On the evolution & equilibration of submesoscale fronts

This dissertation studies regions of the upper ocean where the density drastically changes across length-scales of 0.1 to 10 kilometres due to variations in salinity or temperature. We focus on a specific process called Symmetric Instability and in particular how it influences ocean fronts. Symmetric Instability can efficiently transform energy stored in the currents and density structure of the ocean into turbulence. We design high-performance computational simulations to explore how different factors, such as the strength and width of these density variations, affect this process. These parameters emulate a wide range of ocean fronts, from the Gulf Stream to river plumes to transient fronts in the open ocean. We find that the front strength and width impact how quickly fronts readjust and also what they look like once they reach an equilibrium. However, many global models of the ocean do not have sufficient spatial resolution to represent Symmetric Instability, and instead simply diffuse (numerically or explicitly) away any kinetic or potential energy that builds up at small scales. By doing so, these ocean models do not capture the enhanced vertical transfer of heat, carbon, nutrients, and biota near fronts, nor their cycling or sequestration within earth system and climate models. We therefore propose how these emergent properties of fronts may be incorporated into future front-aware parameterisations of unresolved scales in the upper ocean.