Spectra of acoustic travel time at periods shorter than 33 h at basin‐scales in the North Pacific Ocean: Horizontal diffusion of upper turning points.

Abstract

Previously unexplained fluctuations of acoustic travel time (133 Hz, 0.06 s resolution, 3709 km transmission) at periods between 3.3 and 10 h are likely due to a standard spectrum of internal gravity waves even though the travel times of features arriving 3 to 4 s apart change travel times by nearly the same amount at the same time. This paradox is resolved by appreciating that nearly all acoustic arrivals consist of tens to hundreds of temporally‐unresolved arrivals whose upper turning points diffuse horizontally between the normal 50‐km periodicity of paths. Similar behavior is obtained from numerical calculations with fluctuations based only on internal waves. The horizontal diffusion of acoustic turning points appears to also explain the presence of non‐stationary oscillations at periods up to 33 h. A simple algebraic calculation shows that these longer oscillation periods with amplitudes of about 5 ms are probably due to inertial currents generated by surface winds. If these oscillations are due to i...