Published online by Cambridge University Press: 17 July 2017
Long-term marginal stability of a new family of isolated oceanic vortices is analysed. Sign reversal of the radial gradient of the potential vorticity anomaly, as implied by the isolation requirement, leads to vortex unsteadiness but does not break the coherence of the vortex, which remains marginally stable even for high absolute Rossby numbers $Ro\simeq 0.8$ . The marginally stable vortices are characterized by a zero amount of potential vorticity anomaly on every isopycnal. The marginally stable final state is an unsteady vortex whose inner one-signed potential vorticity anomaly experiences revolution, rotation, precession and nutation.
Time evolution of potential vorticity anomaly isosurfaces PVA=-0.2 (blue) and PVA=0.14 (grey). PVA contours at z=0, from PVA=0.01 to PVA=0.18, are included. The colour distribution corresponds to PVA at z=0.