Oceanic fronts and their effects on underwater acoustics

Abstract

Oceanic fronts are dynamical phenomena with many properties in common with atmospheric fronts. Fronts are the locus of intense horizontal gradients in one or more properties. Oceanic fronts are generated by atmospheric forcing (momentum, heat, and mass transfer), river runoff, and differential advection and mixing. Mixing is intense in fronts; thus, persistent fronts must be maintained through convergent flows. Typically, fronts have strong thermal and haline gradients. Depending upon the relative signs of these gradients, thermohaline fronts are “density-reinforced fronts” (or, simply “density fronts”) or “density-compensated fronts.” These two classes of fronts differ in their dynamics and, thus, flow fields. Sound speed fronts accompany thermal fronts, which leads to strong acoustic refraction across oceanic fronts. Because of the convergent flow, organic and inorganic particles are concentrated at fronts, leading to aggregation of phytoplankton, zooplankton, fish, and marine mammals and birds. Hence, frontal zones are rich in biological scatterers and noise. The microstructure of fluid velocity, temperature, salinity, and thus, sound velocity is intense in frontal zones, which also enhances volume scattering. Due to gravity wave refraction and breaking by frontal flows, marked sea state variations occur across fronts, producing inhomogeneities in surface scattering and ambient noise.