Vortex evolution by straining and development of anticyclonic vortex dominance

A solution for decaying geostrophic turbulence in a fully three dimensional model with finite Rossby number is shown. It exhibits a strong asymmetry in its coherent vortex dynamics: anticyclones on average become stronger and larger, and proceed more rapidly in their merger and alignment interactions. This anticyclonic dominance is also seen in three different vortex experiments on both cyclones and anticyclones: when subjected to either a retrograde or prograde shearing flow, when perturbed from an axisymmetric state, or when engaged in a merger with another like signed vortex. The controlling mechanisms are vortex weakening under straining deformation (with a weakening that is substantially greater for strong cyclones than strong anticyclones) followed by a partially compensating vortex strengthening during a relaxation phase dominated by Vortex Rossby Waves (VRWs) and their eddy-mean interaction with the vortex. The outcome is a net, strain-induced vortex weakening that is greater for cyclones than anticyclones when the deformation radius is not large compared to the vortex radius and the Rossby number is not small. It therefore seems likely that, in a complex flow with many vortices, anticyclonic dominance develops over a sequence of transient mutual staining events due to the greater robustness of anticyclones.