Interannual Coral Delta C-14 Records of Surface Water Exchange Across the Luzon Strait

The Luzon Strait (LS) hosts the largest transport of water between the Western Pacific Ocean (WPO) and the South China Sea (SCS). The transport through the strait, dominated by the westward propagation of the Kuroshio Intrusion, influences the climate and circulation of the SCS. While numerical models have investigated the interannual variability of the transport and subsequent water exchange across the LS, a lack of long-term on-site records prevents a general consensus on the transport rates, variability, and drivers. Corals offer high-resolution, continuous histories of radiocarbon (Delta C-14) content of the seawater dissolved inorganic carbon, allowing us to track changes in ocean transport and circulation through time. Seasonal and annual Delta C-14 samples from Houbihu, Taiwan, and Palaui, Philippines, located on either side of the strait, are compared to the Western Pacific Ocean and SCS Delta C-14 records to examine the spatial and temporal Delta C-14 variability in the region. We calculated the mean transport across the strait using a five-box mixing model and identified its potential drivers. The mean amount of water exchanged across the strait from 1970 to 1999 was 2.2Sv, ranging from -13.4 to 16Sv, where a positive (negative) value indicates net flow into (out of) the SCS. A weaker East Asian Winter Monsoon increases the contribution of the SCS outflow on the Kuroshio Intrusion-dominated LS, while the El Nino-Southern Oscillation primarily drives the intrusion into the SCS. These results provide support to the dominant control of El Nino-Southern Oscillation on the long-term ocean circulation variability in this region. Plain Language Summary The Luzon Strait is a globally critical ocean pathway that links the Western Pacific Ocean and the South China Sea (SCS). Heat, salt, and nutrients are exchanged across the strait, impacting marine ecology and climate in the region. In situ measurements of the surface water processes across the strait are limited, complicating our understanding of its variability and driving mechanism. Here, we present radiocarbon measurements, which represent a record of surface ocean processes including upwelling, extending beyond the instrumental record using two corals collected from either side of the strait, Houbihu, Taiwan, and Palaui, Philippines. Houbihu is impacted by the strength of the East Asian Winter Monsoon winds, affecting the transport of surface waters into and out of the SCS. Palaui is sensitive to surface transports linked to the shifts of the North Equatorial Current bifurcation latitude, primarily influenced by the El Nino-Southern Oscillation. While different processes impact our sites, the difference in the amount of radiocarbon between Houbihu and Palaui and the surrounding seas allows us to further investigate changes to water exchange across the strait. We find that a weaker East Asian Winter Monsoon impedes water inflow into the SCS whereas El Nino-Southern Oscillation facilitates inflow into the SCS.