Anatomies of oceanic fronts

Abstract

Oceanic fronts have attributes in common with atmospheric fronts: for example, strong horizontal gradients in temperature and/or salinity and strong horizontal velocity shears and convergences. Their forcing mechanisms include tidally induced mixing, the advection of plumes, atmospherically driven upwelling, and deformation fields in oceanic flows. Some fronts are confined to the surface or bottom turbulent boundary layers; others exist as internal features of the ocean. Some are thousands of kilometers long, extend to great depth, and are semipermanent; others occur over short horizontal and vertical distances for brief durations. Due to strong convergences often found at fronts, strong vertical velocities can be induced. Also, internal gravity waves are often intense near fronts, and the consequent shear instabilities induce turbulent mixing and fine structure. Thus fronts can be major foci for vertical transfers, including between the surface mixed layer and the ocean interior, especially in situations where a surface front is an outcrop of an oceanic pycnocline. Due to their convergences, fronts are biologically significant: They concentrate nutrients and phytoplankton, and, consequently, zooplankton, fish, marine mammals, marine birds, and probably “marine snow.” Hence, oceanic fronts can be expected to scatter, as well as refract, acoustic energy.