Asymmetry of Submesoscale Instabilities in Anticyclonic and Cyclonic Eddies

The upper-ocean relative vorticity has been found to be cyclonically skewed, but altimetry observations indicate that long-lifespan mesoscale eddies tend to be anticyclonic. We are thus interested in whether cyclonic or anticyclonic eddies are more unstable under similar circumstances. Here we use submesoscale-resolving simulations of idealized mesoscale eddies, incorporating theoretical analyses, to investigate asymmetries of submesoscale instabilities within the anticyclones and cyclones. It is found that submesoscale filaments initiate at regions with the largest horizontal buoyancy gradients for both anticyclones and cyclones, but these filaments subsequently rotate outward in anticyclones while inward in cyclones. Hence submesoscales are more vigorous at anticyclone peripheries and the cyclone center. Such differing distributions and evolutions of submesoscale processes are primarily caused by changes in the background stratification associated with the decaying of mesoscale eddies. The active submesoscales near the cyclone center eventually distort its core structure radically, whereas the anticyclone remains largely unaffected. Previous studies found that flows with large anticyclonic vorticity, which is negative (positive) in the Northern (Southern) Hemisphere, tend to be suppressed from further growing due to developed instabilities. Hence large-magnitude vorticity in the real ocean is more likely to be cyclonic than anticyclonic. However, satellite observations reveal that there are more anticyclonic than cyclonic eddies that live longer in time and also propagate farther in distance. It naturally raises a question whether cyclonic or anticyclonic eddies are more unstable to instabilities. In this study, by conducting high-resolution numerical simulations of idealized mesoscale eddies, we find profound asymmetries in the spatial distributions and time evolutions of smaller-scale features (mainly submesoscale filaments) within the simulated cyclonic and anticyclonic eddies. The submesoscale filaments are first generated at regions with the largest horizontal gradients of density for both eddies, but their subsequent evolutions are different. Submesoscale filaments migrate outward within anticyclonic eddies while inward within the cyclonic eddies. Because of this evolution asymmetry, the core structure of the cyclonic eddies is significantly distorted after a certain period whereas the anticyclonic eddies keep largely unaffected. This is perhaps the reason why anticyclonic eddies tend to live longer in time. Distributions and evolutions of submesoscale features manifest prominent asymmetries in anticyclonic and cyclonic eddiesSubmesoscale filaments are generated in eddy-front area for both types of eddies but migrate outward (inward) for anticyclones (cyclones)Submesoscale asymmetries in anticyclones and cyclones are mainly determined by changes in mesoscale (background) stratification