Seasonal variability of the magnitude and structure of semidiurnal internal tides radiated from the Mariana Arc was examined based on year-long mooring observations. Internal tidal currents, energy density, and energy flux at the focal point of the Mariana Arc were weakest in summer, but typically comparable in the other three seasons. Mode-1 internal tides dominate over the observed period, except in summer when Mode-2 explains a higher proportion. The coherent variance accounts for about 60% of the total semidiurnal motions, with a reduced proportion in summer and winter, generally consistent with the seasonal trend of the barotropic tides. In contrast, the incoherent internal tides show a close temporal consistency with the low-frequency flows. During summer, the background currents are predominantly influenced by energetic mesoscale eddies, whereas the North Equatorial Current (NEC) system dominates in the other seasons. Internal tides influenced by eddies exhibit more standing wave characteristics, higher modal structures, and more incoherent properties compared to those modulated by the NEC. Internal tides are more progressive and propagate outward more easily when modulated by the NEC, whereas those modulated by eddies are more unstable and prone to nearby dissipation, resulting in a significant summer weakening at the Mariana Arc focus.

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