Mooring and glider observations and a high‐resolution satellite sea surface temperature image reveal features of a transient submesoscale front in a typical mid‐ocean region of the Northeast Atlantic. Analysis of the observations suggests that the front is forced by downfront winds and undergoes symmetric instability, resulting in elevated upper‐ocean kinetic energy, re‐stratification and turbulent dissipation. The instability is triggered as downfront winds act on weak upper‐ocean vertical stratification and strong lateral stratification produced by mesoscale frontogenesis. The instability's estimated rate of kinetic energy extraction from the front accounts for the difference between the measured rate of turbulent dissipation and the predicted contribution from one‐dimensional scalings of buoyancy‐ and wind‐driven turbulence, indicating that the instability underpins the enhanced dissipation. These results provide direct evidence of the occurrence of symmetric instability in a quiescent open‐ocean environment, and highlight the need to represent the instability's re‐stratification and dissipative effects in climate‐scale ocean models.

Plain Language Summary

Oceanic submesoscale flows, with typical spatial scales of 1 km, are key to providing a dynamical route from energetic mesoscale eddies (10‐100 km) to turbulent microscales (~1 cm). A submesoscale phenomenon thought to draw kinetic energy from mesoscale currents and transfer it to turbulent dissipative processes is symmetric instability. This mechanism has been abundantly documented in strong and persistent ocean fronts such as those associated with western boundary currents, but its occurrence and impacts in the more extensive, quiescent mid‐ocean regions remain little explored. In this work, we present rare observational evidence of symmetric instability at a transient front in a mid‐ocean area of the Northeast Atlantic, founded on high‐resolution mooring and glider measurements. We show that wind‐driven frictional effects at the front trigger a symmetric instability, which leads to elevated upper‐ocean kinetic energy, re‐stratification and turbulent dissipation. The instability's extraction of kinetic energy from the front quantitatively matches the measured dissipation, which cannot be explained by classical one‐dimensional mixed layer processes. Our findings suggest that submesoscale symmetric instability may occur extensively in the relatively quiescent environment that characterizes the majority of the ocean, and point to the need of representing the instability's effects in climate‐scale ocean models.