Observations of strong current shears in the deep ocean and some implications on sound rays

In the northern Sargasso Sea, recent continuous vertical profiles horizontal velocity and sound speed have revealed strong current shears of large horizontal extent and of inertial frequency in regions of low sound‐speed gradient. The influence of shear on ray paths is particularly evident in restricted regions in which the sound‐speed‐gradient is smaller than the current shear. Stability considerations pertaining to stratified shear flow suggest that shear refraction is most likely to be of importance when the ratio of the vertical gradient of sound speed (CZ) and the Brunt‐Vaisala frequency (N) is of order one. A simple model illustrates that rays emitted or vertexing within a layer of large shear, but small sound‐speed gradient, will be significantly refracted. Moreover, since the interaction is not isotropic, the reciprocity theorem does not apply and the effects of shear on ray paths are dependent on the direction of the ray with respect to the current.