Overturning Instabilities Across a Warm Core Ring From Glider Observations

Abstract

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.

Diurnal Variability of Mixed Layer Overturning Instabilities From Glider Array Observations in the South China Sea

“…This is different from the GI-dominated nature in the winter deep ML (Buckingham et al, 2019;X. Yu et al, 2019) and anticyclonic eddies (Dong et al, 2022;Perez et al, 2022), highlighting SI as the dominant instability type in our observations.…”

Submesoscale Motions Driven by Down‐Front Wind Around an Anticyclonic Eddy With a Cold Core

“…These positive Q MLI had a significant role in the restratification of the frontal region, even though they were overall weaker than the magnitude of Q EBF , which is consistent with the numerical simulation of submesoscale processes around mesoscale eddy by Pérez et al. (2022). As for Q TTW , it was enhanced at Periphery I and Periphery III, exceeding 200 W m −2 and 400 W m −2 , respectively (Figure 10).…”

“…The observation modes of the two missions are traveling and stationary, respectively. The traveling glider navigates over a wide area of space (usually >20 km), capturing mesoscale eddies more proactively (Pérez et al., 2022; Viglione et al., 2018). The stationary glider navigates in a fixed small area (<20 km), can continuously observe the dynamic characteristics of a particular small area (Thompson et al., 2016; Yu et al., 2019).…”

“…Additionally, the deformation of these eddies often leads to strain‐induced frontogenesis, further driving submesoscale motions within the mesoscale eddy region (Siegelman et al., 2020; Zhong et al., 2017). While submesoscale motions are most prominent at the periphery of mesoscale eddies, their distribution varies across different peripheries due to atmospheric forcing, including cooling and wind stress (Adams et al., 2017; Pérez et al., 2022). These atmospheric forcing trigger submesoscale motions that transform vertical geostrophic shear into eddy kinetic energy within the mixed layer (ML) of mesoscale eddies, contributing to turbulent dissipation (Dong et al., 2022).…”

The Energy Decay of Warm‐Core Eddies in the Gulf of Mexico

“…Beyond vortex‐splitting and topographic effects, many processes can contribute to the decay of a mesoscale eddy. These include frontal instability (Brannigan et al., 2017; Pérez et al., 2022), interaction with submesoscale eddies (de Marez et al., 2020; Jouanno et al., 2016; Tedesco et al., 2019), mesoscale straining (Mariotti et al., 1994), layering and double diffusion (Armi et al., 1989; Meunier et al., 2015; Middleton et al., 2021; Schmitt et al., 1986), interaction with internal gravity waves (Joyce et al., 2013; Kunze, 1986; Polzin, 2008), surface heat fluxes (Dewar, 1987), or wind‐current interactions (Dewar & Flierl, 1987; Duhaut & Straub, 2006; Renault, Molemaker, Gula, et al., 2016).…”