Characterizing the Role of Non‐Linear Interactions in the Transition to Submesoscale Dynamics at a Dense Filament.

Ocean dynamics at the submesoscale play a key role in mediating upper‐ocean energy dissipation and dispersion of tracers. Observations of ocean currents from synoptic mesoscale surveys at submesoscale resolution (250 m–100 km) from a novel airborne instrument (MASS DoppVis) reveal that the kinetic energy spectrum in the California Current System is nearly continuous from 100 km to sub‐kilometer scales, with a k−2 spectral slope. Although there is not a transition in the kinetic energy spectral slope, there is a transition in the dynamics to non‐linear ageostrophic interactions at scales of O $\mathcal{O}$(1 km). Kinetic energy transfer across spatial scales is enabled by interactions between the rotational and divergent components of the flow field at the submesoscale. Kinetic energy flux is patchy and localized at submesoscale fronts. Kinetic energy is transferred both downscale and upscale from 1 km in the observations of a cold filament. Plain Language Summary: Ocean dynamics at scales of 100 m–10 km, called the submesoscale, are important because they are associated with large velocity gradients and non‐linear interactions. Large gradients lead to vertical velocity, which facilitates ocean‐atmosphere interactions and ocean biological processes. Velocity gradients and non‐linear processes combine to transfer kinetic energy from the large‐scale flow to small‐scale perturbations. This can lead to instabilities that dissipate energy in the ocean surface layer (rather than the seafloor). Here we analyze novel observations that provide insight into ocean dynamics through the distributions of velocity gradients and energy transfer at 1 km scale. Dynamics at these scales have previously been modeled, but have not been observed directly. We observe a transition where non‐linear dynamics become more important at scales of order 10 km. We also introduce new interpretations of spectral analysis (analysis of energy and correlations across scales). Moreover, we analyze covariance of velocity gradient quantities and flow energetics to demonstrate that energy flux is episodic and localized at fronts. Together, these observations demonstrate that fronts play an important role in boundary‐layer kinetic energy processes and highlight the evolution of upwelling filaments. Key Points: Remote sensing observations reveal a kinetic energy spectrum with a continuous slope from 100 to 1 km in an eastern boundary regionBetween 1 and 10 km, ageostrophic non‐linear interactions become dynamically importantCross‐scale kinetic energy transfers computed from 2D velocity observations are associated with shear strain in the observed front [ABSTRACT FROM AUTHOR]