Stratospheric vacillation and the dynamics of annular mode variability

The leading coupled mode of low-frequency variability in the troposphere and stratosphere during northern winter is the northern annular mode, or NAM. The near-surface manifestation of the NAM is the so-called Arctic Oscillation, or AO. In both positive and negative phases, NAM anomalies have a prevailing tendency to migrate downward from the mid- to lower stratosphere. The tropospheric NAM and its near-surface AO component coincide with the arrival of a downward-migrating NAM anomaly at the tropopause. These observations have inspired an investigation of stratospheric vacillation in a new type of model, a sigma-coordinate primitive equation model run with spectral truncation at a single zonal wavenumber. Disturbances are forced by an interior potential vorticity source intended to simulate either asymmetric heating or wave generation resulting from baroclinic disturbances. The additional degrees of freedom obtained by having the disturbance and mean flow vary in latitude, as well as altitude, allows horizontal wave propagation to play a leading role in the vacillation dynamics. Our model experiments produce poleward and downward propagating mean-flow anomalies associated with stratospheric warmings similar to those observed in the atmosphere. Experiments designed to isolate the various dynamical components show 1) how the vacillation depends on the wave refraction properties of the mean flow in the troposphere and lower stratosphere; 2) that the mean flow variations in the troposphere are driven by wave-flux anomalies in the lower stratosphere, communicated downward to the surface by an induced mean meridional circulation, with nonzero surface pressure anomaly as implied by the theory of Haynes and Shepherd; and 3) wave reflection, from critical layers that form in subtropical and polar regions near the stratopause during the course of a stratospheric warming, modulates wave flux anomalies throughout the lower atmosphere. Transience of wave flux anomalies is largely responsible for the mean-flow evolution seen in the model, and the temporal development of flux anomalies gives rise to the downward migration of mean-flow anomalies. In observations, the tropospheric NAM is accompanied by variations in transient-eddy fluxes of heat and momentum which create a positive feedback, acting to maintain the current state of the NAM. The role of transient-eddy feedback in the simulated quasi-linear vacillation cycle is examined.