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.