Symmetry breaking and nonlinear transformation of two-layer eastward propagating dipoles

We study the evolution of eastward propagating dipoles (modons) in a two-layer quasi-geostrophic β-plane model using high-resolution numerical simulations. Various combinations of background gradients of potential vorticity in the upper and lower layers (which may include sloping topography) shed light on the recently discovered breakdown mechanisms and rich dynamics of dipolar vortices. We consider dipoles with a circular separatrix in the upper layer and, owing to the β-effect in the same layer, the symmetry of the flow breaks due to an exponentially growing, rotating, asymmetric mode of linear instability associated with Rossby wave radiation. Further nonlinear transformation is found to consist of two phases: fast partner separation, resulting in a deceleration of the eastward drift, and subsequent slow separation with a saturated asymmetric mode accompanied by much weaker, shorter Rossby waves. This weakly radiating phase—with pulsating partners and homogenized potential vorticity between the partners and the separatrix—can be considered as a new type of long-lived dipole. Conversely, when no β-effect is present in the upper layer, the dipoles remain nearly symmetric, even when the topographic β-effect is present in the active lower layer. In this case, the development of a weak asymmetric mode is related to a small meridional shift of the dipole center on the numerical grid.