Mechanisms for Wave Packet Formation and Maintenance in a Quasigeostrophic Two-Layer Model

Abstract

A quasigeostrophic, two-layer, ?-plane channel model is used to investigate the dynamics of baroclinic wave packets. A series of experiments are performed in which an unstable flow is maintained by lower-level Ekman friction and radiative relaxation toward a temperature profile that corresponds to a broad parabolic upper-level jet. The final statistically steady state achieved in each experiment is found to depend on the magnitude of the hyperdiffusivity ?0 and the supercriticality, which is controlled by ?. The most important qualitative difference in such states between experiments is found to be the degree to which a waveguide in the upper level is found to develop. The mechanism for this upper-level waveguide development is the mixing effect of the eddies at the flanks of the jet, which leads to a strong potential vorticity gradient at the center of the channel, with well-mixed regions to the north and south. Two distinct regimes with different qualitative behavior are observed and illustrated by two particular experiments. In the first regime strong hyperdiffusivity inhibits the development of the waveguide. Steady wave packets are shown to stabilize the background flow upstream by increasing the meridional shear of the jet. This upstream stabilization is argued to be a mechanism for packet maintenance in this regime. In the second regime the diffusivity is lower, and a well-developed upper-level waveguide results. The wave packets in this regime are unsteady and are shown to stabilize the background flow at, and slightly upstream of, their maxima. Wave activity diagnostics suggest that the most important mechanism in maintaining these packets is the zonal convergence of wave activity, indicating that the wave packets are undergoing a form of nonlinear self-focusing, analogous to that identified in weakly nonlinear models. Finally, results are presented from a 10-level primitive equation model with parameter values relevant to the real atmosphere. In this experiment the nonlinear response of the background flow to the wave packets is shown to be qualitatively very similar to that observed in the low-diffusivity two-layer model experiment.