ELLIPTIC WARM-CORE RINGS IN A 2-LAYER OCEAN WITH AMBIENT SHEAR

The rodon solution for an elliptical vortex with outcropping interface is extended from the reduced-gravity limit to the two-layer model. Motions in the lower layer consist of a reaction to the presence of the upper-layer vortex, or of an independent shear flow, or both. For simplicity and clarity, the analysis is performed in a geostrophic limit, which restricts the attention to vortices somewhat larger than the baroclinic radius of deformation. In a first part, geostrophic equations are established and discussed to identify clearly when the vortex motions induced in the lower layer and the ambient lower-layer shear must be retained or may be neglected. In the second part, the equations are solved for various cases. It is found that lower-layer motions induced by the upper-layer vortex provide a feedback on the latter and that this reaction is a mere increase of the anticyclonic rotation rate of the rodon. Furthermore, this increased rate has a straightforward interpretation in terms of potential vorticity conservation. When an additional shear flow in the lower layer is important, the upper-layer rodon can exhibit one of three behaviors: it can execute complete rotations, albeit at a varying angular rate, it can oscillate within two extreme orientations, or it can be sheared away with time. Results also show that anticyclonic shear flows (same sign as the rodon) have relatively benign effects unless they are very strong, while moderate cyclonic shear flows (opposite sign to the rodon) have very significant effects.