The nonlinear evolution of two surface quasi-geostrophic vortices
Résumé
Using a numerical model of the two-level surface quasi-geostrophic equations, we investigate the evolution of two circular temperature patches, one located at each level. We vary the vortex intensities, radii, and the vertical distance between the two levels. We also study different radial profiles of temperature for each vortex. This paper considers two main situations: (initially columnar) vortices with like-signed buoyancies which are linearly unstable and may break, and initially tilted vortices with opposite-signed buoyancies, which may align vertically. Numerical experiments show that (1) identical contra-rotating vortices break into hetons when initially perturbed by low azimuthal modes; (2) asymmetric vortices stabilise nonlinearly more often and can form quasi-steady baroclinic tripoles; (3) co-rotating vortices can align when the two fluid levels are close to each other vertically, and when the vortices are initially distant from one another by less than three radii; (4) for initially more distant vortices, the two vortices rotate around the plane center;
(5) in all cases, the vortex contours are disturbed by Rossby waves. These results compare favorably to earlier results with internal quasi-geostrophic models, but apply more easily to near surface dynamics in the ocean. Further modelling efforts may extend the present study to fully three dimensional ocean dynamics.