Acoustic propagation in the western Greenland Sea frontal zone

Abstract

A study, utilizing numerical experiments, of the environmental acoustic effects of the western Greenland Sea Frontal Zone in the summer of 1983 along a 185-km west to east transect is presented. The transect begins at the edge of the marginal ice zone in the East Greenland Current, then crosses the East Greenland Polar Front and terminates in the water of the Greenland Sea gyre. The transect contains several mesoscale eddylike features and a sharp front between Return Atlantic Intermediate Water and Greenland Arctic Intermediate Water, whose effects on an acoustic field have not been previously reported. This front can impart >15-dB increase in loss in <10 km. The effects of the frontal zone on acoustic propagation as a function of location and depth of the acoustic source and depth of the receiver for a nominal frequency of 50 Hz are discussed. Significant differences, sometimes greater than 20 dB, in the acoustic field were observed and are explained in the analysis for variations in any of the source and receiver parameters. The sound-speed structure on either side of the frontal zone makes propagation loss and the dominant acoustic modes very dependent on direction and location. The direction of propagation with the lowest loss (as much as 15-dB difference along the entire transect) can change with a modest shift in source from a 10-m depth to 150 m. As compared to the nonfrontal environment, propagation in the presence of the frontal zone, for any of the source and receiver depths studied, can result in changes in the levels and distribution of energy in the acoustic field by as much as 15 dB as well as changes in the mode of propagation (e.g., refracted/surface reflected to SOFAR). It can also result in an increase (>10 dB) in the sensitivity of propagation loss to changes in receiver depth (10–150 m) compared to the nonfrontal zone environment. A significant finding is that a longer transmission range (93 km) through the waters of a frontal zone can reduce the effects of crossing the front itself by as much as 15 dB. It was also observed that 75 km of upslope propagation that included two oceanic fronts was unaffected by a decrease in bottom depth of 2000 m within the 75-km range.