Resistance of a Coherent Vortex to a Vertical Shear

Abstract

The authors study the splitting of a coherent vortex by a large-scale baroclinic background current. A criterion for the splitting of the vortex is defined, and the process is then studied numerically and analytically in a 2½-layer reduced-gravity model in which the vortex is represented by a potential vorticity (PV) patch in each layer. Three effects are important for the process: 1) the vortex ?coherence,? which is a measure of the advective effect induced by the PV patches on each other; 2) the background current shear, which tears the vortex; and 3) the baroclinic ? effect, associated with the background current PV gradient, which is shown to counteract the shear. When the baroclinic ? effect is neglected, it is shown that PV patches oscillate around an equilibrium state, and they separate when the oscillation amplitude is larger than the splitting criterion. This model also shows that vortex core deformations play a (moderate) role when the vortex radius is larger than the first baroclinic radius of deformation. The baroclinic ? effect substantially compensates the advective tearing and drastically reduces the oscillation amplitude. Thus, the vortex is able to resist much higher shear when the current PV gradient is taken into account. On the other hand, the baroclinic ? effect also induces dispersion of the vortex, which is essential when the shear is strong enough. It is shown that, in fact, a vortex is generally scattered by Rossby waves before it is split.