Decadal climate control of circulation regime transitions in a large marginal sea

Marginal seas, as transitional zones, are closely connected to the open ocean and adjacent coastal systems. Their circulations often exhibit strong oscillatory behavior that shapes heat and salt transport, nutrient cycling, and regional ocean-atmosphere interactions. However, the characteristics and underlying dynamics of these oscillations remain insufficiently understood. Using the unique three-layer alternating circulation in the South China Sea as an example, we show that the system undergoes a pronounced regime transition from 1993-2008 to 2009-2018. This transition is closely linked to the phase change of the Pacific Decadal Oscillation. Specifically, upper-layer cyclonic circulation intensifies during the pre-2009 but weakens during the post-2009 period, while the middle-layer anticyclonic circulation exhibits the opposite pattern. In contrast, the deep-layer circulation strengthens substantially during the post-2009 period. These regime transitions arise from the interplay of surface wind forcing, the external exchanging current with the Pacific, and topographically modulated internal vertical coupling. The decadal trend of the upper-layer circulation is primarily wind-driven. The weakening of middle-layer circulation during pre-2009 is governed by pressure torque influenced by the upper-layer, whereas its post-2009 strengthening is attributed to vortex stretching associated with enhanced deep intrusion from the Pacific and a stronger deep-layer circulation. The findings clarify the oscillatory nature of South China Sea layered circulation under climate variability and highlight its role in regulating regional mass transport and ocean-atmosphere interaction.