The sound field disturbance caused by a Mediterranean salt lens

Abstract

Sub-mesoscale coherent vortices of Mediterranean water called meddies can be found in the main sound channel of the Canary Basin. Their positive temperature and salinity anomalies create a +16 m s-1 sound speed aberration at 30oN. The meddy splits the sound channel into a thermal sound channel and a deeper hydrostatic channel. Examination of historical data reveals that the sound speed anomaly caused by the meddy is never large enough to exceed the limiting sound speed of the background main channel. The sound field generated by sources at 1000 Hz and 4 Hz were examined for seasonal and geometric variation in both non-perturbed (meddy absent), and perturbed environments. The presence of a meddy had negligible effect on a shallow source in summer and only a weak effect in winter. Sound fields generated by sources at 500, 1000 and 2500 m experienced large (up to 12 dB) variations in transmission loss. It was found that the meddy removed energy from the convergence zones and redistributed it into the shadow zones within the SOFAR channel. Strong modal coupling caused by large horizontal sound speed gradients at the meddy edge for frequencies above 6 Hz caused ray path instability and created a complex arrival pattern.