Towards a process description of acoustic uncertainty in the upper ocean

The upper ocean is a region that hosts the worlds largest biomass, is the boundary layer mediating the exchange of momentum, energy, heat, gasses, etc. between the air and sea, and it is a region of critical Naval tactical importance. Acoustic modalities are important tools for sensing this region as evidenced by the ground-breaking results of bio-acousticans monitoring whales, fish, zooplankton, and other marine organisms. But while the significance of upper ocean acoustics has been long appreciated, the understanding of key ocean acoustic processes is lacking. Here, we define upper ocean as the region of the water column where the impacts of winter mixing are evident, usually two to three times the depth of the maximum mixed layer. The classical view of the thermohaline structure posits a combination of mostly one-dimensional mixing processes, slow mesoscale stirring and advection, and subsequent subduction towards the main thermocline. However, a number of recent studies points to an important role played by three-dimensional, small-scale, rapidly evolving, submesoscale fronts and their potential interaction with internal waves. A few observational highlights will be reviewed with an emphasis on sound-speed and spice distributions, and implications for deterministic and stochastic sound propagation effects will be discussed.