Acoustic effects of a Northeast Pacific Ocean warm-core eddy

Abstract

A persistent large (200-km-diam) warm core eddy has been reported [S. Tabata, J. Phys. Oceanogr. 12, 1260–1282)] in the Northeast Pacific Ocean off Sitka, Alaska (centered at 57° N Lat., 139° W. Long.). Not associated with a strong boundary current, this eddy differs from a previously observed South Pacific warm-core eddy [P. Scully-Power et al., J. Acoust. Soc. Am. 63, 1381–1388(1978)]. It appears to be formed from an influx of warm water at 100–500 m which moves northward along the North American Coast. At these high latitudes the surrounding water mass is relatively cold with a deep sound channel axis at 200 m and a secondary channel axis at 100 m. It was found, as in other warm core eddys, that there is a broadening of the secondary sound channel. For this eddy there is also a significant change in the depth of the deep sound channel axis (400 m at center of eddy, 200 m outside the eddy). An analysis was conducted using a range dependent propagation modeling program for a sound speed cross-section of the eddy. Results which show the relative effect on sound propagation are given for various source and receiver depths. Computations are also made of the frequency dependence of both the deep and secondary sound channels and their interdependence as suggested by Hall [M. Hall, J. Acoust. Soc. Am. 66, 1102–1107 (1979)].