The South China Sea Throughflow (SCSTF) serves as a critical oceanic conveyor belt for heat and freshwater, mediating water exchanges between the South China Sea (SCS) and the Pacific and Indian Oceans while regulating key processes such as heat and salt budgets, eddy activities, and marine biogeochemical cycles. It also plays a pivotal role in modulating the Indonesian Throughflow (ITF) and shaping climate variability across the Indo-Pacific. However, long-term direct observational data of the SCSTF have long been scarce, leaving its long-term changes under climate change and associated driving mechanisms poorly understood.
To address this problem, a research team from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) reconstructed the record of the SCSTF's volume transport from 1894 to 2022. The study employed oxygen isotope (δ¹⁸O) records from a Porites lobata coral core collected off Palaui Island in the northeastern Philippines—a region directly influenced by the Kuroshio Current, the primary inflow of the SCSTF. Complemented by satellite altimeter observations spanning 1993 to 2022, the coral proxy data yielded a continuous monthly record of Luzon Strait Transport (LST)—the volume of water flowing through the Luzon Strait, the main gateway between the Pacific Ocean and the SCS, and a key measure of SCSTF strength.
The results uncovered distinct patterns in SCSTF variability: in addition to well-documented interannual and decadal fluctuations associated with the El Niño–Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), the SCSTF displayed a strong long-term decreasing trend of −0.14 ± 0.02 Sv per decade. With a mean volume transport of 5.15 Sv over the period, total transport decreased by 1.81 ± 0.26 Sv over the 129—year period—a decline of more than one-third.
Applying the Time-Dependent Island Rule (TDIR) theory, the researchers identified the main driver of this slowdown—the strengthening of trade winds in the tropical western Pacific over the past century. Under global warming, the tropical Pacific has shifted toward Cold Tongue Mode (CTM), characterized by warming in the western Pacific and cooling in the eastern equatorial Pacific. This thermal pattern has intensified western Pacific trade winds, generating negative wind stress curl anomalies in the off-equatorial region. These anomalies trigger downwelling Rossby waves and positive sea-level anomalies east of the Philippines, ultimately forming an anticyclonic circulation anomaly that reduces Kuroshio intrusion into the SCS and weakens the SCSTF.
The researchers also highlighted important questions for future research. The 35% reduction in transport has increased the SCS water renewal time from 2.6 to 3.5 decades, which could alter the basin's thermocline and halocline structures and reshape air–sea interactions related to the East Asian monsoon. The weakening Kuroshio intrusion has already been linked to decreasing chlorophyll-a concentrations and phytoplankton growth in the SCS, threatening marine productivity and fish habitats. Furthermore, the slowdown of the SCSTF may act as a compensatory mechanism for the strengthening ITF, potentially restructuring inter-basin ocean circulation in the Indo-Pacific.
"This study provides the first centennial-scale reconstruction of the SCSTF, establishing a critical baseline for understanding its past changes and predicting future behavior," said NAN Feng, lead author of the study.
Schematic illustration of the dynamics on the SCSTF slowdown driven by the wind variations. (Image by IOCAS)
(Text by NAN Feng)
Media Contact:
ZHANG Yiyi
Institute of Oceanology
E-mail: zhangyiyi@qdio.ac.cn
(Editor: ZHANG Yiyi)