Deep Hydrographic Fluctuations in the North-East Atlantic Mediterranean Outflow: Influence on Acoustic Propagation

Abstract

Some general characteristics of the mesoscale variability of the deep sound velocity profile in North-East Atlantic, due to the intrusion of Mediterranean Water through the Straight of Gibraltar are described, together with its influence on sound propagation.

In the Canarian basin the effect takes the form of isolated lenses (MEDDIES) of about 100 km diameter. In an area off the Portugal Coast and up to 60° N the Mediterranean Water extends as a deep layer, progressively extinguishing as going North and West.

In all cases, the presence of Mediterranean Water introduces a local maximum at about 1 000 m depth on the sound velocity profile. Recent measurement of the SHOM indicates that the value of sound velocity at the deep local maximum does present strong spatial oscillations.

From the acoustic point of view, the presence of a deep maximum divides the principal sound propagation channel into two new subchannels. It is shown that the principal characteristics of sound propagation in such a variable medium can be explained by the exchange of acoustic energy between the various channels resulting from the variations of the value of sound velocity at the maximum. Knowledge of the variations of this sound velocity value on a grid scale of about 25 km would be adequate for good deterministic predictions of sound propagation.

Some general rules are formulated to describe the effect of interchannel energy transfers on sound propagation as a function of the source and receiver depth and their horizontal position with respect to the “fronts”.

Several examples of sound field computations are given.