Sound speed, submesoscale, and spice: The role of rapidly evolving instabilities in setting upper ocean properties

Abstract

Sound speed characteristics in the upper ocean are sometimes thought to be set by a combination of mostly one-dimensional vertical mixing processes and (slow) mesoscale advection and stirring. However, a series of recent projects have showcased the role of small-scale (tens of meters to tens of km horizontally), rapidly evolving (hours to days) submesoscale instabilities in setting both stratification and sound speed properties. The observations also reveal that sound speed structure set by such processes is fundamentally three dimensional, with significant lateral gradients at a broad range of scales. Some relevant processes are frontogenetic and create sometimes rapid secondary circulations near lateral density gradients. Some are associated with wind-driven near-inertial oscillations or internal waves. Some of the more interesting recent observations concern the intersection of submesoscale and internal wave processes. Here, I will review a few highlights from the last decade of observations on unbalanced, nonlinear processes that conspire to set density, spice, and sound speed profiles in several contrasting oceans.