Seasonal variability of the meridional overturning circulation in the South China Sea and its connection with inter‐ocean transport based on SODA2.2.4

Abstract

We have proposed a five‐layer‐scheme to investigate the volume transport through the South China Sea (SCS) based on the updated Simple Ocean Data Assimilation (SODA2.2.4) product. By demonstrating horizontal transport in each layer, we have revealed different formation mechanisms for the meridional overturning circulation (MOC) in winter and summer in the SCS. Our analysis suggests three meridional circulation systems in the SCS: (1) the seasonal monsoon‐driven circulation in the surface layer, i.e., southward circulation in winter and northward in summer, (2) the compensatory transport‐induced seasonal intermediate MOC in the central SCS, and (3) the persistent deep MOC in the southern SCS all year round. By examining vertical velocity distribution, we have identified that the major overturning process of the intermediate MOC is located along the continental slope east and southeast of Vietnam, while the major overturning process of the deep MOC is located along the continental slope northwest of Borneo. The downwelling in the intermediate MOC in winter and upwelling in the deep MOC all year round bring different water masses to the intermediate and subintermediate layers to be mixed in the SCS. We found no evidence to suggest that the strength and extent of the MOC south of 18°N are related to inter‐ocean volume transport. The surface layer transport in the Luzon Strait has been decreasing since the 1960s. However, the causes of the meridionally staggered and interdecadal alternating acceleration/slowdown of the meridional stream function difference are unknown.

Effect of potential vorticity flux on the circulation in the South China Sea

“…The intermediate layer, defined by a potential density range of 26.40–27.62 kg m −3 , lies in an average depth range of 400–1850 m in the Luzon Strait. The mean volume flux is eastward in the Luzon Strait, which infers that the North Pacific Intermediate Water (NPIW, with characteristic density of 26.73 ) is impeded to intrude into the SCS [ Qu et al ., ; Li and Qu , ; Liu et al ., ; Zhu et al ., ]. In this density layer, water flows southward from the SCS to Sulu Sea through the Mindoro Strait, consistent with the bottom‐intensified deepwater overflow there based on satellite‐derived data [ Qu and Song , ].…”

“…The cyclonic and anticyclonic circulations are separated by the zero stream‐function contour line associated with the zero zonally integrated wind stress curl line or the separating point of the western boundary current to the east of Vietnam [ Gan and Qu , ]. Furthermore, the wind driven current flows inward to the SCS in winter but outward to the western Pacific Ocean in summer in the Luzon Strait above the isopycnal surface 24.2 kg m −3 [ Zhu et al ., ], while the current is southward in winter and northward in summer in the Karimata Strait [e.g., Susanto et al ., ]. The seasonal reversal of surface current does not construct a steady state of circulation, and more importantly, it has been extensively analyzed and attributed mainly to the wind stress curl by previous studies [e.g., Qu , ; Gan et al ., ; Gan and Qu , ; Gan et al ., ].…”

“…To some extent, the outward flow might impede the NPIW near the upper interface to intrude the SCS. This agrees with previous studies on weak intrusion of the salinity minimum water based on WOD hydrographic data [ Qu et al ., ; Liu et al ., ] and SODA outputs [ Zhu et al ., ]. As a result of irregular boundary current in the southern SCS, the velocity in the northern boundary, especially along the continental slope south of China, accounts for most of the line integral around the boundary circuit.…”

“…On the other hand, since the Luzon Strait is the only channel of deep water renewal in the SCS, the persistent Kuroshio intrusion in the subsurface and deepwater overflow in the deep layer are of a decisive influence on the circulation pattern in the SCS [e.g., Qu et al ., ]. On the basis of limited observations, a vertically sandwiched flow structure in the Luzon Strait, i.e., inflow in the upper and deep layers and outflow in the intermediate layer, has been reported by previous studies [e.g., Qu , ; Qu et al ., ; Li and Qu , ; Yuan , ; Tian et al ., ; Fang et al ., ; Yang et al ., ; Zhang et al ., ; Xu and Oey , ; Zhu et al ., ]. Corresponding to this flow structure in the Luzon Strait, a sandwiched basin‐scale circulation, i.e., cyclonic in the upper and deep layers and anticyclonic in the intermediate layer, has been proposed by numerical models.…”

Elemental and Sr‐Nd Isotope Geochemistry of Sinking Particles in the Northern South China Sea: Implications for Provenance and Transportation

“…Black and purple stars are sampling locations of this article and Schröder et al (), respectively. The surface and intermediate currents are basically depicted by Zhu et al () and Hu et al (): The surface currents are indicated by light blue arrows (winter) and yellow arrows (summer); the intermediate currents are indicated by dark blue arrows (winter) and orange arrows (summer). The Kuroshio Current is indicated by the black arrows (Fang et al, ).…”