Submesoscale processes and dynamics

Abstract

Increased spatial resolution in recent observations and modeling has revealed a richness of structure and processes on lateral scales of a kilometer in the upper ocean. Processes at this scale, termed submesoscale, are distinguished by order-one (O(1)) Rossby and Richardson numbers; their dynamics are distinct from those of the largely quasi-geostrophic mesoscale as well as fully three-dimensional, smallscale processes. Submesoscale processes make an important contribution to the vertical flux of mass, buoyancy, and tracers in the upper ocean. They flux potential vorticity through the mixed layer, enhance communication between the pycnocline and surface, and play a crucial role in changing the upper-ocean stratification and mixed-layer structure on a timescale of days. In this review, we present a synthesis of upper-ocean submesoscale processes that arise in the presence of lateral buoyancy gradients. We describe their generation through frontogenesis, unforced instabilities, and forced motions due to buoyancy loss or down-front winds. Using the semi-geostrophic (SG) framework, we present physical arguments to help interpret several key aspects of submesoscale flows. These include the development of narrow elongated regions with O(1) Rossby and Richardson numbers through frontogenesis, intense vertical velocities with a downward bias at these sites, and secondary circulations that redistribute buoyancy to stratify the mixed layer. We review some of the first parameterizations for submesoscale processes that attempt to capture their contribution to, first, vertical buoyancy fluxes and restratification by mixed-layer instabilities and, second, the exchange of potential vorticity between the wind- and buoyancy-forced surface mixed layer, and pycnocline. Submesoscale processes are emerging as vital for the transport of biogeochemical properties, for degenerating spatial heterogeneity that is critical for biogeochemical processes and mixing, and for the transfer of energy from meso- to small scales. Several studies are in progress to model, measure, analyze, understand, and parameterize these motions.