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 wh
en sheared environments are present are still not completely understood. Th
e interaction of binary vortices within horizontal environmental shear is e
xplored here through shallow water simulations on a beta plane. Due to nonl
inear feedbacks, the combination of environmental vorticity (or vorticity g
radient) and shear, as well as the multiple vortex situation, results in a
more complicated track than for a storm experiencing any individual compone
nt. Despite the complexity of these vortex-environment interactions, the us
e of previous single-vortex studies greatly aids interpretation. Centroid-r
elative motion of the individual vortices is considered, as well as the pro
pagation of the vortex pair centroid, to understand motion effects of the d
ifferent vortex-environment combinations.
As the vortices interact, vortex Rossby waves are generated through distort
ion of the symmetric vorticity field by the opposing vortex. Initially, the
high-frequency waves have an insignificant effect upon vortex intensity or
propagation, and beta-induced wavenumber one asymmetry dominates as expect
ed. However, as the waves approach a critical radius (zeta = 0), wave poten
tial vorticity filamentation and stretching by the circulation of the adjac
ent vortex leads to a coupling of the two vortices. This vortex coupling re
sults in enhanced propagation speeds of the two vortices proportional to th
e effective size of the dual-vortex system.
The sign of vorticity of the environmental flow can act to enhance or negat
e beta-drift such that single- or dual-vortex propagation is altered. Furth
er, 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.