Observations suggest that vortex dipoles of mesoscale dimensions may provid
e a simple yet realistic representation of the structure and dynamics of je
t streaks in the extratropical upper troposphere. Moreover, the effects of
horizontal divergence in the vicinity of jet streaks often may be of second
ary dynamical importance, suggesting that a nondivergent barotropic framewo
rk may provide a logical starting point for an idealized investigation of j
et streaks.
Analytical solutions of barotropic vortex dipoles are shown to exhibit char
acteristic signatures similar to those identified in observational case-stu
dies of jet streaks. In addition to the dipole of relative vorticity, these
signatures include: (i) a localized maximum in fluid speed (i.e. a jet str
eak), (ii) ageostrophic flow that is directed towards lower pressure in the
entrance region and towards higher pressure in the exit region of the jet
streak, (iii) a four-cell pattern of ageostrophic vorticity that is cycloni
c in the entrance and exit regions and anticyclonic on the flanks of the st
reak, and (iv) a translation speed that is significantly slower than the ma
ximum fluid speed. On the basis of these similarities, it is suggested that
vortex dipoles provide a plausible dynamical representation of the structu
re and motion of jet streaks.
Nevertheless, vortex dipoles in isolation are unable to account for certain
observed features of jet streaks, such as the anisotropy of the wind held
in the along-stream direction and the asymmetry in the relative-vorticity h
eld, in which the cyclonic vortex typically is stronger than the anticyclon
ic vortex. Moreover jet streaks generally are not isolated, but are embedde
d in a larger-scale jet stream, which may be zonally varying or wavelike. A
nalytical and numerical solutions of barotropic vortex dipoles in the prese
nce of a variety of non-uniform background flows characteristic of the larg
e-scale extratropical circulation are shown to account for the above featur
es absent from dipoles in isolation. These solutions are also shown to prov
ide idealized depictions of the Life cycles of jet streaks in the extratrop
ical upper troposphere.