Coupled External‐Internal Dynamics of Layered Circulation in the South China Sea: A Modeling Study. Issue 7 (22nd July 2019)

• Record Type: Journal Article
• Authors: Cai, Zhongya; Gan, Jianping
• Is Part Of: Journal of geophysical research. Volume 124:Issue 7(2019)
• Page Start: 5039
• Page End: 5053
• Publication Date: 2019-07-22
• Subjects: layered circulation -- vertical transport -- vorticity dynamics -- South China Sea; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC014962
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: 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 transport The vertical transport is associated with the cross‐slope motion due to slope current‐topography interaction and links the circulation in the different layers The vorticity induced by the internal vertical transport is the major response to the external vorticity flux in the middle and lower layers