Simulations of dual-vortex interaction within environmental shear

For over a century it has been known that each vortex in a multiple vortex configuration will move in response to the other vortices. However, despite advances since that time, the complexities of multiple vortex scenarios when sheared environments are present are still not completely understood. The interaction of binary vortices within horizontal environmental shear is explored here through shallow water simulations on a β plane. Due to nonlinear feedbacks, the combination of environmental vorticity (or vorticity gradient) and shear, as well as the multiple vortex situation, results in a more complicated track than for a storm experiencing any individual component. Despite the complexity of these vortex-environment interactions, the use of previous single-vortex studies greatly aids interpretation. Centroid-relative motion of the individual vortices is considered, as well as the propagation of the vortex pair centroid, to understand motion effects of the different vortex-environment combinations. As the vortices interact, vortex Rossby waves are generated through distortion of the symmetric vorticity field by the opposing vortex. Initially, the high-frequency waves have an insignificant effect upon vortex intensity or propagation, and β-induced wavenumber one asymmetry dominates as expected. However, as the waves approach a critical radius (ζ = 0), wave potential vorticity filamentation and stretching by the circulation of the adjacent vortex leads to a coupling of the two vortices. This vortex coupling results in enhanced propagation speeds of the two vortices proportional to the effective size of the dual-vortex system. The sign of vorticity of the environmental flow can act to enhance or negate β-drift such that single-or dual-vortex propagation is altered. Further, when environmental vorticity is present, the rate of mutual orbit from Fujiwhara rotation is altered. When the environmental flow is cyclonic, the cyclonic mutual rotation of the vortices is accelerated. Conversely, when the environmental flow is anticyclonic, the mutual rotation of the vortices is substantially decelerated, but remains cyclonic.