We investigated the mean kinetic energy (MKE) and eddy kinetic energy (EKE) in the South China Sea to illustrate the dynamics of the vertically rotating cyclonic–anticyclonic–cyclonic (CAC) circulation in the upper, middle, and deep layers. We found that strong MKE along the basin slope and the associated EKE arising from the vertical shear and stratification of the mean current characterize the circulation. In the upper layer, the external MKE input from the Kuroshio intrusion and wind forcing drive the cyclonic circulation, with the wind forcing providing most of the EKE. External forcing, however, does not directly provide the MKE and EKE of the CAC circulation in the semi-enclosed middle and deep layers, where the internal pressure work near Luzon Strait and the vertical buoyancy flux (VBF) in the southern basin and along the western slope maintain the MKE and EKE. The internal pressure work is formed by ageostrophic motion and pressure gradient field associated with circulation. The VBF is generated by vertical motion induced by the geostrophic cross-isobath transport along the slope where variable density field is maintained by the external flow and the internal mixing. The kinetic energy pathway in the CAC circulation indicates that the external forcing dominates upperlayer circulation and the coupling between internal and external dynamics is crucial for maintaining the circulation in the middle and deep layers. This study provides a new interpretation to the maintenance of CAC circulation from energy prospect.

基于动能路径推断的中国南海分层环流动力学

中文摘要：本文研究中国南海平均动能（MKE）和涡动动能（EKE）以阐明其上层、中层和下层垂向旋转气旋－反气旋－气旋（CAC）式环流动力学。我们发现该环流具有强沿海盆斜坡的MKE和相关EKE来源于平均流动的垂直剪切和层结的特征。黑潮入侵和风应力的外部MKE输入驱动上层气旋式环流，且风强迫为其提供大部分的EKE。但是外界强迫不能直接提供半封闭中层和深层CAC环流的MKE和EKE，中层和深层的MKE和EKE通过吕宋海峡附近的内压力功和南部海盆和西部斜坡的垂直浮力通量（VBF）维持。内压力功是由非地转运动和与环流相关的压力梯度场形成的。VBF是产生自沿斜坡的地转跨等深线输运引起的垂向运动，其密度场的变化是由外部流动和内部混合维持的。CAC环流的动能通路表明内部强迫主导了上层环流，内外动力耦合对维持中层和深层环流至关重要。本研究从能量的角度提供了关于CAC环流维持的新解释。