Acoustic impact of upper ocean models

Abstract

The upper mixed-surface layer of the ocean, with its homogeneous vertical temperature profile, provides a somewhat special acoustic propagation environment where the sound speed will increase (due to pressure) down to the bottom of the mixed layer, and then drop off due to temperature decrease until it reaches the minimum at the main sound channel. Above the velocity maximum at the bottom of the mixed layer, a surface 'duct' will develop, in which for all frequencies above a cut-off the acoustic energy will be trapped mostly near the surface, inside the mixed layer. As an alternative to direct measurement of the mixed-layer profile, so-called upper ocean or mixed-layer models (MLM's) have been developed. The output of these mixed-layer models then becomes the input for an acoustic model that

predicts the transmission loss.

In this memorandum, we examine this two-step modelling process with the

objective of understanding what parameters must be accurately predicted by the MLM. We also consider the improvements that result compared to a simpler alternative (climatology) and, finally, the differences between particular mixed-layer models. The differences are considered not simply in terms of mixed-layer model accuracy, but in terms of the acoustic impact, which is the ultimate objective of the process.