Stratosphere-troposphere coupling in a relatively simple AGCM: The importance of stationary planetary waves

The impact of topographically forced stationary waves in the dynamical coupling between the stratosphere and the troposphere is explored in the simple atmospheric General Circulation Model similar to the one by Polvani and Kushner (2002, GRL). We find that the mean structure and variability of the polar vortex in this model is very sensitive to the amplitude of the topography, and Northern Hemisphere-like variability, with the correct frequency of warming events, occurs only for a relatively narrow range of topographic heights. A variable polar vortex (i.e. one associated with realistically frequent sudden warmings) is less coupled to the tropospheric jet than in the absence of topographic forcing, suggesting that stationary waves play a critical role in stratosphere-troposphere interactions and need to be included, even in idealized models. The frequency of sudden warming events and the timescale at which events descend through the stratosphere -- and sometimes into the troposphere -- is explored, and contrasted with the variability observed in stratosphere only model with a fixed stationary wave at the tropopause.