Semidiurnal variation of the ocean sound velocity associated with the M 2 internal tide

Ocean sound velocity significantly varies at tidal frequency in not only shallow but also deep pert. Unexpectedly large semidiurnal fluctuation of ocean-acoustic waves (T-waves), which propagate through the SOFAR channel (= the low velocity channel in the ocean), is found on the ocean bottom seismometer records for the submarine volcanic swarm in Mariana. The amplitude is one order larger than any previous artificial experiments. Here we report the first in situ evidence that T-wave travel time provides information about vertical movement of seawater due to internal tides. Numerical 3-D modeling shows the internal tide excited by external tidal forcing is particularly large along the IzuBonin-Mariana Ridge because of rough topography. A semidiurnal up-and-down movement associated with the internal tide causes an undulation of the SOFAR channel on the order of 100 m, which causes T-wave travel time variations consistent with the observed ones. Generation of internal tides is an important sink of the external tidal energy so that accurate estimate of conversion of the external to internal tides is essential to close the global tidal energy budget and to understand the Earth-Moon system evolution.