The Role of Planetary Waves in the Downward Influence of Stratospheric Final Warming Events

Stratospheric final warming events are simulated in an idealized atmospheric model by imposing a winter-to-summer transition in radiative equilibrium temperature only in the stratosphere. Large ensembles of events are simulated with different strengths of topographic forcing. It is found that the dates of final warmings become earlier and their downward influence on the troposphere becomes stronger for greater topographic amplitudes. This result is similar to observed differences between the downward influence of the final warming in the Northern and Southern Hemispheres. The mechanisms through which the final warming exerts its influence on the tropospheric circulation are investigated using a zonally symmetric model. It is found that lower-stratospheric wave driving induces a residual circulation that affects the tropospheric circulation. The stratosphere also affects the propagation of planetary waves in the upper troposphere, resulting in a burst of wave activity and a rapid deceleration of tropospheric zonal winds at the time of the final warming. These results highlight the important roles of planetary waves in the downward influence of the stratospheric final warming events.