Tidal effects on long‐range sound transmission

Abstract

Time series of acoustic travel times over long distances have significant tidal components. We discuss three experiments: First, for a 900 km westward transmission from a moored source southwest of Bermuda, the M2 constituent (apart from mooring motion) of 8 ms can be accounted for by a variable propagation with and against the 2 cm/s barotropic tidal current. The situation is different for the K1 and S2 constituents (which have a significant solar component), particularly for rays with shallow turning points, suggesting an effect from a minute daily variation in the temperature of the upper 100 m. Second, for a 500 km transmission from the Bahamas northward and parallel to the continental margin, the measured M2 constituent is not in phase with the computed effect of the barotropic tidal current, unlike the westward transmission mentioned above. Here the mode conversion from barotropic to baroclinic tides plays a crucial role: the principal cause of the travel time oscillations in quadrature with the tidal current is a vertical oscillation by several meters (and attendant temperature and sound speed fluctuations) associated with the baroclinic tide just seaward of the continental margin, in the near‐zone of mode conversion. Third, for 300 km transmissions from a moored source northwest of Bermuda to both a moored receiver and a landbased receiver, the tidal components in the travel time are dominated by tidally induced mooring motions. The tidal amplitude varies in time as the tidal mooring motion is modulated by the variable current (because of the nonlinear drag), and the current can be so estimated. An important part, possibly a predominant part, of the (nontidal) travel time variation of 0.7 s is evidently associated with the mean mooring motion.—The interpretation of the tidal fluctuations in travel time is varied and complex.