Dynamical Forcing of Stratospheric Planetary Waves by Tropospheric Baroclinic Eddies.

The forcing of planetary wave variability in the stratosphere by synoptic-scale baroclinic eddies in the troposphere is considered. Simple forced‐dissipative numerical experiments are performed in a primitive equation model using a deep hemispheric model domain. The flow is thermally relaxed toward zonally symmetric notional wintertime conditions. No zonally asymmetric thermal or topographic forcing is applied. All planetary-scale zonal asymmetry arises solely through the nonlinear wave‐wave interaction of the baroclinic eddies in the troposphere. The numerical experiments indicate that realistic stratospheric planetary wave amplitudes and variability, comparable to those observed in the Southern Hemisphere, can be forced through this mechanism. No evidence is found in these simulations for planetary-scale disturbances arising through in situ instability in the stratosphere. The nonlinear tropospheric forcing mechanism in the numerical simulations is further investigated by reproducing the stratospheric planetary wave response with a linear model that is forced by the nonlinear eddy forcing that acted in the troposphere of the nonlinear simulation. The forced linear model experiments indicate that (i) as anticipated, both the eddy vorticity forcing and the eddy temperature forcing are required to account for the planetary wave response, (ii) only the low-frequency component of the nonlinear forcing is important, (iii) the vertical structure of the eddy forcing is equivalent to a compact source near tropopause level, and (iv) the variability of the planetary wave response in the stratosphere arises primarily from the variability of the nonlinear eddy forcing in the troposphere, rather than from the variability of the wave propagation characteristics associated with the basic-state zonally averaged flow. The eddy vorticity and eddy temperature forcing fields are combined into a single expression by introducing a transformation of the equations that govern the Fourier decomposition of deviations away from the zonally averaged flow, referred to as the transformed Fourier decomposition (TFD). The TFD transformation is essentially a generalization of that used in the transformed Eulerian mean formalism. The spatial and temporal characteristics of the total eddy forcing are then analyzed. The baroclinic eddies in the troposphere of the full simulation show strong organization into wave packets with a dominant wave-2 structure in amplitude. There is a strong, high-frequency, nonlinear wave-2 forcing associated with these packets. However, the propagation characteristics of the background flow in the simulation do not allow upward propagation of wave-2 disturbances with the corresponding frequency and there is little associated signal in the stratosphere. Experiments with a linear model, applying the same nonlinear forcing, show that there are background zonal flows, with plausibly realistic velocity fields, that allow upward propagation of such disturbances. It is therefore suggested that baroclinic wave packets may be an important mechanism for forcing higher-frequency wave-2 disturbances observed in the real Southern Hemisphere stratosphere. The lowfrequency stratospheric disturbances obtained in the nonlinear simulations appear to be associated with more subtle aspects of the baroclinic wave packets such as their spatial and temporal variability.