Warm Eddy Effects on the Refraction of Diurnal Internal Tides in the Northwestern Pacific From PIES Observations

Abstract A tidal model based on altimeter observations reveals that first‐mode diurnal internal tides (DITs) propagate approximately 2,100 km eastward from the Luzon Strait (LS) into the Pacific Ocean. As they radiate over long distances, the DITs refract equatorward due to the beta effect. In this study, we utilize in situ round‐trip acoustic echo time measurements between the seafloor and the sea surface, obtained from an array of 10 pressure‐recording inverted echo sounders (PIES), to investigate the variability of DITs in the eastern Philippine Sea (EPS). The observations conducted over 1‐year and 1.5‐year periods during 2020–2021 reveal a clear weakening of DIT amplitudes in summer, in contrast to the barotropic diurnal tides, which show maximum spring tide amplitudes at the solstices and minimum amplitudes at the equinoxes. The observed seasonal variation in DIT energy flux shows a significant correlation with the relative vorticity averaged over regions of energetic warm eddies. Ray‐tracing using HYCOM ocean model outputs indicates that the warm eddies in the upstream region of the ray path during summer (July to September) enhance the equatorward refraction of DITs. This study suggests that the superposition of the K 1 and P 1 constituents induces a pronounced semi‐annual cycle in the DITs, even over considerable propagation distances. In addition, warm eddies exert a substantial influence on the DIT propagation path. Our results imply that the pronounced temporal variability of DITs should be considered to improve the parameterization of internal‐wave‐induced ocean mixing in oceanic and climate models.