The structure, variability, and dynamics of the deep circulation in the South China Sea (SCS) have long remained inadequately understood due to a lack of comprehensive, long-term in-situ observations. To address this, the largest CPIES array ever deployed in a marginal sea, consisting of 67 Current and Pressure-recording Inverted Echo Sounders (CPIES), was established, providing basin-wide, sustained measurements of abyssal currents and bottom pressure. Our results reveal a continuous, topographically trapped deep western boundary current (DWBC) that flows southwestward along the continental slope from the northeastern to the southwestern SCS. The current is predominantly barotropic, with maximum velocities near the boundary decreasing downstream. Striking seasonal variability emerges: in the central and southwestern basin, the robust southwestward DWBC in summer reverses into a broad northeastward countercurrent in autumn, whereas the northeastern segment remains persistently southwestward. This seasonal reversal, previously undocumented, indicates a regional dipole structure. A regional vorticity budget analysis indicates that the abyssal circulation is primarily maintained by a first-order balance between planetary vorticity advection and bottom pressure torque. This implies that vortex stretching due to flow–topography interaction fundamentally drives the deep current system.
The CPIES array also captured a variety of energetic processes. It documented the propagation of topographic Rossby waves (TRWs) within the deep basin and bottom-trapped TRWs to the west of the Luzon Strait, highlighting their role in deep energy dispersion. Importantly, near-inertial waves excited by Typhoon Mangkhut (2018) were observed to reach the deep basin, demonstrating efficient downward energy transfer and strong vertical coupling. Furthermore, the array-derived bottom pressure data were used to validate GRACE-FO satellite estimates; the good agreement confirms the array’s value as a ground-truth reference and demonstrates the reliability of spaceborne observations in monitoring deep-ocean mass variations in marginal seas. Collectively, these findings, revealed by the CPIES array in a marginal sea, provide transformative insights into SCS deep-sea dynamics, emphasizing the control of bottom topography, wave propagation, storm-induced energy penetration, and seasonal modulation of boundary currents. They significantly advance our understanding of deep-layer material cycling and the marine dynamic regime in large marginal seas.