Submesoscale Instabilities in Mesoscale Eddies

Abstract

Submesoscale processes have been extensively studied in observations and simulations of fronts. Recent idealized simulations show that submesoscale instabilities also occur in baroclinic mesoscale cyclones and anticyclones. The instabilities in the anticyclone grow faster and at coarser grid resolution than in the cyclone. The instabilities lead to larger restratification in the anticyclone than in the cyclone. The instabilities also lead to changes in the mean azimuthal jet around the anticyclone from 2-km resolution, but a similar effect only occurs in the cyclone at 0.25-km resolution. A numerical passive tracer experiment shows that submesoscale instabilities lead to deeper subduction in the interior of anticyclonic than cyclonic eddies because of outcropping isopycnals extending deeper into the thermocline in anticyclones. An energetic analysis suggests that both vertical shear production and vertical buoyancy fluxes are important in anticyclones but primarily vertical buoyancy fluxes occur in cyclones at these resolutions. The energy sources and sinks vary azimuthally around the eddies caused by the asymmetric effects of the Ekman buoyancy flux. Glider transects of a mesoscale anticyclone in the Tasman Sea show that water with low stratification and high oxygen concentrations is found in an anticyclone, in a manner that may be consistent with the model predictions for submesoscale subduction in mesoscale eddies.

The Seasonality of Physically Driven Export at Submesoscales in the Northeast Atlantic Ocean

“…Brannigan () and Brannigan et al () predict that symmetric instabilities will be strongest near the periphery of anticylonic eddies. We use sea level anomaly measurements from AVISO (Archiving, Validation and Interpretation of Satellite Oceanographic data), where positive (negative) values of sea level anomaly correspond to cyclones (anticylones; Figure c, black).…”

“…Increasing (decreasing) will deepen (shoal) the mixed layer, and indeed short-lived deepenings of the mixed layer in May and June correspond to increases in (Figure 9a). Brannigan (2016) and Brannigan et al (2017) predict that symmetric instabilities will be strongest near the periphery of anticylonic eddies. We use sea level anomaly measurements from AVISO (Archiving, Validation and Interpretation of Satellite Oceanographic data), where positive (negative) values of sea level anomaly correspond to cyclones (anticylones; Figure 9c, black).…”

Mesoscale and Submesoscale Contributions to High Sea Surface Chlorophyll in Subtropical Gyres

“…Moreover, the spatial resolution of the data (~4 km) is still not fine enough to resolve all the submesoscale structures. According to the simulation in Brannigan et al (), both the relative vorticity and the surface temperature in eddies show increasing numbers of small‐scale structures as the model resolution increases from 4 to 0.25 km. Thus, with higher‐resolution data, we would expect greater contributions from submesoscale structures to high CHL.…”

High‐Resolution Observations and Tracking of a Dual‐Core Anticyclonic Eddy in the Algerian Basin

“…Alternatively, these observations are consistent with a study that showed, based on measurements and modeling, small‐scale variations in strain, divergence, and vorticity in the eddy core (Zhong et al, ). A numerical study (Brannigan et al, ) showed similar vertical structure of a passive tracer in the eddy core with a decrease in grid size to 250 m. Both studies attributed the small‐scale vertical structures to mixed layer instability and ageostrophic dynamics.…”

“…The mesoscale patterns of eddies has been extensively observed, but the nonlinear dynamics and the interactions with the atmosphere generating finer‐scale motions have mainly been explored only numerically and theoretically. Recent high‐resolution numerical modeling of eddies have described submesoscale processes (Brannigan, ; Brannigan et al, ; Zhong et al, ). Likewise, observed eddies appear often more complex than expected.…”