Incoherent signatures of internal tides in the Tokara Strait modulated by the Kuroshio

Internal tides are ubiquitously generated from disturbances between barotropic tides and bottom topography in stratified oceans. Incoherence is routinely considered as non-stationary phase discrepancy between internal and astronomical tides. Internal tides, especially incoherent constituents, contribute substantial energy to dissipation and turbulent mixing. Using similar to 8-month near-full-depth moored current observations, we investigated internal tides in the Tokara Strait, where the Kuroshio exits the East China Sea. Semidiurnal internal tides, especially the M2 internal tides, dominate the diurnal internal tides by approximately one-order of magnitude. We decomposed the bandpass-filtered tidal currents into coherent and incoherent constituents, both of which presented a spring-neap cycle; however, the former was largely coupled with local barotropic tides, while the latter had an incoherent phase. The coherent semidiurnal internal tides were intensified and had larger variability near the bottom; nevertheless, the intensity and variability of incoherent internal tides decreased with depth. These results suggest that the incoherent internal tides are exogenous and are possibly affected by the background flow field during propagation. The mean incoherence (above 54 % for both semidiurnal and diurnal internal tides) was expectedly higher than the global mean (similar to 44 %), implying a robust effect of nonlinear Kuroshio-tide interactions. Moreover, the relationship between the variability of the incoherence and the north-south shift of the Kuroshio further revealed the impact of the background flow intensity variation on internal tides. A ray tracing model demonstrated different refraction indices during the propagation of internal tides, leading to converged (diverged) ray paths in the mooring when the Kuroshio was at the northern (southern) axis position, thus explaining the differential incoherence caused by the variability of the Kuroshio.