The stratospheric influence on wave-mean flow interaction in the troposphere as seen in a quasi-linear vacillation model

The leading coupled mode of low-frequency variability in the north polar troposphere and lower stratosphere in winter, known as the Arctic Oscillation, has a robust signature in lower-tropospheric geopotential height, as shown by Thompson and Wallace. The near-surface pattern, like the stratospheric AO, is zonally symmetric to a first approximation. It was recently shown by Baldwin and Dunkerton that in northern winter, the warm phase of the Arctic Oscillation is closely related to stratospheric warming events. In both phases, the AO has a prevailing tendency to propagate downward from the mid- to lower stratosphere. The tropospheric AO anomaly and its near-surface component approximately coincide with the arrival of the downward-propagating AO signature at the tropopause.

These observations have inspired an investigation of the dynamics of stratospheric vacillation cycles in a new type of vacillation model. The model used in our experiments is a sigma-coordinate primitive equation model run with spectral truncation at a single zonal wavenumber, and with disturbances 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 that arise by allowing the disturbance and mean flow to vary in latitude as well as altitude allows horizontal wave propagation to play a leading role in the vacillation dynamics.

There are many components to a vacillation's anatomy which fit together in a complex way. How the vacillation depends on such parameters as the structure of the time-mean wind field, the forcing strength and location, and the specification of thermal damping will be explored. The dynamics of the vacillation may be understood in terms of how the mean flow, meridional circulation, and surface pressure respond to time-varying horizontal wave flux divergences, and in terms of how the wave propagation responds to a time-varying mean flow. It will be shown that the wave propagation characteristics throughout the domain are primarily governed by the large mean flow variations in the upper stratosphere. The resulting vacillations in the wave fluxes impose a significant variation in the troposphere flux divergences, and give rise to a mode of variability in the troposphere mean flow that resembles the Arctic Oscillation. The variation in the troposphere winds in turn modulates the horizontal propagation direction of the waves emanating form the source region, and thereby contributes to the timing of the vacillation. The troposphere and stratosphere therefore work together to produce the model vacillation, but the vacillation is primarily controlled in the upper stratosphere via periodic reversals of momentum flux as the waves alternate between poleward and equatorward propagation. The vacillation ceases completely if variability is inhibited through artificial damping above 40 km.