Abstract
A potent mechanism for the generation of low-frequency atmospheric variability on vortex basic states consisting of a single potential vorticity jump, or contour, separating two regions of uniform equivalent barotropic potential vorticity is described. Such basic states represent in a simple manner the potential vorticity distribution of the extratropical upper troposphere. It is shown that the group velocity for stationary waves propagating on such states can vanish for realistic zonal variations in the basic-state flow along the vortex edge, leading to local exponential disturbance growth due to the accumulation of wave action. Further, pseudo-energy stability criteria are derived that suggest that exponentially growing global disturbances are possible for sufficiently strong zonal variations in the flow along the vortex edge.
These predictions are examined using linear and nonlinear initial value problem calculations. For wavenumber-1 flow variations in the basic-state zonal flow along the vortex edge, no global instability occurs. However, strong local disturbance growth in response to weak stationary forcing does occur and can lead to irreversible deformation of the vortex. For wavenumber-2 and higher variations in the basic-state zonal flow along the vortex edge, global instability occurs if the stability criteria is violated. These instabilities have peak dimensional e-folding times on the order of one week, with faster growth rates corresponding to stronger zonal variations in the flow along the vortex edge. Quantization of the zonal scale of amplifying disturbances occurs, indicating disturbance resonance with the underlying zonal variations in the basic-state flow along the vortex edge. In the nonlinear regime, longer wavelength disturbances lead to large amplitude periodic fluctuations of the vortex. Intermediate wavelength disturbances are shown to yield suprisingly realistic blocking events, while short wavelength disturbances saturate at amplitudes too small to change the overall structure of the vortex.
The pervasiveness of instability in this simple system suggests similar processes may be important for blocking transitions and the generation of low-frequency variability in the extratropical atmosphere. Preliminary results from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis show that the climatological 330-K isentropic potential vorticity is accurately characterized as the time average of a fluctuating single-contour vortex. Wave action conservation on basic states constructed using dynamical fields on the 330-K isentropic surface reproduces observed shifts in low-frequency variability that occur during the El Niño cycle. Further, these shifts lead to transient-driven time mean flow anomalies that have a teleconnection pattern-like structure, despite the fact that meridional propagation of waves is forbidden in this system. The ability of this system to accurately simulate diverse atmospheric phenomena as well as explain certain aspects of upper-tropospheric dynamics suggests that it may provide a powerful new paradigm with which to view low-frequency dynamics in the climate system.
Corresponding author address: Kyle Swanson, Department of Mathematical Sciences, University of Wisconsin—Milwaukee, Milwaukee, WI 53201.
Email: kswanson@csd.uwm.edu
