Overturning Instabilities Across a Warm Core Ring From Glider Observations

This work presents new evidence for conducive conditions for the existence of overturning instabilities in the mixed layer across a mesoscale anticyclonic Loop Current Eddy (LCE) and surrounding cyclonic eddies in the Gulf of Mexico (GoM). The LCE was intensively sampled using four gliders during 12 months. The LCE is characterized by a strong anomaly of fresh and warm water in the center and strong strain at the periphery. Mixed layers prone to overturning instabilities are diagnosed from the gradients of buoyancy. In the GoM, the reduction of potential vorticity (PV) in the mixed layer is driven by surface cooling by year‐round persistent negative turbulent heat fluxes, eventually reaching −700 W m−2 during extreme winter wind events. Lateral buoyancy gradients associated with submesoscale filaments and fronts at the periphery of the LCE enhance the frictional torque supplied by the wind stress when aligned with the geostrophic jet current. Negative PV in the mixed layer occurs simultaneously with negative turbulent heat fluxes and events of Ekman buoyancy fluxes that reduce the stratification. These regions are susceptible to mixed‐layer gravitational‐symmetric instability preferentially in winter. Extreme Ekman equivalent heat flux (≤−10,000 W m−2) associated with wind stress of 1.2 N m−2 coincides with the development of inertial‐symmetric instability across anticyclonic vorticity. These observations might have several implications for the restratification processes, kinetic energy budget, and biogeochemical cycles inside LCEs and can contribute to the formation of low‐PV waters that could feed mode water formation inside the GoM. Plain Language Summary Recent computational simulations of ocean circulation have shown intense vertical velocities associated with the so‐called “submesoscale” currents as eddies, fronts, and filaments, occurring at horizontal scales of 1–10 km and life spans of 1 day. Fine‐scale temperature and salinity measurements obtained using autonomous underwater vehicles, called gliders, in the Gulf of Mexico (GoM), are used to identify the times and locations where overturning instabilities modulate the ocean–atmosphere exchanges. These instabilities are enhanced inside eddies ubiquitous in the Gulf and at their periphery forced by strong winds during winter. A high‐resolution model of the circulation of the GoM confirms the observations for contrasting wind conditions. These submesoscale diagnostics might be relevant to understand