Altimetry observations reveal that semi-diurnal internal tides (SDITs) generated at the Mariana Arc in the eastern Philippine Sea (EPS) concentrate at a focal point (17°N, 140°E). This study investigates the variability of SDITs resulting from their interaction with background currents. We utilize in-situ round-trip acoustic echo time measurements between the seafloor and the sea surface, obtained from six pressure-recording inverted echo sounders (PIES) deployed along three latitudes across the SDIT convergence region over approximately 1.5 years (2020–2021). The observational region is located on the southern flank of the north Pacific subtropical gyre, where zonal flow is dominant, including the westward North Equatorial Current (NEC) and the eastward Subtropical Countercurrent (STCC). We find that the SDIT energy flux observed at the focal point exhibits a strong positive correlation with the zonal velocity in the PIES region. Ray-tracing analyses further show that SDIT ray paths vary with changes in zonal flow. In particular, the SDIT focusing weakens when the westward flow, which corresponds to the direction of SDIT propagation, becomes dominant. Variations in zonal velocity are closely linked to meridional shifts of the NEC. During positive phases of the Pacific Decadal Oscillation (PDO), the northward migration of the NEC leads to enhanced westward flow in the PIES region. These results demonstrate that SDIT dynamics are strongly modulated by background currents and underscore the necessity of incorporating realistic, time-varying flow fields into internal tide models.

semi-diurnal internal tides,north Pacific subtropical gyre,pressure-recording inverted echo sounders,pacific decadal oscillation (PDO)