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Circulation, biogeochemistry and carbon cycling in ocean margins
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On dynamics of the vertical exchange among the three-layer circulations in the South China Sea
Monday 7th @ 1510-1530, Concert Hall
Zhongya Cai* , Department of Mathematics, Hong Kong University of Science and Technology
Jianping Gan, Department of Ocean Science & Department of Mathematics, Hong Kong University of Science and Technology
Presenter Email: zycai@ust.hk |
| The South China Sea (SCS) circulation features with three-layer cyclonic-anticyclonic-cyclonic (CAC) structure in the upper (<750m), middle (750-1500m) and deep (>1500m) layers. The vertical exchange dynamically links and sustains the layered circulation in the basin. This study investigated the three dimensional structure and seasonal variability of the water exchange among the layers in the CAC circulation and explored the underlying dynamics. The exchange over the varying slope regions largely determines the basic vertical exchange pattern in the SCS and is mainly controlled by the bottom cross-isobath geostrophic transport (CGT). In the central deep basin (CDB) (> 3000m), the convergence of water induced by deep water intrusion generates the prevailing upwelling among three layers. In summer, the subduction of the upper layer water can occur along the northern and western slope due to off-shore CGT and near the Mindoro Strait due to bottom frictional effect. The deep layer water upwells to the middle layer in the northern CDB and western slope as a result of on-shore CGT. During winter, the subduction of upper layer water is enhanced along the northern slope, while the upwelling of deep water is reduced in northern DSB and western slope due to the stronger/weaker upper/deep intrusion through Luzon Strait. The vorticity dynamics reveal that the bottom CGT are formed by the joint effects of the surface wind stress curl and the nonlinear advection and beta effect of circulation in both three layers. The vertical vorticity fluxes associated with the vertical exchanges play significant role in the vorticity balance in the middle and lower layer. The negative vorticity of the anti-cyclonic circulation in the middle layer is provided by the deep layer and the upper layer in the summer and winter, respectively. |
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