Azimuthal variation of low-frequency acoustic propagation through asymmetric Gulf Stream eddies

Abstract

A numerical modeling study of the azimuthal variation of 25-Hz acoustic propagation through radially asymmetric Gulf Stream eddies is presented. Both a cold-core (cyclonic) and a warm-core (anticyclonic) eddy, which are quasi-elliptical in shape, are investigated. Sources located both inside and outside the eddies are considered with transmission out to 240 km. The source is at a depth of 150 m where there are substantial environmental variations due to eddies. Receivers are at depths of 100, 150, and 300 m. The dimensions of the first four convergence zones (CZ) are utilized as indicators of the azimuthal variation of propagation. Propagation conditions assumed an absorbing bottom and no azimuthal coupling of energy. Sensitivity to change in azimuth is dependent upon the orientation of the eddy relative to the direction of transmission. It is increased for transmission paths parallel to the major axis relative to those parallel to the minor axis. For propagation parallel to the major axis, a 15° change in azimuth can result in a change of as much as 13 km in width and 15 km in range for the third CZ. An irregularity in the shape of a quasi-elliptical eddy is shown to effect an appreciable difference in transmission. A 30° shift in the direction of propagation to the right side of the major axis of a cold-core eddy produced a 20% change in the width of the third CZ, while a 30° shift to the left side of the axis gave a 64% change in the width. Sensitivity can depend upon the direction of propagation along a major axis for a source located within the eddy. Propagation in one direction can produce a sensitivity for a 20° change in azimuth of more than double the shift in range to the third CZ, as the opposite (180°) direction yielded over a 75° change in azimuth.