Modulation of the Pacific Decadal Oscillation on the Weakening of the South China Sea Winter Cold Tongue During the Mature Phase of El Niño

Extensive research has demonstrated that during the mature phase of El Niño, the anomalous southerly wind induced by the western North Pacific anomalous anticyclone (WNPAC) weakens the South China Sea winter cold tongue (SCSCT). This study further examines the decadal effects of the Pacific Decadal Oscillation (PDO) on the weakening of the SCSCT during the mature phase of El Niño. Results suggest that, compared to the positive PDO phase, the weakening of the SCSCT was more pronounced during the negative PDO phase. Upper mixed layer heat budget analysis unveils that the net surface heat flux term dominated the abnormal warming in the northern part of the cold tongue, while the abnormal warming in the southern part is ascribed to both the horizontal heat advection and net surface heat flux terms. The differences between the negative and positive PDO phases (N-P) were that cooler (warmer) sea surface temperature anomalies (SSTAs) in the tropical eastern (central) Pacific induced weaker (stronger) WNPAC (Southeastern Tropical Indian Ocean anomalous anticyclone [SEIOAC]) through anomalous Walker circulation. Subsequently, for the N-P, the anomalous Hadley circulation between the SEIO and the SCS led to cross-equatorial southerly wind anomalies, which were the fundamental cause of the anomalous warming of the SCSCT. Combining the effects of both the tropical Indian Ocean and Pacific Ocean to explain the decadal differences in the interannual relationship between El Niño–Southern Oscillation (ENSO) and the SCSCT, this work provides new insights for predicting the climate variability in East and Southeast Asia.