Submesoscale Dynamics in the Upper Ocean

Oceanic motions with spatial scales of 200 m–20 km, called submesoscales, are ubiquitous in the upper ocean and serve as a key intermediary between larger-scale balanced dynamics and unbalanced turbulence. Here, we introduce the fluid dynamics of submesoscales and contrast them with motions at larger and smaller scales. We summarize the various ways in which submesoscales develop due to instabilities that extract potential or kinetic energy from larger-scale balanced currents; some instabilities have counterparts at larger scales, while others are distinct to the submesoscale regime. Submesoscales modify the density stratification in the upper ocean and redistribute energy between scales. These energy transfers are complex, having both up-scale and down-scale components. Submesoscale eddies and fronts also contribute to a spatially heterogeneous distribution of shear and restratification that leave an imprint on upper ocean turbulence. The impact of submesoscales on the Earth's climate remains an exciting frontier. Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 55 is January 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.