Revisiting the Dynamics of Interannual Variations of the Upper Luzon Strait Transport

Abstract The Luzon Strait Transport (LST) plays an important role in the heat and salt budgets, circulation variations, eddy generation, and biogeochemical cycles in the South China Sea (SCS). Due to lack of long-term observations, the LST interannual variations and the underlying dynamics remain ambiguous. Using satellite altimeter data, the long-term upper LST (upper 500 m) over the past 30 years (1993–2022) was first estimated in this study. The mean value of the LST is estimated at 4.18 Sv (1 Sv ≡ 10 6 m 3 s −1 ) with a standard deviation of 1.03 Sv at the interannual time scale. The LST interannual variations are not correlated with the upstream Kuroshio changes against the well-known “teapot effect” adopted by most previous studies but well correlated with the migration of the North Equatorial Current bifurcation ( Y b ) and wind stress curl anomaly (WSCa) east of the Philippines. It is found that positive/negative WSCa in the off-equatorial region associated with westerly/easterly wind anomalies in the western tropical Pacific could induce westward upwelling/downwelling Rossby waves, corresponding to negative/positive sea level anomalies (SLAs). When approaching the eastern coast of the Philippines, negative/positive SLAs were transferred along the Philippines coast as coastal Kelvin waves through the Mindoro–Sibutu pathway. Thus, cyclonic/anticyclonic circulation anomaly formed to the east and around the Philippines, resulting in strong/weak LST. Using the time-dependent island rule theory, the LST interannual variations driven by large-scale and regional wind were quantified. This dynamic framework can interpret approximately two-thirds of the LST interannual variations, providing new insights into the dynamics of low-frequency variations of the Kuroshio intrusion into the SCS. Significance Statement The strong Kuroshio intrudes into the South China Sea (SCS) through the Luzon Strait. The LST plays an important role in the physical oceanography and biogeochemical cycles in the SCS. Due to lack of long-term observations, dynamics on the interannual variations of the LST remain ambiguous. In this study, we first calculated long-term LST over the recent three decades. Using the time-dependent island rule theory, interannual variations of the LST were quantified by linking large-scale wind and oceanic waves (Rossby and Kelvin waves). The detailed dynamical processes controlling the LST interannual variations are illustrated. This dynamic framework can interpret most of the LST interannual variations, providing new insights on the dynamics of the western boundary current leaping across a gap.