eddies on the continental shelf of northern SCS propagated southeastward with a maximum speed of 0.09 m s−1, and those to the east of Vietnam coast had the largest kinetic energy, both of which imply strong interaction between eddy activity and WBC in the SCS. Thirdly, strong intraseasonal variability (ISV) of sea surface temperature (SST) near the WBC regions was found, and the ISV signal of SST in winter weakens the ISV signal of latent heat flux by 20%. Fourthly, the long-term change of SCSTF volume transport and its connection with the ocean circulation in the Pacific were discussed.
Highlights
We discuss the variability of the SCS WBC (SCSWBC), interaction between eddy and SCSWBC, air-sea interaction, and the SCS throughflow (SCSTF)
Funded by the National Natural Science Foundation of China in 2010, the multi-disciplinary northern SCS (NSCS) open cruise is executed under the ship-sharing policy
During the NSCS open cruise, the long-term observation section of 18°N is included in the Climate Variability and Predictability (CLIVAR) International Climate and marine planning framework, which promotes the Indo-Pacific warm pool and South China Sea (SCS) to become one of the key research areas of the “Asian Monsoon Years”
Summary
The studies on the SCSWBC are limited. In 1961, Wyrtki [1] pointed out for the first time that there is a western intensification in the SCS. Liu et al [13] calculated the basin-scale Sverdrup transport in the SCS by neglecting the bottom topographic effect, and explored the impact of wind stress on the seasonal circulations in the SCS. The thermocline change caused by the wind stress curl can adjust the ocean state to the quasi-steady Sverdrup balance; it establishes the seasonal-mean large-scale circulation and the corresponding strong SCSWBC [14]. Zhai et al [15] used numerical experiments to study the dynamics of barotropic circulation caused by summer and winter monsoons. Their results showed that the water that flows through the Luzon Strait into the SCS has an important contribution to the boundary current on the northern slope and the SCSWBC. The other effects in the SCSWBC region, such as baroclinicity, terrain, and flow, and buoyancy flux impact are all ignored, which leads to the difference between the idealized circulation characters and the reality
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