The southward winter coastal current (SWCC) along southeast Vietnam is a notable seasonal characteristic of the South China Sea (SCS) cyclonic circulation in boreal winter (December–February, DJF). The SWCC is driven by the northeast monsoon and regulated by the El Niño–Southern Oscillation (ENSO). Over three decades, La Niña winters (e.g., 1998–99, 2010–11, and 2020–21) strengthen northeasterly anomalies, enhancing SWCC under this cold-phase climate condition. However, the recent cold-phase SWCC in January 2021, accompanied by the beginning of negative Pacific Decadal Oscillation (PDO) regime, exhibited anomalous intensification (+2.5 m⋅s⁻¹ northeasterly anomaly, 1.3 times greater than the climatology). How did this impact SCS carbon dioxide (CO₂) budget, a CO₂ outgassing hotspot (+0.5 mol C⋅m⁻²⋅yr⁻¹⋅decade⁻¹ from 1982 to 2020). Using reanalysis and satellite datasets, results show that the intensified 2020–21 cold-phase SWCC was also modulated by a northeast-to-southwest propagating mesoscale cyclonic eddy (SWCE) along its eastern flank. The SWCE’s persistent southwestward flow on its western flank enhanced the 2020–21 cold-phase SWCC by considerably 0.11 m⋅s⁻¹ stronger than the January climatology in mid-January. Meanwhile, this increased CO₂ uptake by 2.70 mol C⋅m⁻²⋅yr⁻¹, 2.5 times greater than the climatology, as downwelling suppresses oceanic dissolved inorganic carbon (DIC) at surface. These findings highlight oceanic modulation of the 2020–21 cold-phase SWCC, advancing ocean-atmosphere carbon cycle understanding and the United Nations 2030 Sustainable Development Goal 13 (Climate Action).