Analysis of Unforced Symmetric Instability in Mesoscale Eddies Using In Situ Observations

Symmetric instability (SI) is known to be effective in extracting kinetic energy from currents in frontal regions. This energy cascade involving SI has previously been observed at fronts at the sea surface, with wind forcing as the chief catalyst for destabilization. In this article, using in situ observations collected during the Elucidating the Role of Clouds–Circulation Coupling in Climate-Ocean–Atmosphere (EUREC4A-OA) cruise, we argue that North Brazil Current (NBC) rings could have been subject to symmetric instabilities using classical mathematical criteria even if they lie below the pycnocline. In particular, we demonstrate that the boundaries of mesoscale eddies could be the loci of unforced centrifugal–symmetric instabilities (CSIs) depending on the slope of isopycnal surfaces or, equivalently, the relative magnitudes of vertical and lateral stratifications. Assuming a gradient wind balance, this ratio implicitly accounts for curvature. We additionally analyze their primary sources of kinetic energy, accounting for the curvature. This analysis suggests that horizontal shear is likely the main driver of the disturbance’s development. Finally, we estimate the characteristic time scale for the development of CSI and find that unforced symmetric modes may emerge within less than 1.5 days. Together, our results suggest that mesoscale eddies in the NBC may be intrinsically subject to symmetric instabilities owing to the centrifugal force, with potential implications for the life cycle of mesoscale eddies and transport of active/passive tracers by these oceanic phenomena.