The linear interference effect in stratosphere-troposphere coupling: analyzing the role of standing and travelling waves

Variations in the relative zonal phasing between a wave anomaly and the background stationary wave pattern—the “linear interference” effect—is known to explain a large part of planetary wave driving of the polar stratosphere in both hemispheres. While the linear interference effect is robust across observations, models of varying degrees of complexity, and in response to various types of perturbations, it is not well understood dynamically. In particular, an important open question is whether the interference is dominated by standing waves that amplify and attenuate the background pattern, or by travelling waves shifting into and out of phase with the background. Furthermore, the relatively long time-scale of the planetary waves involved in linear interference merits closer investigation. These questions are addressed using wavenumber-frequency spectral decompositions of the wave field into standing and travelling components. It is found that in observations there is a consistent relationship between the phase of the stationary wave pattern and the dominant standing waves, with the standing waves shifted eastward to the stationary waves, and having a more barotropic structure than the stationary waves. This finding is found, consistently, in high-top and low-top atmospheric models. The long timescales of the standing waves suggests possible routes to enhanced predictability of the stratospheric zonal wind anomalies involved in stratosphere-troposphere coupling. The implications for better understanding the wave driving events initiating stratosphere-troposphere coupling are discussed.