CORRESPONDING AND PRESENTING AUTHOR:
Philip Cunningham
TITLE:
Graduate Student
AFFILIATION:
Department of Atmospheric Science,
State University of New York at
Albany, 1400 Washington Avenue, ES-329, Albany, NY 12222
PHONE NUMBER: (518) 442-4923
EMAIL:cunning@atmos.albany.edu
An analytical solution for a point-vortex dipole in a jet-like zonal background flow with piecewise uniform potential vorticity suggests that the presence of this background flow enhances the anisotropy of the dipole streamfunction. This suggestion is supported by numerical integration of vortex dipoles in a jet-like zonal flow. In this case, approximately steady dipole solutions are shown to exist that are significantly more anisotropic than the vortex dipoles in isolation. Since the large-scale flow often is wavelike, however, we also examine numerical simulations of symmetric vortex dipoles in a large-scale Rossby wave. These simulations reveal that as the dipole travels through the wave, its component vortices are strengthened selectively depending on whether the dipole is located in the trough or the ridge. This behaviour suggests a simple way to view the asymmetry of the relative vorticity field: as with the anisotropy of the wind field, the relative vorticity asymmetry is controlled by the background flow. Furthermore, these simulations depict idealised jet-streak life cycles that exhibit similarities to the evolution of jet streaks in the atmosphere.
While it is instructive to view the relative vorticity asymmetry as being controlled by the background flow, observations suggest that the vortex dipoles associated with jet streaks may be significantly asymmetric even in symmetric background flows, and in some cases the relative vorticity field may be monopolar. Numerical simulations of asymmetric vortex dipoles in jet-like zonal background flows that are themselves asymmetric are shown to exhibit complex evolutions. Despite this complexity, the characteristic structures in these evolutions appear to have observational support in atmospheric water vapour imagery. Future work addresses the relevance of monopolar vortices to jet streaks, the interpretation of jet-streak life cycles as details of balanced turbulence, and possible extensions to a stratified baroclinic atmosphere.