A Shift of Multidecadal Trend in Coastal Upwelling Off Southern Vietnam in the South China Sea Around 1999

To explore multidecadal trends of the Vietnamese Southern Coastal Upwelling (SCU), we employed an SCU index (Southern Coastal Upwelling index [SCUI]), defined as the difference between basin‐wide SST () and local minimum SST (). During 1982–2024, SCUI shifted markedly around 1999, weakening before 1999 (B99) and intensifying thereafter (A99). Empirical Orthogonal Function (EOF) analyses identify two key dynamical processes: (a) a persistent La Niña–like SST pattern accompanied by intensified Pacific trade winds, enhancing northern South China Sea (SCS) anomalous anticyclonic circulation and reducing SCS Throughflow; and (b) multidecadal variability in interhemispheric thermal contrast, resulting in cross‐equatorial winds. Under their joint influence, the B99 weakening was dominated by a reduced southwesterly summer monsoon affecting regulated by trade winds and cross‐equatorial winds, while the A99 intensification was attributable to warming induced by enhanced northern SCS negative wind stress (WS) curl anomaly and weakened SCS Throughflow rather than enhanced local upwelling itself.