Refraction of sound waves at polar latitude

Abstract

Horizontal refraction in the ocean sound channel is a function of the acoustic mode number and frequency (chromatic aberration), and may lead to wide separations of long‐range transmission paths. We consider the 1960 antipodal transmission from Perth, Australia, to Bermuda. The path has a southernmost point in the Indian Ocean, which depends sensitively on horizontal refraction associated with the north‐to‐south shoaling of the sound axis across the Antarctic Circumpolar Current. (Refraction by the current velocity is relatively small.) This southernmost point is at about 40°S for low modes of relatively high frequency, and at about 50°S for high modes of low frequency.

Bermuda shadow

“…We prescribe the adiabatic approximation, that is to say, that the transition in the axis depth of the sound channel with latitude is sufficiently gradual that no significant interaction occurs between different modes and different frequencies: the energy within a narrow frequency band of a fixed mode is conserved. This model is equivalent to an action-conserving ray model (Munk, 1991). In practice the adiabatic approximation implies a small fractional change in the channel over one ray loop span (order 50 km), and this condition is not grossly violated.…”